HARVARD UNIVERSITY Library of the Museum of Comparative Zoology ... coMP.-0OL IRE AT BASIN NATURALIST MEMOIR ber 4 Brigham Young University 1 FEI Soil-Plant-Animal Relationships Bearing on Revegetation and Lane Reclamation in Nevada Deserts GREAT BASIN NATURALIST Editor. Stephen L. Wood, Department of Zoology, Brigham Young University, Provo, Utah 84602. Editorial Board. Kimball T. Harper, Botany; Wilmer W. Tanner, Life Science Museum; Stanley L. Welsh, Botany; Clayton M. White, Zoology. Ex Officio Editorial Board Members. A. Lester Allen, Dean, College of Biological and Agricul- tural Sciences; Ernest L. Olson, Director, Brigham Young University Press, University Editor. The Great Basin Naturalist was founded in 1939 by Vasco M. Tanner. It has been published from one to four times a year since then by Brigham Young University, Provo, Utah. In gener- al, only previously unpublished manuscripts of less than 100 printed pages in length and per- taining to the biological and natural history of western North America are accepted. The Great Basin Naturalist Memoirs was established in 1976 for scholarly works in biological natu- ral history longer than can be accommodated in the parent publication. The Memoirs appears irregularly and bears no geographical restriction in subject matter. Manuscripts are subject to the approval of the editor. Subscriptions. The annual subscription to the Great Basin Naturalist is $12 (outside the United States $13). The price for single numbers is $4 each. All back numbers are in print and are available for sale. All matters pertaining to the purchase of subscriptions and back num- bers should be directed to Brigham Young University, Life Science Museum, Provo, Utah 84602. The Great Basin Naturalist Memoirs may be purchased from the same office at the rate indicated on the inside of the back cover of either journal. Scholarly Exclianges. Libraries or other organizations interested in obtaining either journal through a continuing exchange of scholarly publications should contact the Brigham Yoimg University Exchange Librarian, Harold B. Lee Library, Provo, Utah 84602. Manuscripts. All manuscripts and other copy for either the Great Basin Naturalist or the Great Basin Naturalist Memoirs should be addressed to the editor as instructed on the back 10-80 1.7M 46937 '22 JREAT BASIN NATURALIST MEMOIR Brigham Young University Soil-Plant-Animal Relationships Bearing on Revegetation and LanC Reclamation in Nevada Deserts CONTENTS Preface. Arthur Wallace 1 Phenology of desert shrubs in southern Nye County, Nevada. T. L. Ackerman, E. M. Romney, A. Wallace, and J. E. Kinnear 4 Residual effects of supplemental moisture on the plant populations of plots in the north- ern Mojave Desert. R. B. Hunter, E. M. Romney, A. Wallace, and J. E. Kinnear 22 The pulse hypothesis in the establishment of Artemisia seedlings at Pahute Mesa, Ne- vada. E. M. Romney, A. Wallace, and R. B. Hunter 26 The role of pioneer species in revegetation of disturbed desert areas. A. Wallace and E. M. Romney 29 Frequency distribution of numbers of perennial shrubs in the northern Mojave Desert. A. A. El-Ghonemy, A. Wallace, and E. M. Romney 32 Further attributes of the perennial vegetation in the Rock Valley area of the northern Mojave Desert. S. A. Bamberg, A. Wallace, E. M. Romney, and R. E Hunter 37 Multivariate analysis of the vegetation in a two-desert interface. A. A. El-Ghonemy, A. Wallace, and E. M. Romney 40 A phytosociological study of a small desert area in Rock Valley, Nevada. A. A. El-Gho- nemy, A. Wallace, E. M. Romney, and W. Valentine 57 Socioecological and soil-plant studies of the natural vegetation in the northern Mojave Desert-Great Basin desert interface. A. A. El-Ghonemy, A. Wallace, and E. M. Romney 71 Frequency distribution of three perennial plant species to nearest neighbor of the same species in the northern Mojave Desert. A. Wallace, E. M. Romney, and J. E. Kin- near 87 Relationship of small washes to the distribution of Lycium andersonii and Larrea triden- tata at a site in the northern Mojave Desert. A. Wallace, E. M. Romney, and R. B. Hunter 92 Regulative effect of dodder (Cuscuta nevadensis Jtn.) on the vegetation of the northern Mojave Desert. A. Wallace, E. M. Romney, and R. B. Hunter 96 Photosynthetic strategies of two Mojave Desert shrubs. G. E. Kleinkopf, T. L. Hartsock, A. Wallace, and E. M. Romney 98 Transpiration and C02 fixation of selected desert shrubs as related to soil-water poten- tial. S. B. Clark, J. Letey, Jr., O. R. Lunt, A. Wallace, G. E. Kleinkopf, and E. M. Romney 108 Effect of certain plant parameters on photosynthesis, transpiration, and efficiency of water use. H. M. Mork, A. Wallace, and E. M. Romney 115 Carbon fixed in leaves and twigs of field Larrea tridentata in two-hour exposure to KX)2. A. Wallace, E. M. Romney, and R. B. Hunter 1 19 The role of shrubs on redistribution of mineral nutrients in soil in the Mojave Desert. E. M. Romney, A. Wallace, H. Kaaz, and V. Q. Hale 122 Ecotonal distribution of salt-tolerant shrubs in the northern Mojave Desert. E. M. Rom- ney and A. Wallace 132 Parent material which produces saline outcrops as a factor in differential distribution of perennial plants in the northern Mojave Desert. A. Wallace, E. M. Romney, R. A. Wood, A. A. El-Ghonemy, and S. A. Bamberg 138 Frequency distribution and correlation among mineral elements in Lycium andersonii from the northern Mojave Desert. A. Wallace, E. M. Romney, G. V. Alexander, and J. E. Kinnear 144 Mineral composition of Atriplex hymenelytra growing in the northern Mojave Desert. A. Wallace, E. M. Romney, R. B. Hunter, J. E. Kinnear, and G. V. Alexander 154 Field studies of mineral nutrition of Lama tridentata: importance of N, pH, and Fe. R. B. Hunter, A. Wallace, and E. M. Romney 161 Retranslocation of tagged carbon in Ambrosia dumosa. A. Wallace, J. W. Cha, R. T. Mueller, and E. M. Romney 166 Persistence of 14C labeled carbon in Larrea tridentata up to 40 months after photo- synthetic fixation in the northern Mojave Desert. A. Wallace, E. M. Romney, and J. W. Cha 170 14C distribution in roots following photosynthesis of the label in perennial plants in the northern Mojave Desert. A. Wallace, R. T. Mueller, J. W. Cha, and E. M. Romney 175 Distribution of photosynthetically fixed 14C in perennial plant species of the northern Mojave Desert. A. Wallace, J. W. Cha, and E. M. Romney 190 Depth distribution of roots of some perennial plants in the Nevada Test Site area of the northern Mojave Desert. A. Wallace, E. M. Romney, and J. W. Cha 199 Rodent-denuded areas of the northern Mojave Desert. R. R. Hunter, E. M. Romney, and A.Wallace 206 Fencing enhances shrub survival and growth for Mojave Desert revegetation. R. R. Hunter, A. Wallace, and E. M. Romney 210 The challenge of a desert: revegetation of disturbed desert lands. A. Wallace, E. M. Romney, and R. R. Hunter 214 Great Basin Naturalist Memoirs Soil-Plant-Animal Relationships Bearing on Revegetation and Land Reclamation in Nevada Deserts No. 4 Brigham Young University, Provo, Utah 1980 PREFACE Arthur Wallace' Disturbed lands in desert ecosystems may require decades or centuries for natural re- turn to their original condition. The fragile nature of deserts due to hostile climate par- tially explains this reclamation problem that investigators and developers are now faced with because of new governmental regu- lations. This series of 30 papers relates to efforts to develop information which can be used ei- ther to prevent needless destruction of desert systems or to help restore disturbed lands to their original condition. The studies involved cover a period of sev- eral years. Included were those years during which the International Biological Program, through the National Science Foundation, participated in desert ecosystems studies. The goals of that program included those of pres- ervation, use, and restoration of deserts. The Nevada Operations Office of the Department of Energy (formerly Atomic Energy Commis- sion and Energy Research and Development Administration) for the past decade has been vitally concerned about problems related to cleanup of some soils contaminated with ra- dionuclides. Any cleanup operation would drastically alter natural ecosystems, possibly resulting in problems more difficult to solve than the original ones. Ongoing environmen- tal and ecological studies at the Nevada Test Site have been made by members of our group since 1960. The present 30 papers resulting from those studies can be divided into six groupings. The first group consists of a single paper that de- scribes the amazing amount of variability en- countered from year to year in the phenolo- gical events of the perennial plant species at the Nevada Test Site. This variability is of concern to those who would attempt to plant or manipulate any native desert species. The second group of 11 papers describes how the plant communities are put together and explains some of their attributes. An un- derstanding of plant sociological relationships in any ecosystem is prerequisite to any sub- sequent management. These papers concern Laboratory of Nuclear Medicine and Radiat jlogy, University of California, Los Angeles, California 90024. Great Basin Naturalist Memoirs No. 4 distribution, interactions, turnover, habitat preferences, longevity, and other topics. The third group of five papers relates to the carbon cycle under desert conditions. More specifically, the papers are concerned with below-ground aspects of plant commu- nities in the desert areas studied. The below- ground contributions to biomass under desert conditions are poorly understood, and these studies, made with the help of the 14carbon isotope, provide some answers. Information of the type contained in these five papers is particularly useful in land-cleanup pro- cedures where soil is only partially removed. The fourth group, consisting of six papers, relates to soil-plant relationships of desert vegetation and mineral composition of plants. Knowledge of soil preferences for plants is of prime importance for any at- tempt at revegetation and land reclamation. Almost as important is knowledge concerning the reasons for soil preferences for plants. These six papers provide some needed infor- mation in these areas. The introductory pa- per discusses the subject of how plants modi- fy desert soils and redistribute mineral nutrients in them. This, without question, points out one of the most important prob- lems associated with restoration of vegetation on disturbed desert land, that is, the destruc- tion of the fertile spots in the desert created by long-time plant activity. The fifth group, with four papers, concerns photosynthesis and transpiration processes. The first paper touches on the subject of C3 and C4 plants in regard to mechanisms ol photosynthesis and shows relationships with water-use efficiency, which itself concerns transpiration. Attempts of man to manipulate and regulate deserts to achieve restoration or revegetation must consider the important as- pects of adaptive and survival characteristics imparted by photosynthetic mechanisms, which in turn can be influenced by soil mois- ture conditions. These phenomena induce competitive effects among plant species. These studies contribute to understanding of deserts and will lead to more efficient man- agement of then i. The sixth and last group (three papers) re- lates to practical aspects of desert revegeta- tion. The first two papers of this group dis- cuss the all-important interaction o\ native animals with new vegetation obtained either bv natural reinvasion or by transplanting specimens onto disturbed lands. The prob- lems caused by native animals and the one as- sociated with "fertile islands" discussed in the fourth grouping of papers constitute formi- dable obstacles to certain types of desert land restoration. The final paper of the group and of the series is a summary chapter of the challenges involved in being able to comply with governmental regulations involved with southwestern deserts. Some synthesis of the total project is attempted in the final chap- ter. Some important omissions from this series of 30 papers relate to soil characteristics, the nitrogen cycle, and water relationships. These were not purposely overlooked, and some publications on these topics have been made elsewhere as follows: Farnsworth, R. B., E. M. Romney, and A. Wallace. 1978. Nitrogen fixation by microfloral-higher plant as- sociations in arid to semiarid environments. Chapter 2. pages 17-19 in Nitrogen in desert eco- systems. US/IBP Synthesis Series 9. Dow den. Hutchinson j;es 232-243 in Nitrogen in desert ecosystems. IS IBP Syn- thesis Series 9. Dowden, Hutchinson & Boss. Inc. Stroudsburg, Pennsylvania. Wallace. A.. E. M. Romney, and R. B. Hunter. 1978. \i trogen cycle in the Northern Mojave Desert: im- plications and predictions. Chapter 14, pages 2(17 2IS in Nitrogen in desert ecosystems. US/IBP S\nthesis Series '). Dowden. Hutchinson & Hoss. Inc.. Stroudsburg, Pennsylvania. Wallace, V. P.. M. Romney, G. E. Kleinkopf, and s. \l. Soufi. 1978. Uptake ot mineral tonus ot nitrogen b) desert plants. Chapter1), pages 130 1 ~> 1 in \i trogen in desert ecosystems. US IBP Synthesis Series l) Dowden. Hutchinson l\ Boss. Inc., Stroudsburg, Pennsylvania. A recent suggestion that desert sands cata- lyze photochemical formation of ammonia iChein. Eng. News, 13 Nov. 1978) at rates of from 2 to 25 kg /ha per year could provide new insight into the desert nitrogen cycle. A more complete listing of environmental 1980 Nevada Desert Ecology 3 studies conducted previously at the Nevada A. Emery. Ecology of the Nevada Test Site: Test Site is given in the Nevada Applied A narrative summary and annotated bibliog- Ecology Group Publications. 1978. raphy. NVO 167. U.S. Department of ORNL/EIS-127 and in T. P. O'Farrell and L. Energy, Las Vegas, Nevada. PHENOLOGY OF DESERT SHRUBS IN SOUTHERN NYE COUNTY, NEVADA T. L. Ackerman', E. M. Romney . V Wallace', and J. E. Kinnear1 Abstract.- This study was done to document the variability in time of phenological events at different locations on the Nevada Test Site. Phenological events for desert shrubs were recorded, and rainfall and temperature data were gathered for four to six years at eight sites that are located within the northern Mojave Desert, the southern Great Basin Desert, and the transitional zone between them. Results have been graphically displayed to show the variability in phenological activity encountered during the study period and also to show the general correlation between these events and the environmental regime that triggered a particular phenological stage among different species. For a given location a four-to-six-week range in beginning events from sear to year was common. In addition to the usual spring activity that normally followed winter rain and snow, most shrub species resumed new growth, and six species were observed to flower and fruit following rare summer or early fall rains. In comparison to sur- rounding locations, the closed drainage basins within the study area have lower temperatures at night that result in a delay of phenological events in most shrubs. Faithful participants in the annual pilgri- mage to view desert wildflowers and shrubs are cognizant of the extreme variability in time of phenological events at different desert locations. Each person visits his favor- ite location hoping that it will be a "good year" of splendorous color. Often he is re- warded, but some years fall short of expecta- tions. Phenological events are triggered mainly by rainfall and suitable temperature, al- though photoperiod is probably important for some species. The environmental regime that triggers a particular phenological stage varies among species: annual plants will not germinate and develop unless moisture and temperature conditions are suitable. Shrubs can survive long periods of low moisture and high or low temperature by dormancy or in- activity. Desert areas are characterized by in- frequent, low rainfall and extremes in tem- perature. Desert plants are adapted to these conditions. They are capable of a rapid growth response when conditions are favor- able, and they rapidly become dormant or in- active when soil moisture is low or temper- atures arc extreme. This study reports phenology of desert shrubs on the Nevada Test Site in southern Nevada for eight sites in five vallevs for a four-to-six-year period. The eight sites lie across a gradient within the northern Mojave Desert, the southern Great Rasin, and the transition zone between them. Rlaisdell (1958), Wein and West (1972), and West and Gasto (1978) related environ- mental data to phenology in the Great Rasin desert. Tueller et al. (1973) reported pheno- logy for the Great Rasin desert, and Reatley (1975) related climate to vegetation patterns across the Mojave /Great Rasin desert transi- tion of southern Nevada. Reatley (1974a) de- scribed the effects of rainfall and temper- ature on the distribution of Larrea tridentata i.Sesse & Moc. ex DG.) Cov. on the Nevada Test Site, where L. tridentata is at its north- ern limits. Reatley (1974b) also developed a generalized word model relating phenology of desert plants to environmental triggers (temperature and rainfall) in the Mojave and transition desert portions of the Nevada Test Site. The purpose of this study was to determine the influence of environmental factors on the phenology of desert shrubs along the transi- tion from the Mojave Desert to the Great Ra- sin desert. This study was part of a project to determine the climatic factors that cause the vegetative composition to change across this area. Because of the wide vear-to-vear varia- .1 n.i. leai Medii inc ami lia.li.iii.ni Hiolugv, University ..I ( alifomia, I os tageles, ( alifomia 1khi21 1980 Nevada Desert Ecology bilitv in desert weather patterns, many more years of data probably will be required to de- rive a complete understanding of the climate- phenology relationships. Table 1. Species for which phenology was recorded (common names in parentheses). Acamptopappus shockleyi A. Gray (C.oldenhead) Ambrosia dumosa (A. Gray) Payne (Bur-Sage) Artemisia spinescens D. C. Eat. (Bud-Sage) Artemisia tridentata Nutt. (Big Sagebrush) Atriplex canescens (Pursh) Nutt. (Four-Winged Salt- brush) Atriplex confertifolia (Torr. & Frem.) S. Wats. (Shad- scale) Ceratoides lanata (Pursh) J. T. Howell (Winterrat) Coleogyne ramosissima Torr. (Blackbrush) Ephedra nevadensis S. Wats. (Mormon Tea) Eriogonum kearneyi Tidestr. Grayia spinosa (Hook.) Moq. (Spiny Hop-Sage) Hymenoclea salsola Torr. & Gray (Cheesebush) Krameria parvifolia Benth. (Bange Ratany) Larrea tridentata (Sesse & Moc. ex DC.) Cov. (Creosote Bush) Lycium andersonii A. Gray (Desert-Thorn) Lycium pallidum Miers var. oligospermum C. L. Hitchc. (Box-Thorn) Oryzopsis hymenoides (Boem. & Schult.) Bicker (Indian Bice-Grass) Spliaeralcea ambigua A. Gray (Desert mallow) Yucca schidigera Boezl ex Ortgies (Mojave Yucca) Materials and Methods The species for which phenology was re- corded are given in Table 1. Characteristics of the eight study sites are given in Table 2. Phenological and environmental measure- ments were made weekly in the spring and early summer when plants were most active, and every two to four weeks during other seasons of the year. For each species we re- corded the date of "first" observed leaf bud, leaf stage, flower bud, flower, fruit, seed or fruit fall, leaf fall, and dormancy. Within the time of these events there was a great varia- tion from shrub to shrub. Environmental data collected at the time of each observation included rainfall, max- imum and minimum temperatures 30 cm aboveground and soil temperature at 15, 30, and 45 cm depths (Fenwall KA31L4 thermis- tors). Soil moisture was measured gravi- metrically on samples taken at monthly inter- vals from 7 to 15 cm and 30 to 38 cm depths from 1968 to 1970. Thermocouple psy- chrometers (Wescor) were used from 1971 to 1973 for soil moisture measurements. Results and Discussion Phenology, rainfall, and temperature data for the years 1968-1970 for the Mercury Val- Table 2. Characteristics of the eight phenology study sites. Mean Distance annual N from rainfall Elev. Mercurv during Site location (m) Valley site Desert type Vegetation type Years studied study, mm Rock Valley 1020 19.2 km (west) Mojave Larrea- Ambrosia- Lycium 1968-1973 172.5 Mercury Valley 1100 Mojave Larrea-Ambrosia- Lycium L968-1973 157.6 W. Frenchman Flat 1000 11.3 km Mojave Larrea-Lycium- Ambrosia 1968-1973 1 19.') N. Frenchman Flat 950 20.1 km Mojave Larrea-Atriplex canescens 1968-1973 136.1 Yucca Flat 1 1200 33.6 Transitional Atriplex confertifoIia-Liicium 1969-1973 187.3 Yucca Flat 2 1225 33.6 Transitional Atriplex confertifolia-Lijcium 1969-1973 199.0 Yucca Flat 3 1300 33.6 km Transitional Coleogyne-Yucca 1969-1973 185.7 Pahute Mesa 1720 52.6 km Great Basin Artemisia tridentata 1970-1973 149.7 Great Basin Naturalist Memoirs No. 4 ley, Rock Valley, and Frenchman Flat sites have been reported (Wallace and Romney 1972). A correction should be made on page 286 of that report in the rainfall reported for Rock Valley in August 1969. The graph should show 0.35 cm of rain instead of 3.5 cm. The environmental data for these same sites for the years 1968 to 1970 were report- ed by Romney et al. 1973. Results for 1968-1973 for Rock Valley were reported by Ackerman and Bamberg (1974). Data for 1971 to 1973 are given in the diagrams of Figures 1-8 of this report. Four-to-six-year summaries of ranges of beginning dates of phenophases, with means, are given in Fig- ures 9-16. General Response to Moisture and Temperature In this area the gentle winter and early spring (October to March) rains are more im- portant for growth in the spring than the in- frequent intense rains of summer. Usually from late November through early January the night air temperatures are near or below freezing, so most of the moisture from rains during this period is stored in the soil until favorable conditions permit budding and leafing out of plants. Low moisture or low temperatures may result in a delayed start <>t the growth season, i.e., phenology of cadi species is determined by the right moisture and temperature range. A case in point fol- lows. Compared to 1971, the spring growth season for the early species was delayed one to three weeks by low temperatures in 1972, and that of L973 was delayed two to four weeks by cool, cloudy, rainy weather from [anuary through March. For a given species, leafing out and flower- ing started earlier at Mercury Valley than at the Frenchman Flat sites (which are more than 11 km farther north). The timing varied from one week to more than a month in dif- ferent years. Phenologies at the Frenchman Flat sites were usually the same as at Yucca Flat Station 3, which is 100 in higher in ele- vation and more than 13.5 km farther north. but sometimes plants were slower m leafing out and flowering. The Frenchman Flat sites had lower minimum spring air temperatures (means ranged from 2-5 C lower for all years) and higher maximum spring air tem- peratures (means ranged from 3-4 C higher for all years). The lower minimums are prob- ably due to temperature inversion. The Yucca Flat Station 1, near Yucca Playa, gen- erally was a week or more behind Station 3 in phenology. The latter site is 100 m higher in elevation upon the Bajada and 1.9 km SW of Station 1. Station 1 had lower minimum air temperatures (means ranged from 2.4 to 5.6 C for all years) probably again due to an inversion of cooler air. Beatley (1975) men- tions that Frenchman Flat and Yucca Flat are both closed drainage basins with low temper- ature inversion layers. Flowers and flower buds of Atriplex canescens (Pursh) Nutt. at the north Frenchman Flat station appeared from 10 days to a month earlier than at the Pahute Mesa station, which is 770 m higher in elevation and 22.5 km farther north. Effects of Summer Rains Summer rains are local, infrequent, and of such intensity that much water runs off and little penetrates the soil to become available for plant growth, except in drainage chan- nels. The effect of these rains varies depen- ding on the amount and penetration in the soil. The Mojave Desert sites appeared to need rains greater than 2 cm in the summer for any effect on the shrubs. Shrubs respond- ed after such rains in August 1972 at Mer- cury Vallev. Rock Valley, Frenchman Flat, and Yucca Flat. Summer rains, however, can vary in the amounts of moisture deposited in different \ alleys. In 1970 north French Flat had 61 mm of rain in August; shrubs respond- ed with new growth or breaking of dor- mancy, and some flowered. Other sites in west Frenchman Flat and sites in Rock Val- ley and Mercury Valle) received less than 20 nun rain from the same storm system, and no shrubs responded. In 1971, only shrubs at Rock Valley (50 mm of rain) and Yucca Flat Station 1 (24 mm of rain) responded to sum- mer rains. Active growth of shrubs alter sum- mer rains usually occurs over a two-to-three- week period. Plants usually then become in- active and deciduous species soon start drop- ping their leaves, probably because of the high summer air temperatures (above 35 C), which cause high evaporation and trans- 1980 Nevada Desert Ecology MERCURY VALLEY ACAMPTOPAPPUS_ shockleyi AMBROSIA dumosa CERATOIDES lanata EPHEDRA nevadensis GRAY I A spinosa KRAMERIA parvifolia LARREA tridentata LYCIUM andersonn 0RYZ0PSIS hymenoides SPHAERALCEA . YUCCA schidigera ■ ■ ft I . . I H m\ • ■ ■ -H -*■ -'t I | | 1 • • •• e -T- I . . I JFMAMJJASONDJFMAMJJASONDJFMAMJJASOND 1971 J L 1972 1973 J L 60 () III 40 ca ~> t- ?() < ir iii n U > in i- -20 cr < J I I I u JFMAMJJASONDJFMAMJJASONDJFMAMJJASOND • Bud -B-Leaf -©-Flower -▼-Fruit — Leaf fall — 1 Dormant Fig. 1. Phenological events, rainfall, and maximum-minimum air temperature data for Mercury Valley during the period 1971, 1972, and 1973. Great Basin Naturalist Memoirs No. 4 AMBROSIA dumosa AT Rl PL EX conferti folia CERATOIDES lanata EPHEDRA nevadensis GRAY I A spmosa KRAMERIA _ parvifoiia LARREA tridentata LYCIUM andersonii LYCIUM pallidum ROCK VALLEY -» — ▼ •» — » T— T- JFMAMJJASONDJFMAMJJASONDJFMAMJJASOND J FWAMJJ ASONDJFMAMJJASONDJFMAMJJASOND • Bud -«-Leaf -e-Flower -^-Fruit ■—Leaf fall — I Dormant Fig. 2. Phenological events, rainfall, ami m temperature data for Rock Valley during the p 1973. lOd 19, 1. 19,2. and 1980 Nevada Desert Ecology WEST FRENCHMAN FLAT ACAMPTOPAPPUS shockleyi AMBROSIA dumosa ARTEMISIA spmescens CERATOIDES lanata GRAY/A spmosa LA R RE A LYCIUM ondersonii ORYZOPSIS hymenoides JFMAMJJ ASONDJFMAMJ J ASONDJFMAMJJ AS0N0 JFMAMJ J ASONDJFMAMJJ ASONDJFMAMJ J ASOND • Bud -B-Leaf -e-Flower ■▼•Fruit ■—Leaf fall — | Dormant Fig. 3. Phenological events, rainfall, and maximum-minimum air temperature data for west Frenchman Flat during the period 1971, 1972, and 1973. 10 Great Basin Naturalist Memoirs No. 4 ACAMPTOPAPPUS shockleyi AMBROSIA dumosa AT R/ PL EX conescens CERATOIDES lanala GRAY/A spinosa LARREA tndentata LYCIUM andersonii 0RYZ0P5IS hymenoides SPHAERALCEA _ ambigua • ••«..*-*- ■ ■ ■ ■ — I , , 1 , , I NORTH FRENCHMAN FLAT > ■ ■ ■ I . . I • •«■ -V V- • ■ ■ ■ J FMAMJJ ASONDJ FMAMJJ ASONDJ F M A M J J ASOND JFMAMJ JA SOND JFMAMJ JASONDJ FMAMJJA SOND • Bud -«-Leaf -e-Flower •▼-Fruit ■—Leaf fall — I Dormant Fig. I. Phenological events, rainfall, and maximum-minimum ail temperature data foi north Frenchman Rat during the period L971, L972,and 1973. Nevada Desert Ecology 11 jiration rates. Grayia spinosa (Hook.) Moq. md Artemisia spinescens D. C. Eat. shrubs lave not been observed to break dormancy liter summer rains; they do, however, break dormancy after fall rains. Different Species Response Each species has a temperature range in which it will grow if adequate soil moisture is present. The two Lycium species were the YUCCA FLAT STATION 1 ACAMPTOPAPPUS_ shockleyi ARTEMISIA spinescens ATRIPLEX confertifolio CERATOIDES I i-e- -▼-» — r — v — t- ■ » » » IT » » I ■ ■ I ■ ■ I JFMAMJJASONDJFMAMJ J A S 0 ND J F MAM J J A S 0 N D JFMAMJJ ASONDJ FMAMJJ ASO NDJ FMAMJJ ASOND • Bud -B-Leaf -©•Flower -▼-Fruit •—Leaf fall — | Dormant Fig. 5. Phenological events, rainfall, and maximum-minimum air temperature data for Yucca Flat Station 1 during the period 1971, 1972, and 1973. 12 Great Basin Naturalist Memoirs No. 4 YUCCA FLAT STATION 2 ACAMPTOfWPUS. shockleyi ARTEMISIA spinescens ATRIP LEX confertifolia Ceraloides lonata PSOROTHAMNUS _ fremontia HYMENOCLEA . so/so/a LYCIUM andersonii ORYZOPSIS hymenoides SPHAERALCEA ambiguo J FMAMJ JASONDJ FMAMJJASONDJ FMAMJJ ASOND JFMAMJJASONDJFMAMJJASONDJFMAMJJASOND Bud ♦ Leaf Flower ▼ Fruit Leaf fall — | Dormant Fig. 6. Phenological events, rainfall, ami maximum-minimum ai temperature data for Yueea Flat station 2 during the period 1971 L972,and 1973. L980 Nevada Desert Ecology L3 YUCCA FLAT STATION 3 AT Rl PL EX confertifolia CERATOIDES lanata COLEOGYNE ramosissimo PSOROTHAMNUS fremontii GRAY/A spinosa HYMENOCLEA so/so/a LYCIUM andersonii SPHAERRALCEA ambigua YUCCA b rev it • ■ ■ ■ ■ -H I ■ ■ I - *-•- ■- ImImI I, ,1 J F MAM J J AS ON D J F M A M J J AS ONDJFMAMJJASOND 1973 JFMAMJJASONDJFMAMJJASONDJFMAMJJASOND • Bud -B-Leaf -©-Flower -▼-Fruit -—Leaf fall — I Dormant Fig. 7. Phenological events, rainfall, and maximum-minimum air temperature data for Yucca Flat Station 3 during the period 1971, 1972, and 1973. 14 Great Basin Naturalist Memoirs No. 4 first to break dormancy, and this occurred when night air temperatures were around freezing and maximum air temperatures av- eraged 15 C (Ackerman and Bamberg 1974). Krameria parvifolia Benth. was the last spe- cies to break dormancy in the spring in the Mojave Desert, and this occurred when max- imum air temperatures were over 27 C and minimum air temperatures over 6 C (Acker- man and Bamberg 1974). Krameria parvifolia was never dormant in the summer, no matter how dry the soil. Larrea tridentata was the last species to flower (usually in May) in the Mojave Desert. Next to the last was A. canes- cens. Bamberg et al. (1975) found that L. tri- dentata and K. parvifolia in Rock Valley had net photosynthesis during the summer when their plant tissue water potentials were -65 bars and -72 bars, respectively. Larrea triden- tata put out new leaves 4 October 1973, after maximum air temperatures of 25-34 C and minimum air temperatures of 10-15 C. There had been only one slight summer rain on 4 August of 2.3 mm. ARTEMISIA tridentata ATRIPLEX cantscens ERIOGONUM ttaorneyi ORYZOPSIS hymenoidas FftHUTE MESA •<► w v w v JFMAMJJASONDJFMAMJJASONDJFMAMJJASOND • Bud -B-Leaf •©-Flower -▼•Fruit ■—Leaf fall — | Dormant Fig. 8. Phenological events, rainfall, and maximum-minimum air temperature data for Pahute Mesa during the period 1971, 1972, and 1973. 1980 Nevada Desert Ecology 15 MERCURY VALLEY 1100 m (MSL) ACAMPTOPAPPUS shockleyi AMBROSIA dumosa CE RAW IDES lanata EPHEDRA nevadensis GRAY I A spinosa KRA MERIA parvifo l/a LARREA tridentata LYCIUM andersonii ORYZOPSiS hymenoides SPHAERALCEA ambigua YUCCA _,. schidigera a = 4years b = 5 years + = mean -V-b - + - STEM - + ■♦- J F M A leaf bud leaf --h -l-b -I- ' -i-b -f -b 4-b -I— a i i— J F M A M flower bud flower Fi, 9. Snn,„,aries o, ,he average da.e and ranges o, beginning dates nf phenophases, a, Mercnrv VaUev d.ning six years, except where otherwise noted. L6 Great Basin Naturalist Memoirs No. 4 Flowering Shrub species which flowered only in the spring (probably determined by photo- periodisin) arc: Ephedra nevadensis S. Wats., Atriplex confertifolia (Torr. & Frem.) S. Wats., Coleogyne ramosissima Torr., G. spin- osa, Hymenoclea salsola Torr. 6c Gray., Psorothamnus fremontii (Torr.) Barncby, and Yucca schidigera Roezl ex Ortgies. Artemisia tridentata Nutt. flowered only in the fall. Species that flowered anytime during the spring, summer, and fall, if they received enough rain, were: Larrca tridentata, Cera- toides lanata (Pursh) J. T. Howell. Lycium andersonii A. Gray, and Ambrosia dumosa (A. Gray) Payne. Lycium pallidum Miers var. oligospermum C. L. Hitchc. produced flower buds after the fall rains of 1972. Onjzopsis hymenoides (Roem. & Schult.) Ricker pro- ROCK VALLEY 1020 m (MSL) AMBROSIA dumosa ATRIPLEX confertifolia CE RAW IDES lanata EPHEDRA nevadensis GRAY I A spinosa KRAMER! A par vi folia LARREA tridentata LYCIUM andersonii LYCIUM pa llidum +- -a -4 H STEM H- - + J I I - H- ■l- -+ - -4- H— b -H -b -f- J M A M M a ~- 4 years b r 5 years t = mean leaf bud leaf flower bud flower Fig, in. Summaries ol tin aw rage date and ranges oi beginning da years, except where otherw ise noted .pit. .vs. at Rock \ ,,11, 1980 Nevada Desert Ecology 17 duced flowers after rains in the fall of 1972 at Yucca Flat Station 1, as well as in the spring, but not after summer rains, which probably indicates it needs lower temperatures to flower than are present in the summer. The amount of moisture in the soil at the start of the spring growth period seemed to determine initial flowering response. Lack of moisture at this time resulted in a species producing few or no flowers. For example, at north Frenchman Flat in 1971, where the lowest rainfall was recorded in the spring for all areas and all years, Lycium andersonii, Ambrosia dumosa, and Acamptopappus shockleyi A. Gray produced no flowers. Mois- ture later in the spring growing season may result in either renewed growth or late flow- ering or reflowering. In 1971 at west French- man Flat, L. andersonii and O. hymenoides, which usually produce flower buds in Febru- ary or March, did not produce any until after a May rain of 37 mm. Lycium andersonii in Mercury Valley also had the same delayed flowering. This May rain caused Ceratoides Janata to reflower at both places. In Rock Valley it caused reflowering in A. dumosa and Krameria parvifolia. NORTH FRENCHMAN FLAT 950 m (MSL) ACAMPTOPAPPUS shockleyi AMBROSIA dumosa ATRIPLEX canescens CERATOIDES lanata 6 RAY I A spinosa LARREA tridentata LYCIUM andersonii ORYZOPSIS hymenoides SPHAERALCEA ambigua ■+ -a — h- -h- -h- - + -b ^ a - ♦- - -«--b - 1-- -4-b 4- •b - -h - .-t- j i M J F M A M a =4 years b =5 years t =mean leaf bud leaf flower bud flower Fie;. 11. Summaries of the average date and ranges of beginning dates of phenophases, at north Frenchman Flat during six years, except where otherwise noted. 18 Great Basin Naturalist Memoirs No. 4 Dormancy Dormancy in desert shrubs is a complex phenomenon involving physiological changes, drought, and temperature responses. As the growing season progresses to early summer, soil moisture is depleted by rapid evaporation and transpiration as daily air and soil temperatures rise. During this time de- ciduous shrubs start to shed their leaves and soon became dormant. An interaction of high temperatures and low soil moisture probably results in summer dormancy for some species. Artemisia spinescens and Grayia spinosa al- ways became dormant when daytime air temperatures were over 40 C, and they did not break dormancy even after summer rains. Wallace and Romney (1972) found that G. spinosa would break dormancy only after a low temperature period (5 C or less) or after an application of gibberellin. Dormancy in G. spinosa is probably induced by an internal physiological mechanism triggered by either heat or photoperiod. In 1972, G. spinosa WEST FRENCHMAN FLAT 1000 m (MSL) ACAMPTOPAPPUS shock ley i AMBROSIA dumosa CERATOIDES lanata GRAYIA spinosa LARREA tridentata LYCIUM andersonii LYCIUM pallidum -h +- -ho 1 -H - f- - 4-b H-b b M A MAM a = 4 years b = 5 years t = mean leaf bud leaf flower bud flower Fig. 12. Summaries oi the average date and ranges oi beginning dates <>t phenoph during six years, except where otherwise noted. ;st F. •iK-iiiiiaii Nevada Desert Ecology 19 broke dormancy on 10 November after fall rains when night air temperatures were be- low 0 C. Artemisia spinescens always broke dormancy after fall rains. Krameria parvifolia started going dormant in the fall only when air temperatures fell below 4 C at night. In addition to having a heat dormancy period in the summer, some species can have an in- duced dormancy during the winter because freezing night temperatures kill their leaves. This happened to Ambrosia dumosa during the winter of 1968 in Rock Valley and Mer- cury Valley, in 1970 and 1972 at north Frenchman Flat, and in 1972 at west French- man Flat, when all of its leaves froze.. This species escaped the freeze during the winter of 1972 in Mercury Valley and Rock Valley. Leaves of Lycium andersonii were killed on 14 December 1972, at Yucca Flat Station 3, but not in Rock Valley. Ephedra nevadensis has only small scalelike leaves, so it is diffi- cult to determine when it is physiologically active. This shrub, however, grows new stems after summer and fall rains and has a stem color change from green to brown during pe- riods of apparent inactivity. Shrub species that never drop all leaves (therefore, are considered evergreens) and which put out new growth after spring, sum- mer, and fall rains are Larrea tridentata, Co- leogyne ramosissima, Atriplex canescens, A. confertifolia, Ceratoides lanata, and Arte- misia tridentata. During the dry summer with high air temperatures, these shrubs may lose most of their leaves but they retain enough to justify their classification as ever- green. In extreme conditions during dry years, either stem ends or complete stems died in some specimens of these species. A partial summary of some of the phenolo- YUCCA FLAT STATION 1 1200 m (MSL) ACAMPTOPAPPUS shockleyi ARTEMISIA spinescens ATRIPLEX confertifolia CERATOIDES lanata LYCIUM andersonii ORYZOPSIS hymenoides j i i i —ha - + - - 4- -f J F M A MAM a = 4 years + = mean leaf bud leaf — flower bud — flower Fig. 13. Summaries of the average date and ranges of beginning dates of phenophases, at Yucca Flat Station 1 during six years, except where otherwise noted. 20 Great B.^ Naturalist Memoirs No. 4 gical events for six plant species at three lo- cations is shown in Figure 17. The three loca- tions were chosen from the eight studied to show a north-south gradient of 33.6 km in which considerable variation in climate oc- curs. There is also an elevation gradient of 1100 to 1200 m going from south to north for the three locations. Phenological events occurred progressivelv later for Acamptopappus shockleyi in the three sites going from south to north. The range in appearance was a month or more. Lycium andersonii was the only other of the six species chosen that appeared in all three locations, and its behavior was similar to that of A. shockleyi except that this species had a somewhat narrower range in appearance of leaf buds, leaves, flower buds, and flowers. Lycium andersonii seems to be under less precise control than A. shockleyi because of its narrow range for appearance of new leaves, etc. This species will also leaf out YUCCA FLAT STATION 2 1225 m (MSL) ACAMPTOPAPPUS shockleyi ARTEMISIA spinescens AT Rl PL EX conferti folia CERATOIDES lanata PSOROTHAMNUS fremontii HYMENOCLEA salsola LYCIUM andersonii ORYZOPSIS hymenoides SPHAERALCEA ambigua 4- -4- J F M A ■ -Jlz i J F M A M a = 4years + = mean leaf bud leaf — flower bud flower Fig II. Summaries of the average date and ranges oi beginning dates of phenophases, during six years, excepl where otherwise noted Yucca Flat Station 2 1980 Nevada Desert Ecology 21 late in the summer when rains occur after plants have entered dormancy. Three of the six species shown in Figure 17 occur in only two of the three stations. They are absent at Yucca Flat Station 1, the north- ernmost of the three stations. Two of the three species are considered as Mojave Desert species (Ambrosia dumosa and Larrea tridentata). The third, Grayia spinosa, grows in both southern Great Basin and northern Mojave deserts. Leaves and flowers appeared later on L. tridentata than on A. dumosa. The range of events was more narrow for L. tri- dentata than for A. dumosa. Grayia spinosa leaves also appeared earlier than L. triden- tata, and its flowers appeared earlier than both L. tridentata and A. dumosa. Krameria parvifolia occurred only at the southernmost of the three stations. Its phe- nological events occurred late in comparison with other species at that site. It is of interest that species with late-phenological events YUCCA FLAT STATION 3 1300m (MSL) ATRIPLEX confertifolia CERATOIDES /a not a C0LE06YNE ramosissima GRAYIA spinosa HYMENOCLEA sal so I a LYC/UM andersonii SPHAERALCEA ambigua YUCCA brevifolia - + - - + - -+- - + + ra i F M J F M A M a =4years + = mean leaf bud leaf flower bud flower Fig. 15. Summaries of the average date and ranges of beginning dates of phenophases, at Yucca Flat Station 3 during six years, except where otherwise noted. 22 Great Basin Naturalist Memoirs No. 4 ARTEMISIA tridentata ATRIPLEX canescens ERIOGONUM kearneyi PAHUTE MESA 1720 m (MSL) i i l I i J F M A M J L - + J L J I J FMAMJ JASO = mean leaf bud leaf flower bud flower Fig. 16. Summaries of the average date and ranges of beginning dates of phenophases, at Pahute Mesa during six years. LEAF APPEARANCE ACAMPTOPAPPUS AMBROSIA shockleyi dumosa MERCURY VALLEY WEST FRENCHMAN FLAT YUCCA FLAT STATION 1 J F M A J F M A GRAY I A spinosa J F M A KRAMER I 'A LARREA parvifolia tridentata J F M A J F M A LYCIUl andersc J F M i FLOWER APPEARANCE MERCURY VALLEY — i -' ----*' ■+■ ^- ~h- +b -f-b WEST FRENCHMAN FLAT -+ -H- - ♦- - -t- -b^b +b YUCCA FLAT STATION 1 --►-"to -Tt F M A M FMAM FMAM FMAM FMAM F M A + mean a = 4 Years - Leaf bud or flower bud b = 5 Years — Leaf or tie wer Fie,. 17. Summary oi phenological data tor leaf and flower appearance lor six of the plant species at three of th locations (Mercury Valley; west Frenchman Flat 13.3 km north of Mercury Valley, anil Yucca Flat Station 1 33.6 kn north of Mercury Valley). Six years of data are involved unless otherwise stated. 1980 Nevada Desert Ecology 2:5 were absent from one or two of the sites. A wide amplitude is possibly characteristic of plants that leaf and flower under cool condi- tions. Acknowledgment This work was supported partially by Con- tract EY-76-C-03-0012 between the U.S. De- partment of Energy and the University of California and partially by the International Biological Program /Desert Biome (National Science Foundation Grant GB-32139). Literature Cited Ackerman, T. L., and S. A. Bamberg. 1974. Phenology studies in the Mojave Desert at Rock Valley (Ne- vada Test Site). Pages 215-226 in Lieth, ed. Phenology and seasonality modeling. Ecological Studies, Vol. 8. Springer-Verlag. New York, New York. Bamberg, S. A., G. E. Kleinkopf, A. Wallace, and A. Vollmer. 1975. Comparative photosynthetic production of Mojave Desert shrubs. Ecology 56:732-736. Blaisdell, J. P. 1958. Seasonal development and yield of native plants on the upper Snake River Plains and their relation to certain climatic factors. U.S. Dept. Agric. Tech. Bull. 1190. Beatley, J. C. 1974a. Effects of rainfall and temperature on the distribution and behavior of Larrea triden- tata (Creosote-bush) in the Mojave Desert of Ne- vada. Ecology 55:245-261. 1974b. Phenological events and their environ- mental triggers in Mojave Desert ecosystems. Ecology 55:856-863. 1975. Climates and vegetation pattern across the Mojave/Great Basin Desert transition of southern Nevada. Amer. Midi. Nat. 93:53-70. Romney, E. M., V. Q. Hale, A. Wallace, O. R. Lunt, J. D. Childress, H. Kaaz, G. V. Alexander, J. E. Kinnear, and T. L. Ackerman. 1973. Some char- acteristics of soil and perennial vegetation in northern Mojave Desert areas of the Nevada Test Site. UCLA Report 12-916. Tueller, P. T., A. D. Bruner, and J. B. Davis. 1972. Ecology of Hot Creek Valley— vegetation and soil reponses to underground detonation. Univ. Ne- vada Agric. Expt. Sta. Rept. NVO-409-1. Wallace, A., and E. M. Romney. 1972. Radioecology and ecophvsiologv of desert plants at the Nevada Test Site. USAEC Report TID-25954. Wein, R. W., and N. E. West. 1972. Phenology of salt desert plants near contour furrows. J. Range Management 24:299-304. West, N. E., and J. Gasto. 1978. Phenology of the ae- rial portion of shadscale and winterfat in Curlew Valley, Utah. J. Range Management 31:43-45. RESIDUAL EFFECTS OF SUPPLEMENTAL MOISTURE ON THE PLANT POPULATIONS OF PLOTS IN THE NORTHERN MOJAVE DESERT R. B. Hunter1. E. M. Romney', A. Wallace1, and J. E. Kinnear1 Abstract.- Residual effects of sprinkle irrigation from 1968-1970 on populations oi Mojave Desert shrub commu- nities were observed in late 1974. The sprinkle-irrigated plots showed a residual increase in density of four species, but other species either failed to reproduce in significant numbers or lost all gains made during the years following treatment. The seven-year change for the irrigated plots was equivalent to a gain of 1178 perennial plants per ha, but the nonirrigated plots lost an average of 1050 plants per ha equivalent during the same period. The biomass gain after seven years was equivalent to 1000 kg/ha for irrigated plots and 310 for nonirrigated plots. This study was made on plots which were established in early 1968 (Wallace and Rom- ney 1972b). The objective of the work re- ported herein was to measure changes in bio- mass and density of plant species during a seven-year period that included three years of sprinkle irrigation followed by four years of natural rainfall. Materials and Methods The site of the study area is Mercury, Ne- vada, near the waste water ponds from the local sewage processing system. Close prox- imity to a source of irrigation water was one prerequisite for the overall research program. The soil at this site is underlain by virtually impervious hardpan at depths varying from 15 to 75 cm. The thickness of the hardpan layer is usually greater than 10 cm. Perennial plants grow both singly and in clumps, sepa- rated by bare areas of desert soil. The size and spacing of the clumps are irregular. As many as 10 different species may grow to- gether in a single clump. A census was made in early spring of 1968 of all perennial plants (including shrubs, grasses, herbs, and their seedlings) in 25 cir- cular experimental plots, each plot being 30.5 m in diameter. Each plant was cate- gorized by species position and dimension analysis. This census effort involved more than 19,000 individual plants representing 28 different species. A special method was devised for the pur- pose of locating and cataloging each plant in each plot. A permanent standpipe for mount- ing a surveyor's transit was installed at the center of each plot, with a marker located on magnetic north at a distance of 15.25 m. Ori- entation for each vegetational unit (clump) was the measured distance from the plot cen- ter to the vegetational unit center. The azi- muth to each unit was measured from mag- netic north (0°) to the center of the vegetational unit. The unit's greatest and smallest width and its species content were recorded. Each species within a unit was measured in like manner, and it was further identified by height. These data were record- ed and transferred to punch cards for com- puter processing. For both sprinkler-irrigated and control plots, the height and two widths of each shrub were recorded. From this information shrub volume and biomass were calculated using a dimension measurement regression line (Wallace and Romney 1972a). In No- vember 1974, the census in several plots was repeated and dimensional measurements were also made. Abiotic data obtained from Rock Valley located about 20 km west of the study plots are given in Table 1. 'Laboratory of Nuclear Medicine and Radiation Biology, University oi California, Los \ngeles, California VXX12-J. 21 1980 Nevada Desert Ecology 25 Results and Discussion Changes induced by supplemental sprinkle irrigation were still apparent four years after the last application of water. In particular, there remained a net gain in population on the watered plots and a net loss on the dry plots after seven years (Table 2). Population changes depended significantly on species. Ceratoides lanata (Pursh) had a much more rapid turnover during the course of the study than did most other species, los- ing 12 to 36 percent of its population on dry plots while its population increased on irri- gated plots. Sphaeralcea ambigua A. Gray lost an even higher proportion, 55 to 93 per- cent on the dry plots, and nearly as much, 47 to 79 percent, on watered plots. Grazing rab- bits are especially hard on S. ambigua, a pre- ferred food source; therefore, survival data may have little relationship to earlier plot treatment. The population of Acamptopappus shockleyi A. Gray increased on all plots, wa- tered or not. Gains on dry plots ranged from 6 to 55 percent and on watered plots from 8 to 105 percent. Numbers of Ambrosia dumosa (A. Gray) Payne increased slightly on watered plots and decreased slightly on dry plots. This species, especially, showed visible increases in new biomass in response to supplemental mois- ture. Other species generally showed negligible changes in populations. For several species this low turnover rate is considered to be sig- nificant. Krameria parvifolia Benth., Ephedra funerea Cov. & Mort, Ephedra nevadensis S. Wats., Yucca schidigera Roezl ex Ortgies, and Salazaria mexicana Torr. must have unusu- ally long life spans if our data are representa- tive. Similarly, their invasion of disturbed sites must be very slow. The biomass changes by species on these plots are reported in Table 3. In the seven- year period of this study, biomass increased more than 25 percent for 6 species and de- creased more than 25 percent for 2 species Table 1. Abiotic factors in the general environs of Valley about 20 km west of Mercury. ndy plots. Data are from the USWB station located in Rock 1963- 1964- 1965- 1966- 1967- 1968- 1969- 1970- 1971- 1972- 1973- 1974- 1975- 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 Rainfall, inn (USWB) July — 20.0 9.1 6.8 7.1 51.0 3.0 10.2 0.5 0.0 0.0 25.6 0.0 August 19.5 8.1 4.8 3.8 50.2 4.3 1.0 22.6 33.3 21.3 3.5 1.6 1.8 September 34.2 0.0 0.0 1.0 10.1 0.0 0.2 0.0 0.0 9.0 0.0 0.0 5.3 October 3.3 4.0 0.7 0.2 0.0 11.6 7.0 0.0 0.0 34.2 3.6 25.9 0.5 November 18.5 3.3 49.2 2.5 28.1 2.5 17.0 19.6 0.8 32.8 15.0 2.5 4.8 December 0.7 0.0 62.4 10.6 10.1 4.3 0.0 17.8 40.6 0.0 12.4 32.8 0.0 January 1.7 10.6 8.8 32.7 1.5 68.0 0.3 0.0 0.0 26.0 34.0 1.0 0.0 February 0.7 0.0 16.2 0.0 30.7 103.0 44.5 8.1 0.0 49.3 1.0 4.1 39.0 March 0.6 12.4 1.2 0.0 7.3 19.0 14.0 1.3 0.0 73.3 6.4 26.9 1.0 April 10.6 60.1 0.7 26.6 5.5 3.0 1.8 0.0 2.0 14.9 0.0 9.9 6.0 May 0.5 2 2 8.6 5.3 0.0 1.0 0.0 19.3 0.0 10.7 0.2 8.1 3.0 June 5.0 0.5 1.7 11.4 3.0 12.0 0.8 0.0 14.5 4.2 0.0 0.0 0.0 Mean air temperatures (C) USWB July 27.2 28.6 26.6 29.1 29.7 28.6 31.1 29.2 30.1 34.1 31.5 30.1 30.6 August 25.S 27.2 25.2 29.4 31.4 23.6 29.7 30.3 28.3 28.3 29.5 28.7 28.8 September 23.9 23.3 21.1 23.6 23.9 23.0 24.9 22.2 22.1 23.9 24.5 27.5 26.8 October 20.5 20.5 19.4 16.9 18.6 17.5 17.2 14.7 16.7 14.2 18.0 17.5 20.3 November 12.7 9.2 12.5 11.1 13.3 12.8 8.4 9.5 6.1 7.9 9.0 10.2 9.0 December 8.3 8.4 8.6 6.1 2.0 3.3 8.4 2.6 4.6 3.8 6.5 3.8 8.1 January 5.3 8.4 0.6 6.7 7.3 7.8 6.4 8.8 6.5 3.1 2.7 4.3 7.8 February 5.0 8.3 5.9 9.5 12.2 2.8 10.3 6.1 11.7 6.9 7.4 5.7 8.9 March 11.7 11.4 13.1 10.8 13.1 13.6 13.1 9.0 19.3 7.0 12.3 7.6 9.9 April 13.6 15.0 16.1 10.0 13.6 16.7 11.1 12.0 16.1 14.4 14.4 9.4 13.1 May 16.4 17.2 20.8 19.4 20.3 22.7 19.2 14.9 20.8 22.5 22.6 19.5 22.8 June 23.0 21.4 25.0 20.3 25.8 23.6 23.9 24.3 27.1 28.0 29.5 26.6 - 26 Great Basin Naturalist Memoirs on nonirrigated plots. The number of species showing gains in the seven years for the non- irrigated plots was 9; 2 showed neither gain nor loss and 5 species showed losses. In the seven-year study period, biomass increased more than 25 percent for 14 species and de- creased more than 25 percent in one species on the irrigated plots. The number of sped showing gains in seven years for the irrigat< plants was 16, and one showing a loss. It very clear that irrigation in 1968, 1969, ai 1970 resulted in a larger biomass persistii through 1974 (P < 0.01 by significance test Table 2. Population changes (averages and standard errors of the means for three plots) in sprinkle-irrigated pk and control plots over a seven-year period (1968-1974). Water was applied to plots in 1968. 1969, and 1970. Area each plot is 730 m2. Species Start Number SEM New Number SEM Died Number SEM Seven-vear change Number SEM Perce A. shackle i/i A. dumosa A. confertifolia Cactus spp. ('. lanata C. ramosissima E. funerea E. nevadensis G. spinosa K. parvifolia L. tridentata L. fremontii L. andersonii M. tortifolia M. spinescens O. hymenoides S. mexicana S. ambigua S. pauciflora S. speciosa Y. schidigera A. shockleyi A. dumosa A. confertifolia ( lactus spp. ('. lanata (',. ramosissima E. funerea E. nevadensis (.'. spinosa K. pan ifolia I., tridentata I. fremontii I., andersonii M. tortifolia \1 spinesa ns ( >. hymenoides s. in, dcana S. ambigua S. pauciflora S, speciosa ) schidigera III i gated plots 198 57.7 88.7 19.0 35.0 16.2 + 53.7 19.3 178 3.7 16.3 4.4 4.3 1.2 + 12.0 5.5 15.3 7.9 1.7 1.7 9.3 4.8 -7.7 4.6 3.3 1.5 2.0 1.5 0.0 0.0 + 2.0 1.5 129.3 34.9 54.3 23.3 24.7 11.6 + 29.7 14.5 1.0 1.0 0.0 0.0 0.7 0.7 -0.7 0.7 21.0 7.9 2.0 2.0 0.3 0.3 + 1.7 1.7 26.3 7.2 3.0 0.6 1.0 0.0 + 2.0 0.6 21.0 7.6 13.3 8.5 2.0 0.6 + 11.3 8.1 80.3 3.0 2.0 1.0 3.3 1.5 -1.3 2.4 45.7 8.9 0.7 0.3 1.7 0.3 -1.0 0.6 4.0 4.0 27.0 13.3 0.7 0.7 + 26.3 13.3 60.3 6.4 1.0 0.6 0.7 0.6 + 0.3 0.7 9.3 6.4 10.0 2.7 1.3 1.3 + 8.7 3.8 3.0 2.0 5.0 5.0 0.0 0.0 0.0 0.0 12.3 4.3 2.0 1.5 8.0 2.1 -6.0 1.2 5.7 2.0 0.0 0.0 0.0 0.0 0.0 0.0 72.3 14.7 22.7 16.3 71.0 2S.2 -48.3 14.S 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.7 0.7 3.0 0.6 0.3 0.3 + 2.7 0.7 6.7 0.9 0.7 0.3 0.0 0.0 + 0.7 0.3 Nonirrigated plots 61.0 17.2 21.0 2.3 11.3 5.2 + 9.7 2.9 83.7 10.4 3.3 1.9 4.7 1.8 -1.3 0.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.3 0.7 1.0 1.0 0.7 0.3 + 0.3 0.9 273.3 44.2 36.3 12.4 100.3 13.5 -64.0 25.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 3.7 1.9 1.3 0.3 0.0 0.0 + 1.3 0.3 4.0 3.1 2.3 0.9 0.0 0.0 + 2.3 0.9 26.3 3.5 9.3 4.1 2.0 1.2 + 7.3 2.9 22.0 4.6 0.3 0.3 0.3 0.3 0.0 0.6 50.0 10.7 1.0 1.0 1.3 0.9 -0.3 1.8 0.7 0.7 1.0 0.6 0.0 0.0 + 1.0 0.6 36.0 5.1 0.3 0.3 1.0 0.6 -0.7 0.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.7 0.3 0.0 0.0 0.0 0.0 0.0 0.0 6.7 10 2.0 0.6 3.0 2.1 -1.0 1.5 9.0 3.7 0.0 0.0 0.7 0.7 -0.7 0.7 lid 7.1 2.0 1.0 30.7 1.5 28.7 0.7 0.7 0.7 0.3 0.3 0.3 0.3 0.0 0.6 207 SI I I 1.4 5.7 2.3 -1.3 1.9 1.0 1.0 0.7 0.7 1.3 0.3 -0.7 0.3 Nevada Desert Ecology 27 Table 3. Biomass and changes in kg/ha (averages and standard errors of the means for three plots) in sprinkle- rigated and control plots over a seven-year period (1968-1974). 1968 1974 Mean SEM Mean SEM Seven-year change Mean SEM Percent Irrigated ( n = 3) 104.9 29.6 146.6 33.8 + 41.7 11.7 + 39.8 328.1 66.1 560.9 64.3 + 237.8 42.5 + 72.5 38.5 22.8 74.4 59.6 + 36.0 36.8 + 93.7 152.0 56.0 310.8 111.0 158.8 57.0 + 104.5 312.1 134.6 372.9 167.8 61.8 48.0 + 19.8 41.1 11.4 68.2 13.4 + 27.1 5.18 + 66.0 40.6 23.2 241.1 149.8 + 200.5 126.6 + 494.2 0.13 0. 13 2.9 2.9 + 2.8 2.8 + 213.9 91.8 18.3 117.4 15.4 + 25.8 4.2 + 28.1 211.8 86.5 289.2 133.0 + 77.4 48.7 + 36.6 0.00 11.6 4.9 + 11.6 4.9 + oo 383.2 53.8 493.9 57.2 110.4 8.0 + 28.8 9.7 8.5 16.3 13.3 + 6.5 4.9 + 66.7 0.24 0.16 0.003 0.003 -0.24 0.16 -100 2.2 0.4-4 2.52 1.47 0.28 1.7 + 12.5 0 0 0.07 0.07 + 0.07 0.07 + oo Nonirrigated (n = 3) 42.0 14.4 53.5 19.2 + 11.4 4.8 27.2 201.5 33.0 160.1 53.2 58.7 22.1 29.1 0 0 0 0 0.0 557.5 30.2 516.4 66.9 -41.1 37.6 -7.9 25.9 13.4 44.4 23.0 + 18.5 9.7 + 71.2 33.5 32.2 62.2 59.7 + 27.8 27.5 + 83.0 59.2 18.7 108.8 42.9 + 70.0 17.8 + 118.0 0 0 0 0.0 44.3 14.4 51.5 15.9 + 7.3 2.50 + 16.4 550.4 56.5 571.7 161.9 + 120.3 32.6 + 21.9 0.03 0.03 0.45 0.27 + 0.42 0.24 + 1400 334.3 30.1 372.9 27.6 + 38.6 15.7 + 11.5 1.12 0.72 0.93 0.67 -0.19 1.0 -17.0 0.04 0.02 0 -0.04 0.02 -100 1.81 0.27 0.31 0.04 -1.50 0.31 -82.9 0.09 0.05 0.07 0.04 -0.02 0.03 -22.2 Acknowledgment This study was supported by Contract EY- 76-C-03-0012 between the U.S. Department of Energy and the University of California, Los Angeles. Literature Cited Wallace, A., and E. M. Romney. 1972a. A study of a measure of species association between pairs of perennial plants in desert hardpan soil. Pages 205-299 in A. Wallace and E. M. Romney, eds. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Technical In- formation Services, USAEC Report TID-25954. 1972b. Response of Mojave Desert vegetation to nitrogen fertilizer and supplemental moisture. Pages 349-351 in A. Wallace and E. M. Romney, eds. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Techni- cal Information Service, USAEC Report TID- 14943. THE PULSE HYPOTHESIS IN THE ESTABLISHMENT OF ARTEMISIA SEEDLINGS AT PAHUTE MESA, NEVADA E. M. Romney1, A. Wallace', and R. B. Hunter Abstract.- New Artemisia seedlings are not established each Near. Many that are established fail to survive be cause of unfavorable rainfall in succeeding years. A total of 184 young plants was examined for the number of annua growth rings to ascertain the year of establishment after all vegetation had been killed near the time of a nuclear tes event in 1965. The three most important recent years for establishment and survival of new seedlings (as of 1976 an< based on a sample of 184 plants) were 1966 (9 percent), 1969 (29 percent), and 1973 (36 percent) A total of 2 percent was established in the other years from 1965 to 1976. These three years were also the years with high raintal input during preceding winter and spring months. If old plants are killed, seeds germinate with mud, lower n.put of precipitation. Many seedlings germinated in 1968 at a site where old ones had been burned off even though th rainfall was not favorable. Plants of a given age varied greatly in size according to then competition. Seedlings gei minating in old stands grew little in comparison with those germinating in areas where old plants had been killec One exception was an area where intense competition occurred due to large numbers ot new plants, resulting r growth restriction on all plants. It is generally considered that favorable rainfall years are necessary for the estab- lishment of perennial plants under desert conditions (Beatley 1975, Wallace and Rom- ney 1972). There is some question about the need for more than one favorable year in suc- cession for establishment of new plants, at least under some circumstances (Wallace and Romney 1972). Studies made of ages or size of desert perennial plants most often indicate a rather uniform distribution of the input of new perennial plants (El-Ghonemy et al. 1979, this volume). Such studies, however, are obscured by the fact that differences in shrub size tend to disappear after a few years. The present study was undertaken be- cause data for precipitation for recent years are available for Pahute Mesa, and because Artemisia can be dated by counting annual growth rings (Ferguson 1960). Materials and Methods The Pahute Mesa area of the Nevada Test Site is located at an elevation of about 2000 m. The predominant vegetation in many areas of it is Artemisia tridentata Nutt. an( Artemisia nova A. Nels. (Beatley 1975, 1976) Revegetation studies following nuclear test ing have been conducted there previously and the age of many of the plants in stud; plots is determined by knowledge of whei they germinated (Wallace and Romne; 1972). The past 12 years of the history of th area is fairly well known. By 1976 some o the known seedlings had attained the size o many other plants in the population, evei though they were much younger. This infor mation was useful in determining the plant which should be sampled. On 15 July 1976, total of 184 plants was measured by dimen sion analysis using methods reported (Wal lace and Romney 1972), and then cut and ex amined for ring count (Ferguson I960' Sampling was done at five different sites es tablished to track vegetation recovery fron the Palanquin and Cabriolet plowshare test (Rhoads et al. 1969). Dates of these nnclea tests were April 1965 and January 196£ Plant weight by dimension analysis was cal culated from the regression of weight = 3479V + 0.081. V is volume in m3; weight i in g dry weight. Laboratory ol Nu< leai Medi< Ine and Radiation Biology, Universit) ol < alifbrnia, Los Vngeles, ( alifornia 90024. 2S L980 Nevada Desert Ecology 29 Results and Discussion Seedlings that initially germinated in areas where vegetation was killed by the 1965 Pa- lanquin test were essentially all destroyed in the drought of 1968. It was determined at that time that rabbits had eaten the plants in a desperate attempt for their own survival (Wallace and Romney 1972). Results obtained from the 1976 sampling are presented in Table 1. The first year of most pronounced establishment of seedlings after disturbance was in 1969, except at Cabriolet where germination occurred one year earlier (1968). The Cabriolet event was in January 1968 and by spring and summer months the pressure of old plants using the available soil moisture was absent in the new- ly killed area. Fallout radiation destroyed nearby standing vegetation, but not the seed supply in soil. As a result, the old seeds were available for germination in the spring of 1968, when soil moisture became more plentiful due to death of the old plants. The year 1969 was one of high rainfall in February, resulting in extensive germination Table 1. Number, size, and age of 184 young Artemisia plants from Pahute Mesa accord measurements and annual ring counts. ing to dimensional No. of plants No. of rings 1976 mean above-ground dr wt per plant g dry weight Coefficient of variation weight Percent Year of germination Normal vegetation (control) 15 59.9 10 14.76 7 12.28 6 5.57 3 0.37 2 0.16 17.64 18.76 1.42 0.16 0.10 1961 1966 1969 1970 1973 1974 Total Total Total Total Adjacent to roadside in control area 10 154.4 7 49.9 3 0.24 1 0.08 . (1965) 67.6 1966 57.9 1969 0.02 1973 1975 13 7 244.6 244.8 1969 3 5 64.4 34.3 1971 37 3 8.2 9.7 1973 1 2 0.17 - 1974 54 Palanquin totally killed area (1965) — 5 10 567.3 218.3 1966 3 7 185.4 58.7 1969 1 2 0.42 — 1974 19 3 4.55 7.09 1973 1 4 188.1 - 1972 29 Cabriolet totally killed area (1968) 22 8 7.82 2.18 1968 4 7 7.88 2.27 1969 3 3 8.10 2.15 1973 29 30 Great Basin Naturalist Memoirs No. 4 of new seedlings that survived in large num- bers. The years 1966, 1969, and 1973-74 were also good years for establishment of seedlings in both control and disturbed areas. Area 12 Mesa station recorded 24.7 cm of rainfall in the 1968-1969 season and 18 cm in the 1972-1973 season. The November and December rainfall for 1965 was about 9 cm, and the relatively cool spring months made 1966 a favorable year. Tueller and Clark (1976) reported similar precipitation data for the Pahute Mesa area. Most seedlings established on a scraped roadside installed in 1965 were related to the high rainfall year of 1969. The percentages of the total new plants for all areas for the three most important years were 9 (1966), 29 (1969), and 36 (1973). Re- sults confirm the idea that new seedlings in this ecosystem truly come in pulses related either to rainfall or to disturbance that kills old plants and makes more favorable soil moisture for the seedlings. The sizes of the plants in Table 1 are of considerable interest. They differ consid- erably for given ages, a fact related to natural competition in the environment. Even after 7 and 10 years, plants in old, nondisturbed areas were still small and had survived with difficulty. They were usually close to old plants even though in disturbed areas most germination was between old plants (Wallace and Romney 1972). These new plants may re- main small until old ones die naturally. In disturbed areas plants of the same age were much larger, except in the Cabriolet area, where so many seedlings germinated that competition kept them small. Seedlings also remained small in the control area, where a normal stand of vegetation was present. There seems to be a niche among old, es- tablished plants where new ones become es- tablished and await the chance to replace the older plants. Acknowledgments This study was supported by Contract EY- 76-C-03-0012 between the University of Cal- ifornia and the U.S. Department of Energy. Additional support was provided by the Ne- vada Applied Ecology Group, Nevada Oper- ations Office, Las Vegas, Nevada. Literature Cited Beatley, J. C. 1975. Climates and vegetation patterns across the Mojave/Great Basin Desert transition of southern Nevada. Anier. Midi. Natur. 93:53-70. 1976. Vascular plants of the Nevada Test Site and central-southern Nevada. Tech. Information Cen- ter, Office of Tech. Information, ERDA Report TID-16881. El-Ghonemy, A. A., A. Wallace, and E. M. Romney. 1980. Frequency distribution of numbers of per- ennial shrubs in the northern Mojave Desert. Great Basin Nat. Mem. 4:32-36. Ferguson, C. W. 1960. Annual rings in big sagebrush, Artemisia tridentata. Unpublished dissertation. University of Arizona. Tucson. Rhoads, W. A.,R. B. Platt, R. A. Harvey, and E. M. Romney. 1969. Ecological and environmental ef- fects from local fallout from Cabriolet. 1. Radi- ation doses and short-term effects on the vegeta- tion from close-in fallout. USAEC Report, PNE 956. Tueller, P. T., and J. E. Clark. 1976. Nonradiation ef- fects on natural vegetation from the Almendro underground nuclear detonation. USERDA Re- port NVO-409-3. Wallace, A., and E. M. Romney. 1972. Radioecologv and ecophysiology of desert plants at the Nevada Test Site. National Tech. Information Service. USAEC Report TID-25954. THE ROLE OF PIONEER SPECIES IN REVEGETATION OF DISTURBED DESERT AREAS A. Wallace' and E. M. Romney' Abstract.- The northern Mojave Desert, as are many deserts, is characterized in part by small "fertile islands" in which exist individual shrub clumps each containing two or more plants. These fertile sites promote characteristic organization of both plant and animal activity in the desert. Destruction of these fertile sites makes revegetation extremely difficult because most seedlings germinate in these sites. Some pioneer species do, however, germinate and survive in the bare areas between the fertile sites. Four such species in the northern Mojave Desert are Acamptopappus shockleyi Gray, Lepidium fremontii Wats., Sphaeralcea ambigua Gray, and Atriplex confertifolia (Torr. & Frem.) Wats. These four species may have a role in starting new fertile islands. Mojave Desert Characteristics Revegetation procedures have been stud- ied for some years by our group as a means of land reclamation of disturbed desert areas (Romney et al. 1971, Wallace and Romney 1975, 1976, Wallace and Romney, 1972a, 1972b, Wallace et al. 1977). The revegetation process is ordinarily slow, partly because the low rainfall in many years just will not sup- port the establishment of new seedlings. A more important reason, however, is that the soil surface becomes organized as a result of prior plant activity, which results in micro- watersheds. When this basic structure is de- stroyed, revegetation is extremely difficult to achieve either naturally or by manipulation. Rainfall in the Mojave Desert is enough to support only a small amount of vegetation. An interesting feature of this desert is that just part of the soil surface (10 to 20 percent) is generally occupied by clumps of growing plants, and the other 80 percent to 90 per- cent serves mainly as watershed for the 10 to 20 percent of area supporting vegetation (Charley 1972, Romney et al. 1977, Garcia- Moya and McKell 1970). The land surface structure has been in place for decades, or centuries (Wallace and Romney 1972b), and has resulted in the soil beneath shrub clumps becoming very fertile areas that compare fa- vorably with agricultural soils or those of grassland or forest ecosystems. The fertile areas are high in soil organic materials and available nutrients. Roots of plants in the fer- tile shrub clumps extend outward into the bare areas so that the soil moisture of the to- tal land area potentially becomes available to the clumps. This system is much more ef- ficient in sustaining plants than would a sys- tem in which the soil organic matter and readily available nutrients are uniformly dis- tributed throughout the whole soil area but at a much lower level. That condition would result in much nitrogen deficiency. Role of Pioneer Shrur Species The bare desert soil between shrub clumps generally is low in organic materials, and it characteristically has an unfavorable soil structure (less aeration) that tends to inhibit the establishment of new seedlings as well as the growth of other plants. Plant species that are capable of invading bare areas, especially when sufficient soil moisture is available, must be adapted to conditions related to poor soil structure and low organic matter. When land disturbance destroys these fer- tile islands, it is well known that the natural revegetation problem is formidable (Wallace and Romney 1975, Wallace et al. 1977). There are some common perennial species in the northern Mojave Desert, however, which 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 90024. 31 32 Great Basin Naturalist Memoirs No. 4 are able to pioneer by growing in the less fer- tile bare areas. Somehow they obtain suf- ficient N, but likely not through fixation of atmospheric N2 (Hunter et al. 1977). They also obtain sufficient other nutrients, and they must be adapted to growth in soil of poor structure. Four such species are Acamptopappus shockleyi Gray, Lepidium fremontii Wats., Sphaeralcea ambigua Gray, and Atriplex confertifolia (Torr. & Frem.) Wats. The latter is ubiquitous in the western deserts. The encouragement of these species to grow on disturbed sites and studies to help make this possible are of urgent priority if revegetation required by land restoration leg- islation is to be achieved successfully in the northern Mojave Desert. It is of considerable importance to learn the nutrient status, water and oxygen requirements, and other ecologi- cal behavior characteristics of these four pio- neer species that can help solve land recla- mation problems. A Case History In 1967, at Mercury, Nevada, a site 18 m in diameter was cleared of vegetation. The main crown roots of the plants were re- moved, but the fertile islands were not de- stroyed. The original purpose was to change the soil moisture status. After nine years, on 28 May 1976, the following numbers of in- vading perennial plants were counted in the plot: 14 Ceratoides lanata (Pursh) J. T. How- ell, 4 Lycium andersonii A. Gray, 33 A. shockleyi, 26 L. fremontii, 4 Ambrosia du- mosa (A. Gray) Payne, 3 Machaeranthera tor- Hfolia (A. Gray) Cronq & Deck, 6 Oryzopsis hymenoides (Roem. & Schlt.) Ricker, 18 S. ambigua, 2 Sitanion jubatum J. G. Sin., and 2 Krameria parvifolia Benth. No /,. fremontii were involved in the adjacent nondistnrbed area, but plants for seed stock were located in a nearby wash. Seventy-three percent of the invading perennial plants in this plot were of the species defined above .is pioneer Species, and they were the only ones that were found in the original bare areas be- tween the fertile islands. Most of the other new plants had invaded the old fertile island sites. Yerv lew annuals were present in the nondistnrbed areas, but some were found in the cleared area. Atriplex confertifolia was not found at or near this particular test plot area, but we have observed its pioneering capabilities at a number of disturbed sites located elsewhere around the Nevada Test Site. Acamptopappus shockleyi can survive for a number of years. On a particular site where 29 seedlings were observed eight years ear- lier, 21 of them were still surviving in 1976. The mortality after the eight years was 28 percent. Acknowledgments This study was supported by Contract EY- 76-C-03-0012 between the U.S. Department of Energy and the University of California. Literature Cited Charley, |. L. 1972. The role of shrubs in nutrient cy- cling. Pages L82-203 in C. \1. McKell, J. P. Blais- dell, and J. R. Goodin, eds. Wildland shrubs— their biology and utilization. USDA Forest Ser- vice, General Tech. Report INT-1. Intermountain Forest and Range Exper. Sta.. USDA Ogden, Utah. Garcia-Moya, F.. wn C. M. McKell. 1970. Nitrogen economy ol a desert wash plant community. Ecology 5:81-88. Hunter, R. B.. A. Walla< i . vnd F. M. Romney. 1977 Nitrogen transformations in Rock \ alle) ami ad- jacent areas of the Mohave Desert. Is [BF Desert Biome Res. Memo. 70-2S. Utah State Uni- versity, Logan. Romney, E. \F. \. Wallace, wn J. D.Childress. 1971. Revegetation problems following nuclear testing activities at the Nevada Test Site. Proc. 3rd Natl. Symp. on Radioecology, Oak Ridge. Tennessee. 2:1015-1022. (CONF-710501-P2). Romney, F. \1 . \ \\ u 1 \< i . H. k\ \/. V, O. Hale, wn J. I). Childress. 1977. Effect of shrubs on redist- ribution ot mineral nutrients in zones near roots m the Mojave Desert. The below-ground ecosys- tem: a synthesis ol plant associated processes. Range Science Series No. 20. Colorado State Uni- versity, Fort ( ollms. Wallac i. \.. wn E. M. Romney. 197"). Feasibility and alternate procedures tor decontamination and post-treatment management of Pu-contaminated areas in Nevada. Pages 2~>l 337 in The radio- ecolog) ol plutonium and other transuranics in desert environments. Nevada Applied Ecolog) Group Report NVO 153 . 1972a. Revegetation in areas In close m fallout from inn leu detonations. Pages 75 86 in Radio- ecolog) ami ecophysiolog) ol desert plants at the Nevada Test Site. I SAE< Office ol Information Services, TID-25954. 1980 Nevada Desert Ecology 33 . L972b. Approximate age <>l shrub clumps at the Nevada Test Site. Pages 307-309 in Radioecolog) and ecophysiology of desert plants at the Nevada Test Site. USAEC Office of Information Services. TID-25954. 1976. Initial land reclamation procedures related to possible Pu-cleanup activities at the Tonopah Test Range. Pages 65-77 in Environmental pluto- nium on the Nevada Test Site and environs. Ne- vada Applied Eeologv Group, Report NVO-171. FREQUENCY DISTRIBUTION OF NUMBERS OF PERENNIAL SHRUBS IN THE NORTHERN MOJAVE DESERT A. A. El-Ghonemy', A. Wallace', and E. M. Romney- Abstract.— Frequency distribution according to plant size as measured by dimensional analysis on different mathematical bases were determined for 10 common perennial plant species from Rock Valley in the northern Mo- jave Desert in Nevada. A total of 4282 individual plants was measured. The data provide information concerning the stability and prosperity of the natural vegetation as judged by the relative proportions of individuals in the size-class spectrum, as well as show graphically the relative abundance of the different species in the study area. On the species level, the populations were close to normally distributed on the logo basis, but with remarkably negative skewness due to better segregation of the small-sized individuals into many segmental units. On the arith- metic basis, three categories of frequency pattern were recognized, but all with marked positive skewness due to better segregation of large-sized individuals into many segmental units. The feature common to all species studied is the preponderance of young individuals, which in many cases could have an abundance many times that of large individuals. The natural vegetation in Rock Valley, therefore, represents a reasonably active stage. Assembling and comparing observations on the size classes of the component species of the natural vegetation are of primary con- cern for understanding the stability and pros- perity of the vegetational cover. The size- class spectrum may also reveal valuable in- formation in relation to climatic features oc- curring in the past during the early life of the existing vegetation, as well as the possible segregation of the individual species into two or more ecotypes. The purpose of this study is to demonstrate through two different mathematical ap- proaches the size-class frequency distribution for 10 common perennial species in the Rock Valley area of the northern Mojave Desert. The site involved and the original data ob- tained are part of the US/IBP Desert Biome studies. Material and Methods A system random-number-generating func- tion (Wallace and Romney 1972) was used to select the random number pairs whose inter- section identify sample location within the study area. A total of 4282 randomly selected individuals from 190 sampling plots (50 X 2 m) have been considered. The sampling tech- nique was designed so that sampling points could be randomly distributed over the entire area. Each individual was identified by spe- cies and measured for height and width (mean of dimension) in 1971 at the Rock Val- ley IBP validation site. Calculations using these dimensional measurements were made to estimate shrub biomass, using previously calculated regression equations of dry weight on volume indices for the different species in- vestigated (Wallace and Romney 1972). Frequency for size-class distributions were made for shoot weights on arithmetic and log,, bases. A computer program was devel- oped in which the class interval on the X axis and the scale of abundance on the Y axis were kept constant for all species, when data were illustrated on the loge basis. In the arith- metically illustrated data, the scale on both Y and X axes varies from one species to the oth- er. 'University of Tanta, Tanta, Egypt. 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 90024. 34 Nevada Desert Ecology 35 Results and Discussion The results of the size-class distribution on the loge basis are illustrated graphically in Figure 1. The frequency distribution for all species is close to normal, with some negative skewness, the degree of which differs from 440n °> A.dumosa b) K.parvifolia rC c) G. spinosa 2 4 6 8 10 12 16 SIZE CLASSES (ON LOG 2 4 6 8 10 12 14 E BASIS) Fig. 1. Size distribution of 10 shrubs based on stem dry weights. Size-class interval is constant for all species (975 g). Shaded bars represent size classes with individuals that are close to the average weight for the species. 36 Great Basin Naturalist Memoirs No. 4 one species to the other. This negative skew- ness is attributed to the fact that the transfor- mation of data into natural log values has re- sulted in a better segregation of small-sized individuals into many classes. This can be readily observed when examining the number of size classes on both sides of that class rep- resenting those individuals with an average weight. The abundance of the different species, as reflected by their absolute frequencies (which represent one aspect of the species impor- tance), is given in Table 1 and can also be easily detected from Figure 1. Ambrosia du- mosa (A. Gray) Payne is the most frequent species in the study area. It has been ran- domly recorded 1183 times out of 4282 re- cords for all species. The species poorly rep- resented are those of Atriplex confertifolia (Torr. & Frem.) Wats, and Acamptopappus shockleyi Gray, represented by 37 and 23 in- dividuals, respectively. Another point of interest is the fact that all species exhibit large standard deviation (Table 1) relative to the means. This is par- ticularly remarkable for A. dumosa, Larrea tridentata (Sesse & Moc. ex DC.) Cov., Ly- cium pallidum Miers, and A. shockleyi. The standard deviation for these species exceeds the mean weights. The relation between size classes and the total number of individuals, irrespective of their taxonomic position, is given in Table 2. Examination of these data shows that size class 11 with log,, ranging from 4.169 to 5.144 (arithmetic range from 64.63 to 171.4 g/plant) embraces the higher number of indi- viduals (1,361 out of 4,282). In this class, A. dumosa, Krameria panifolia Benth., Ephedra net adensis Wats., and A. confertifolia attain their maximum abundance. On either side of this class the number of individuals progres- sively decreases. The preponderance of younger individuals, as reflected in Figure 1, is an indication of vi- tality and prosperity of the species; it is most- ly due to successful germination of seeds and survival of seedlings. Bainfall in the year 1969 was above normal and may be the source of the seedlings observed. The size-class frequencv distribution on the arithmetic basis shows another interesting picture. Figure 2 shows the frequencv distri- bution for three representative shrubs, each with its specific pattern. Three shapes of fre- quency distribution have been recognized; the j-shape, asymmetric, unimodal, positively skewed shape, and the asymmetric, poly- modal shape. Categorizations of the 10 spe- cies studied according to the shape of their frequency distribution are given in Table 3. It is obvious that seven species belong to cat- egory (a), i.e., with a J-shape distribution. These species are L. tridentata, A. dumosa. Grayia spinosa, E. nevadensis, Ceratoides Ja- nata (Pursh) J. T. Howell, L. pallidum, and A. confertifolia. A similar pattern of size-class distribution for L. tridentata has been pre viously demonstrated by Chew and Chew (1965). In category (b) the representative spe cies are K. panifolia and L. andersonii. Cate- gory (c) is represented by A. shockleyi. : Tutu 1 Some statistical attributes reflet :ting abundance and distrib ution 8.069 > 3193.9 - 38 Great Basin Naturalist Memoirs No. 4 tata individuals into different ecotypes has not been demonstrated. The different eco- types of L. tridentata might differ phys- iologically and consequently might be spa- tially separated. These two distinct life forms of L. tridentata are comparable in many re- spects to what is known in the Australian semiarid regions as Whip-stick and Bull Mal- lee, two distinctly different forms of Eu- calyptus oleoca (Beadle 1948, El-Ghonemy 1967). It can be concluded from the above study that the perennial species in the Rock Valley area are in a reasonablv active state. The cur- Table 3. Frequency distribution of individuals of dif- ferent plant species in relation to the number included in the size class that includes individuals of the average weight for the species. 250 L tridentola 200 150 co 100 CO < M CO I o o < UJ z 200 ^^ K. porvifolio CO < 150 Q 1 100 z U_ ° 50 rr UJ °D 5 0 " "Tn 6 A shockleyi 4 2 n i n r 2 4 6 8 10 12 14 SIZE CLASSES (on arithmetic basis) Fig. 2 Size distribution oi three representative shrubs based on stem dr) weight. Size ( Kiss intervals are 316 g for Lam a tridentata, 64.8 g foi Krameria pan ifolia, and 11.2 g tor Acamptopappus shockleyi. Shaded bars repre scut size-classes thai contain individuals that are close to the average weight for the species. Number of indivi duals in relation to their weight Species < average average > average (a) Species with J-shaped distribution curve Larrea tridentata 280 117 120 Ambrosia dumosa 632 314 237 Grayia spinosa 386 104 101 Ephedra nevadensis 232 58 97 Ceratoides lanata 163 7 64 Lycium pallidum 127 41 59 Atriplex confertifolia 20 4 13 (b) Species with unimodal. positively skewed distribution curve Krameria parvifolia 412 159 161 Lycium andersonii 165 87 99 (c) Species with asymmetric, polymodal distribution curve Acamptopappus shockleyi 14 1 8 rent rate of entrance of new plants into the ecosystem generally exceeds the rate of loss, with a consequent preponderance of younger individuals. Acknowledgments This study was supported by Contract EY- 76-C-103-0012 between the U.S. Department of Energy and the University of California and by the US/ IBP Desert Biome program. Dr. A. A. El-Ghonemy is a visiting scholar from Tanta University, Egypt. Literature Cited Bl \iiii. \ (' W. 1948. The vegetation and pasture of western South Wales. Govt. Printer, Sydney, Aus- tralia. Chew, H. M . and A. F. Chew. 1965. The primar) pro- ductivity of a desert shrub Larrea tridentata* community. Ecol. Monog. 33(4):355-375. El-Ghonemy, V \. ll)67 Soil-vegetation relationships for some major plant communities in south- western New South Wales. Australia. Unpub- lished dissertation. Universit) (it New England, Armidale. Australia Wallace, V., wi> E. M. Romney. 1972. Radioecologj and ecophysiology of desert plants at the Nevada Test Site. National Technical Information Ser- vices. USAEC Report TID-25954. W| LKOWITZ, J. R., R. B. Ewen, \M> J. Com \. 1971. 1 1 1 - troductor) statistics tor the behavioral sciences. \< aJi inn Press, New York. FURTHER ATTRIBUTES OF THE PERENNIAL VEGETATION IN THE ROCK VALLEY AREA OF THE NORTHERN MOJAVE DESERT S. A. Bamberg1, A. Wallace1, E. M. Romney', and R. E. Hunter1 Abstract.- Above-ground and below-ground biomass, percent dead shrubs by species, and percent of dead stems of living species were determined for a site in the northern Mojave Desert. The Rock Valley area of the northern Mo- jave Desert was used as an International Bio- logical Program (IBP) Desert Biome valida- tion site (Turner, 1973, 1975, 1976, Turner and McBrayer, 1974). Some characteristics of the vegetation of that site are described else- where in this volume (El-Ghonemy et al. 1980, Wallace et al. 1980). This report sum- marizes some other aspects of the vegetation on this site. Materials and Methods The techniques used are described in El- Ghonemy et al. (1980). Briefly, a total of 4282 randomly selected individuals over the large plot was used in the study. Each indi- vidual was identified and subjected to various measurements. Live and dead plants were also determined. Abiotic data for the area have been record- ed in the validation site reports (Turner, 1973, 1975, 1976, Turner and McBrayer 1974). The zone numbers (20 to 25) are de- fined in these reports. Results and Discussion The dry weight estimates of the stem por- tions of the plants are in Table 1. These val- ues obtained by dimension analysis (Wallace and Romney 1972) were used to calculate the aboveground stem biomass per unit area (Table 2). The ratios of root/stem obtained in one study (Wallace et al. 1974) and corrected in another (Wallace et al. this volume 1980) were used to calculate below-ground stand- ing biomass for this area (Table 2). The pro- portion of below-ground biomass is greater than that obtained with our 14C techniques, but does seem to be a bit lower than that ob- tained for the Great Basin desert (Caldwell and Camp 1974). Carcasses of many dead shrubs were on the site, and numbers were determined for each of the major perennial species as percent of dead to live plus dead numbers (Table 3). On the average, from 10 to 15 percent of the in- dividuals for each species were dead. The correlation coefficients between the number of plants per hectare and percent dead were not significant (Table 3). It can be expected that there is some rela- tionship between the percent of dead plants and longevity. Species with the largest span of life most likely would show the smallest percentage of dead plants at any one time. This hypothesis, of course, would be in error if any species took several times as long as another to decompose and disappear from the system. This does not seem to be the case, however. Ephedra nevadensis S. Wats, and Atriplex confertifolia (Torr. & Frem.) S. Wats, then would have the shortest life span of the shrubs represented. Krameria parvi- folia Benth. and Lycium andersonii A. Gray would have the longest. Each plant had a portion of dead wood, and an estimate of it for each species is also given in Table 3. It is noted that half or more of the stems of Ambrosia darnosa, Atriplex 'Laboratory of Nu Medicine and Radiation Biology. University of California, Los Angeles, Califor 39 40 Great Basin Naturalist Memoirs No. 4 conferti folia, Ceratoides lanata, and Larrea tridentata were dead. No species had more than 26 percent of its stems dead on the aver- age. Results from this investigation indicate that vast areas of the northern Mojave Desert support stands of vegetation in which as much as one-fourth of the standing crop may be dead wood. This represents a large reser- voir of organic material that eventually must undergo breakdown, decomposition, and mineralization. The fact that so much of this dead wood remains standing above ground for decades after death suggests that either the woody material has little value as food for existing insect populations, it is resistant to breakdown by insects or microbes, or spe- cies are not present to serve the role in stem tissue breakdown that exists in less arid eco- systems. Acknowledgments This study was supported in part bv Con- tract EY-76-C-03-0012 between the U.S. De- partment of Energy and the University of California and by the US/IBP Desert Biome, Utah State University, Logan. Table 1. Mean dry stem weight (g) and its standard deviation per plant determined by random quadrat count and dimensional analysis in Rock Valley validation site. Species 20 Zone 21, 22 23 24 25 1. Acamptopappus shockleyi Standard deviation Number 48.8 59.2 10 - - - - 60.9 45.1 9 2. Atriplex confertifolia Standard deviation Number - - - - 48.5 33.9 20 16.4 21.6 17 3. Ephedra nevadensis Standard deviation Number 109.5 225.5 182 46.7 67.9 41 138.9 132.1 41 126.7 88.0 33 150.9 345.2 20 162.5 204.5 70 "■ Ceratoides lanata Standard deviation Number 59.7 91.2 33 51.1 53.6 46 54.9 64.8 4 46.9 103.2 50 80.0 109.4 32 80.0 111.3 69 8. Ambrosia dumosa Standard deviation Nu'mber 108.8 98.0 390 73.0 78.3 120 133.8 160.4 107 100.4 102.2 89 124.2 127.8 182 107.1 104.7 295 9. Grayia spinosa Standard deviation Number 108.8 L23.3 44 89.3 106.5 34 162.3 234.7 9 82.1 112.6 42 70S 73.7 160 65.7 6(5.6 502 11. Kramaria parvifolia Standard deviation Number 134.5 87.8 419 129.1 97.1 46 126.8 97, S 75 170.2 128.5 32 156.0 117.9 56 133.6 106.5 104 12. Larrea tridentata Standard deviation Number 455.9 IS 1.2 240 133.2 424.1 44 386.5 39:5.6 43 386.3 348.0 24 511.7 517.5 7:5 372.8 385.8 9.3 14. Lycium andersonii Standard deviation Number 386.0 250.3 243 393.2 253.8 22 362. 1 212.1 35 224.4 220.9 17 362.8 215.0 6 320.5 218.7 28 15. Lycium pallidum Standard deviation Number 275.4 L66.1 19 322.8 227.1 34 395.7 253.3 8 191.5 1.57.6 16 325.0 250.6 59 202.3 182.9 91 1980 Nevada Desert Ecology 41 Table 2. Stem and root dry weight per hectare (kg/ha) in the Rock Valley validation site. Zone 20 21 22 23 24 25 Species Stem Root Stem Hoot Stem Root Stem Root Stem Root Stem Root Acamptopappus shockleyi 2.2 2.1 - - 1.0 0.8 4.6 4.2 0.9 0.9 6.2 5.8 Atriplex confertifolia — - - — - - — — 13.4 10.0 3.1 2.3 Ephedra nevadensis 93.0 138.1 39.5 57.9 163.5 239.6 182.2 266.9 41.5 60.9 127.6 186.9 Ceratoides lunula 9.3 14.7 48.6 76.7 6.7 10.5 102.2 161.5 35.4 56.0 61.8 97.6 Ambrosia dumosa 200.6 406.6 180.6 366.1 401.1 81.3.1 389.3 789.0 311.0 630.4 364.4 738.7 Grayia spinosa 22.1 27.7 62.7 78.6 40.9 51.4 150.2 188.5 155.9 195.7 222.6 279.4 Krameria parvifolia 263.2 364.5 122,8 215.5 266.7 369.3 237.3 328.5 120.6 167.1 155.8 215.7 Larrea tridentata 511.5 1113.2 392.9 689.5 465.6 1013.2 404.1 879.4 513.7 1117.0 388.8 846.1 Lycium andersonii 4.38.5 642.5 177.7 260.4 355.5 520.9 160.0 234.5 30.1 44.1 100.6 147.4 Lycium pallidum 24.2 69.8 227.9 658.3 88.6 255.5 133.5 385.6 264.9 765.0 206.3 596.0 1564.6 2779.2 1252.7 2403.0 1789.6 3273.5 1763.4 3238.1 1487.4 3047.1 1637.2 3115.9 Total root + stem 506.3. 1 5001.5 4534.5 4753.1 Table 3. Standing dead plant material on the Rock Valley validation site in 1971 separated into the percentage of dead shrubs out of the total shrubs calculated from random quadrat counts (given in columns 1-7) and into the percentage of dead wood as a portion of living shrubs derived from destructive whole shrub sampling given in Col-- umn 8. Correlation coefficient Dead wood (percent as percent dead X no. Zone of standing of shrubs 20 21 22 23 24 25 Total live and per ha) Species Dead shru )s, percent site° dead stem r Ambrosia dumosa 6.8 14.4 7.8 17.1 13.2 13.4 11.2±4.0 66.5 + 0.70 Atriplex confertifolia - - - - 29.3 21.0 25.4 ± 5.9 54.9 - Ephedra nevadensis 26.1 20.8 29.9 6.8 12.8 19.4 22.5 ±8.5 29.4 -0.10 Ceratoides lunula 1.4 3.7 6.9 7.9 8.6 9.1 6.5 ±3.1 66.3 + 0.31 Grayia spinosa 7.6 16.2 21.4 7.0 1.3.7 12.6 11.4 ±5.4 47.9 -0.24 Krameria parvifolia 0.9 1.7 1.9 3.1 0.6 1.4 1.2 ±0.9 32.8 + 0.19 Larrea tridentata 7.2 6.0 4.4 11.7 4.8 9.3 7.1 ±2.8 68.6 -0.13 Lycium andersonii 1.3 5.0 10.0 2.2 0.0 4.9 2.7 ±3.6 29.2 + 0.32 Lycium pallidum 1.4 5.6 21.3 2.0 5.7 6.0 5.7 ±7.3 26.1 -0.29 ' ± is standard deviation. Literature Cited Caldwell, M. M., and L. B. Camp. 1974. Below ground productivity of two cool desert communities. Oecologia 17:123-130. El-Ghonemy, A. A., A. Wallace, and E. M. Romney. 1980. Frequency distribution of numbers of per- ennial shrubs in the northern Mojave Desert. Great Basin Nat. Mem. 4:32-36. Turner, F. B., ed. 1973. Rock Valley validation site re- port. US/IBP Desert Biome, Research Memo 73- 2. 1975. Rock Valley validation site report. US/ IBP Desert Biome Res. Memo 75-2. 1976. Rock Valley validation site report. US/IBP Desert Biome Res. Memo 76-2. Turner, F. B., and J. F. McBrayer, eds. 1974. Rock Valley validation site 1973 progress report. US/IBP Desert Biome Res. Memo 74-2. Wallace, A., and E. M. Romney. 1972. Radioecology and ecophysiologv of desert plants at the Nevada Test Site. National Technical Information Ser- vice, USAEC Report TID-25954. Wallace, A., S. A. Bamberg, J. W. Cha. 1974. Quan- titative studies of roots of perennial plants in the Mojave Desert. Ecology 55(5): 1160-1 162. Wallace, A., E. M. Romney, R. A. Wood, A. A. El- Ghonemy, and S. A. Bamberg. 1980. Parent ma- terial which produces saline outcrops as a factor in differential distribution of perennial plants in the northern Mojave Desert. Great Basin Nat. Mem. 4:138-14.3. MULTIVARIATE ANALYSIS OF THE VEGETATION IN A TWO-DESERT INTERFACE A. A. El-Ghonemy1, A. Wallace-, and E. M. Romnev- Abstract.— This report further describes the distribution and ecological characteristics of the natural vegetation at the Mojave Desert-Great Basin Desert interface. The region studied is one of extraordinary biological interest because of its geographic location straddling the boundaries of two large deserts of the western United States, and because of the kind and manner of its past land use (atmospheric and underground testing of nuclear devices). The present analysis determines the magnitude of variations in the phytosociological structure in this region and eval- uates some relationships between its vegetation and environment. Vegetation and soils were sampled in 66 stands representing many possible physiographic variations. Relative density and relative coverage were determined for each perennial species and summed to provide an estimate of its importance value (I.V.). Importance values were used to ordinate stands to provide a synthesis of the phytosociological data and to portray the compositional relation- ships of species. The results of this study indicate that the area is dominated by several interrelated vegetational groupings. Correlations between the vegetational groups and the different environmental variables indicate that the distributional pattern of the vegetation is controlled largely by soil physical properties, salinity, and fertility levels. The landscape of the Nevada Test Site is one of the most intensively studied and best understood deserts in the United States. Its potential is unique for studies critical to bet- ter understanding of arid lands. Previous phy- tosociological studies in this area are largely descriptive (Wallace and Romney 1972, Romney et al. 1973, Beatley 1976). The objective of this study was to use some multivariate methods to analyze sociological relations among plant communities of the natural vegetation on the Nevada Test Site. This study is closely related to that pre- viously carried out by the authors (El-Gho- nemy et al. 1980), in which an account is giv- en on the location, physiography, climate, vegetational groupings, and community di- versity. Materials and Methods Procedural details involving selection of stands and sampling techniques for soil and plants at 66 sites (Table 1) have been report- ed by Wallace and Romney (1972) and Rom- ney et al. (1973). For treatment of data, one classification and two ordination techniques were used to analyze a data matrix consisting of the importance values for each of the pe- rennial species encountered in each of the stands. The classification technique involves the unweighted pair-group agglomerative clus- tering, using arithmetic averages to compute the similarity between a cluster and a stand which is a candidate for entry into a cluster (Sneath and Sokal 1973). The Euclidean dis- tances (Ed) were used as the measure of sim- ilarity between stands. The first ordination technique is that of Wisconsin (Gray and Curtis 1957) as modi- fied by Beals (1969). The raw data were nor- malized by row and column, and interstand similarities were calculated using the formula 2w -j- (a + b), where w represents the sum of the smaller values for common species; a and b represent the sum of all species in stands A and B, respectively. The maximum dissimilarity value was set equal to the max- imum similarity value found in the similarity matrix. Ten percent of the i'th axis was searched to locate the second end stand for the (i + l)'th axis. The second ordination technique involves principal component analysis of the matrix of interstand correlation coefficients (Sneath and Sokal 1973). Eigenvectors (normalized to the eigenvalues) were not rotated. 'University of Tanta, Tanta, Egypt. 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 90024. 42 1980 Nevada Desert Ecology 43 Results 1. Classification of the Vegetation Data (a) The clustering units.— The program cluster was used and Ed was selected as a measure of similarity. It should be mentioned that with this method of classification the stands are clustered into cells regardless of whether they form discrete groups in nature or whether they are merely parts of a contin- uum. The dendrogram shown in Figure 1 is derived from this cluster analysis. Because the paired and grouped stand clusters in the dendrogram result in linkages at various lev- els of similarity, an ecologically meaningful classification is not automatically indicated. Classification can be obtained, however, by setting more or less arbitrary threshold val- ues. Threshold values used were set at 10, 15, 20, and 29 Ed. These are indicated by hori- zontal dash lines on the dendrogram. At Threshold Line 4 there are three main clusters. Cluster 1 links together 58 stands (1 through 62) at Ed distance of about 28. Clus- ter II links together 5 stands (21 through 24) at Ed of 20. Cluster III links together stands 33, 39, and 35 at Ed of about 16. At Threshold Line 3 clusters II and III re- mained unaltered, but cluster I became dis- tinguishable as three subclusters: subcluster IA that links together 50 stands (1 through 65), subcluster IB that links together stands 54 and 62. At this level of similarity stands 19, 31, and 11 became so dissimilar to other stands that they remain rather isolated. At Threshold Line 2 the subcluster IA be- came distinguishable as five vegetational groupings. The first grouping, IAa, links to- gether 11 stands (3 through 34). The second grouping, IAb, links together 12 stands (6 through 51) at Ed of 12. The third grouping, IAc, links 8 stands (4 through 15) at Ed of about 14. The fourth grouping, IAd, links 6 stands (12 through 48) at Ed of about 14.5. The fifth grouping, IAe, links together stands 50, 64, 66, and 65 at Ed of about 16. At this level of similarity many individual stands or 63 56 59 5 20 45 17 52 6 37 7 55 40 49 22 53 13 10 15 12 43 47 50 66 23 25 31 54 2J 38 24 39 2 42 36 3 41 26 29 18 34 30 60 8 56 44 51 4 9 57 14 16 61 46 48 64 65 27 19 II 62 28 32 33 35 STAND NO. , SEQUENCE OF GROUPING Fig. 1. Dendrogram resulting from the application of the agglomerative clustering analysis. The dotted lines de- note the levels at which the dendrogram yields different vegetational groupings. 44 Great Basin Naturalist Memoirs No. 4 T\ble 1. Relative dominance of major species in each of the 66 stands ithose 2 percent or more for a given stand See Table 2 for plant abbreviations.! Staid Want species no. As Aco Ef Lt Vs La Ms Ad Cs kp CI Pfr En Lp Mercury Valley 1 2.3 1-4.4 25.-1 31.1 25.4 2 7.1 37.6 10.3 13.8 3 6.3 46.0 14.9 15.4 S 8 3.6 3.6 4 14.4 2.3 21.9 6.7 19.4 11.1 13.0 8.9 5 42.(1 13.7 15.3 12.9 2.2 5.9 5 4 4.5 fi 2.5 46.7 2.3 54 6 10.1 2.4 7 9.2 2.5 71.9 16.3 8 IDS 26.7 38.2 7.4 15.0 9 2.3.1 22.5 14.9 31.6 10 6.8 36.8 3.4 40.0 1 1 10.9 12 9.8 6.0 2.6 22.6 16.1 13 31.6 13.3 15,8 23.1 5.-5 5.0 Frenchman Flat 14 6.3 20.3 8.5 3.4 16.8 8.6 50.8 15 3.2 33.8 20.9 1 8 25.3 1 1 8 16 3.7 15.1 4.9 24.2 117 17 2.3 84.3 49 6.4 18 81.1 8.1 2.3 4.4 19 20 62.3 7.0 3.5 7.8 17.5 21 4.1 33.6 22 20.4 S.O 7.4 22.3 43.4 23 5.2 4.4 37.9 24 8.4 25 13.8 4.5 4.4 5.8 9.7 6.3 2.9 26 53.9 2.5.0 5.1 15.3 27 2.5 7.4 53.6 28 29 4.6 50.7 2.1 9.1 5.3 5.7 30 2.8 71.7 15.5 6.9 2.5 31 .39.4 54.0 54 32 315 33 100.0 34 91.3 3.5 65.2 2.0 5.7 36 12.3 6.7 8.9 24.5 47.2 37 3.7 64.6 13.4 V.~i 14.0 38 39 97.6 2.4 10 28.6 22 8 311 10.9 41 57.4 11.9 5.9 8.3 118 42 2.2 50.3 16 29.6 2.3 3.4 2.9 Rock Valley 43 6.3 7.5 II 7 20.3 28.6 2.8 6.2 16.4 11 7.6 11.3 5.8 31.3 20.0 7.0 3.3 11.8 45 46.3 27.7 13.6 8.5 2.1 46 15.6 30.0 12 I 23 ! 16.8 17 21.1 39.0 12.0 111 1.2 2.2 6.6 18 115 31.5 6.6 5 1 12 7S 9.7 49 24.0 1.5.4 2.4 22.2 10.8 11.9 7S .50 8.6 2.1 12.5 vi i 5.6 11.7 /«, 1«m Flat* 51 26.0 21.0 15.9 ss 2.6 :\:i 52 84.1 1.0 lit- 53 14.6 29.6 5.6 13.6 5 7 119 111 51 39.5 55 3.6 19.4 1.5 3.9 26.5 2.2 6.0 7.3 .56 2.2 3.3 5.4 19.9 12.8 27.9 57 28.7 12 9 2.6 16.2 SO .58 27 5 26.5 7.0 16 6 59 19.0 19.0 115 1.8 15 7 60 16.4 31.3 II .3 i7 i 1.2 Yucca Flat 5.2 63 11.5 41.4 64 10 8 16.5 23 o W 9 Nevada Desert Ecology 45 Table 1 continued. IK Li Sp Mt Mp Ta Ssp Aca Asp Pp Ls Cr VI) He 36.1 52.7 3.4 5.6 6.5 .3.3 4.3 [2.9 3.8 18.6 2.3 2.0 24.1 60.9 61.5 52.0 32.3 57.4 36.9 36.3 98.4 3.3 4.6 60.2 29.4 8.2 34.9 26.7 5.0 62.1 3.8 140 3.3 4.9 46 Great Basin Naturalist Memoirs No. 4 couples of stands become mostly dissimilar to the well-defined clusters so that they remain isolated (Fig. 1). Classification at Threshold Line 1 resulted in many fragmentary units of limited general- izable value. Sociological significance of the vegetation groupings.— For purposes of discussion each of the clusters identified, irrespective of its hierarchal level on the dendrogram, was called a vegetational grouping and named af- ter the most abundant species, that is, the species with the highest average importance value. Table 2 includes the average importance values for the different species in the various vegetational groupings. Inspection of this table gives the following explanation of the results of the cluster analysis. Vegetational Grouping IAa—Larrea triden- tata (Sesse & Moc. ex DC) Cov.: This group- ing is represented by stands from Frenchman Flat (7 stands), Mercury Valley (2), Jackass Flats (1), and Rock Valley (1). The clustering seems to be based on the presence of L. tri- dentata as a leading dominant in all the stands in the cluster. The average importance value of L. tridentata (Table 2) is 93.4 (out of 200). The associated species are generally of minor importance, except Ambrosia dumosa (A. Gray) Payne (I.V. = 23.8). Stand 36 (Fig. 1), although dominated by L. tridentata (I.V. = 112), is the last to join the cluster. This is primarily due to the presence of Atriplex ca- nescens (Pursh) Nutt. in substantial amounts (I.V. = 55). Vegetational Grouping IAb—A. dumosa: This cluster does not appear to be a very nat- ural unit, and on the basis of the subleading dominant species there would be grounds for the recognition of three smaller clusters (Fig. 1). The most influential species responsible for the segregation of this vegetational grouping into smaller clusters are L. triden- tata (in stands 6, 30, 37), Krameria parvifolia Benth and Coleogyne ramosissima Torr. (in stands 7, 8, 55, 56), Grayia spinosa (Hook.) Moq. (in stands 40, 44, 49, 51), and Oryzopsis hymenoides (Roem. & Schult.) Ricker (in stand 60). The average importance value for A. dumosa in this grouping is 77. Associated species of pronounced significance are L. tri- dentata (I.V. = 37) and K. parvifolia (I.V. = 29.4). Vegetational Grouping IAd— Transitional: Stands of this grouping also show little re- semblance and mostly fuse in pairs above an Ed of about 10. Dominance is shared by many species, among them Lycium ander- sonii A. Gray (I.V. = 22.2), Ephedra neva- densis S. Wats. (I.V. = 22.1), O. hymenoides (I.V. = 17), and C. ramosissima (I.V. = 15). Vegetational Grouping IAe—G. spinosa: The relatively high similarity between stands 64 and 68 (Fig. 1) is not only due to the dom- inance of G. spinosa, but also due to the pre- ponderance of L. tridentata, E. nevadensis, and C. ramosissima. The average importance value of G. spinosa in this grouping is 71, fol- lowed by 23 for L. andcrsonii and 22.5 for C. ramosissima. Vegetational Grouping IB— Lycium shockleyi A. Gray (L. rickardii) C. H. Mull.: All stands of this cluster are also dominated by L. shockleyi, with an average importance value of 88. Other important species are L. tridentata (I.V. = 35) and Atriplex confer- tifolia (Torr. & Frem.) S. Wats. (I.V. = 27). Vegetational Grouping IC— C. ramosis- sima: All of the five stands representative of this cluster are dominated by A. canescens. Stands 28 and 38, which fuse at very high similarity levels, are overwhelmingly domi- nated by A. canescens (I.V. = 188 and 185, respectively). The average importance value for A. canescens in this cluster is 150. Species of some importance are Stanleya pinna ta (Pursh) Britt. (I.V. = 18) and Lycium palli- dum Miers (I.V. = 8.4). Vegetational Grouping III— A. confer- tifolia: This is the last cluster to join the den- drogram, and it enters at a very low level of similarity (Ed = about 33). All stands are dominated by A. confcrti folia. Stand 33 (I.V. = 200) fuses with 39 (I.V. = 193) at the highest recorded level of similarity (Ed = 0.8). Stand 35, which is also dominated by A. canescens, has a high (45.6) importance value for A. canescens as well as for Ceratoides la- nuta (Pursh) J. T. Howell (I.V. = 22). This results in its fusion with stands 33 and 39 at a relatively low similarity level (Fig. 1). Other species in this grouping are of little signifi- cance. 1980 Nevada Desert Ecology 47 2. Ordination of the Vegetation Data (a) Ordination of Stands: According to the Bray and Curtis (1957) technique, the two-di- mensional ordination of stands (Fig. 2) has re- sulted in three distinct hyperspheres which correspond to the vegetation groupings iden- tified at Threshold Line 4 on the dendrogram derived from the cluster analysis and super- imposed on the ordination plane. Smaller vegetational groupings identified at higher similarity levels (Threshold 2) are mostly in- terconnected on the ordination plane (see dashed lines), but are still distinguishable in the form of successive groups of stands segre- gated along the primary X axis. In the application of principal component analysis (PCA), five components or axes were extracted that account for 65.3 percent of the total variation (Table 3). Plotting of stand scores (Figs. 3 and 4) on the two axes that showed a greater number of significant corre- Table 2. Average importance values for 35 perennial species in the different vegetational groupings (see Fig. 1). Lt Ad As Trans ° Gs Ls Cr Aca Aco (IAa) (IAb) (IAc) (IAd) (IAe) (IB) (IC) (II) (HI) Larrea tridentata (Lt)°° 93.4 39.2 30.3 41.8 13.5 38 24.0 _ Ambrosia dwnosa (Ad) 23.8 77.4 15.8 14.5 14.0 2 0.3 0.5 1.3 Oryzopsis hymenoides (Oh) 7.0 7.2 10.1 1.0 3.3 3 _ 3.1 2.2 Ceratoides lanata (CI) 5.0 3.6 23.7 4.0 2.9 _ 0.6 1.0 9.3 Sphaeralcea ambigua (Sa) 2.6 1.2 2.0 1.2 - 2 3.3 2.2 - Lycium andersonii (La) 16.0 5.3 17.4 38.0 23.0 _ 11.0 2.4 Mirabilis pudica (Mp) 0.1 - - 2.6 - 1 _ 3.0 _ Hymenoclea sabola (Hs) o.s — 2.5 11.2 5.1 1 8.1 _ 2.7 Acamptopappus shockleyi (As) 6.8 7.4 44.9 _ _ 21 _ _ _ Ephedra funerea (Ef) 2.0 1.1 2.3 - - 4 - - - Psorothamnus fremontii (Pfr) 5.6 0.7 0.1 9.2 Krameria parvifolia (Kp) 5.2 29.4 3.9 22.2 _ 5 _ _ _ Yucca schidigera (Ys) 3.3 _ 0.8 _ _ _ _ _ _ Yucca brevifolia (Yb) 1.8 - _ 5.0 _ 2 _ _ _ Lycium pallidum (Lp) 3.1 3.1 9.0 10.3 - - - 8.4 - Lepidium fremontii (Lf) 0.3 0.4 0.5 1.9 _ Ephedra nevadensis (En) 5.5 6.8 3.9 22.1 20.0 _ 6.3 _ _ Menodora spinescens (Ms) - 1.3 0.5 0.5 _ 4 0.8 _ _ Grayia spinosa (Gs) 11.1 10.3 37.4 17.0 71.5 - _ 0.9 0.6 Coleogyne ramosissima (Cr) - 4.5 - 15.0 22.5 6 137.0 - - Machaeranthera tortifolia (Mt) _ 1.0 0.6 5.1 _ _ 2.4 Cactus spp. (Ca) 1.0 - - -' 0.2 - 0.4 _ _ Stipa speciosa (Ssp) — - 0.9 — 22.0 _ 0.8 _ _ Tetradymia axillaris (Ta) — - 1.0 0.3 0.4 _ 3.3 _ _ Atriplex confertifolia (Aco) - 1.0 - - - 23 0.4 12.8 188.0 Atriplex canescens (Aca) 5.0 1.0 _ _ _ _ _ 150.0 15.0 Stanleya pinnata (Sp) - - - 0.3 - - - 18.0 _ Hilaria rigida (Hi) _ 0.1 0.3 _ _ _ _ _ _ Lycium shockleyi (Ls) - - - - _ 88 _ _ _ Artemisia spinescens (Asp) - - 0.6 - - - - - - Primus fasciculata (Pf) Salazaria mexicana (Sm) Thamnosma montana (Tm) Haplopappus cooperi (He) Psorothamnus polyadenitis (Pp) 'Transitional "Letters in parentheses indicate abbreviations for species' names. Table headings < respond to these abbreviations. 48 Great Basin Naturalist Memoirs No. 4 lations with environmental variables, viz., Axes I and III, show that the application of PCA technique has resulted in much better segregation of stands into groups comparable with those derived from the clustering tech- nique at Threshold Line 2 on the dendro- gram. The two vegetation groupings of A. canes- cens and A. confertifolia are separated at the negative side of the first axis. Separation of the two groupings from one another is affect- ed by the third axis. Stand 35, which joins the A. confertifolia grouping at a relatively low level of similarity (Fig. 1), appears on the or- dination plane to be more associated with the A. canescens grouping. The groupings of L. tridentata and A. dumosa, which have more or less overlapping scores on the first axis, are 80 _ 'P\ 70 - 60 50 j! 40 40 1 <^„ • ,' © r • /0$ v _ • ;-«• \f_ „] o 6 A -'„--' o ,' 20 10 0 " i s / / *© I I o ;' / / / i i i ■ j i 10 20 30 40- 5 50 60 70 BO 90 Fig. 2. Ordination of the stands on the first axes derived from the Wisconsin ordination (Bray & Curtis) technique. The classification of stands according to the cluster analysis has been superimposed on the ordination diagram. Dash- ed lines connect some of the vegetational groupings. Empty circles are stands of /.. tridentata (lAa), crossed circles are stands of A. dumosa (IAb), vertically half-blocked circles are stands of (.'. spinosa (IAe), horizontally half-blocked circles are stands of A. canrscen.s (II), large blocked circles are stands of A. shockleyi IAc), blocked triangles are stands of C. ramosissima (IC), empty triangles are stands of L. shockleyi (IB), crosses are stands of A. confertifolia (III), and small blocked circles are transitional stands (OAd). 1980 Nevada Desert Ecology 49 clearly separated on the third axis. Along this axis A. dumosa scores positive values, but those of L. tridentata are negative. Along the first axis the groupings of L. tridentata, A. sJwckleyi, L. shockleyi, and C. ramosissima are also separated from one another. The grouping of G. spinosa is primarily separated from other groupings along the third axis. It is also apparent (Fig. 4) that the groupings of L. shocklei/i and C. ramosissima occupy a more or less central position between other groupings. (b) Behavior of species along the environ- mental gradients: The behavioral pattern of eight common species as expressed by their importance value is represented separately on the ordination derived from the first and third axes of the PCA (Fig. 5:A-H). Larrea tridentata and A. dumosa attain their high importance values at the high and medium positions of the positive side of the axis. Along the third axis the two species behave rather differently: the high values of A. du- mosa are on the positive side and the high values of L. tridentata are on the negative side. Grayia spinosa and L. andersonii also exhibit definite patterns, with their high im- portance values at the medium position on nr 26 3fl o 35 24 o o©2l 33 39 • o 03 12 62 0.8 16 o 63 o25 "19 o7 56 53 08 o44 60 °55 • o51 o36 22 o 31 o 43 40 06 37 ° ♦ 49 o34 59 2> o^7 " *? A i°i 61 o58 _ o45 o4 G|4 02 26 °° 54 o ,3<2 09 3 o5 o2918 1°5 217*0 o°41 64 66 © o 10 °4 5 -1-0.6 n Fig. 3. Plotting of stands on axes I and III from the principal component analysis (PCA). 50 Great Basin Naturalist Memoirs No. 4 the first axis. Acamptopappus shockleyi shows its high values at medium to lower positions on the positive side of the first axis. On the third axis most of the high values are on the negative side. More definite patterns are those of A. canescens and C. ramosissima. They have attained their high importance values at the extreme positive and the ex- treme negative end of the third axis, respec- tively. Atriplex confertifolia attained its high importance values at the central positions of the first axis and at medium positions with regard to the positive side of the third axis. (c) Ecological significance of phytosociolo- gical gradients: Simple correlation coefficients between the phytosociological gradients represented by the five axes ex- tracted from the PCA and the various envi- ronmental variables are given in Table 3. Most of the correlations are extremely low. The lowest negative correlation (-0.39) is that between the first axis and field capacity for water relations in soil. This axis is also sig- nificantly correlated with both sodium (nega- tive) and iron (positive). Axis 2 shows no sig- nificant correlations with any of the variables studied. Axis 3 shows significant correlations and most of the variables correlated signifi- cantly with the first axis, but with opposite trends. This axis also shows significant corre- lations with both potassium (positive) and ni- trogen (negative). Axis 4 shows significant positive correlations with electrical con- ductivity and potassium, and axis 5 shows sig- nificant positive correlations with soil mois- ture retention capacity, electrical conductivity, calcium, magnesium, copper, and nitrogen. It is apparent from these correlation stud- ies that the segregation of the vegetation cov- er into distinct groupings on the ordination plane is largely attributed to variations in soil properties. The vegetational grouping of A. canescens and A. confertifolia occupy sites poor in phosphorus, organic matter, and ni- trogen, but rich in sodium, potassium, cop- per, and percent of clay. The grouping of A. dumosa reflects sites rich in phosphorus, iron, and to some extent in sodium, but poor in ni- trogen. The Grayia spinosa grouping oc- cupies sites decidedly poor in sodium, potas- sium, and fine particles, but rich in nitrogen and iron. The groupings of L. shockleyi and C. ramosissima occupy sites with more or less intermediate soil characteristics. (d) Correlation among species and species ordination: The spatial pattern of one species may be modified bv another. This leads to the question of interspecific correlation. The causes of these correlations are, however, varied. The most common cause is, no doubt, the mutual response to varying environments. There also are interactions between species that do not involve independent environmen- tal factors (allelopathic effect, competition, or amelioration or degradation of environ- mental conditions). It is, however, difficult to reach firm conclusions as to the cause of cor- Table 3. Simple linear correlation coefficients (r) between five principal components and the various environmen- tal parameters" Proportion of Accumulated Parameter total variance variance F. Lime PCA axes Percent Percent Cap. PH EC Percent 1 31.1 31.1 -0.39000 -0.13 0.04 -0.20 2 11.1 42.2 0.05 0.21 o.os 0.21 3 8.1 50.3 0.10 0.07 0.09 -0.06 4 7.9 58.2 0.22 -0.03 0.27° -0.07 5 7.1 65.3 0.38° °° 0.07 0.31 ' ° 0.08 'A single asterisk denotes a significant correlate and 0.1 percent probability levels. F. Cap. is water retention at field capacity. EC is electrical conductivity. Cations are exchangeable. Fe, Zn, and Cu are from 0.005 M DTP A extract. N is total soil nitrogen. at 5 percent probability level. Double and triple asterisks denote a significant correlation at 1 percent 1980 Nevada Desert Ecology 51 in t0.8 I / -0.3 %& '© ©\ ■• ©\ V © \ \® e ' o v^/ 1.0 \*\» •• \'': \» A/ ocb. ^r\ I ;»»; |»'; N»' •p. -0.6 IE Fig. 4. Ordination of stands on axes I and III from the principal component analysis. The classification of stands according to the agglomerative clustering analysis has been superimposed on the ordination diagram. For identi- fication of groupings see Fig. 2. Table 3 continued. Ca Na K plus Mg Na P Mg/g Fe Zn Cu N % me/ 100 g ixg/g DTPA ext Elevation -0.34 °° 0 -0.19 -0.17 -0.30°° -0.32°°° 0.31°° -0.06 -0.28° 0.10 0.18 0.12 0.08 -0.10 0.15 -0.17 -0.21 0.05 -0.22 -0.13 0.03 0.35° ° 0.28° -0.14 0.34°°° -0.16 -0.37°°° -0.21 -0.15 -0.31°° -0.10 0.23 0.34° °° 0.17 0.17 0.05 0.07 0.14 -0.21 -0.10 -0.1 -0.12 0.21 0.25° -0.20 0.18 0.11 0.13 0.31°° 0.27° 0.03 52 Great Basin Naturalist Memoirs No. 4 relations by simply observing the spatial dis- tribution of the two species in nature; how- ever, if neither species separately shows any patterning but the two random distributions are coincidental (or countercoincidental), a direct relationship between the species seems the most likely explanation (Goodall 1970). In Table 4 a partial simple linear correla- tion matrix is given for 35 common species showing the positive and negative relation- ships present. The species constellation based on correlation values is illustrated in the form of a three-dimensional diagram in Figure 6. The components involved in the construction of this diagram were extracted using the principal component analysis of the matrix of interspecific coefficients (Sneath and Sokal 1973). Five groupings of species are appar- ent-a central group and four peripherals. The arrangement of species in this diagram (M 0.8 L0« • •* • 15 ,' H»dmm Q3 •• > »6 i .5 •' «4i "6 '•■.V5 •79 »J6 • ' .• « •46 • *32 46 l?w. «28 »2S*3. »>25 * *66 0.8 nr (c) m -0.3 0.6 i »24 , m«4iu* 1.59' ' ,'16 10 >23 -0.6 (B) 1O.8 -03 I ' 7T .15 ^.*> \ *J ,23 V.92 •1 34 ;•*• 1.0 — - • 1 1-0.8 1 (0) » ,-03 08 •j • ,7 ,2C*> Jui *23 N -0-6 1.0, Fig. 5. (A-H). The behavior of eight species as expressed by their importance values along the phytosociological gradients expressed by the first and third axes of the principal component analysis. The importance value for each are superimposed upon a common ordination plane. High, medium, and low values are indicated where appropriate. A is L. tridentata, B is A. dumosa, C is G. spinosa, D is L. andersonii, E is A. shockleyi, F is A. canescens, G is C. ramosissima, and H is A. con ferti folia. 1980 Nevada Desert Ecology 53 reflects positive correlations in the main, the general position of each grouping of species being related to the negative correlations also present. The following are the five groups identified: Group 1 (upper left-hand side of the dia- gram): Grayia spinosa, L. andersonii, L. palli- dum, Stipa speciosa. Group 2 (lower left-hand side): Ambrosia dumosa, Hilaria rigida (Thurb.) Benth. ex Scribn., Yucca schidigera Roezl ex Ortgies, Ephedra funerea Cov. & Mort., Salazaria mexicana Torr., E. nevadensis, K. pa rvi folia. Group 3 (upper right-hand side): Hyme- noclea salsola Torr. & Gray, Tetradymia ax- illaris A. Nek, A. shockleyi, Machaeranthera tortifolia A. Gray, Sphaeralcea ambigua A. Gray. Group 4 (lower right-hand side): Yucca brevifolia Engelm. in Wats., Thamnosma montana Torr. & Frem., Psorothamnas fre- montii (Torr.) Barneby, Primus fasciculata (Torr.) A. Gray, Cactus species. Group 5 (central): Psorothamnus poly- adenitis (Torr.) Rydb., A. canescens, A. con- fertifolia, Mirabilis pudica Barneby, O. hyme- Fig. 5 continued. (E) 0-3 08 *3^ .«Vjb v.1?'*?, *50 0.6 (6) r08 .. • • * „ • .'S •• "49 o . * 1 0 3 • ' 2. 10 • • •• . . • * • •*• Hiqh i^ .16 . 22 14*9 lt'^.57 21 • -05 (F) * *1MU6 1.0 -0.3 -hi r i— 1 0.5 10 -i I I I (H) | 08 .."3 "•;• . High 208> 193 • 12^ * •35*35 t? •30 .5» 1.( -03 . .12 \ •• .1ft "-• • .11 . • •• 1 • * '•' • 06 i I 54 Great Basin Naturalist Memoirs No. 4 noides, Mendora spinescens A. Gray, L. nor significance in vegetation structure (very tridentata, S. pinnata, P. fremontii, Hap- low importance values), and consequently lopappus cooperi (A. Gray) Hall, L. shockleyi, such a group is not detectable in nature. On C. lanata, Artemisia spinescens D. C. Eat., C. the other hand, group 1, dominated by G. ramosissima. spinosa, L. andersonii, S. speciosa, and C. It is equally clear that none of these five ramosissima, is well defined in nature and groups is an isolated entity; each is linked to floristically structurally comparable with the the adjacent group by correlations of differ- G. spinosa grouping previously defined by ent magnitude between member species of stand classification and ordination tech- the representative groups or through inter- niques. mediate species. The deduction that these correlated groups Discussion of species represent significant communities in nature may not necessarily hold. The very The data obtained by measuring the obvious group on the lower right-hand side of amount of a species in each of n stands can the diagram (Group 4) includes species of mi- be represented by a scatter diagram of n Table 4. Part of simple linear correlation matrix for 35 perennial specimens for the northern Mojave Desert show- ig positive and negative species* relationships. Acamptopappus shockleyi (As) + 0.34° 00Oh, + 0.29°Mp Atriplex confertifolia (Aco) -0.28° La Ephedra funerea (Ef) + 0.75 Ys, +0.29°Hr Ceratoides lanata (CI) + 0.27°Lp, +0.35oooTa, + 0.63° ° "Asp Ambrosia dumosa (Ad) + 0.26 + Kp, +0.28"Hr Krameria parvifolia (Kp) + 0.50°°°En, +0.27"Pfr, +0.25°Ca, +0.29°Pf, +0.26pAd Lama tridentata (Lt) -0.27°Ta, -0.25° Aca, +0.36oooPp Lycium andersonii (La) + 0.38°ooGs, + 0.25°Oh, -0.60oooPp, -0.28"Aco Menodora spinescens (Ms) Yucca schidigera (Ys) + 0.75oooEf Sphaeralcea ambigua (Sa) + 0.33°°°Mt, +0.25°Hs Ephedra nevadensis (En) + 0.50oooGs Grayia spinosa (Gs) + 0.33°°°Ssp, +0.3 + 0.63oooCl) +0.33oooHs, +0.61oo,,Ta Psorothamnus polyadenitis (Pp) + 0.36°°°Lt Cactus sp. (Ca) + 0.25°Kp, + 0.63° " Pfr, +0.53oooPf, +0.45oooYb, +0.72ooTm I'runus fasciculata (Pf) + 0.29°Kp, +0,S2oooPl. +0.53oooCa Lycium shockleyi (Ls) + 0.35oooSm ) '«< rca brevifolia (Yb) + 0.45oooCa, +0.51oooTm Salazaria m&cicana (Sm) + 0.35° "Ls, +0.51oooTm Thamnosma montdna (Tm) + 0.27°Pfr, +0.72°°°Ca. +0.51oooYh Haplopappus cooperi (He) 'Denotes a correlation at 5 percent probability level, "at 1 percent probability . ami ° ° ° .it (I 1 percent probability. 1980 Nevada Desert Ecology 55 -P<0.001 -P<0.01 •P<0.05 Or'"" SSp V" AdQ. Fig. 6. Special ordination on the first three positive correlations are interconnected. Lette: 1. As = Acamptopappus shockleyi 2. Aco = Atriplex confertifolia 3. Ef = Ephedra funerea 4. CI = Ceratoides lanata 5. Ad = Ambrosia dumosa 6. Kp = Krameria parvifolia 7. Lt = Larrea thdcntata 8. La = Lycium andersonii 9. Ys = Yucca schidigera 10. Ms = Mendora spinescens 11. Sa = Sphaeralcea ambigua 12. En = Ephedra nevadensis 13. Gs = Grayia spinosa 14. Lf = Lepidium freniontii 15. Oh = Oryzopsis hymenoides 16. Mt = Machaeranthera tortifolia 17. Pfr = Psorothamnus freniontii axes derived from the principal component analysis. Species showing s beside each circle indicate abbreviations for species' names. 18. Cr = Coleogi/ne ramosissima 19. Lp = Lycium pallidum 20. Hs = Hymenoclea salsola 21. Hi = Hilaria rigida 22. Sp = Stanleya pinnata 23. Mp = Mirabilis pudica 24. Ta = Tetradymia axillaris 25. Ssp = Stipa speciosa 26. Aca = Atriplex canescens 27. Asp = Artemisia spinescens 28. Pp = Psorothamnus polijdenius 29. Ca = Cactus sp. 30. Pf = Prunus fasciculata 31. Ls = Lycium shoeklcyi 32. Sm = Salazaria mexicana 33. Yb = Yucca brevifolia 34. He = Haplopappus cooperi 56 Great Basin Naturalist Memoirs No. 4 points in an s-dimensional coordinate frame. Classification consists of subdividing the swarm of points into a number of disjointed sets. If the points chance to fall into several compact, widely separated groups, no diffi- culty arises and formal rules for effecting a classification are scarcely needed. According to Pielou (1969) this ideal result is rarely ob- tained when vegetation is randomly sampled. More often than not, the points representing the stands are diffusely scattered and any classification procedure is largely arbitrary. A way out of this difficulty is to ordinate the stands rather than to classify them. The purpose as in classification is still to simplify and condense the mass of raw data yielded by vegetation sampling in the hope that rela- tionships among the plant species, and be- tween them and the environmental variables, will be manifested. Ordination consists of plotting n stands in a space of fewer than s dimensions in such a way that none of the important features of the original s-dimensional pattern is lost. Or- dination has two great advantages over classi- fication. It obviates the necessity for setting up arbitrary criteria for defining the classes, and there is no need to assume that distinct classes, if there are any, are hierarchically re- lated. However, the compatability of the two approaches, viz., the classification and the or- dination techniques, was pointed out by An- derson (1965) and Goodall (1970), and in re- cent years it has become more common for classification and ordination to be used on the same data (Gray and Bunce 1972, Wil- liams and Walker 1974, Ayyad and El-Gho- nemy 1976). The importance of the study area from the phytogeographical point of view may be due to its position straddling the boundaries of the Great Basin to the north and the Mojave Desert to the south. The behavior of the biot- ic communities of the Mojave Desert in gen- eral, or in some of its sectors, has attracted the attention of many biologists. Shreve and Wiggins (1964) have described the Mojave Desert as showing its most distinctive devel- opment between 600 and 1200 m elevation (2000-4000 ft.). When it is followed thence toward the northeast or southeast, it loses some of its characteristic vegetational fea- tures and much of its distinctive flora. The basic structure of the vegetation throughout the Mojave Desert is open stands of L. triden- tata and A. dumosa. On the western edge the plants are joined and to some extent replaced by A. tridentata, G. spinosa, T. axillaris, and other perennials; and at high elevations on the north, C. ramosissima, G. spinosa are dominant. In the northern sector of the Mojave Desert, particularly in the Nevada Test Site and its surroundings, phytosociological stud- ies have been carried out by many authors. Among these are Beatley (1963, 1969, 1974, 1975, 1976), Allred et al. (1963), Rickard and Beatley (1965), Brown and Mason (1968), Wallace and Romney (1972), Romney et al. (1973), Tueller et al. (1974), and El-Ghonemy et al. (this volume). In these studies some of the vegetational units have been identified and named by various terms as communities, associations, types, subtypes, and vegetational groupings. In the most recent work by El- Ghonemy et al. (1980) the correlation be- tween the various vegetational groupings identified and the environmental variables have been demonstrated, as well as the vege- tation diversity and the successional trends among the different groupings. In the present study the application of the agglomerative clustering technique has proved useful in classifying stands into sever- al vegetational groupings. However, most of these groupings are not distinct. The mem- bers of each pair of groupings are, in most cases, linked together by having one or more of the dominant species in common. This, as Goodall (1954) mentioned, does not preclude the possibility of the classification for par- ticular ends, but it is generally more appro- priate to ordinate stands. In the present study the application of both Wisconsin and PCA ordination tech- niques emphasizes this idea and indicates that the vegetational groupings yielded by the clustering technique are generally inter- connected. However, the PCA technique has proved more efficient in segregating stands in a manner more or less similar to that achieved by the clustering approach. The better segregation of stands along the PCA ordination plan makes the description and correlation of the vegetational grouping more efficient. 1980 Nevada Desert Ecology 57 The vegetational groupings identified in the study area are more or less similar to those previously identified by El-Ghonemy et al. (this volume) with stands clustered into groupings according to their leading domi- nant species (i.e., species with highest impor- tance values). Because in the multivariate analysis the similarity between stands in- volves the use of similarity functions that take into account the whole number of spe- cies involved in community structure, the stand's characteristics for a given vegetation unit may not have the same leading dominant species. Consequently, the detailed structure of comparable vegetation groupings derived from the two approaches may not be the same. The spatial arrangement of stands along the different vegetational gradients provides evidence that variations in vegetation com- position are expressed by more than one axis of the ordination components. This implies that the distribution of vegetation in the study area is controlled by complexes of in- terrelated factors. These fall into three main groups. The first group relates to soil fertil- ity, as reflected by the concentrations of phosphorus, nitrogen, potassium, and other nutrient elements. The second complex ex- presses soil salinity, and the third complex re- lates to soil texture and water-retention ca- pacity. The role of soil fertility as a factor in the delimitation of the natural plant commu- nities has been stressed by several authors (Beadle 1954, 1962, El-Ghonemy 1966, Ayyad and El-Ghonemy 1976). In the present study the association of cer- tain vegetational groupings, for example, that of A. dumosa, with phosphorus-rich soil, has been demonstrated. The role of total soil salinity or of particular salt has been criti- cally reviewed by Chapman (1960), Gates et al. (1956), Ayyad and El-Ghareeb (1972), and Beatley (1976). The Atriplex communities at the Nevada Test Site have been described by Beatley (1976) as occupying sites at the high end of the soil salt content and fine particle gradients. These results are in full agreement with the results obtained in the present study. Conclusions (1) The application of the multivariate analysis resulted in the segregation of the vegetational cover into more or less distinct groupings. (2) The application of the principal com- ponent analysis for the ordination of the veg- etation data resulted in the segregation of stands into more or less distinct sets, com- parable in structure to the vegetation group- ings derived from the cluster analysis. (3) The physical and chemical properties of the soil play a definite role in the delinea- tion of the vegetational groupings. (4) The ordination of species scores may result in groupings of correlated species that could be floristically different from those ob- tained if ordination is based on stand scores. Acknowledgments The multivariate analyses were carried out using a Burroughs B-6700 at the Utah State University Computer Center. The authors are indebted to Dr. W. Valentine for his assis- tance in data processing. Support for this study was furnished in part by Contract EY-76-C-03-0012 between the U.S. Department of Energy and the Uni- versity of California and an IBP Desert Biome subcontract with Utah State Univer- sity and the University of California. Literature Cited Allred, D. M., D. E. Beck, and C. D. Jorgenson. 1963. Biotic communities of the Nevada Test Site. Brig- ham Young Univ. Sci. Bull., Biol. Ser. 2(2):52. Anderson, D. J. 1965. Classification and ordination in vegetation science: controversy over a nonex- istent problem? J. Ecol. 53:521-526. Ayyad, M. A., and R. El-Ghareeb. 1972. Micro- variations in edaphic factors and species distribu- tion in a Mediterranean salt desert. Oikos 23(1): 125- 131. Ayyad, M. A., and A. El-Ghonemy. 1976. Phytosociolo- gical and environmental gradients in a sector of the western desert of Egypt. Vegetation 31(2):93-102. Beadle, N. C. W. 1954. Soil phosphate and delimitation of plant communities in eastern Australia. I. Ecology 35:370-375. 1962. Soil phosphate and delimitation of plant communities in eastern Australia. II. Ecology 43:281-344. Beals, E. W. 1960. Forest bird communities in the Apostle Islands of Wisconsin. Wilson Bull. 72:156-181. Beatley, J. C. 1963. Vegetation and environment of the Nevada Test Site. Ecol. Soc. Amer., Bull, (abstr.) 44:123. 58 Great Basin Naturalist Memoirs No. 4 1969. Vascular plants of the Nevada Test Site, Nellis Air Force Range, and Ash Meadow (north- ern Mojave and southern Great Basin deserts, south central Nevada). USAEC Report, UCLA 12-705. 1974. Effect of rainfall and temperature on the distribution and behavior of Larrea tridentata (creosote bush) in the Mojave Desert of Nevada. Ecology 55:245-261. 1975. Climate and vegetation pattern across the Mojave Great Basin Desert transition of southern Nevada. Amer. Midi. Natur. 93(l):53-70. 1976. Vascular plants of the Nevada Test Site and central southern Nevada. ERDA Technical Infor- mation Center. TID-26851. 803 p. Bray, J. R., and J. T. Curtis. 1957. An ordination of the upland forest communities of southern Wiscon- sin. Ecol. Monogr. 27:325-349. Brown, K. W., and B. J. Mason. 1968. Range survey area 18, Nevada Test Site. U.S. Dept. of Health, Education and Welfare, Public Health Service. SWR 116-52. Chapman, V. J. 1960. Salt marshes and salt deserts of the world. Interscience Pub., Inc., New York. 352 P- El-Ghonemy, A. A. 1966. Soil-vegetation relationships for some major plant communities in south- western New South Wales, Australia. Unpub- lished dissertation, University of New England, Armidale. El-Ghonemy, A. A., A. Wallace, and E. M. Romney. 1980. Socioecological and soil-plant studies of the natural vegetation in the northern Mojave Desert-Great Basin desert interface. Great Basin Nat. Mem. 4:71-86. Gates, D. H., A. L. Stoddart, and W. C. Cook. 1956. Soil as a factor influencing the plant distribution on salt deserts of Utah. Ecol. Monogr. 26:155-173. Goodall, D. W. 1954. Objective methods for the classi- fication of vegetation. III. An essay on the use of factor analysis. Aust. J. Bot. 2:304-324. 1970. Statistical plant ecology. Ann. Rev. Ecol. Syst. 1:99-124. Gray, E. J., and G. H. Bunce. 1972. The ecology of Morecambe Bav. VI. Soils and vegetation of salt marshes: A multivariate approach. J. Appl. Bot. 9:111-234. Pielou, E. C. 1969. An introduction to mathematical ecology. Wilev (Interscience), New York. Rickard, W. H, and J. C. Beatley. 1965. Canopy cov- erage of the desert shrub vegetation mosaic of Nevada Test Site. Ecology 47:524-529. Romney, E. M., V. Q. Hale, A. Wallace, O. R. Lunt, J. D. Childress, H. Kaaz, G. V. Alexander, J. E. Kinnear, and T. L. Ackerman. 1973. Some char- acteristics of soil and perennial vegetation in northern Mojave Desert areas of the Nevada Test Site. AEC Report UCLA 12-915. Biomed. and Environ. Res. Report TID-4500. Shreve, F., and W. Wiggins. 1964. Vegetation and flora of the Sonoran Desert. Vols. 1 and 2. Stanford University Press, Stanford, Calif. Sneath, P. H. A., and R. R. Sokal. 1973. Numerical tax- onomv. Freeman, San Francisco. Tueller, P. T., D. B. Allen, R. Everett, and J. B. Davis. 1974. The ecology of Hot Creek Valley, Nevada, and nonradiation effects of an under- ground nuclear detonation. U.S. Atomic Energy Commission, R 96, NVO 409-2. Wallace, A., and E. M. Romney. 1972. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Technical Information Ser- vices, USAEC Report TID-25954. Williams, C. H., and H. B. Walker. 1974. The vegeta- tion of tropical African Lake: classification and ordination of the vegetation of Lake Chilwa (Ma- lawi). J. Ecol. 62(3):831-853. A PHYTOSOCIOLOGICAL STUDY OF A SMALL DESERT AREA IN ROCK VALLEY, NEVADA A. A. El-Ghonemy1, A. Wallace2, E. M. Romney-, and W. Valentine' Abstract.— The aim of this study was to gain more understanding of the compositional structure of vegetation in the US/ IBP Desert Biome validation site located in Rock Valley, Nevada. The vegetation data collected from 85 stands, randomly distributed to cover all physiographic variations in the study site, permitted categorization of the vegetation units either by coordinates or by class membership. The vegetational groupings so identified were then used for constructing a more reliable vegetation map for the Rock Valley validation site. Multivariate statistical methods have been increasingly used in an attempt to reduce the complexity of plant ecological data and pro- vide a clearer understanding of the under- lying pattern. This in turn can form the base of a second, more rewarding phase of phy- tosoeiology, i.e., the causal nature of this pat- tern. Two basic approaches have been used to simplify the complex ecological data: 1. Classification: In this approach the stands or the sampling units are arranged in groups, the members of which have certain common properties. 2. Ordination: Such a technique attempts to find the major axes of variation. Each sample unit can then be related to one or more of these axes so as to convey maximum information about its composition and rela- tionships with other sample units. As Goodall (1970) points out, any particular piece of vegetation can be categorized either by coordinates or by class membership, the lat- ter being less precise but more convenient. The initial inventory of Rock Valley began in 1971. The US/IBP Desert Biome Program, in seeking to understand the functioning of the arid land ecosystem, has established re- search areas in each of four major arid land types in western North America. One of them was in Rock Valley, Nevada. The Desert Biome research program de- sign embraced two types of endeavors. One involved the investigation of specific abiotic and population processes and the devel- opment of models of these processes and of the function of large systems. The other in- volved the testing of these models by com- paring their prediction with actual measure- ments of changes in the states of the desert ecosystem. The validation of a system model required, then, an exhaustive initial inventory of the system followed by periodic eval- uations of extensive arrays of state variables and the external influences impinging upon them. During the spring of 1971, the IBP valida- tion site in Rock Valley was delimited. The site is about 0.46 km2 in extent. In July 1970 the site was being photographed at two scales, 1:2400 and 1:600. These photographs are being kept as a permanent record and could be used to evaluate changes brought about by continued use of the area. Other de- scriptions of the site are reported by Turner (1973, 1975, 1976) and Turner and McBrayer (1974). The plant taxonomy of the area is giv- en by Beatley (1976). The objective of this work was twofold: (1) conduct initial inventory of the micro- variations in vegetational structure, and (2) present such variations in the form of a vege- tation map delimiting the boundaries of the identified vegetational units. Such informa- tion is prerequisite for future assessments in vegetational changes. 'University of Tanta, Tanta, Egypt. ^Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles. California 90024. 'Utah State University, Logan, Utah 84322. 59 60 Great Basin Naturalist Memoirs No. 4 This study is closely related to those pre- viously carried out by El-Ghonemy et al. (1980a, 1980b, this volume) in the northern Mojave Desert, in which full account is given on the location, physiography, climate, vege- tational groupings, successional trends, and community diversity. Methods Selection of stands and sampling technique: Sampling of perennial vegetation was carried out in 190 stands in quadrats of 50 X 2 m size. The coordinates of these quadrats were generated by a computer pro- gram designed to insure random dispersion (Wallace and Romney 1972). Density mea- surements of each species at each site were determined. Shrubs with canopies over- lapping the quadrat boundaries were counted inside only when their root crowns were in- side the boundary line. Detailed characterization of soil was devel- ped from four soil profiles excavated at each of the four corners of the validation site. These profiles were dug to the respective hard pan layer underlying the area. The soil profiles were described and characterized ac- cording to the USD A 1960 soil classification and seventh approximation system. Soil chemical analysis was according to the U.S. Salinity Laboratory Staff (1954) procedures. Multivariate analysis of the vegetation data: One classification and one ordination technique were applied. The clas- sification technique is the unweighted pair- group method of the agglomerative cluster- ing technique, using the arithmetic averages to compute the similarity between a cluster and a stand which is a candidate for entry into a cluster (Sneath and Sokal 1973). The Euclidean distance (ED) was used as a mea- sure of similarity among stands. The ordination technique is that of the principal component analysis of the matrix of interstand correlation coefficients (Sneath and Sokal 1973). Eigenvectors (normalized to eigenvalues) were not rotated. To facilitate data processing, the number of stands (190) was reduced through random selection to 85 stands. Results and Discussion Classification of the vegetation data: Figure 1 shows cluster analysis dendrograms with the dotted horizontal lines denoting the levels at which clusters were distinguished 1 y 7 12 18 15 3 20 5 64 60J4 35 J7 81 65 66 68 56 85 61 63 78 45 44 51 55 57 84 80 74 32 43 4 49 29 2 31 28 48 14 23 21 8 10 H 17 19 25 13 39 40 53 69 82 73 38 70 83 72 42 56 54 " 67 30 41 36 71 62 77 79 59 7 5 76 33 46 27 47 24 26 5C 6 22 16 Fig. 1. Dendrogram resulting from the application of the agglomerative clustering analysis. The pecked lines de- note the levels at which the dendrogram yields meaningful vegetational groupings. 1980 Nevada Desert Ecology 61 Table 1. Estimated densities (in on the Rock Valley validation site. ' mber of individuals /ha) of the perennial species in seven vegetational dth relative density as percentage given in parentheses. groupings Species Ba Vegetational groupings0 Bb CD Ambrosia dumosa" ° Grayia spinosa Lyciwn pallidum Kramcria pan ifolia harreo tridentato Ephedra nevadensis Ceratoides lanata Lijcium andersonii Machaeranthera tortifolia Acamptopappus shockleyi Oryzopsis hymenoides Psowthainnus fremontii Coleogyne ramosissima Scdazaria mexicana Mirabilis pudica Opuntia exhinocarpa Encelia virginensis 2635 1008 2635 4026 2050 4758 3806 (29.5) (18.6) (36.5* (36.0) (15.9) (32.6) (24.6. 2.342 132 176 630 2196 .3118 4978 (28.2) (2.2) (2.4) (5.6) (17.0) (21.3) (32.2) 878 220 410 659 1244 878 1025 (10.0) (3.7) (5.7) (5.8) (9.6) (6.0) (6.6) S7S 1470 1318 878 1771 1098 1756 (10.0) (25.0) 18.2) (7.8) (13.7) (7.5) (11.3) 732 1180 849 732 805 7.32 1610 (8.2) (20.0i 11.7 (6.5) (6.0) (5.0) (10.4) 439 (350 805 1464 658 1171 644 1.9 1 1.0 ill.l) (13.1) (5.0) (8.0) (4.2) 4:30 15 88 1830 3806 1171 790 (4.0! (0.2) (1.2) (16.3) (30.4) (8.0) (5.0) 293 052 732 322 366 483 205 0.0 20 37 37 0.0 293 76 0.5) (0.5) (0.3) (2.0) (1.2) IS 20 0.0 0.0 132 29 (0.2) 0.0 (0.4) (1.0) (0.9) 40 102 148 146 0.0 293 132 0.6 (1.7i (2.0) (1.4) (2.0) (0.9) 0.0 0.0 0.0 73 (0.7) 0.0 22 (0.1) 0.0 0.0 0.0 6 (0.1) 415 (4.2) 0.0 0.0 0.0 73 0.8 0.0 0.0 0.0 0.0 0.0 3 (0. 1 ) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 17 (0.2) 0.0 0.0 0.0 0.0 0.0 22 (0.1) 0.0 0.0 °A = A. dumosa-C.. spinosa. Ba = K. partifulia-L. tridentata. Bb = A. dwnosu-k. parvifolia. C = A. dumosa-C. lanada. D = C. lanata-G. spinosa. E = A. dumosa-C. spinosa-E. nevadensis. F = C spinosa-A. dumosa. ""Nomenclature according to Beatlev 1976. 62 Great Basin Naturalist Memoirs No. 4 and identified. At threshold line 2, six clusters (A-F) were identified and named after two or more of the species with the highest density values. Stand number 6, dominated by Gray- ia spinosa (Hook.) Moq. (density = 41 per- cent), being dissimilar to all other stands, re- mained as a separate unit. At a slightly higher level of dissimilarity this stand, how- ever, fused with a neighboring grouping co- dominated by C. spinosa and Ambrosia du- mosa (A. Gray) Payne. The following is a description of the vege- tational groupings: Grouping A (A. dumosa-G. spinosa): This grouping is represented by 12 stands (Fig. 1) covering most of the northern part of the study area (Fig. 2). The two most abundant species are A. dumosa and G. spinosa. The area occupied by this grouping represents about 21 percent (10 ha) of the whole area. The soil supporting this grouping is char- acterized by deeper horizons and a relatively more favorable moisture regime. Detailed physical and chemical attributes of the soil profile sampled within one of the representa- tive stands of this grouping (Table 2) indicate predominance of coarse materials, relatively high percentage of water-soluble cations and anions, and low exchangeable sodium per- centages. Grouping B (A. dumosa- Krameria parvi- folia Benth.): Most of the stands (50) belong to the grouping. Inspection of Figure 1 shows that this grouping is not a natural one, and at a slightly higher similarity level (threshold line 1) there would be grounds for the identi- fication of two subgroupings, Ba and Bb (Table 1). Subgrouping Ba [K. parvifolia-Larrea tri- dentata (Sesse & Moc. ex DC.) Cov.]: The area occupied by this subgrouping covers about 19 ha representing about 40 percent of the study area (Fig. 2). Properties of soil pro- files collected from two representative stands within this community indicate high lime content, low values for water-soluble cations, and a moderate exchangeable sodium per- centage (Tables 3 and 4). Table 2. Physical and chemical attributes of soil profile at the northwest corner of the study area. Elev itioi Slope Area fe et % Aspect Ph ysiography Erosion Rock Valley 3340 2 NE Ba ada Moderate Depth Color Co or ( '(insistence Horizon cm dry we Phase dry Al 000-009 10YR5/4 L0YR4/3 Smooth Soft A2 009-021 10YR' 73 L0YR5/4 Smooth Sltl) hard CI • 021-032 10YR7/3 10YR5/4 Gravelly Soft C2 032-050 10YR" 73 L0YR6/4 Cobb cky lcky iek\ 11.6 10.0 21.0 24.4 77.9 65.6 64.1 62.7 5.8 16.5 8.6 6.6 4.7 7.9 6.2 6.3 Sat. extract soluble Cations and anions Percent Na K Ca Mg CI N03 S04 Sat. Ext. Boron (<2.0mm) (MEQ/1 ter) (MEQ/liter) ppm 5.0 13.5 21.7 19.0 1.00 0.70 0.90 3.00 4.10 0.29 0.65 0.13 20.13 4.69 3.35 4.02 13.77 1.83 1.87 1.26 1.60 0.60 1.50 1.40 0.00 0.00 0.00 0.00 0.40 0.10 0.15 0.16 0.00 0.00 0.00 0.00 P DTPA-extractable micronutrients Organic N Str (NaHC03) ppm Fe ppm Zn ppm Cu ppm Mn ppm icture 3.04 0.52 0.52 0.06 2.0 0.7 0.7 1.2 1.20 0.70 0.80 0.70 0.24 0.32 0.20 0.15 14.50 2.75 5.60 5.90 .120 .030 .032 .030 Wk.Fine Str.Med. Wk.Fine No Str. Platy Platv Sub.Ang.Bl Single.Gr 66 Great Basin Naturalist Memoirs No. 4 Subgrouping Bb (A. dumosa-K. parvi- folia): This subgrouping links together 17 stands in five patches scattered in a mosaic fashion. The total area occupied by this sub- grouping represents about 21 percent of the area. Inspection of Figure 2 indicates that parts of the vegetational zones constituting this subgrouping occupy transitional posi- tions between the southern and northern halves of the study area. Grouping C [A. dumosa-Ceratoides lanata (Pursh) J. T. Howell]: This grouping com- prises four stands occupying intermediate po- sitions between most of the identified group- ings. The most significant difference in floristic composition between grouping C and the neighboring groupings is the very low density of Coleogyne ramosissima Torr. in these later groupings, though its density in grouping C exceeds 400 plants per ha. The area occupied by grouping C represents about 2 percent of the study area. Grouping D (C. lanata-G. spinosa-A. du- mosa): This is also a transitional grouping comprising two stands and occupying a tiny area covering about 1 percent of the study site. The distinction between this grouping and the neighboring ones is principally based on the relatively high abundance of C. lanata (Table 1). Grouping E (A. dumosa-G. spinosa-Eph- edra nevadensis S. Wats.): This grouping comprises seven stands, mostly linked togeth- er at relatively low similarity levels. The area representing this grouping covers about 12 percent of the studv area in two patches (Fig. 2). The properties of soil collected from one of the representative stands of this grouping (Table 5) are characterized by relativelv high phosphorus and low lime content. Grouping F (G. spinosa-A. dumosa): This grouping comprises two patches covering about 3 percent of the north-eastern part of Table 5. Physical and chemical attributes of soil profile at the southeast corner of the stuck area. Elevation SI ope \rea feet % Aspect PI biography Erosion Rock Valley 3360 3 NE Bajada Slighl Depth Color Col Con sistence Horizon cm dry wet Phase dry All 000-006 L0YR5/3 10YH4/2 Grav ellv Loo A 12 006-012 10YR6/3 L0YR4/3 Sinn, .th Soft CI 012-023 10YR7/3 L0YR4/3 SlIKH ith Soft C2 023-034 10YR7/3 10VR4/4 Smoi >th Soil C3C ' 034-057 10YR6/3 10VR4/4 Col.l »&Gravl Solt Percenl moisture retenl ion PH Sat. 1 ■:< 25 0 1 /3 1 15 pH iiinihos Horizon Sat. Bar Bar Bar Paste Ext. cm) All 31.5 8.9 7.7 5.8 8 ! 8.9 1.37 \12 28.8 8.1 6.9 5.2 8.7 8.9 0.61 CI 27.6 10.5 9.3 5.6 8.8 9.0 0. H C2 27.5 12.2 9.3 5.4 8.8 9.0 0.12 ( 3C 27.1 12. (i 9.2 5.7 8.7 S.S o 12 Organic carbon Exchangeable cations Exch. Na ( ai n hi Ml u loo j ;„, Exch. Cap. Horizon % \a K ( :a + M g % MEQ lOOgm Ml 0.87 0.83 1 .34 7.33 8.7 9.5 V12 0.49 0.71 L.36 7.99 7.4 10.0 CI 038 0.97 L.56 S.72 8.6 1 1 .3 C2 0.32 1.07 lis 6.95 11.3 9.5 C3C 0.30 1.29 1.67 7.04 12.9 10.0 1980 Nevada Desert Ecology 67 the study area (Fig. 2). The big patch oc- cupies an intermediate position between groupings A, D, and F, and the small patch represents a small island within grouping A. The most important species of this group- ing are G. spinosa (4978 plants/ha) and A. dumosa (3806 plant/ha). Subordinate species are those of K. parvifolia and L. tridentata. It is obvious that the application of the ag- glomerative clustering technique in vegeta- tion analysis has resulted in identifying dis- tinct vegetational groupings. Although interconnected, they are quite recognizable in the field and could be used in drawing a reliable vegetation map for the study site (Fig. 2). Ordination of the vegetation data: The ordination of stands along the second and third principal component axes is illus- trated in Figure 3. The groupings and sub- groupings derived from the clustering analy- sis exhibit a clear pattern on the ordination plane. On this plane three major vegetational zones are immediately obvious, a central zone and two lateral ones. The central /one includes subgrouping Bb and Grouping C; the right-hand side zone includes sub- grouping Ba and the left-hand side zone in- cludes groupings A, D, E, and F. The separa- tion between these three vegetational zones is effectuated along the second principal axis. On the other hand the distinction between groupings A, C, E, and F is expressed by the third principal axis (Fig. 3). It is worth noting that groupings A, C, D, E, and F, which exhibit fusion between then- stands at remarkably low similarity levels (Fig. 1), occupy the left-hand side of the ordi- nation plane (Fig. 3) and cover in mosaic fashion the northern half of the study area (Fig. 2). On the other hand, subgroupings Ba and Bb, whose stands fuse together at rela- Table 5 continued. % Surfae Soil stiiniiit's origin Relict Drainage Permeability 40-60% Limestone Smooth Well Moderate Consb Particle size distribution (mm) % Consistence tence ( loarse sam Fine sand Silt Clay moist wet 2-0.2.5 0.25-0.05 0.05-0.002 < 0.002 Friable Nonst cky 9.6 82.8 3.9 3.7 Friable Nonstick v 8.6 84.5 4.3 2.7 Friable Nonst cky 8.0 80.2 7.2 4.6 Friable Nonst cky 10.0 78.8 7.2 4.1 Friable Nonst cky 26.8 63.8 5.6 3.8 Sat. extract soluble Cations and anions Percent Na K Ca Mg CI N03 SU4 Sat. Ext. lime (<2.0mm) (MEQ/1 iter) (MEQ/liter) ppm 11.4 0.60 1.25 13.42 3.53 1.20 0.00 0.35 0.00 5.5 0.25 0.70 8.05 1.08 0.70 0.00 0.16 0.00 15.0 0.30 0.60 3.87 1 .34 0.50 0.00 0.14 0.00 UK) 0.40 0.73 4.02 1.19 1.5(1 0.00 0.14 0.00 36.5 0.45 1.00 2.68 1.23 1.10 0.00 0.14 0.00 P DTPA-e.xtraet ible micronn rients Organic N (NaHC03) Fe Zn Cu Mn ppm ppm ppm ppm ppm % Structure 2.36 1.7 2.75 0.20 12.00 .091 Wk.Fine Platy 1.64 1.2 1.45 0.15 4.00 .047 Wk.Fine Sub.Ang.Bl 0.40 0.6 1.10 0.20 3.85 .041 Wk.Fine Sub.Ang.Bl 0.00 0.6 0.85 0.18 4.30 .034 Wk.Fine Sub.Ang.Bl 0.00 0.7 1.70 0.18 4.10 .032 Wk.Fine Sub.Ang.Bl 68 Great Basin Naturalist Memoirs No. 4 tively high similarity levels, occupy the right- hand side of the ordination plane and cover extensive patches in the southern half of the study area. In Figure 4 (A-F) an indication of the abundance of some common species is plotted on the stand ordination to illustrate some aspects of their phytosociological be- havior. For each species, the range of density values was divided into quartiles (I-IC) in or- der of increasing density. Stands in which a given species concur with densitv values in the fourth quartile are surrounded by pecked line. For some species these stands occur in one grouping (e.g., C. Janata and G. spinosa), but for others they are distributed among two or more groupings (e.g., L. tridentata, L. an- dersonii, and K. parvi folia). It is equally clear that none of these species can be considered as leading dominant (species with the highest densitv value) for the whole sectors of the study area. Instead, each species exerts local dominance or is distinctly more important in certain grouping of stands. In a previous study (Turner and McBrayer 1974), The Rock Valley validation site was subjectively divided into six vegetational zones. These zones, although differing in the relative abundance of the various species, were all characterized by having A. dumosa as a leading dominant species. Five of these vegetation zones occupy the northern half of I 0.7" • ,«s, / IF _, / \ - - " " x /' s — 4. — ** N , - ' x / ^« • \ A • s'~ X x / R , X X / x x / • \ X xx vx . XX, X, IT i ! ii W! / ** */ , -Li- 1 0.8 (A i\ !

0i ^ a ; \ o / •N" (' © " • \l ° V v® •A± o o / "c -0.6- IE Fig. 3. Ordination plan.- ,>l stands ol the Rock Valle) valida component axes. Pe( ked lines encircle stands belonging to ea< agglomerative clustering analysis i Fig. 1 ). the plane of the (1 third principal Nevada Desert Ecology 69 (A) I 11 • 0.7 XI I.! I • • 11 • KM*? i. • J J. J #1 :i T. .T I.tl #I 0.8 \ T^IH. I At. t i • • i • i. •1 •I !• .j. "1.1 !• 4-1 rfi* 0.9 - trJ n m 0.8 (B) i Li GT 07 --. I ^•/••' i i. \ r I-*]E> .If / *i«i I ' L J , 0.9 1 0.9 Fig. 4 (A-F). Stand ordination showing density quartiles (I-IV) on an increasing scale of density for selected spe- cs. Peeked lines surround stands in which the species is represented with a density value in the fourth quartile. (A) mbrosia durnosa, (B) Lycium andersonii, (C) Lama tridentata, (D) Grayia spinosa, (E) Ccratohles lanata, (Ft Kra- cn'« parvifolia. 70 Great Basin Naturalist Memoirs Fig. 1 continued. 0.8 (C) r j r"i.**..?? *ii ° IE 0.7 0.5- .131 -nr ,1L in. jr t. -TTT ^^ / /ir 'ur ?0 n. • m. ■ • I 'J'f •IL././ HI 0.9 (D) n 0.7 - I \ -1 \ 3> - j . i i • i. • r7. E* i • 5 \ l ' I .1 I. I i. i. 0.6. i" • • 0 < I r Nevada Desert Ecology Fig. I continued. (E) I I 0.7- i2 , .J — i Q9 08 I# v& i i. I JL I i * T •r n 0.6' (F) \¥ r • • ■*• • I 0.7 t t / h; Vl-1* J"* 1 i \ ^ Hi 0.6 1 I 0.9 72 Great Basin Naturalist Memoirs No. 4 the validation site; the southern half is occu- pied by zone six. In the present study the ap- plication of the agglomerative clustering ap- proach in vegetation analysis, substantiated by the principal component analysis, resulted in the identification of seven vegetational groupings segregated among 20 vegetation zones (Fig. 2). In these zones the leading dominant species is not necessarily A. du- mosa. Other species such as K. parvifolia and G. spinosa are also leading dominants in fair- ly extensive patches of the vegetational cov- er. We arrived at the following conclusions: 1. Classification and ordination techniques have proved to be compatible, at least in a general way, and have resulted in better analysis for the vegetation date collected from the Rock Valley validation site. 2. Each species has its own distributional pattern; certain species may have similar pat- terns, but no two are identical. 3. Improved vegetation mapping for the study area was possible, based on vegeta- tional groupings identified through the appli- cation of the agglomerative clustering analy- sis. Acknowledgments This study was supported by Contract EY- 76-C-03-0012 between the U.S. Department of Energy and the University of California and by the US/IBP Desert Biome Program, Logan, Utah. Literature Cited Beatley, J. C. 1976. Vascular plants of the Nevada Test Site and central-southern Nevada. Tech. Informa- tion Center, Office of Tech. Information, ERDA Report TID- 16881. El-Ghonemy, A. A., A. Wallace, and E. M. Romney. 1980a. Socioecological and soil-plant studies of the natural vegetation in the northern Mojave Desert-Great Basin desert interface. Great Basin Nat. Mem. 4:71-86. 1980b. Multivariate analysis of the vegetation in a two-desert interface. Great Basin Nat. Mem. 4:40-56. Turner, F. B., ed. 1973. Rock Valley validation site re- port. 1972 progress report. US/IBP Res. Memo 78-2. 1975. Rock Valley validation site report. US/IBP Desert Biome Res. Memo. 75-2. . 1976. Rock Valley validation site report. US/IBP Desert Biome Res. Memo 76-2. Turner, F. B., and J. F. McBrayer. eds. 1974. Rock Valley validation site 1973 progress report. US/IBP Desert Biome Res. Memo 74-2. Goodall, D. W. 1970. Statistical plant ecology. Ann. Rev. Ecol. Syst. 1:99-124. Sneath, D. H. A., and R. R. Sokal. 1973. Numerical taxonomy. W. H. Freeman, San Francisco. Wallace, A., and E. M. Romney. 1972. Radioecologv and ecophysiology of desert plants at the Nevada Test Site. National Technical Information Ser- vices. USAEC Report TID-25954. U.S. Salinity Laboratory. 1954. Methods for soil char- acterization. Pages 83-126 in L. S. Richards, ed. Diagnosis and improvement of saline and alkali soils. USDA Handbook 60. SOCIOECOLOGICAL AND SOIL-PLANT STUDIES OF THE NATURAL VEGETATION IN THE NORTHERN MOJAVE DESERT-GREAT BASIN DESERT INTERFACE A. A. El-Ghonemy1, A. Wallace-, and E. M. Romney2 Abstract.— The purpose of this study is to further describe the distribution, habitats, and ecological character- istics of the natural vegetation in the northern sector of the northern Mojave Desert. Sixty-six stands were classified on the basis of shared leading dominant species. Each of these groupings is well defined and represents a sociolo- gically distinct entity quite recognizable in the field. The relationships between each vegetational grouping and sev- eral environmental variables were statistically analyzed. Significant differences were found among plant groupings with respect to soil moisture tension, absolute and relative amounts of exchangeable Na, exchangeable K, cation exchange capacity, and elevation. The analvsis of the relationship between the phytosociological behavior of the major leading dominant species and the environmental variables shows that some of the simple, or multiple, linear correlations obtained with regard to Larrca tridentata (Sesse & Moc. ex DC.) Cov. were highly significant. Atriplex conferti folia (Torr. & Frem.) S. Wats. and Atriplex canescens (Pursh) Nutt. showed the highest number of significant correlations obtained. Diversity varies from one vegetational grouping to the other as well as between stands of the same grouping. The grouping of L. tridentata has proved to be the most widespread, diversified, and, consequently, the most stable vege- tation cover in the study area; it, therefore, represents a climax community. The vegetational grouping dominated fay A. confertifolia, on the other hand, appears not to be a climax community. The patchiness of the earth's surface in terms of climate, edaphic factors, and phys- iography extends from large areas to minute areas; that is, the difference may be major, as, for example, between desert and grassland ecosystems, or minor, as between the soil sur- face under a shrub and the surface a few cen- timeters away. The most basic relation between the patch- iness of the environment and the forms and distribution of organisms is that of plants. In given climate and soil conditions, certain plant species can survive. In addition, all plants that can survive a particular set of en- vironmental conditions themselves contribute to local climate and microclimatic condi- tions; all interact to form a characteristic rec- ognizable ecological system. The relationship between the biotic and abiotic components of a given ecosystem is complicated, and data available that relate the behavior of the whole biotic components to a biotic factor are scarce. This paucity of data is understandable in view of difficulties involved in collection, particularly when matters are related to such large-scale ecosys- tems as deserts. Additional ecological investigations in the northern Mojave Desert areas destined for various exploitations are needed. Many im- portant studies have already been made in the general area (Allred et al. 1963, Beatley 1963, 1969, 1974, 1975, 1976, Wallace and Romney 1972, Romney et al. 1973), but much yet remains to be explained. Special stress should be laid on the study of the natural vegetation and its synecology, with special attention to soil-vegetation rela- tionships. This paper has an aim of providing a quantitative description of the vegetation and environment for certain sites in the northern sector of the Mojave Desert. It in- cludes an assessment of the relationship be- tween the distributional behavior of some vegetational groupings and local environ- mental variations. 'University of Tanta, Tanta, Egypt. ^Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 90024. 73 74 Great Basin Naturalist Memoirs No. 4 Study Area Physiography and Soil Characteristics The physiography of the study area is de- scribed in the above references. Briefly it is characterized by low-lying, sparsely vegeta- ted, rugged mountain ranges and intervening valleys into which the erosional material has been deposited over ages, creating extensive alluvial fanlike deposits. These deposits, ex- tending from the bases of the mountains and hills, comprise the bajadas, or foothills. The composition of the bajadas is reflected in the source from which the erosional material was derived and the degree of incline and deposi- tion. Another characteristic feature of the north- ern Mojave Desert is those valleys in which moisture is trapped when runoff occurs from the surrounding terrain. The silt-laden wa- ters, which eventually reach the lowest eleva- tion in such valleys, concentrate as ephem- eral bodies which, upon elevation, leave a deposit of fine silt and clay that becomes very hard when dry. These lake beds are termed playas and for the most part lack con- spicuous vegetation. These physiographic features of the Mojave Desert are in some re- spects similar to the Arabian inland deserts in Syria, Jordan, Iraq, and Saudi Arabia. The principal geographic areas in which the present study was conducted are located in two closed and three open drainage basins, located on the Nevada Test Site in southern Nevada. Two of the five valleys in this area, Frenchman and Yucca valleys, are closed catchment basins in which large playas exist. They, particularly Yucca, represent transition to Great Basin vegetation. The other valleys, generally known as Rock Valley and Jackass Flats and Mercury Valley, are not landlocked basins. They all have transitional Great Ba- sin-Mojave Desert vegetation. They drain to the southwest into the Amargosa drainage system terminating in Death Valley. The northernmost flat in the study is Yucca Flat, south of which is Frenchman Flat, separated from the former by a low ridge called Yucca Pass. The alluvial thickness toward the cen- tral part of Yucca Flat is about 250-300 m, and about 200 m in the playa in Frenchman Flat (Allred et al. 1963). Most of the soils in the different study sites are calcareous, but some are low in lime because of the influence of alluvial materials of volcanic origin. Climate The climate of the area is also described in the above references. The study sites lie to the east and leeward side of the Sierra Ne- vada, which forms a massive barrier to the prevailing winds from the west. This barrier to moist air has resulted, at least in part, in a vast desert region of which the northern Mo- jave Desert is a part. Annual rainfall averages from 10 to 15 cm, most of which occurs in late winter. Summer rainfall is principally due to convective showers associated with thunderstorms, which in turn are induced by high humidity. This phenomenon results in considerable annual variation in precipi- tation, with some microgeographical varia- tion within a given year. There is also a pro- nounced difference in mean annual rainfall. Snowfall is sparse in the lower valleys and usually only present for short winter periods. The average temperatures vary with the location, from 18 to 25 C, maximum, and 4 to 11 C, minimum. The highest temperature recorded was 44 C in July 1959 at Jackass Flats; the minimum was -16 C in January 1955 at Yucca Flat. The relative humidity varies from 2 per- cent to approximately 90 percent, the highest occurring in predawn hours and the lowest during daylight hours. The average is about 21 percent in summer as contrasted with 30 percent in winter. Materials and Methods Selection of Stands and Vegetation Sampling To encompass a broad spectrum and diver- sity of vegetation types, 66 stands were se- lected along the various environmental gradients encountered in the different study sites. Maps prepared by Beatley (1969) assist- ed in the selection. The number of stands var- ied according to the complexity of the vege- tation from 27 in Frenchman Flat to 6 in Yucca Flat. In selecting each stand, a reason- able degree of physiognomic and phys- iographic homogeneity was secured by visual 1980 Nevada Desert Ecology 75 judgment. Some cogent ecological attributes of perennial vegetation at each stand were determined by nondestructive, dimensional measurements. Procedural details and calcu- lations involving this method have been re- ported (Wallace and Romney 1972, Romney et al. 1973). Briefly, two 2 X 25 m quadrats were laid out in undisturbed vegetation at right angles to each other. All perennial plants within each quadrat were identified to the species level and measured for height and width and size (mean of two dimensions). Shrubs with canopies overlapping the quad- rat boundary were counted inside only when their root crown was inside the boundary line. Calculations using these dimensional measurements were made for each species to estimate absolute density, cover, and volume. The corresponding relative values for density and cover were then calculated and summed to give a stand-importance value ranging be- tween 0.0 and 200. Treatment of the Vegetation Data When a many-species population is sam- pled, it is interesting to inquire whether the units are naturally classifiable into distinct groups. Pielou (1969) suggested that it was al- ways possible to subdivide a collection of quadrats in one way or another (i.e., classify them), but it does not follow that the vegeta- tion they represent is classifiable into well- defined separate parts. From the theoretical point of view, there are two main concepts concerning the nature of the vegetation: the association concept and the continuum con- cept. According to the association concept, vegetation is composed of well-defined, dis- crete, integrated units that can be combined to form abstract associations reflective of nat- ural entities in the real world. According to the continuum concept, on the other hand, vegetation changes continuously and is not differentiated, except arbitrarily, into sociolo- gical entities. In the present study, it was found that a fruitful way to proceed with vegetation study was to apply a simple approach to account for vegetation structure and then evaluate jthe consequences of this approach by more sophisticated mathematical arguments. The technique adopted was originally used by Brown and Curtis (1952) as an approach for expressing the continuum nature of the upland conifer-hardwood forests of northern Wisconsin. More recently Karboush et al. (1975) have applied the same approach for classifying the fungal flora in some parts of the Egyptian desert. According to this technique, an impor- tance value was calculated for each species in each of the 66 stands examined. By in- spection of the importance values, each stand in turn is assigned a leading dominant, i.e., that species with the highest importance val- ue. Stands with the same leading dominant are then grouped. Obviously, some of the subordinate species in any one stand will be the leading dominants of other stands. Aver- age importance values are then calculated for each species in each group of stands. The group of stands (or group of species) with the same leading dominant is conveniently re- ferred to as a vegetational grouping some- what like the ecological grouping of Whitta- ker (1967). Soil Sampling and Analysis At each stand a trench was dug extending across a representative shrub clump for a giv- en site and out into the bare area between shrub clumps. This was done to permit an ex- amination and sampling soil profile under both shrub and bare areas in order to in- vestigate the modifying influence of per- ennial vegetation on desert soils. The depth of each trench was to the caliche hardpan, or, if no restricting layer existed, to an arbi- trary depth well into the C horizon. Soil sam- ples were taken from each profile horizon under both shrub and bare areas. These sam- ples were screened in the field to pass a 6.3 mm sieve, and the rock and gravel contents were estimated and discarded. The remaining samples were transported to the laboratory, where they were oven dried and then further screened to pass a 2 mm sieve. Available phosphorus was extracted with sodium bi- carbonate and determined colorimetrically using the method of Olsen et al. (1954) as de- scribed by Chapman and Pratt (1961). Lime content was determined by the manometric method of Williams (1948). The available iron, zinc, copper, and manganese were ex- 76 Great Basin Naturalist Memoirs No. 4 tracted with DTPA (diethylene triamine pen- taacetic acid) chelate and determined by atomic absorption as described by Lindsay and Norvell (1969). Organic nitrogen analysis was by the Kjeldahl method (Bremner 1965). Analytical methods used to determine other physical and chemical properties were those of the USDA Salinity Laboratory Staff (1954). In the present paper, unless otherwise mentioned, correlation is made between properties of soils collected under the shrub and plant. Data for all soil profile variables have been adjusted through a computer pro- gram to mean values for the 2.5 to 30 cm soil depth. Results Spatial Variations in Vegetation Cover The data from the phytosociological analy- sis that aim at providing a picture of the gen- eral composition of the perennial vegetation of the study areas are given in Table 1. Thirty-five species were encountered. None of these species can be considered as a lead- ing dominant of the whole study area; in- stead, some exhibit local dominance or are distinctly more important in certain groups of stands. Six major vegetational groupings have been defined (Table 1). The number of stands for each grouping varies between 15 for the most representative species (L. triden- tata) and 5 for the least (Grayia spinosa (Hook.) Moq. and Acamptopappus shockleiji A. Gray). Each of the leading dominant spe- cies of these groupings, i.e., L. tridentata, Ambrosia dumosa (A. Gray) Payne, G. spin- osa, A. shockleiji, Atriplex conferti folia, and Atriplex canescens, attained a maximum stand-importance value of more than 100 out of 200 and an average group-importance val- ue, based on the structure of the stands in each group of that particular species, of more than 66 (Table 1). Another five vegetational groupings of mi- nor representation have also been identified. Each of the leading dominant species of these groupings, viz., Coleogyne ramosissima Torr., Lycium shockleyi, Menodora spinescens A. Gray, Ephedra nevadensis S. Wats., and Kra- meria parvifolia Benth. attained an absolute maximum importance value of more than 60 and an average value of more than 50. Each of these groupings is represented by a min- imum of two stands and a maximum of four. Some other species, such as Lepidium fre- montii S. Wats., Ceratoides lanata (Pursh) J. T. Howell, Hymenoclea salsola Torr. and Gray, Lycium andersonii A. Gray, Psoro- thamnus fremontii (Torr.) Barneby, Lycium pallidum A. Gray, and Oryzopsis hymcnoides (Roem. & Schult.) Ricker, though important and common species, are not dominant and accordingly are not included in the provi- sional arrangement of leading dominant spe- cies. An ecotonal grouping has also been identi- fied (Table 1). It includes six stands in which the dominance is shared by two or more spe- cies of the nonleading dominants. Thus, al- most 10 percent of the stands did not fit into the system. It is clear from Table 1 that, in the vegeta- tional grouping dominated by L. tridentata (I.V. = 87) (I.V. is importance value), the sub- ordinate species are A. dumosa (I.V. = 31) fol- lowed by L. andersonii (I.V. = 17). Other spe- cies associated with L. tridentata are mostly of minor importance because their average importance values are generally below 10. In the A. dumosa grouping (I.V. = 80) the next highest importance value to that of A. du- mosa is 31 for L. tridentata and 15 for G. spinosa. Other species of some importance are K. parvifolia (I.V.= 12) and E. nevadensis (I.V. = 11). In the G. spinosa grouping (I.V. = 79), the subordinate species were L. andersonii (I.V. = 27) and L. tridentata (I.V. = 20). Other important species in this grouping are A. dumosa (I.V. = 16) and E. nevadensis. In the A. shockleyi grouping (I.V. = 66), the species second in importance is L. tridentata (I.V. = 31) followed by G. spinosa (I.V. = 22), L. andersonii (I.V. = 16), and A. dumosa (I.V. = 15). Ceratoides lanata (I.V. = 13) is also of some significance in this grouping. The vegetational groupings of A. confertifolia (I.V. = 129) and A. canescensi (I.V. = 137) are of some interest. In the first' grouping only one species, L. tridentata, seems of some importance (I.V. = 25). All oth- er species are of very low importance value and consequently of minor significance in community structure. In the vegetational 1980 Nevada Desert Ecology 77 grouping, the species second in importance is A. conferti folia, but its very low importance value of 13 makes it of minor significance in this grouping. Another point of interest is the relatively very low number of species in these two groupings. In either grouping the number of species does not exceed 10, com- pared with about 20 in the other vegetational groupings. It is also obvious from Table 1 that L. tridentata, which is an integral com- ponent of all other vegetational groupings identified, is missing from the A. canescens grouping. The structure of the vegetational group- ings defined as being of minor representation is also given in Table 1. Larrea tridentata can be considered as a common species in all groupings. It is, however, particularly well represented in the M. spinescens and A. shockleyi groupings. Its importance values in these two groupings are 47 and 31, respec- tively. The high (112) importance value of C. Table 1. Average importance values of plant species in provisionally defined vegetational groupings. Vegetational Groupings Major Minor L. A. G. A. A. A. C. L. M. E. K. Eco- Species trid dum. spin, shack, conf. can. ram. shock spin. nev. par. tonal A. Leading dominants Lanca tridentata (15)° 87 31 20 31 25 - 17 31 47 22 19 17 Ambrosia dumosa (10) 31 80 16 15 4 0.6 0.2 8 3 15 29 18 Grayia spinosa (5) 10 15 79 22 - 0.5 7.5 - 1.5 - 23 18 Acamptopappus shockleyi (5) 6 9 8.6 66 2 — — 21 3.5 — — 6 Atriplex confertifolia (6) 5 - 129 13 _____ 4.4 ^triplex canescens (6) 3 1 - - 9 137 3.2 33 2 15 Coleogyne ramosissima (4) 7 — 112 6 — — — — Lycium shockleyi (3) — — 88 — — — — Mendora spinescens (2) 3 5 — — 0.6 — 1.2 5 79 26 — 0.2 Ephedra nevadensis (2) 5 11 10 5 - - 16 - 16 73 25 9 Krameria parvifolia (2) 6 12 - 5 - - - 3.6 11 38 54 11 B. Common Lepidium frcmontii (\) 1 0.2 0.4 0.6 _______ 17.3 Ceratoides lanata 5 3.5 9 13 7 7 1.8-5 5.5 4.6 23 Hymenoclea salsola 1 — 4 3 4 4 — 0.3 — — — 4 Lycium andersonii 17 8.7 27 16 — — 15 — 4 - 32 21 Dalea frcmontii 1 1.4 - 0.1 - — 0 1.2 - - - 25 Lycium pallidum 3 5 5 3 ______ 15 14.4 Oryzopsis hymenoides 6 9 9 11 2 2 - 2.7 9 5 1.9 6.4 C. Infrequent or of minor importance Sphaeralcea ambigua 3 1.4 - 1.4 0.5 0.5 1.3 1.5 3.5 - - 1 Yuccd schidigera 4 — — 1.3 ________ Ephedra fancied 1.4 1 — — — — — 1.5 0.3 — — — Hikirid rigidd __ _ _________ Stanleya pinnata ___________ 15.8 Mirabilis pudica _______ 0.2 — — — 0.3 Tetrodymia axillaris — — 1 0.6 — — 2.2 — — — — 1.2 Stipa speciosa — — 5.2 — — — 11 — — — — 1.5 Artemisia spinescens — — 1 — — — — — — — —1.1 Dalea polyadenia 1 ___________ Cactus sp. 0.7 - 0.1 - - - - 0.8 - - 1.1 Primus fasciculata ___________ 0.5 Salazaria mexicana _ 0.1 — — — — — 0.9 — — — — Yucca brevifolia 1.5 — — — — — 7.5 0.7 — — — — Thamnosma montana 0.2 ___________ Haplopappus cooperi — — 3.1 1.2 — — — — — — — — Machaeranthera tortifolia — — 1 — 1 __3 ____ "Number of stands in which the species is the leading dominant. 78 Great Basin Naturalist Memoirs No. 4 ramosissima reflects the nonsignificance of other associated species except for L. triden- tata (I.V. = 17) and E. nevadensis (I.V.= 16). In the A. shockleyi (I.V. = 88) grouping the subdominance is shared by A. canescens (I.V. = 33), L. tridentata (I.V. = 31), and A. shockleyi (I.V. = 21). In the M. spinescens (I.V. = 79) grouping, which is only represent- ed by two stands, L. tridentata plays an im- portant role in community structure (I.V. = 47), followed by E. nevadensis (I.V. = 16) and K. parvifolia (I.V. = 11). Other species are of minor significance. In the vege- tational groupings of C. ramosissima, A. shockleyi, and M. spinescens, the number of species is quite high. Each community is rep- resented by at least 14 species. On the other hand, in the two minor groupings of E. neva- densis and K. parvifolia the number of spe- cies in either community is generally below 10. In these last two groupings subdominance is shared by many species. In the E. neva- densis grouping the subdominant species are K. parvifolia (I.V. = 38), M. spinescens (I.V. = 26), L. tridentata (I.V. = 22), A. du- mosa (I.V. = 15), and A. canescens (I.V. = 15). In the K. parvifolia grouping (I.V. = 54), Ly- cium andersonii is of great significance in community structure (I.V. = 32), followed by A. dumosa (I.V. = 29) and E. nevadensis (I.V. = 25). In fact, the relatively low impor- tance value of K. parvifolia brings this group close to certain ecotonal communities. In the arbitrarily designated ecotonal grouping (Table 1) the dominance is shared by many species. This grouping is characterized by a number of species, a character generally asso- ciated with transitional zones or effects. Nature of Relationships between Groupings Relations between different vegetational groupings may be expressed in terms of sim- ilarity indices for pairs. Some of these depend on the number of species common to the two groupings in relation to the total number present, and others depend on quantities present. In the present study, the similarity be- tween different groupings identified was as- sessed with the use of Spatz's (1970) formula as given by Dombois and Ellenberg (1974). "Index of similarity (I.S.) = ' Mc R x Ma + Nib + Mc x 100 The first component (R) is an expression of the relative similarity of the two groupings being compared, and is calculated bv divid- ing the smaller quantitative value of the spe- cies common to the two groupings bv the greater quantitative value. The resulting frac- tions for each of the species in common are added and divided by the total number of species in the two groupings. Mc is the sum of importance values of the species common to both groups, Ma is the sum of importance values of species restricted to grouping (a) and Mb is the sum of importance values of species restricted to grouping (b)." According to Dombois and Ellenberg (1974) this index has greater sensitivity to quantitative difference than any other index. In Table 2 are represented the coefficients of similarities between the different group- Table 2. Similarity indices between different vegetational groupings. Vegetational Groupings L. tridentata A. dumosa A. canescens A. confertifolia C,. spinosa A. shockleyi < '. ramosissima />. shockleyi M. spinescens E. nevadensis /, A. A. A. G. A. C. L. \l. E. K. tri. (him. can. conf. spin. slt,',\ ram. slice. spin. nev. par. _ 38 6 16 31 H 13 13 29 17 22 9 9 34 33 21 18 32 17 24 4 6 2 1 1 2 o 4 7 8 6 14 IS 16 4 43 11 3 3 15 3 18 17 9 11 12 9 24 33 23 8 5 16 23 1980 Nevada Desert Ecology 79 ings identified. It is clear that similarity coefficients between groupings are generally low. Among the highest coefficients are those between the L. tridentata grouping and those of A. shockleyi (44), A. dumosa (33), and G. spinosa (31). The A. dumosa grouping also has some relatively high similarity indices with those of G. spinosa (34), A. shockleyi (33), and A/, spinescens (32). Grayia spinosa grouping has high similarity indices with those of A. shockleyi (43) and K. parvifolia (33). It is interesting to notice that the lowest similarities obtained are those between the grouping of A. canescens and all other group- inns. Distribution of Stands with Leading Dominants among the Different Study Sites In Table 3, stands are grouped according to leading dominant species and the different sites are investigated, viz., Mercury Valley, Frenchman Flat, Rock Valley, Jackass Flats, and Yucca Flat. The average density of the leading dominant species in each of these sites is also presented. Larrea tridentata was dominant in 15 stands, 9 of which were in Frenchman Flat (Table 3). The average den- sity of L. tridentata within its stands in this site is 1244 plants/ha. The only site in which L. tridentata is not represented by stands in which it is a dominant species is Yucca Flat, although L. tridentata does occur in the tran- sitional basin (Beatley 1974). Ambrosia dumosa, on the other hand, is poorly represented in Frenchman Flat and is not dominant in Yucca Flat. In Jackass Flats, where A. dumosa is well represented (4 stands out of 10), its average density is 5875 plants/ha. Grayia spinosa is also well repre- sented in Yucca flat with an average density of 5600 plants/ha. Acamptopappus shockleyi shows its domin- ance in Mercury Valley (3 stands) and Jackass Flats (2 stands). Its average density in these two sites is 7400 and 6300 plants/ha, respec- tively. Atriplex confertifolia and A. canescens reach maximum dominance in Frenchman Flat. One stand with A. confertifolia as a leading dominant has been reported in Mer- cury Valley. The average density per hectare for either of the two species (in Frenchman Flat) is about 3400 plants. Grayia spinosa is mostly represented in Yucca Flat, and L. shockleyi is represented only as a leading dominant species in French- man Flat. Stands in which M. spinescens and E. nevadensis are dominants are equally dis- tributed between Frenchman flat and Jackass Flats, but the stands in which K. parvifolia is dominant are highly localized. Soil Characteristics in Relation to Major Vegetational Groupings Horizontal variations in some of the soil characteristics among different major vegeta- tional groupings are summarized in Table 4. Table 3. Distribution of stands with leading dominant species among the different study sites, and the average absolute density for each species (no. of plants/ha). (No. of stands in which each species is dominant.) Site Leading dominant species Mercury Frenchman Rock Jackass Yucca Valley Flat Valley Flat Flat 3 (1233)" 9 (1244) 2 (1850) 1 (700) - 3 (3962) 1 (4800) 2 (4400) 4 (5875) — 1 (3500) 1 (3900) 1 (4800) - 2 (5600) 3 (7400) - — 2 (63(H)) — 1 (2900) 4 (3370) - - - — 6 (3430) — — — - - - 1 (14HX 3 (5370) — 3 (5130) — — — — 1 (7700) — 1 (4400) — - 1 (3300) - 1 (600) - Larrea tridentata Ambrosia dumosa Grayia .spinosa Acamptopappus shockleyi Atriplex confertifolia Atriplex canescens Coleogyne ramosissima Lycium shockleyi Menodora spinescens Ephedra nevadensis Krameria parvifolia 2 (2500) "Number between parentheses indicates average density (plants/ha). 80 Great Basin Naturalist Memoirs No. 4 A general idea about the magnitude of varia- tion in each of these soil variables can be gained if the average and range are exam- ined. A more precise judgment, however, may be achieved by examining the results of table the ordered means of soil variables that show significant variations among major veg- etational groupings are presented. Six soil variables out of 16 (given in Table 4) show significant variations: soil moisture retention analysis of variance given in Table 5. In this capacity at 0.0 bar, exchangeable Na, ex- Table 4. Average and range of soil variables in different see Table 5). major vegetational groupings (for statistical difference Vegetational groupings Soil variable L. tridentata A. dumosa G. spinosa A. shockleyi A. confertifolia A. canescens Soil moisture retension at 0.0 bar 12.7 7.6-20.6 9.7 4.8-16.0 12.4 8.8-17.8 12.5 5.1-18.9 13.5 10.4-17.0 19.3 8.8-33.5 pH 8.64 8.3-8.8 8.5 7.8-8.8 8.5 8.4-8.7 8.3 8.4-8.8 8.66 8.3-8.9 8.65 8.5-8.8 E.C. mmhos/cm 1.84 1.0-3.3 1.17 0.39-2.5 1.65 1.2-2.6 1.15 1.1-2.6 1.45 0.3-3.0 1.88 0.55-3.2 Lime % 13.3 0.9-30 6.0 0.1-14.3 8.1 1.0-22.9 10.0 0.2-20.7 12.5 5.7-24.4 13.8 2.4-30 Ex. Na+ meq/100 gm 0.41 0.2-1.0 0.33 0.22-0.6 0.41 0.2-0.73 0.42 0.3-0.53 2.22 0.2-0.4 0.55 0.4-0.8 Ex. K+ meq/100 gm 4.47 2.5-8.1 3.38 2.1-5.6 4.66 2.8-8.7 2.89 2.6-3.6 7.56 3.2-13.5 5.66 0.9-9.1 Ex. Ca++ + Mg++ meq/100 gm 7.86 4.1-11.8 7.52 2.6-13.4 9.11 7.4-11.3 8.72 3.5-12.9 9.08 2.6-18.0 7.31 5.7-10.9 Ex. Na+ % 3.39 1.7-8.4 3.18 1.5-7.6 4.76 1.7-3.8 3.68 2.5-5.6 12.1 2.6-22.1 3.42 1.9-6.2 Cat. Ex. Cap. meq/100 gm 12.8 9.5-15.8 11.3 6-17 14.2 11.2-18.8 12.4 6.9-16.0 15.3 8.4-19.3 17.2 12.6-22.5 (NaHCQj) phosphorus ppm 1.88 0.2-4.3 1.56 0.5-3.3 2.2 1.1-5.7 1.8 0.7-3.4 0.52 0.2-1.2 1.5 0.8-2.3 (DTPA) Iron ppm 0.33 0.1-0.7 0.31 .02-0.6 0.33 0.3-0.6 0.38 0.2-0.5 0.16 0.1-0.3 0.23 0.1-0.3 (DTPA) Zinc ppm 0.67 0.3-1.33 0.46 0.2-0.67 0.52 0.31-0.8 0.6 0.3-1.3 0.51 0.3-0.7 9.44 0.16-0.73 (DTPA) Copper ppm 0.13 0.1-0.22 0.126 0.05-0.2 0.23 0.13-0.45 0.15 0.1-0.26 0.16 0.1-0.2 0.22 0.1-0.27 (DTPA) Manganese ppm 2.58 1.2-3.8 2.51 0.9-5.1 2.6 1.6-3.8 3.0 1.3-6.1 1.96 1.2-3.0 2 1.0-2.55 Org. nitrogen % 0.07 0.04-0.095 0.062 0.026-0.12 0.07 0.04-0.1 0.09 0.02-0.13 0.03 0.02-0.04 0.06 0.03-0.08 Elevation m 1030 960-1134 1007 910-1085 1199 991-1463 1070 1037-1104 997 939-1207 955 945-979 1980 Nevada Desert Ecology SI changeable K, exchangeable Na expressed as percentage of the total cations, cation ex- change capacity, and elevation. The vegeta- tional grouping of A. dumosa occupies the stands with the lowest level of soil moisture retention. Groupings of A. canescens, on the other hand, occupy stands with the highest moisture retention. Other groupings occupy intermediate positions along the soil moisture retention gradient. Along the exchangeable sodium gradient, the grouping of A. confer- tifolia occupies a significantly high position. The response of the different vegetational groupings to exchangeable K is rather signifi- cant. The two groupings of A. shockleyi and A. dumosa occupy the lower end of the gradient; A. canescens occupies the highest portion, and the other groupings are of inter- mediate positions. In exchangeable Na, ex- pressed as percentage of the total cations, the grouping of A. confetti folia occupies soils having the highest levels. It is of interest to notice that the grouping of A. canescens oc- cupies an intermediate position along the so- dium percent gradient. On the other hand, when we consider the gradient of the total cation exchange capacity, we find that the grouping of A. canescens occupies the high- est level. Atriplex canescens does populate low-lying areas that have accumulated clay and silt. The distribution of the different veg- etational groupings along the elevation gradient presents another point of interest. There is a stepwise segregation of groupings with altitudinal zonation. The grouping of A. canescens occupies the lower level of the gradient, and G. spinosa occupies the highest level. Behavior of Leading Dominant Species along an Environmental Gradient According to Dagnelle (1965), correlation of species with particular environmental var- iables has much potential as a tool for expla- nation of plant distribution though, as always with correlation or regression studies based on observational data, conclusions regarding causation must be hedged with reservation. In this case, the reservations are most likely to concern doubts whether the environmental variable studied is directly responsible or whether it is merely associated with the vari- able to which the effect observed should be properly ascribed. In the present study the simple linear cor- relation between the soil variables and both the importance values and the absolute den- sities (abundance) for the six major leading dominant species has been calculated (Table 6). The number of significant correlations ob- tained are very few for most of the species. The results are to be interpreted with cau- tion. The importance value of L. tridentata shows one positive correlation (P<0.05) with electrical conductivity (E.C.). This correla- Table 5. Ordered means for soil parameters that showed significant variations among different major vegetational groupings, according to the analysis of variance for unequal cell sizes. Soil variables Soil moisture retention 9.7 (Ad) 12.4 (Gs) 12.5 (As) 12.7 (Lt) 13.5 (Ao) 19.4 (Ac)0 Ex. Na+ meq/lOOgm 0.33 (Ad) 0.41 (La) 0.41 (Gs) 0.42 (As) 0.55 (Ac) 2.22 (Ao) Ex. k+ meq/lOOgm 2.87 (As) 3.38 (Ad) 4.47 (La) 4.66 (Gs) 5.66 (Ao) 7.56 (Ac) Ex. Na + ( 3.18 (Ad) 3.39 (Ld) 3.42(Ac) 3.68 (As) 4.76 (Gs) 12.1 (Ao) Cation ex. cap. meq/lOOgm 11.3 (Ad) 12.4 (As) 12.8 (La) 14.2 (Gs) 15.3 (Ao) 17.2 (Ac) Elevation m 955 (Ac) 977 (Ao) 1007 (Ad) 1030 (Ld) 1070 (As) 1199 (Gs) "The means parenthesed are not different from each other; those over different parentheses are. La = Larrea tridentata. Ad = Ambrosia dumosa, Gs Grayia spinosa. As = Acamptopappus shockleyi, Ao = Atriplex confertifolia, Ac = Atriplex i 82 Great Basin Naturalist Memoirs No. 4 tion, however, does not necessarily mean that L. tridentata is really increasing its abun- dance with the progressive increase in E.C. Examining the correlation of the absolute density of the same species with the same soil variable shows that there is no significant correlation and that the trend of correlation present is even negative. Accordingly, the positive correlation of the importance value of L. tridentata with E.C. is actually due to Table 6. Simple correlation coefficients (r) between the importance value (a) and the absolute density (b) of major leading dominant species and soil variables. A single asterisk denotes a significant correlation at the 5 percent proba- bility level, and double, triple, and quadruple asterisks denote a highly significant correlation at the 2. 1. and 0.1 percent probability levels. Species L. A. C. A. A. A. Soil variable tridentata dumosa spinosa shockleyi confertifolia canescens Soil moisture retention a -0.036 -0.34°° 0.11 0.01 -0.16 0.61° at 0.0 bar b 0.223 -0.39°°° -0.18 0.01 0.16 0.91°°°° pH a 0.19 -0.15 -0.03 -0.41°° 0.05 0.31 b -0.10 -0.17 -0.16 -0.16 -0.07 0.18 E.C. mmhos cm a 0.326° -0.20 0.23 0.03 0.5°° 0.23 b -0.071 -0.21 -0.14 0.02 0.15 -0.12 Lime % a 0.078 -0.27 -0.18 -0.09 -0.25 0.58° b 0.22 -0.33° -0.15 -0.03 -0.lt) 0.91°°°° Ex. Na meq/ a -0.245 -0.31° -0.17 -0.17 0.45° -0.35 100 g b -0.09 -0.28 -0.10 -0.15 0.63° °° -0.31 Ex. K meq/ a 0.226 -0.19 -0.002 -0.21 0.23 -0.01 100 g b -0.027 -0.19 -0.05 -0.23 -.10 -0.39 Ex. Ca + Mg meq/ a -0.17 -0.11 0.08 0.06 0.40 0.45 100 g b -0.06 -0.06 -0.004 0.07 0.13 0.78°°° Ex. Na+ % a -0.27 -0.31° -0.21 -0.05 0.49°° -0.4 b -0.089 -0.27 -0.14 -0.06 0.56° °° -0.44 Cat Exc Cap meq/ a 0.24 -0.12 0.03 -0.15 0.1 0.32 100 g b 0.04 -0.13 0.01 -0.15 0.35 0.38 (NaHCOj) P ppm a 0.20 -0.01 0.16 0.19 -0.35 0.02 b 0.14 -0.02 0.10 0.23 -0.60°°° -0.06 (DTPA) Fe a -0.21 -0.14 0.14 0.19 -0.40 0.5° b -0.10 -0.02 0.31 0.21 -0.053°° 0.43 Zn a 0.004 -0.33° -0.17 0.13 -0.16 -0.26 b 0.009 -0.35°° -0.11 0.17 0.08 -0.31 Cu a -0.02 -0.15 0.39°° 0.06 0.16 0.58° b -0.21 -0.22 0.33° 0.07 0.09 0.55°° Mn a 0.02 -0.06 0.04 0.33 0.07 0.06 b 0.03 -0.0.3 0.09 0.4°° 0.31 0.34 Total N % a 0.09 -0.19 0.02 0.35° -0.31 0.69°°° 1) 0.10 0.02 0.04 0.36° -0.49° 0.51 Elevation m a -0.1 -0.28 0.8 0.17 -0.18 -0.68°°° b -0.2 -0.14 0.39°° 0.20 0.05 -0.7°°° 1980 Nevada Desert Ecology 83 the decrease in the abundance of its associ- ated species. In fact, all species, including L. tridentata, are decreasing their abundance along the E.C. gradient, but L. tridentata has the slowest rate. The correlation between A. dumosa and exchangeable Na shows similar results. On the other hand, the abundance of A. dumosa shows decidedly significant correlation (nega- tive) with soil moisture retention (r = -0.39; P<0.01) and lime (r = -0.31; P<0.02). How- ever, the coefficient of determination (r2) is actually a very low percentage. The abun- dance and the importance value of G. spin- osa are positively correlated with copper at 2 percent and 5 percent levels of significance for the two parameters, respectively. The abundance of G. spinosa is also positively correlated with altitude (r = 0.39; P<0.02). The importance value and abundance of A. shockleyi are correlated negatively with pH and positively with total nitrogen, but most of these correlations are at the 5 percent lev- el of significance. The abundance of this spe- cies shows also a positive correlation with ex- tractable Cu at the 2 percent level of significance. The highest number of signifi- cant correlations obtained are those between Atriplex species and the abiotic variables. The abundance of A. confertifolia shows posi- tive correlations with both absolute and rela- tive amounts of exchangeable Na at the 0.1 percent level of significance. On the other hand, it shows negative correlations with phosphorus (P<0.01), iron (P<0.02), and to- tal nitrogen (P<0.05). The importance val- ues of A. confertifolia show only two positive correlations with both absolute and relative amounts of exchangeable Na at the 5 percent and 1 percent levels of significance. Atriplex canescens behaves in a different manner; it shows no correlation with any of the soil var- iables associated with A. confertifolia (Table 6). Four soil variables, namely, soil moisture retention, lime, exchangeable Ca + Mg, and Cu show strong positive correlations with the abundance of A. canescens. The high values for the coefficient of determination (r2) of 0.83 for soil moisture, 0.3 for lime, 0.63 for Ca + Mg, and 0.42 for Cu indicate the sig- nificance of the role of these soil variables in affecting the distribution and abundance of A. canescens. The negative correlation of the same species with altitude is also highly sig- nificant. This may be related to drainage in that the species does occur at the bottom of drainage basins. The importance value of A. canescens also shows positive correlations (but mostly at low significance) with soil moisture retention, lime, copper, and total nitrogen. The multiple linear correlations (R) relat- ing the abundance of the different leading dominant species and the different com- binations of soil variables are given in Table 7. Atriplex confertifolia, A. canescens, and A. shockleyi show the greatest number and the highest magnitude of significant relationships with the different combinations of soil varia- bles. Larrea tridentata, A. dumosa, and G. spinosa show smaller numbers of significant correlations. The following are multiple regression equations relating the absolute density (num- ber of plants/ha of the three species highly correlated with the different combinations of environmental variables: (for the level of sig- nificance see Table 7). A) Atriplex canescens Absolute density = 1-04 moisture re- tention % + 1.0 lime % - 2.72 (R = 0.9ooo°) Absolute density = 51.3 - 5.5 pH + 1.92 lime % (R = -0.92° °°°) Absolute density = 0.19 - 3.1 Na % + 0.26 K% + 3.32 (Ca+Mg)% (R = 0.82ooo°) Absolute density = 15.3 + 58.2 Fe ppm - 26.7 Zn ppm + 148.4 Cu ppm - 6.7 Mn ppm (R = 0.72oo°) B) Atriplex confertifolia Absolute density = 10.1 pH - 10.5 P% - 199 N%- 50.1 (R = 0.7° °°) Absolute density = 9.53 Na% - 0.7 K% + 1.05 (Ca + Mg)% - 0.157 (R = 0.7oo°) C) Acamptopappus shockleyi Absolute densitv = 365.7 - 34.3 pH + 1.6 P% + 359.6 N% (R = 0.6°'°°) Diversity in Different Vegetational Groupings The various ways of defining and measur- ing diversity have been reviewed and dis- 84 Great Basin Naturalist Memoirs No. 4 cussed by Pielou (1969). According to Pielou's definition, diversity is a single statis- tic in which the number of species and the evenness (uniform distribution of individuals among species) are combined. A collection is said to have high diversity if it has many spe- cies and their abundance is fairly even. Con- versely, diversity is low when the species are few and their abundance uneven. It should be noted, however, that diversity is sometimes used as a synonym for a number of species; that is not the sense in which it is used here. In the present study, the Simpson's (1949) measure of diversity as quoted by Pielou (1969) has been used for calculating the di- versity in the different major vegetational groupings. This index reads as follows: D = 1 N(N-l) ^NYN-l) where D is Simpson's measure of diversity in an S species collection containing N individ- uals, of which Nj belongs to the jth species (j = 1, 2, 3, -, S; SjNj = N) The results of this study are given in Table 8. The vegetational groupings studied can be classified into two main categories. The first category is characterized by low diversity (<0.4) as well as by a wide range of varia- tion. This category includes the vegetational groupings of A. confertifolia and A. canes- cens. In one of the stands of A. confertifolia the whole population is A. confertifolia with a consequent zero diversity. The second cate- gory includes the groupings L. tridentata, A. dumosa, G. spinosa, and A. shockleyi. The di- versity in these groupings is fairly high (>0.7) and with a narrow variation range. These results obviously indicate the poverty in species and unevenness in the distribution of individuals among species in stands repre- sentative of the vegetational groupings of the first category. Conversely, there are a rich- ness in species and an evenness in individual distribution in stands representative of the vegetational groupings in the second cate- gory. Di SCUSSION Since climatic, topographic, edaphic, and biotic conditions vary to a greater or lesser degree within a landscape, numerous habitats and plant communities are formed and be- come manifest in a mosaic or zonation of vegetation. An important feature of vegeta- tion, therefore, is change. The causal factor or factors behind this change is of primary concern to biologists, soil scientists, and even to geologists. Environmental variations generally occur in the form of gradients on different scales. Some of those gradients are described as mi- crogradients (Hanson and Churchill 1965). They may be caused by soil variations in mi- crorelief, texture, organic content, phos- phorus content, or any other conditions that might be of direct or indirect influence on plant life and existence. The soil sampling program was not intensive enough to reflect all these. Table 7. Multiple correlation (R) between the absolute density of each of six leading dominant species and varia- tions in multiple combination of soil variables. A single asterisk denotes a significant correlation at the 5 percent probability level, and double, triple, and quadruple asterisks denote high significant correlation at 2, 1, and 0.1 per- cent probability levels. Spec IES Combined soil L. A. G. A. A. A. VARIABLES tridentata dumosa spinosa shockleyi canescens confertifolia All variables 051o... 0.67° °°° 0.74° °°° q93ooo. 10o.,o 0.950000 Fe, Zn, Cu, Mn 0.37 °° 0.31 0.42° ° 0.49*" 0.72° O0 0.62° °° pH, P, N 0.19 0.30° 0.18 0.60° °°° 0.67° ° 0.70"... Na, K, Ca + Mn 0.15 0.27 0.16 0.23 0.82°°°° 0.7"" E.C., Na%, Cat. ex. cap 0.11 0.31° 0.2 0.18 0.64° ° 0.6° °° Moisture, lime 0.32° 0.34° 0.18 0.05 093. ... 0.29 pH, lime 0.28 0.38° ° 0.15 0.39° Q92. ... 0.09 pH, P 0.18 0.17 0.18 0.4400 0.11 0.63 "" 1980 Nevada Desert Ecology 85 The behavior of the biotic communities in the Mojave Desert in general or in some of its sectors has attracted the attention of many biologists. Shreve and Wiggins (1964) have described the Mojave Desert as showing its most distinctive development between 600 and 1200 m elevation (2000-4000 ft). When it is followed thence toward the northeast or southeast, it loses some of its characteristic vegetational features and much of its dis- tinctive flora. The basic structure of the veg- etation throughout the Mojave Desert is very open stands of L. tridentata and A. dumosa. On the western edge these plants are joined and, to some extent, replaced by Artemisia sp., G. spinosa, Tetradymia, and some suffru- tescent perennials, and at higher elevations on the north C. ramosissima and G. spinosa are dominant. In the northern sector of the Mojave Desert, particularly in the Nevada Test Site and its surroundings, detailed phytosociolo- gical and autoecological studies have been carried out by manv authors. Among them are Beatley (1963, 1969, 1974, 1975), Allred et al. (1963), Rickard and Beatley (1965), Brown and Mason (1968), and Wallace and Romney (1972). In these studies some vegeta- tional units have been defined and named by various terms as associations, types, and sub- types. The correlation between these vegeta- tional units and certain environmental varia- bles has been discussed also. In the present study it has proved useful to segregate the stands into several vegetational groupings according to leading dominant species (species with the highest importance values). However, these groupings are not ab- solutely discrete. The members of each pair of groupings are, in various degrees, linked together by having one or more of the domi- I nant species in common. This, however, does not preclude the fact that these vegetational : groupings are well defined and represent so- ciologically distinct entities quite recogniz- able in the field. Six major and five minor vegetational groupings have been defined. In a wider ecological study these minor group- ings may prove to be of major importance. Several edaphic factors were analyzed sta- tistically in relation to the distribution of each of the major vegetation types. Signifi- cant differences were found among the plant groupings with respect to soil moisture ten- sion, absolute and relative amounts of ex- changeable Na, exchangeable K, cation ex- change capacity, and elevation. However, no one grouping is restricted in its distribution by a narrow tolerance range for any specific soil factor. Overlapping of the vegetational groupings occurred for all the soil variables measured. This overlapping, however, does not preclude the fact that, within the overall structure of the vegetation dealt with, each vegetational grouping studied has a signifi- cant association with a particular com- bination of environmental variables. The analysis of the relationship between the phytosociological behavior of the major leading dominant species and the environ- mental variables studied is of certain interest. Only one of the simple linear correlations be- tween the relative or absolute abundance of /.. tridentata and any of the environmental variables is highly significant. This might be explained by the fact that L. tridentata may be so well adapted to the conditions in the study area that its behavior is not noticeably affected by the changes within the limits of any of the factors studied. Other factors could be more important. The coefficient of determination (r2) of 0.25, as determined from the multiple correlation analysis, in- dicates that only 25 percent of the total vari- ation in abundance of L. tridentata along its range of distribution in the study area is due to the combined effect of the different envi- ronmental variables investigated. Table 8. Simpson's measure of diversity in different vegetational groupin Uv Vegetational Grouping Diversity L. tridentata A. dumosa G. spinosa A. shockleyi A. canescens A. confertifolia Mean 0.75 0.72". 0.706 0.703 0.390 0.33 Ran i;e 0.61-0.84 0.64-0.84 0.7-0.8 0.64-0.81 0.18-0.55 0.0-0.73 S.D. ± 0.067 ± 0.05 ±0.048 ± 0.075 ±0.206 ±0.34 S.E. ±0.017 ±0.0175 ±0.0214 ± 0.034 ±0.08 ±0.155 Great Basin Naturalist Memoirs No. 4 That L. tridentata has a very wide range of distribution has been discussed by many au- thors. As reviewed by Barbour (1968), it dom- inates a desert area of 358,000 km2 in the southwestern United States; occurs in 183,000 km2 in adjacent vegetation; and cov- ers a range that differs widely in climates, soils, elevations, and communities. However, within this distribution range, L. tridentata is so ubiquitous that Benson and Darrow (1954) have used its range limits to define the boundaries of the warm desert. More recent- ly Beatley (1974) has suggested that the pre- vailing low minimum air temperatures and their extremes in the lowlands of drainage basins of Nevada are inferred to be the pri- mary cause of the absence of L. tridentata in three discrete vegetation zones. Wallace and Romney (1972) have suggested that the lack of L. tridentata in dry lake areas of closed ba- sins in southern Nevada is due to periodic flooding. This species is sensitive to poor root aeration (Lunt et al. 1973). The analysis of correlation between species behavior and site variables also shows that the concentration of the different ions has a significant role in determining the abundance of some of the different species studied. On the other hand, total salinity as reflected by electrical conductivity or as total soluble ca- tions is of limited significance as a controlling factor. Similar results are obtained by Gates et al. (1956) and Ayyad and El-Ghareeb (1972). In a study on some of the alkali desert soils in Utah, Gates et al. (1956) concluded that total salinity is not a wholly satisfactory criterion and that future work should involve specific ions. Chapman (1960) concluded that the roles of the cations and anions or their combination in relation to vegetation zona- tion may have far greater importance than has previously been suggested. On the basis of subjective and qualitative argument, it was for many years believed by the majority of ecologists (Odum 1959, Han- son and Churchill 1965, Pielou 1975) that the more complex a community (that is, the more numerous its species and the more intricate their relationships), the greater the commu- nity's system stability would be; for, if each species would rely on many rather than few food sources and be regulated by many rather than few predators, the eggs-in-basket effect would be minimized. As a result, a high di- versity would cause a high community stabil- ity. However, acceptance of this theory has wavered since May (1973) pointed out that community stability is not a mathematical consequence of high species diversity and that the contrary is true. May's theory is still, so far, in its infancy, is a matter of con- troversy, and may not apply to deserts. As Pielou (1975) suggests, many species models that make no allowance for certain vegeta- tional parameters, such as spatial hetero- geneity, are totally unrealistic. In the present study, the relative stages of community stability for the different vegeta- tional groupings has been discussed on the basis of the assumption that community sta- bility and high diversity are positively corre- lated (Pielou 1975). It is also based on the as- sumption that the diversity of an abstract community should be expressed as an average of the diversity measures for the different concrete units of that particular community, and consequently the lower the standard er- ror of the diversity measure, the greater the homogeneity of the community. The vegeta- tional grouping of L. tridentata, showing both high diversity and greater homogeneity, may be considered, therefore, as the most stable community in the study area, con- sequently representing its climatic climax vegetational cover. The vegetational group- ing of A. dumosa is also characterized by both high diversity and homogeneity, but it has a relatively narrower distribution range. The view that the L. tridentata grouping represents the most stable community in the study area has been supported by the study of Beatley (1969), who described the L. triden- tata type as the one with the highest floristic diversity in the region, and by Shelford (1963), who described the L. tridentata com- munity as representing the bush desert cli- max. However, the position of the L. triden- tata community along the successional ladder is a matter of great controversy, and views vary according to phytogeographical regions. According to Stebbins and Major (1965), L. tridentata has been described in the Mojave Desert as representing a relict species. On the other hand, in Sampson Valley in south- eastern Arizona, Chew and Chew (1965) 1980 Nevada Desert Ecology 87 have described L. trident at a as a pioneer spe- cies that has been recently dispersed in the stndv area at the expense of Flourensia. Gardner (1951) concluded also that L. triden- tata is expanding its distribution and domin- ance into areas occupied by Flourensia in the Rio Grande Valley, New Mexico, probably as the result of changes in the complex of soil factors, especially the loss of surface soil. Acknowledgments This study was supported by Contract EY- 76-C-03-1200 between the U.S. Department of Energy and the University of California, CETO of the Nevada Test Site, and the U.S. International Desert Biome Program. Literature Cited Allhed, 1). M., D. E. Beck, and C. D. Jorgenson. 1963. Biotic communities of the Nevada Test Site. Brig- ham Young Univ. Sri. Bull., Biol. Ser. 2(2):] 52. Ayyad, M. A., and R. El-Ghareeb. 1972. Micro- variations in edaphic factors and species distribu- tion in a Mediterranean salt desert. Oikos 23:25-131. Barbour, \1. C. 1968. Germination requirements of desert shrub I. mien divericata. Ecology 49:915-929. Beatley, |. C. 1963. Vegetation and environment of the Nevada Test Site. Ecol. Soc. Am.. Hull. 14:123 (abstract). _ . 1969. Vascular plants of the Nevada Test Site. Wilis \ii Force Range, and \sh Meadow (north- ern Great Basin desert, southcentral Nevada). UCLA 12-705. Laboratory of Nuclear Medicine and Radiation Biol.. University of California, Los Angeles. 122 pp. 1974. Effect of rainfall and temperature on the distribution and behavior of Larrea tridentata (creosote bush) in the Mojave Desert ol Nevada. Ecology 55(2):245-261. 1975. Climate and vegetation pattern across the Mojave/Great Basin desert transition of southern Nevada. Amer. Mdl. Natur. 93(l):53-70. 1976. Vascular plants of the Nevada Test Site and central-southern Nevada: ecologic and geogra- phic distributions. U.S. Technical Information ( enter. Office of Technical Information, ERDA. Benson, L., and R. A. Darrow. 1954. The trees and shrubs of the southwestern deserts, 2d ed. Uni- versity of Arizona Press, Tucson, Arizona. 437 pp. Bremner, J. M. 1965. Total nitrogen. Pages 1149-1178 in C. A. Black, ed. Methods of soil analysis, Part 2. Amer. Soc. Agron., Inc., Madison, Wisconsin. Brown, R. T., and T. T. Curtis. 1952. The upland eon ifer-hardwood forests of northern Wisconsin. Ecol. Monogr. 22:217-234. Brown, K. W., and B. J. Mason. 1968. Range survey area 18, Nevada Test Site. U.S. Dept. of Health, Education and Welfare, Public Health Service, SWRHL-52. Chapman, H. D., and P. F. Pratt. 1961. Methods of analysis for soils, plants and water. Div. Agr. Sei.. University of California., Riverside. Chapman, V. J. 1960. Salt marshes and salt deserts of the world. Inter-science Pub., New York. Chew, B. M., and A. E. Chew. 1965. The primary pro- ductivity of desert shrub (Larrea tridentata) com- munities. Ecol. Monogr. 35:355-375. Dagnelie, P. 1965. L'etude des communantes vegetale par l'analyse statistique des liaisons entre les es- peees et les variables eeologiques: Principes fond- amentaux. Biometrics 21:349-361. Dombois, D. M., and H. Ellenberg. 1974. Aims and methods of vegetation ecology. John Wiley and Sons, New York. Gardner, ]. L. 1951. Vegetation of the creosote bush of the Rio Grande Valley in New Mexico. Ecol. Monogr. 21:379-403. (.mis. I). II., A. L. Stoddart, and W. C. Cook. 1956. Soil as a factor influencing the plant distribution on salt marshes of Utah. Ecol. Monogr. 26:155-173. Hanson, H., and E. D. Churchill. 1965. The plant community. Reinhold, New York. Karboush, \1. A., A. A. El-Ghonemy, and G. Ragheb. 1975. Socioecological studies of the fungal flora of the Egyptian desert: 1. The sociological relations ol the fungal flora to the silicious sand deposit north of Wadi-El Natrun. Zbl. Bakt. Abt. 11 (Bd. 130):131-143. Lindsay, W. 1... and W. A. Norvell. 1969. A micro- nutrient test for Zn, Fe, Mn, and Cu. Agron. tf>sts. p. 84. I.i nt, O. R.. J. Letey, and S. B. Clark. 1973. Oxygen requirements for root growth in three species of desert shrubs. Ecology 54:1356-1362. May, R. M. 1973. Stability and complexity in model eco- systems. Princeton University Press, Princeton. Odum, E. P. 1959. Fundamentals of ecology, 2d ed. W. B. Saunders, Philadelphia. Olsen, S. R., C. V. Cole, F. S. Watanabe, and L. A. Dean. 1954. Estimation of available phosphorus in soils by extraction with sodium bicarbonate. USDA Circ. 939. Pieloi . E. C. 1969. An introduction to mathematical ecology. John Wiley and Sons, New York. 1975. Ecological diversity. A. Wiley Interscience Publication. John Wiley and Sons, New York. Rickard, W. H., and J. C. Beatley. 1965. Canopy cov- erage of the desert shrub vegetation mosaic of the^Nevada Test Site. Ecology 47: 524-529. Romney, E. M., V. Q. Hale, A. Wallace, O. R. Lunt, J. D. Childress, H. Kaaz, G. V. Alexander, J. E. Kinnear, and T. L. Ackerman. 1973. Some char- acteristics of soil and perennial vegetation in northern Mojave Desert areas of the Nevada Test Site. UCLA 12-915, UC-48 Biomedical and Envi- ronmental Res. TID-4500. Shelford, V. E. 1963. The ecology of North America. University of Illinois Press, Urbana. 88 Great Basin Naturalist Memoirs No. 4 Shreve, F., and I. Wiggins. 1964. Vegetation and flora of the Sonoran Desert. Vols. 1 and 2, Stanford University Press, Palo Alto. Simpson, E. H. 1949. Measurement of diversity. Nature 163:188. Spatz, G. 1970. Pflanzengesellsehaften, Liestungen and Leistungspotential fon Allgauer Alpweiden in Abhangigkeit von Standort und Bevvirtchaftung. Dissertation (Dr. Agr.), Gech. Univ.. Munich, Ger. 160 p. + 20 looseleaf tables and 1:5000 map. Stebbins, G. L., and J. Major. 1965. Endemism and speciation in the California flora. Eeol. Monogr. 35(1): 1-35. U.S. Salinity Laboratory. 1954. Diagnosis and im- provements of saline and alkali soils. U.S. Dept. of Agriculture Handbook 60. Wallace. A., and E. M. Romney. 1972. Radioecology and eeophvsiologv of desert plants at the Nevada Test Site. TID-25954. AEC Office of Information. Libr. of Congr. 72-600110. Whittaker, R. H. 1967. Gradient analysis of vegetation. Biol. Rev. 42:207-264. Williams. D. E. 1948. A rapid manometric method for the determination of carbonate in soils. Soil Sci. Soc. Amer. Proc. 13:127-129. FREQUENCY DISTRIBUTION OF THREE PERENNIAL PLANT SPECIES TO NEAREST NEIGHBOR OF THE SAME SPECIES IN THE NORTHERN MOJAVE DESERT A. Wallace1, E. M. Romney', and J. E. Kinnear' Abstract.— Frequency distribution patterns were developed for distance to nearest neighbor of the same species for Lama tridentata (Sesse & Moc. ex DC.) Cow, Ephedra nevadensis S. Wats., and Acamptopappus shockleyi A. pray. The distances between shrubs had been determined previously in another study. About one-third or more of the nearest neighbor of its own kind was within less than one meter for each species, indicating that it was usually within the same shrub clump, which in turn is indicative of an aggregating effect. For L. tridentata and E. neva- densis much of this could be from the same original plant by crown diffusion (L. tridentata) or underground spread- ing (E. nevadensis). None of the three gave evidence of spacing at regular intervals when the nearest neighbor of a single individual within a shrub clump was outside that clump. Rather, they appeared to be randomly distributed under this condition, except possibly for A. shockleyi. Nearest neighbor information among per- ennial plants is of considerable importance in desert environments. Involved is the tenden- cy of a given system to have regulatorv mechanisms that can space plants at quite regular intervals. This is observed with Lar- rea tridentata (Sesse & Moc. ex DC.) Cov. with limited rainfall (Barbour 1969); the more sparce the rainfall, the greater is the spacing. In our previous studies of plant pop- ulations in the northern Mojave Desert some entire populations had been subjected to a census, and from the data nearest neighbor relationships had been calculated (Wallace and Romney 1972d). In the previous work, the mean distance and standard deviation to the nearest neighbor of any species and of the same species were reported for 23 perennial shrubs. In general the coefficient of variation for the distance to nearest neighbor of the same species of the perennial plants was around 100 percent. Barbour's (1969) work indicated that L. tri- dentata spacing could be random, clumped or at regular intervals depending on the cli- mate. The purpose of this report was to de- termine how this species was spaced in part of the Nevada Test Site and to get similar data for other species. Materials and Methods The site of the study area is Mercury, Ne- vada, near waste water ponds from the local sewage processing system. The soil at this site is underlain by a virtually impervious hard- pan layer at depths varying from 15 to 75 cm. Perennial plants grow both singly and in clumps, separated by bare areas of desert soil (Fig. 1). The size and spacing of the clumps is irregular, and several different species may grow together in a single clump (Fig. 2). A census was made in the summer of 1968 of all perennial plants (including shrubs, grasses, herbs, and their seedlings) in 25 circular ex- perimental plots, each plot being 30.5 m in diameter. Each plant was categorized as to its species and its vegetational unit member- ship. This census effort involved more than 19,000 individual plants representing 28 dif- ferent species. A special method was devised for locating and cataloging each plant in each plot. A permanent standpipe for mounting a sur- veyor's transit was installed at the center of each plot, with a marker located on magnetic north at a distance of 15.25 m. Orientation for each vegetational unit was the measured 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 90024. 90 Great Basin Naturalist Memoirs No. 4 Fig. 1. View of northern Mojave Desert study site with typical shrub clumps separated by bare areas of desert pavement. • Fig. 2. Typical clumps oi shrubs in group associal vegetational unit). 1 'resent are Acamptopappus shockleyi (3) Ambrosia dumosa (8), ( 'eratoides lanata (5), ( Wayia spinosa i 1 . Krameria parvifolia ( 1 ) and Lycium andersoni 1 1 1. 1980 Nevada Desert Ecology 9] distance from the plot center to the vegeta- tional unit center. The azimuth for each unit was measured from magnetic north to the center of the vegetational unit. The unit's greatest and smallest width and its species content were recorded. Each species within a unit was measured in like manner, and it was further identified by height. These data were recorded and transferred to punch cards for computer processing. The method for calculating distance to nearest neighbor is given in detail in the pre- vious publication (Wallace and Romney 1972d). Results Discussion Frequency distribution histograms for dis- tance to nearest neighbor of the same species for L. tridentata Sesse & Moc. ex DC. (1241 individuals), Ephedra nevadensis S. Wats. (386 individuals), and Acamptopappus shockleyi A. Gray (3470 individuals) are in Figures 3, 4, and 5. The three species were chosen for their different growth habits. All three, however, tend to exist in clumps with individuals of other species as well as with other individuals of the same species, as is ob- served in each of the histograms. A high pro- portion of the nearest neighbor of the same species lies within a distance of 1.5 meters. A near normal type of frequency distribution existed within the clumps for the first 1.5 me- ters or so according to each of the figures. About one-third of the L. tridentata plants were within about two-thirds meter distance of one another. Part of this may be due to the breakup of crowns into more than one plant (Wallace and Romney 1972b). No attempt was made in the census to identify these as one plant, so a crown diffusion phenomenon existed within the first meter in the histo- gram in Figure 1. Beyond the first clump or beyond a dis- ,l,l,UilTrWiCWT*Tn>T«rrPi it t»i m rvj ID H X m pj T U1 r- m m Fig. 3. Histogram of frequencies of distance to nearest neighbor of the same species in meters of 1241 individual Larrea tridentata plants in the northern Mojave Desert. 92 Great Basin Naturalist Memoirs No. 4 tance of one meter there was almost a con- stant number of individuals within each cell width out to 4 or 5 meters. There was a very slight tendency for a number of nearest neighbors to occur in the next adjacent clump, but data generally show distribution with different distances up to about 5 meters to the nearest neighbor within the study plots. This would imply that the clumps are very randomly distributed. The mean distance of one L. tridentata to another L. tridentata was 2.15 meters. The mode was at 0.6 meters and, being left of the mean, is indicative of some aggregation. The skewness was -0.27 and the kurtosis was -15.77. The E. nevadensis had the smallest popu- lation of the three species studied, and it was chosen for this reason. About one-half of the individuals were nearest neighbor within less than one meter. This may be related to the habit of propagation by underground roots (Wallace and Romney 1972a). These groups would be aggregated. In the census no at- tempt was made to separate such plant groups from those that were truly individual, so this may account for the large proportion of close neighbors. Beyond one meter the dis- tribution appeared to be mostly uniform with distance and therefore random. There did seem to be a small distribution peak at about 3.6 meters, however. The overall mean to nearest neighbor of E. nevadensis of the same species was 3.35 me- ters, with a skewness of 1.36 and kurtosis of 4.68. The mode was at 0.6 meters, which is to the left of the mean and would indicate ag- gregation as explained above. The largest population of the three species was with Acamptopappus shockleyi. It has the tendency to grow both in groups within clumps of other species and as individuals in the space between clumps (Wallace and Romney 1980, this volume). This latter habit is the reason for its more negative association with other species (Wallace and Romney 70 .. eia .. S0 .. H0 .. 30 .. 20 .r 1 0 . n jf F? i i il i i i i iJ.jMrMMtuMl mUuw?^ . .m. .m.m. .m.m. . .m. m LD X — n i/i r- m w IH.H : IE. 2 Fig. 4. Histogram of frequencies of distance to nearest neighbor of the same species in meters oi 386 individual Ephedra nevadensis plants in the northern Mojave Desert. 980 Nevada Desert Ecology 93 972c). The frequency distribution for A. hockleyi is almost exponential, with numbers apidly dropping off with distance. This is, of ourse, related to its relatively dense popu- ition. The data can be interpreted as the secies being to a large extent aggregated, 'he mean distance between neighbors was .18 meters. The mode was at 0.6 meters, 'he skewness of the frequency distribution 'as 2.47 and kurtosis was 12.54. The varied nature of the distribution of the idividuals for each species may indicate that ie site in question is not extremely limited i rainfall. Other sites in the northern Mojave )esert can be found that are more limited in rinfall, and spacing at regular intervals may e more likely at such sites. Because the udy site involves a mixture of vegetation, it quite unlikely that the forces that result in sgular spacing have been in operation in this udy area. 700 .. GS0 G00 55:0 S00 HS0 H00 3S0 300 2S0 200 1 S0 .. I 00 .. S0 ACKNOWLEDGMENTS This study was supported by Contract DE- AMO3-76-SF00012 between the U.S. Depart- ment of Energy and the University of Cali- fornia. Literature Cited Barbour, M. G. 1968. Germination requirements of the desert shrub Larrea divaricata. Ecology 49:915-923. Wallace, A., and E. M. Romney. 1972a. Character- istics of Ephedra species (Mormon tea). Pages 102-109 in Radioecology and ecophysiology of desert plants at the Nevada Test Site. USAEC Report TID-25954. 1972b. Characteristics of Larrea divaricata (creo- sote bush). Pages 115-132 in Radioecology and ecophysiology of desert plants at the Nevada Test Site. USAEC Report TID-25954. 1972c. A study of a measure of species association between pairs of perennial plants in desert hard- pan soil. Pages 205-230 in Radioecology and ecophysiology of desert plants at the Nevada Test Site. USAEC Report TID-25954. 1972d. Ecological attributes of perennial plants in the northern Mojave Desert. Pages 247-257 in Radioecology and ecophysiology of desert plants at the Nevada Test Site. USAEC Report TID- 25954. 1980. The role of pioneer species in revegetation of disturbed desert areas. Great Basin Nat. Mem. 4:29-31. 1 1 U I L 'i 'i Ui irMTVnr'rTnr^n Fig. 5. Histogram of frequencies of distance to nearest neighbor of the same species in meters of 3470 Acampto- ippus shockleyi plants in the northern Mojave Desert. RELATIONSHIP OF SMALL WASHES TO THE DISTRIBUTION OF LYCIUM ANDERSONII AND LARREA TRIDENTATA AT A SITE IN THE NORTHERN MOJAVE DESERT A. Wallace1, E. M. Romney, and R. B. Hunter Abstract.— At a site near Rock Valley, Nevada, dominated by volcanic rocks, both Larrea tridentata (Sesse & Moc. ex DC.) Cov. and Lycium andersonii A. Gray were restricted in distribution. Larrea tridentata did not grow in the many small washes in the area, but L. andersonii grew only in the washes. Ambrosia dumosa (A. Gray) Payne was more dense and more dominant in wash areas than in nonwash areas. The vegetation mosaic of the Rock Valley area of the northern Mojave Desert has a high degree of variability and changes con- siderably from site to site (Beatley 1976, Romney et al. 1973, Turner and McBrayer 1974, Turner 1975, 1976). The dominant spe- cies are Larrea tridentata (Sesse & Moc. ex DC.) Cov., Ambrosia dumosa (A. Gray) Payne, and Lycium andersonii A. Gray on some sites and L. tridentata, Lycium palli- dum Miers, and Grayia spinosa (Hook.) Moq. on others. Ambrosia dumosa and L. pallidum are of lesser importance on these latter sites. The study was made because of the impres- sion that the small washes in the area were free of L. tridentata and that L. andersonii grew only in the washes. In other studies con- ducted here, L. tridentata and L. andersonii have been highly associated, whereas, L. tri- dentata and L. pallidum tend to be negati- vely associated (Romney and Wallace 1980, Wallace and Romney 1972). Materials and Methods The study site was located off Road 40 near the east entrance to Rock Valley at the Nevada Test Site. It is near Site No. 58 of the soils-plant study made by Romney et al. (1973). The area is above the main part of the valley and near Skull Mountain (Beatley 1976). It has a slope of 2 percent to the south and the area is crossed by many small washes, often 10 to 15 m apart. Two belt transects, each 50 m X 2 m, were sampled in both the wash and nonwash areas. An inventory was made of all plants falling more than 50 percent in the transect in order to determine numbers and relative dominance (Wallace and Romnev 1972). Mineral analyses were made of the plants to determine if the location differences could be explained by variations in nutrient ele- ment distribution. Results and Discussion The numbers and relative dominance of plant species are reported in Table 1. The high species diversity seen elsewhere in Rock Valley (Beatley 1976, Romney et al. 1973) is apparent. No L. tridentata were observed in the transects in the washes and no L. ander- sonii were observed in the transects in the nonwash areas. The density of L. pallidum was not different in and out of washes. There were more total plants (greater density) in the wash than out of the wash area, primarily due to variations in the density of A. dumosa. Four possible reasons for the vegetation pattern differences are (1) more water in the washes, (2) different soil texture in the wash- es, (3) soluble salts had been leached out along the washes, and (4) positive effect of 'Laboratory of Nuclear Med - .mil Radiation Biolog) , University of < alifomia, Los Vngeles, Califc 94 1980 Nevada Desert Ecology 95 the wash on the seed germination of the L. andersonii. No seedlings of any of the species were observed either in or out of the washes when sampled in 1976. Soil texture is sandy (Beatley 1976, Romney et al. 1973, Wallace and Romney 1972). A question of most interest was the salt status of the plants, but the mineral element contents generally did not vary significantly between locations. Some of the mineral anal- yses are in Table 2. Lycium pallidum is known to be more adapted to salt than is L. andersonii (Beatley 1976, Romney et al. 1973, Wallace et al. 1973). Lycium pallidum is not an obligate halophyte and this may ac- count for its being equally distributed in wash and nonwash areas. The chlorine con- centration in L. pallidum and G. spinosa var- ied inversely in and out of washes (3.68 per- cent and 2.73 percent in and out of washes for L. pallidum and 1.84 percent and 2.02 percent in and out of washes for G. spinosa). An attempt was made in 1976 to deter- mine differential leaf water potentials in plants in and out of washes as determined with a Scholander bomb (Scholander et al. 1965). Results were inconclusive. Leafwater potentials of the species involved were re- ported earlier (Wallace and Kleinkopf 1974). Given repeated and prolonged measurements in different types of rainfall years, this tech- nique probably could vield important infor- mation on the problem distribution. of differential plant Acknowledgments This study was supported by Contract EY- 76-C-03-00i2 between the U.S. Department of Energy and the University of California. Literature Cited Beatley. J. C. 1976. Vascular plants of the Nevada Test Site and central-southern Nevada. Tech. Informa- tion Center, Office of Tech. Information, ERDA Report TID-16881. Romney, E. M., V. Q. Hale, A. Wallace, O. R. Lunt, J. D. Childress, H. Kaaz, G. V. Alexander, J. E. Kinnear, and T. L. Ackerman. 1973. Some char- acteristics of soil and perennial vegetation in northern Mojave Desert Areas of the Nevada Test Site. USAEC Report UCLA # 12-916. Romney, E. M., and A. Wallace. 1980. Ecotonal distri- bution of salt-tolerant shrubs in the northern Mo- jave Desert. Great Basin Nat. Mem. 4:132-137. Scholander. P. F., H. T. Hammel, E. D. Bradstreet, and E. A. Hemmincsen. 1965. Sap pressure in vascular plants. Science 148:339-346. Turner, F. B. 1975. Rock Valley validation site report. US/IBP Desert Biome Res. Memo. 75-2. 1976. Rock Valley validation site report. US/ IBP Desert Biome Res. Memo. 862. Turner, F. B., and J. F. McBrayer. 1974. Rock Valley validation site. US/ IBP Desert Biome Res. Memo. 75-7. Utah State University, Logan. Table 1. Numbers of shrubs and their relative dominance in wash and nonwash areas0. Wash 1 Rel. Wash 2 Rel. Hill 1 Rel. Hill 2 Rel. Species No. dom. No. dom. No. dom. No. dom. Psorothamnus fremontii 1 0.4 3 0.8 2 3.1 7 5.1 Ephedra nevadensis 1 2.1 0 0.0 0 0.0 0 0.0 Ceratoides lanata 1 0.1 0 0.0 0 0.0 5 1.5 Ambrosia dumosa 57 30.4 72 27.6 40 14.0 29 9.6 Grayia spinosa 34 39.2 33 50.6 29 37.0 31 39.5 Hymenoclea salsola 1 0.9 0 0.0 0 0.0 1 0.6 Larrea tridentata 0 0.0 0 0.0 10 24.0 4 16.7 Lycium andersonii 5 5.6 4 4.7 0 0.0 0 0.0 Lycium pallidum 15 18.6 10 15.8 16 21.7 16 26.9 Tetradymia axillaris 2 2.3 0 0.0 0 0.0 0 0.0 Machaeranthera torti folia 2 0.4 3 0.4 1 0.2 1 0.1 Oryzopsis h ymenoides 0 0.0 3 0.1 0 0.0 0 0.0 119 100.0 128 100.0 98 100.0 94 100.0 "Relative dominance is calculated as Total basal area of species Total basal area all species p Na K Ca Mg Cu % % % % % Mg/g 0.288 0.954 4.393 3.643 1.312 6.6 0.250 1.193 3.955 4.497 1.275 4.5 0.232 0.0269 3.650 10.695 1.111 3.5 93.76 22.50 3.68 8.46 16.01 14.68 0.248 0.0497 7.711 2.440 1.462 4.0 0.312 0.0667 8.618 2.231 1.351 3.4 96 Great Basin Naturalist Memoirs No. 4 Table 2. Mineral composition of leaves of plants in and out of washes. Species and location Lycium pallidum In wash— mean Out of wash— mean In wash vs. out of wash F value 2.529 0.424 1.917 8.606 0.167 3.85 Lycium andersonii All in wash— mean F value between species Graijia spinosa In wash— mean Out of wash— mean In wash vs. out of wash F value 0.582 1.030 0.829 0.406 0.985 Wallace, A., and G. E. Kleinkopf. 1974. Contribution of salts to the water potential of woody plants. Plant Sci. Letters 3:251-257. Wallace, A., and E. M. Romney. 1972. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Tech. Information Service, USAEC Report TID-25954. Wallace, A., E. M. Romney, and G. V. Alexander. 1974. Variation in the simultaneous analysis by emission spectrography of twenty-four elements in plant material. Comm. Soil Sci. Plant Anal. 5: 45-50. Wallace, A., E. M. Romney, and V. Q. Hale. 1973. So- dium relations in desert plants: 1. Cation con- tents of some plant species from the Mojave and Great Basin deserts. Soil Sci. 115: 284-287. Wallace, A., E. M. Romney, R. A. Wood, A. A. El- .Ghonemy, and S. A. Bamberg. 1980. Parent ma- terial which produces saline outcrops as a factor in differential distribution of perennial plants in the northern Mojave Desert. Great Basin Nat. Mem. 4:138-143. 980 Nevada Desert Ec< 3LOGY 97 Table 2 continued. Fe Mn B Al Si Mo Sr Ba Li Mg/g Mg/g Mg/g Mg/g Mg/g Mg/g Mg/g Mg/g Mg/g 296 63.3 33.5 271 1056 2.4 426 23.3 25.4 259 55.4 38.2 231 954 2.1 414 19.8 34.6 2.364 1.774 1.339 0.862 0.927 0.575 0.133 3.386 311 48.9 28.3 299 958 1.6 751 48.7 56.6 6.76 26.65 2.3.01 8.87 5.99 146.0 5.41 15.98 93.3 .342 250 44.1 463 1529 1.0 268 28.2 - 327 202 36.4 378 1416 1.2 213 22.3 - 0.729 0.830 4.218 2.318 0.478 1.810 1.865 6.399 _ REGULATIVE EFFECT OF DODDER (CUSCUTA NEVADENSIS JTN.) ON THE VEGETATION OF THE NORTHERN MOJAVE DESERT A. Wallace', E. M. Romney1, and R. B. Hunter1 Abstract.— On two separate transects in the Rock Valley area of the northern Mojave Desert in the spring of 1976, 4 percent to 17 percent of the perennial plants were infested with the parasite Cuscuta nevadensis Jtn. (dod- der), and dead pieces of dodder from previous years were on dead plants equivalent to another 5 percent, indicating that the dodder had a regulating effect on the plant population and may he an important cause of perennial plant death. Dodder (Cuscuta nevadensis Jtn.), a yel- low-orange parasitic vascular plant, is com- mon in the northern Mojave Desert (Beatley 1976). A previous study indicates its presence on 16 different perennial plant species and its ability to kill the host plant and consequently influence the ecology of an area was recog- nized (Wallace and Romney 1972). The bio- logical characteristics of the genus Cuscuta have been recently reviewed (Ashton 1976). In a 50 X 2 m transect made near Rock Valley, Nevada, in the spring of 1976, 93 liv- ing perennial plants were present: 16 of these were infested with dodder (17 percent). Some of them were so badly infested that death of the host was certain (Fig. 1). Two dead plants were also observed in the transect with pieces of dead dodder attached. The numbers infested by species were: Grayia spinosa (Hook.) Moq. (6), Ambrosia dumosa (A. Gray) Payne (3), Ceratoides lanata (Pursh) J. T. Howell (1), Eycium pallidum Miers (5), and Psorothamnus fremontii (Torr.) Barneby (1). All these species were found to be infested in the previous study. Another transect (100 X 2 m) contained 8 plants with live dodder and 10 dead plants apparently killed by dodder because pieces of dead dodder were attached to them. This transect had approximately 4 percent in- festation and a 5 percent kill from a previous year. A 5 percent shrub mortality per year is greater than average if many of the plants live 20 to 100 years, as believed (Wallace and Romney 1972). The effect of dodder then, at least in some years, is important wherever in- festations occur. The prevalence of dodder is related to the soil-moisture conditions; there- fore, its impact varies from year to year. It grows abundantly in spring seasons with rela- tively cool temperatures. We had postulated that an intensive kill of shrubs could revert such areas into grassland, at least until the perennial shrubs became reestablished. The incidence of dodder then may favor the es- tablishment of grasses. The relative impor- tance of dodder and rodents as regulators of perennial plant populations is a subject of continuing interest. A( KNOWLEDGMENTS This study was supported by Contract EY- 76-C-03-00l2 between the University of Cal- ifornia and the U.S. Department of Energy. 'Laboratory of Nuclear Medic ii og) , University ol < lalifo 1980 Nevada Desert Ecology 99 Literature Cited Ashton, F. M. 1976. Cuscuta spp. (dodder): a literature review ot its biology and control. Div. of Agric. Sci., Univ. of Calif. Bull. 1880. Beatley. J. C. 1976. Vascular plants of the Nevada Test Site and central-southern Nevada. Tech. Informa- tion Center, Office of Tech. Information, EBDA Report TID-16881. Wallace, A., and E. M. Romney. 1972. Characteristics of Cuscuta nevadensis (dodder). Pages 162-163 in A. Wallace and E. M. Romney, eds. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Technical Information Ser- vices. USAEC Report TID-25954. * *£ life; Fig. 1. Lycium pallidum plant heavily infested with C. nevadensis in the spring of 1976. PHOTOSYNTHETIC STRATEGIES OF TWO MOJAVE DESERT SHRUBS G. E. Kleinkopf, T. L. Hartsock1, A. Wallace1, and E. M. Romnev1 Abstract.— Photosynthetic production of two Mojave Desert shrubs was measured under natural growing condi- tions at UCLA. Measurements of photosynthesis, transpiration, resistances to water vapor flux, soil moisture poten- tial, and tissue water potential were made. Atriplex canescens (Pursh) Nutt, a member of the C4 biochemical carbon dioxide fixation group was highly competitive in growth rate and production during conditions of adequate soil mois- ture. As soil moisture conditions declined to minus 40 bars, the net photosynthetic rate of Atriplex decreased to zero. However, the C3 shrub species Larrea tridentata (Sesse & Moc. ex DC.) Gov. was able to maintain positive net pho- tosynthetic production during conditions of high temperature and extreme low soil moisture through the major part of the season. The comparative advantages of the C4 versus the C3 pathway of carbon fixation was lost between these two species as the soil moisture potential declined to minus 40 bars. Desert plants have different strategies for survival, one of the strategies being the C4 biochemical carbon fixation pathway. However, many of the plants are members of the Gj group. In this instance, the C4 fixation pathway does not confer an added advantage to the pro- ductivity of the species in the Mojave Desert. Species distribution based on comparative photosynthetic production is discussed. Desert plant species have evolved special- ized strategies for coping with extreme envi- ronmental conditions. Drought avoidance and drought resistant plant species exist in the same area, although growth and repro- duction may occur at different times during the season. In the Mojave Desert, plant spe- cies growth response and productivity is gov- erned principally by moisture relationships (Bamberg et al. 1975, 1976). Photosynthetic production is also related to species differ- ences between age, leaf type, and distribution (Cunningham and Strain 1969, Strain 1969, Bjorkman 1971, Wallace and Romney 1972). In addition, desert plants possess special physiological traits such as low leaf tissue moisture and high osmotic pressure (Koz- lowslq 1968, 1972, Solbrig and Orians 1977) and temperature adaptation (Bjorkman et al. 1971, Pearcy 1977). Many desert plants carry out most of their photosynthesis during favor- able periods of the year when moisture rela- tionships are conducive to growth (Hatch and Slack 1970, Jarvis 1971, Caldwell et al. L972). Three biochemical pathways for carbon dioxide fixation have been documented rather extensively (Hatch et al. 1971, Burris and Black 1976). These three pathways in- clude C3, C4, and CAM photosynthesis. Atri- plex canescens (Pursh) Nutt., one of the plant species of interest, is a member of the C4 photosynthesizing group. The second plant, Larrea tridentata (Sesse & Moc. ex DC.) Co v., has the C3 pathway of photosynthesis. There is some consensus of opinion that the C4 pathway of photosynthesis has conferred some adaptive advantage to species possess- ing it, enabling them to be more competitive under extreme conditions such as exist in desert environments. In C4 species, carbon dioxide is first fixed by PEPcarboxylase into aspartate or malate and then transferred to specialized bundle sheath cells for fixation by ribulose diphosphate carboxylase. In C3 plants, which lack the specialized bundle sheath tissue, carbon is fixed by ribulose 1 :5- diphosphate carboxylase. The affinity of the PEPcarboxylase for carbon dioxide is greater than is the affinity of carboxylase for carbon dioxide in the C4 pathway. Another advan- tage is a high water use efficiency intrinsic to those plants that have the C4 pathway. This higher rate of photosynthesis and higher wa- ter use efficiency, coupled with higher light saturation and lack of photorespiration, should confer upon those plant species a bet- i.ahoratory ol Nnrlt-ai Midic in.- .mil Kaili.ihiui Nn>lot;\ I niv.-rsit) "I I alifornia, 1 OS Vngeles, ( .ilitoini.i 'KKI^ 1 1(H) 1980 Nevada Desert Ecology 101 ter adaptive strategy for survival in extreme conditions of the desert (Hatch et al. 1971, Solbrig et al. 1977). It became of interest to study the photosynthetic strategy of two shrubs, one of the C4 group, A. canescens, and one of the C3 group, L. tridentata. The morphology, distribution, and density of the species have been described earlier (Wallace and Romney 1972, Solbrig et al. 1977). Materials and Methods This work was done on species from the Nevada Test Site located in a transition zone between the Great Basin desert and the Mo- jave Desert. Climatic conditions in this area are characteristic of both regions, with ex- treme summer heat and winter cold. The pre- cipitation generally is less than 125 mm yearly. Both plant species are native to this area, with L. tridentata being of higher den- sity than A. canescens. Plant materials as cuttings or whole plants were removed from the desert and trans- ported to the UCLA facility for study. Plants taken from the desert were removed during winter dormancy and transplanted directly into cement-lined growth beds where total soil water availability could be controlled. Plant material, as cuttings, was rooted in a glasshouse and then transplanted into the beds for study. The cement-lined beds were 1 X 4 m and 40 cm deep. These growing con- ditions provided a means for establishing and monitoring plant growth during several sea- sons. Four beds were used; 6 to 8 plants of each species were used in the study, and nu- merous photosynthetic measurements were taken on each plant. Soil moisture was mea- sured with psychrometers purchased from Wescor, Logan, Utah. Plant moisture poten- tial was measured with a pressure bomb (Scholander et al. 1965). Gas exchange was measured using a Seamens Null-point cham- ber as described by Koller 1970). The Seam- ens equipment was designed to measure C02 exchange and transpiration at controlled or ambient conditions. Plant materials pre- viously established in the beds were main- tained in a well-watered condition before measurements were taken. Soil water depl- etion occurred by allowing the plants to uti- lize the available soil water. Photosynthesis and transpiration measurements were fol- lowed during several drying cycles. Results and Discussion Data presented here are averages of the photosynthetic rates of the two shrub species in two years. Figures 1 and 2 show the com- parison of photosynthetic rate and resistance to water vapor diffusion as plotted versus in- creasing soil water potential for 1974 and 1975. At higher water potentials and higher water availability, A. canescens showed high- er maximum net photosynthetic rates than L. tridentata for both years. The net photo- synthetic rate of A. canescens was maximum at high soil water content and decreased from near 50 mg C02 per square decimeter per hour to near zero as the soil moisture de- creased to minus 45 bars. At high soil mois- ture these data show the A. canescens re- sponse to be consistent, with C4 photosynthesis being greater than C3; how- ever, the C4 advantage is not as apparent at decreasing soil moisture. Data for L. triden- tata for the two years show the initial lower maximum rate of photosynthesis, but mainte- nance of a small but positive net C02 uptake as the soil moisture decreased to minus 50 bars. Larrea tridentata is capable of small positive net photosynthesis during portions of the day to minus 65 bars of soil water poten- tial (Bamberg et al. 1975). Figure 3 shows the net carbon dioxide up- take of A. canescens and L. tridentata during morning and afternoon conditions. The C4 plant, A. canescens, shows a higher maximum and a broader range of morning fixation (Fig. 3b) than the C3 plant, L. tridentata. A de- creasing rate of photosynthesis and increasing resistance values characterized both plants as soil moisture decreased. The afternoon car- bon fixation by A. canescens showed a differ- ent pattern, i.e., a decrease from an initial high rate at high water content of the soil to a rather low rate. Larrea tridentata, on the other hand, showed very little difference be- tween morning and afternoon fixation rates, starting at a maximum of 30 to 35 mg C02 per square decimeter, decreasing with de- creasing water potential of the soil, but main- taining a positive net fixation to minus 50 102 Great Basin Naturalist Memoirs No. 4 bars. Morning measurements of leaf resist- ance to water vapor flux showed an increase in the afternoon as temperatures gradually increased. Afternoon temperature measure- ments are commonly 30 to 40 C at UCLA, where the measurements were made. These data show the opposing photosynthetic strat- egies of the two desert shrubs. The C4 plant, A. canescens, had a higher photosynthetic rate during conditions of lower morning tem- peratures and higher soil water potentials. However, the C3 plant, L. tridentata, was ca- pable of maintaining a positive net photo- synthetic rate at higher stress levels. In Figure 4, data are plotted which de- scribe the net carbon dioxide uptake at two temperatures, 25 and 35 C. In both species at 25 C, photosynthesis and transpiration paral- leled each other as tissue water potential de- clined. At 35 C transpiration increased pro- 60 — • Atriplex canescens a Larrea tridentata C02 Uptake ~ 50 Rwv 1 •^. .C \ CM \ 1 40 \ • E \\ / / 5 30 w / D AV \ / / Q. Z> V ' / ■ .,_ /V / * / K/ 10 / y ^ \ ^^ .-•^^ -"'A «V * 0 i 1 1 ^T-» 24 20 16 § O 01 12 > 3 or 10 20 30 40 Y Tissue (-bars) 50 -8 -4 60 Fig. 1. Daily average rate ol photosynthesis and stomatal resistance versus tissue water potential ol Atriplex canes{ cens and Larrca tridentata, 1974. Plants were well established in cement lined beds containing native desert soil. Data represent averages of 20 or more measurements on six plants. 1980 Nevada Desert Ecology 103 portionately greater than the photosynthetic increase in both species. As the tissue water potential decreased to minus 40 bars at 35 C, the net photosynthetic rate decreased to zero in A. canescens. The evergreen shrub, L. tri- dentata, was able to maintain a small but positive net photosynthetic rate as the tissue potential decreased below minus 50 bars. The photosynthesis to transpiration ratio, as plotted in Figure 5, shows some interesting 70 - Atriplex canescens /• rl4 -a- Larrea tridentata / Fig. 2. Daily average rate of photosynthesis and stomatal resistance versus soil water potential of Atriplex canes- cens and Larrea tridentata, 1975. Conditions were as described in Figure 1. 104 Great Basin Naturalist Memoirs No. 4 l x CM E E o 3 CsJ O (J CD 2 (b) Morning 20 30 40 • Atriplex canescens a Larrea tridentata C02 Uptake Rwv 10 20 30 Y Soil (-bars) Fig. 3. Photosynthesis and stomatal resistance of two desert shrubs. Data represent averages of plants to morning (cool) conditions and (warm) afternoon conditions as soil moisture declines. Conditions were as described in Figure 1. 1980 Nevada Desert Ecology 105 (a)25°C 30- 1 20 X >> 10 o> E 0 1 D 3 CM O O 30 Tr^N, 4V\ (b)35°C • A. canescens a L. tridentata a> 20 o Q. c o 20 30 40 Y Tissue (-bars) 0 60 Fig. 4. Daily average rate of photosynthesis and transpiration of two desert shrubs versus tissue water potential at two temperatures. Conditions were as described in Figure 1. 106 Great Basin Naturalist Memoirs No. 4 •~ Atriplex canescens —a- Larrea tridentata 30 40 50 Y Tissue (-bars) Fig. 5. Water use efficiency versus (issue water pot< described in Figure I. il ol two desert sh Experi 1980 Nevada Desert Ecology 107 differences between the C4 species, A. canes- cens, and the C3 species, L. tridentata. At moderate tissue water potential between minus 10 and minus 30 bars, A. canescens showed an increasing water use efficiency at both temperatures 25 C and 35 C. Such is characteristic of a C4 shrub. However, as the tissue water potential decreased below minus 30 bars, the ratio decreased rapidly. The strategv displayed by the C3 plant, L. triden- tata, was somewhat different. The water use efficiency as shown by the photosynthesis: transpiration ratio decreased rather gradually as tissue water potential declined to minus 50 jars. Figures 6 and 7 show the relationship be- tween the milligrams carbon dioxide fixed on an area basis and a dry weight basis. These two curves indicate that it is possible with a high degree of confidence to make a dry weight measurement on the leaves and con- vert that to an area base measurement for re- sistance calculation. These data also imply that the specific leaf weight of A. canescens 60 50 -40 o> 30 CM o (J o> 20 10- - Atriplex canescens - a _ a - y^ /& D ^B t i i i 10 20 30 40 mg C02 dm h 50 60 Fig. 6. Photosynthesis of Atriplex canescens. Data are plotted to show the correlation between dry weight and leaf surface measurements for a photosynthetic base. Leaf area determinations of numerous small leaves can be time consuming. 108 Great Basin Naturalist Memoirs No. 4 and L. tridentata do not change as the photo- synthetic rates decline due to decreasing wa- ter potential of the soil. These two plant species, one a C4 carbon fixer and one a C3 fixer, showed differing strategies in coping with the extreme envi- ronment of the desert. The C4 species, A. ca- nescens, appeared to have the higher photo- synthetic rate during conditions of moderate moisture and temperature stress. Higher wa- ter use efficiency is shown by the C4 species under conditions of moderate water stress. However, the evergreen shrub, L. tridentata, is capable of maintaining small but positive net photosynthetic rates throughout the ma- jor portion of the growing season. These two plant species differ in their bio- chemical mechanism of photosynthesis and show contrasting strategies for survival in the desert. Atriplex canescens is capable of pro- ductivity and growth during a more favor- able moisture climate and is not competitive 30- 25 -' 20 ' 15 CM O u 10 E 5- Larrea tridentata 10 20 30 40 mg CO. drrf2-h"1 50 60 Fig. 7. Photosynthesis of Larrea tridentata. Data surface area. e plotte< show the correlation between dry weight and lea 1980 Nevada Desert Ecology 109 under soil moisture conditions of less than minus 35 bars. Distribution of the two shrubs in the various desert climates has been de- scribed by Wallace and Romney (1972). Atri- plex canescens appears to be more suitable to the colder, wetter climates provided by the Great Basin desert than does L. tridentata. The distribution of L. tridentata into the more northern part of the Mojave Desert and into the Great Basin desert appears to be lim- ited by the cold winter temperatures. Acknowledgments This study was supported by Contract EY- 76-C-03-0012 between the U.S. Department of Energy and the University of California. Literature Cited Bamberg, S. A., G. E. Kleinkopf, A. Wallace, and A. Vollmer. 1975. Comparative photosynthetic production of Mojave Desert shrubs. Ecology 56:732-736. Bamberg, S. A., A. T. Vollmer, G. E. Kleinkopf, and T. L. Ackerman. 1976. A comparison of seasonal primary production of Mojave Desert shrubs dur- ing wet and dry years. Amer. Midi. Nat. 95:398-408. Bjorkman, O. 1971. Comparative photosynthetic CO2 exchange in higher plants. Pages 18-32 111 M. D. Hatch, C. B. Osmond, and R. O. Slayter, eds. Photosynthesis and photorespiration. New York. Wilev-Interscience. Bjorkman, O., R. W. Pearcy, A. T. Harrison, and H. A. Mooney. 1971. Photosynthetic adaptation to high temperatures: a field study in Death Valley, California. Science 175:786-789. Burris, R. H., and C. C. Black, eds. 1976. CO2 meta- bolism and plant productivity. Proc. 5th Annual Harry Steinbock Symposium, University Park Press, Madison, Wisconsin. Caldwell, M. M. 1972. Adaptability and productivity of species possessing C3 and C4 photosynthesis in a cool desert environment. Pages 27-29 in L. E. Rodin, ed. Ecophysiological foundation of ecosys- tems productivity in arid zone. Int. Symposium, Leningrad. Cunningham, G. L., and B. R. Strain. 1969. An ecologi- cal significance of seasonal leaf variability in a desert shrub. Ecology 50:400-408. Hatch, M. D., C. B. Osmond, and R. O. Slayter, eds. 1971. Photosynthesis and photorespiration. Wiley-Interscience, New York. Hatch, M. D., and C. R. Slack. 1970. Photosynthetic CO2 fixation pathways. Ann. Review Plant Phys- iol. 21:141-162. Jarvis, P. G., and J. Catsky. 1971. General principles of gasometric methods and the main aspects of in- stallation design. In Plant photosynthetic produc- tion-manual of methods. Dr. W. Junk N. V. Pub- lishers, The Hague. Koller, D. 1970. Determination of fundamental plant parameters controlling carbon assimilation and transpiration by the Null-point compensating sys- tem. Lab. of Nuc. Medicine and Rad. Biology, University of California, Los Angeles. Report. 12- 797. Kozlowski, T. E., ed. 1968. Plant water consumption and response, Vol. 2. Academic Press, New York- San Francisco-London. 1968. Development, control, and Measurement. Vol. 1. Academic Press, New York-San Francisco- London. 1972. Plant responses and control of water bal- ance, Vol. 3. Academic Press, New York-San Francisco-London. Peabcy, R. W. 1977. Acclimation of photosynthetic and respiratory carbon dioxide exchange to growth temperature in Atriplex lentiformis (Torr.) Wats. Plant Physiol. 59:797-799. Scholander, P. F., E. D. Bradstreet, H. T. Hammel, and E. A. Hemmingson. 1965. Sap pressure in vascular plants. Science 148:339. Solbrig, O. T., and G. H. Orians. 1977. The adaptive characteristics of desert plants. American Scien- tist 65:412-421. Solbrig, O. T., M. Barbour, J. Cross, G. Goldenstein, C. Lowe, J. Morello, and T. W. Tang. 1977. The strategies and community patterns of plants. In G. H. Orians and O. T. Solbrig, eds. Con- vergent evolution in warm desert ecosystems., Dowden, Hutchinson, and Ross, Stroudsburg, Pennsylvania. Strain, B. R. 1969. Seasonal adaptations in photo- synthesis and respiration in four desert shrubs growing in situ. Ecology 50:511-513. Wallace, A., and E. M. Romney. 1972. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Technical Information Ser- vices, USAEC Report TID-25954. TRANSPIRATION AND CO, FIXATION OF SELECTED DESERT SHRUBS AS RELATED TO SOIL-WATER POTENTIAL S. B. Clark1, J. Letey, Jr.:, O. R. Lunt\ A. Wallace1, G. E. Kleinkopf', and E. M. Romney' Abstract.— In desert plants, transpiration rates decreased before photosynthetic rates when plants were entering a period of water stress. This may have adaptive consequences. A difference of -5 bars in the soil-moisture potential had considerable importance in reducing the rate of transpiration. In Helianthus annuus L. (sunflower) the photo- svnthetic rate decreased before the transpiration rate in contrast to Great Basin-Mojave Desert plants, and the changes occurred with a -1 bar difference in soil-moisture potential. Morphological changes in three desert plant species [Artemisia tridentota Nutt., Ambrosia dumosa (Gray) Payne, Larrea tridentata (Ses. Moc. ex DC) Cov.] as the soil-moisture potential decreased are given. With a mesic species, //. annuus, 20 percent reduction in photosynthesis and transpiration was reached at higher soil-moisture potentials than with the desert plants. Loss of net photo- synthesis occurred in A. dumosa (a summer deciduous shrub) as ^soil reached -48 bars in the field, whereas L. tri- dentata (an evergreen shrub) at the same time was able to maintain a water potential difference betwen soil and plant of -10 to -15 bars and continue net COg gain well into the summer months. Plants growing in arid regions obviously have the capability of surviving conditions of low soil-water content. The mechanisms differ (Cooper 1975). Transpiration and pho- tosynthesis are two important plant processes that must adapt to the dry conditions for sur- vival. Both processes involve gas exchange between the plant and the atmosphere, and both are known to decrease as soils become drier or as moisture stress increases (Babalola et al. 1968, Cox and Boersma 1967, El-Rah- man 1969, Fischer 1970, Heichel and Mus- grave 1966, Pallas et al. 1967, Schneider and Childers 1941, Schratz 1937, Shinn and Lem- on 1968). Obviously, plants which have very low transpiration rates, thus prolonging the water supply provided that leaf turgor re- mains in a satisfactory state, would be fa- vored under arid conditions. Also, a relatively high rate of photosynthesis could be advanta- geous, especially if it could be maintained with decreasing transpiration rates due to drying of soil. Studies of quantitative relationships among transpiration, photosynthesis, and soil-water potential for desert shrubs have generally been difficult in the past because of technical difficulties. Desert shrubs grow and survive under conditions where the soil-water poten- tial is usually much lower than can be accu- rately monitored by soil-water potential monitoring devices such as tensiometers or resistance blocks. However, recent devel- opments of thermocouple psychrometers for measuring water potential extended the range that can be measured in soil. More so- phisticated instruments that measure water vapor and C02 exchange between plants and the surrounding atmosphere have also been developed (Roller 1970, Mork et al. 1972). In particular, a null-point system that maintains a given atmospheric condition is desirable be- cause the exchange is not greatly influenced by the measuring technique (Koller 1970, Mork et al. 1972). The purpose of the research reported here- in was to measure the relationship between transpiration and photosynthesis and soil-wa- ter potential of various desert shrubs. One study was conducted on sunflowers to pro- vide comparison with a plant not adapted to desert conditions. 'Present address: Fallbrook, California 92028. 2University of California, Riverside, California 92502. 'Laboratory of Nuclear Medicine and Radiation Biology, Uni' 'Present address: University of Idaho, Kimberly, Idaho 83341. of California. Los Angeles, California 9002-1. 110 980 Nevada Desert Ecology 111 Materials and Methods Studies were conducted under glasshouse conditions in containers so that the soil-water condition throughout the root zone could be determined. The plant species studied were Artemisia tridentata Nutt, Ambrosia dumosa (Gray) Payne, Larrea tridentata Ses. Moc, and Helianthus annuus L. (sunflower). Rooted cuttings of A. tridentata (Wieland et al. 1971), along with seedlings of the other plant species, were grown in either 900 ml (10.5 cm diameter and 15 cm deep) or 2700 ml (10.5 cm diameter and 45 cm deep) con- tainers. A Yolo silt loam soil treated with "krilium" to maintain aggregate stability was used as a growth medium. All containers were watered to approximately 20 percent soil-water content (^ soil = -1) and main- tained until roots were established through- out the container. After roots were fully es- tablished, some of the containers were watered on a regular basis, whereas others were allowed to dry out. Soil-water potential (^ soil) was measured by thermocouple psychrometers. (Com- mercial psychrometers manufactured by Wescor Inc., Logan, Utah were used. Read- out was on a Keithley Nanovoltmeter.) One psychrometer was placed at 10 cm depth in the 900 ml container, and three psycho- meters were placed at 10, 28, and 42 cm depths in the 2700 ml containers. Soil-water potential measurements were made early in the morning after plants were taken from the greenhouse into the head- house to prevent rapid increase in soil tem- perature that interferes with potential mea- surement. Plants were allowed to stand in the headhouse for approximately one-half hour before measuring the potential, and then the containers were returned to the greenhouse. Transpiration and apparent photosynthesis were monitored on both watered and unwa- tered plants. These measurements were made by a null-point compensating system de- scribed by Koller (1970) and further modified by Mork et al. (1972). Briefly, the measuring procedure was as follows: The plant was en- closed in a chamber that was maintained at constant relative humidity, temperature, and carbon dioxide concentrations. The amount of water removed to maintain constant rela- tive humidity was measured and represented the transpirational loss from the plant. The amount of COa added to the chamber to maintain the constant concentration was also measured and represented apparent photo- synthesis. The plant chamber was used in the greenhouse; thus light intensity was not maintained constant and represented an un- controlled variable. Light intensity measure- ments were made and recorded for the time transpiration, and apparent photosynthesis measurements were made. The plant cham- ber was maintained at a relative humidity of 28 percent, a CO, concentration of 316 jul/ liter and a temperature of 25 C. The time required for the plant to remain in the chamber depended upon the rate of transpiration and apparent photosynthesis. Sufficient water and C02 had to be ex- changed for significant results. A 20-minute period generally was sufficient when trans- piration and apparent photosynthesis were quite rapid, as occurred for nonstressed plants. Monitoring periods of up to an hour were often required for the stressed plants because of the low transpiration and appar- ent photosynthesis of these plants. Because of the time involved in making measurements, not many plants could be measured. Because both transpiration and ap- parent photosynthesis are dependent upon light intensity, day to day variations in light intensity were partially compensated for by reporting only points where major changes occurred. Representative data, however, are shown in Figure 1. Artemisia tridentata plants were grown in both 900 ml and 2700 ml containers. Appar- ent photosynthesis and transpiration were measured over two drying cycles. Larrea tri- dentata plants were grown in 2700 ml con- tainers, and the transpiration and apparent photosynthesis were monitored for one drying cycle on two plants. The A. dumosa and H. annuus plants were grown in 900 ml containers and monitored for one drying cycle. Gas exchange measurements were made on L. tridentata and A. dumosa during the months of May and June in the Mojave Desert. Soil-water potential and plant-water potential were monitored simultaneously. The latter was monitored by the pressure 112 Great Basin Naturalist Memoirs No. 4 Z o < X •— • TRANSPIRATION. ,0 \ \ •"• C02 \ "\ ■--■ POTENTIAL 0 i ■v o m 20 z 30 ~ i w ■HO * 0) 50 60 3 5 7 TIME (DAYS) 10 12 14 Fig 1 Representative plot of data obtained, in this case for A. dumosa. Data are plotted as a ratio of stressed to unstressed plant conditions. Part of the variation in the ratio for zero time is due to differences in ind.v.dua plants. Light intensity in 1000s M Einsteins rrr* seer* in stressed plants for 0, 3, 5, 7, 10, 12, and 14 days, respectively were 0.4, 1.4, 1.5, 1.2, 1.5, and 1.5, respectively. For unstressed plants for the same number of days they were 0.4, l.Z, Lb, 1.1, 0.5, 1.3, and 1.5, respectively. 1980 Nevada Desert Ecology 113 bomb technique (Scholander et al. 1965). Conditions for measuring photosynthesis and transpiration were the same as for green- house experiments. Results and Discussion The results for A. tridentata are described in Table 1. At the beginning of the measure- ment, plants were in a vigorous state of vege- tative growth; the stems were green and supple, the leaves were approximately 3 cm long, and the internodes approximately 0.5 cm apart. A shoot approximately xk to 1 cm long was developing at each node. The lobes of the trees were deeply cleft, and the color was green with a slight shade of gray both on the upper and lower surfaces. First visual indication of water stress on the plant was a gradual yellowing of the leaves attached to the main stem. As soil-wa- ter potential decreased over an 11 -day peri- od, the leaves turned necrotic and eventually fell off. The earlier symptoms occurred at a ^ soil of -15 to -20 bars. A sharp reduction in transpiration and COa fixation occurred when the soil was -5 to -10 bars. When the soil was between -20 and -25 bars, the re- maining leaves wilted, and the overall color became more gray than green. No measure- ments on transpiration or apparent photo- synthesis were made when ^ soil was -25 bars; therefore, the plant in the dried soil was rewatered to start a second cycle. Although the specific plant under test was not carried beyond -25 bars ^ soil before watering, other plants were carried to lower potentials. The symptoms which occurred were that the shoot tips wilted and, if kept under stress, eventually died. However, the tips can be killed and the plant can recover if it is not held under stress too long. Having the study plant subjected to 11 days without addition of water caused a no- ticeable change in plant appearance and characteristics even after irrigation. The stems were woody; internodes were quite close, giving the appearance of a whorl or ro- sette rather than single leaves alternately spaced as on the first cycle. The longer leaves were still attached, but were either yellow or necrotic. There was an increase in the relative trans- piration and C02 fixation rate following irri- gation. The transpiration was initially high relative to the C02 fixation during the recov- ery period; however, the C02 fixation rate eventually was higher than transpiration. Upon starting the second drying cycle, de- crease in transpiration appeared at a higher ^ soil than did a decrease in C02 fixation. There was a steady decrease in transpiration as the ^ soil decreased below about -2.5 bars. On the other hand, there appeared to be little effect of ^ soil on C02 fixation until the soil-water potential was lower than about -16 bars. Further decrease in ^ soil caused a decrease in CO, fixation. Within limits, a more rapid decrease in transpiration than de- crease in C02 fixation as ^ soil decreases may be an important factor in survival of plants adapted to arid environments. The Table 1. Soil-moisture potential at which various changes occurred in the plants (-bars). H. A. L. L. A. annuus dtimosa tridentata tridentata tridentata Plant No. Plant No. 1 2 20 percent reduction of photosynthesis rate 20 percent reduction of transpiration rate Stress at COj reduction of 75 percent Stress at minimum CC\ exchange 2.5 3.5 L3 38 20 10 02 26 30 54 30 52 114 Great Basin Naturalist Memoirs No. 4 plant can be conserving water while still ef- fectively producing carbohydrates. A recent review by Fischer and Turner (1978) on pro- ductivity in arid and semiarid environments suggests that plants tend to maintain high gas exchange rates as long as possible, thus max- imizing both photosynthesis and trans- piration. Following are visual descriptions of the stressed plant as the ^ soil decreased after the second irrigation. There were no visible symptoms of stress until ^ soil was about -22 bars. This observation related quite closely to the COa fixation but not to transpiration, as there had been a curtailment of transpiration well before visible symptoms occurred. The changes noted in the plant at -22 bars were an inward curling of the outer edge of the leaves; added stress induced by ^ soil of -27 bars caused a very slight wilting. The change in plant appearance at -32 bars was a pro- nounced wilting and leaf color change from greenish to gray. When ^ soil reached -52 bars, no apparent photosynthesis was measur- able. Leaf tips became darker in color and se- verely curled. Soil-water potentials lower than about -61 bars were not measurable by the psychrometers in use (psychrometers more recently developed can measure below -61 bars). Thus, further observations on the plant refer to number of days following the time ^ soil reached -61 bars. After 5 days, the plant was under very se- vere stress, but there were no signs of necr- osis except on leaves that had been damaged during the previous irrigation cycle. The method for measuring apparent photo- synthesis was to monitor the amount of C02 required to maintain a constant C02 content in the plant chamber. Because there was no net C02 use at this high stress, the C02 con- centration in the chamber was monitored with no air flow. With this change, the C02 concentration increased slightly and covering the chamber caused a threefold increase in rate of C02 production, indicating that some photosynthesis occurred at this time, even though it was not equal to the rate of respira- tion. Seven days following the -61 bar potential readings, visible symptoms were about the same. C02 did increase slightly in the cham- ber, indicating respiration; however, the rate of C02 production did not change when the chamber was covered to eliminate light, thus indicating that photosynthesis had com- pletely ceased. Necrosis started to appear on lobes and margins of the leaves nine days following the -61 bar stress. Twelve days following the -61 bar ^ soil reading the plant had a dull slate- gray appearance, the lower leaves were gen- erally necrotic, and the upper leaves were flaccid. There was no change in C02 concen- tration within the chamber, indicating respi- ration had ceased. The plant was then irri- gated, but it did not recover. It appears, therefore, that the plant can recover follow- ing irrigation at any time that measurable metabolism such as respiration can be found. Apparently, after the plant reaches a stage in which respiration completely ceases, it is ir- reversibly damaged. In general, there were similarities between the results observed for plants grown in the 2700 ml container as compared to the 900 ml container even though the drying times to achieve the same level of stress were differ- ent. Transpiration decreased at a higher ^ soil than did apparent photosynthesis during both the first and second irrigation cycles. However, during the second irrigation cycle, more time was required for the plant to re- cover in both photosynthesis and trans- piration after having been subjected to water stress. The increase in apparent photo- synthesis and transpiration following irriga- tion cannot be attributed completely to an increase in plant size because of growth. Al- though the drying period extended over a longer period of time in the 2700 ml contain- er, the visual symptoms of the plant were very similar to those reported for the 900 ml container. During the first drying cycle, the longer leaves on the main stem curled and turned yellow. As drying continued, the other leaves changed to a grayer color with some curling. During the second drying cycle, there was no great appearance of chlorosis or necrosis of the leaves, but the leaves became a dull color and rolled. The results for A. dumosa are also summa- rized in Table 1. This plant was carried over one drying cycle only. As was observed with A. tridentata, the transpiration appeared to decrease at a higher ^ soil as compared to 1980 Nevada Desert Ecology 115 C02 fixation. Transpiration noticeably de- creased at ^ soil less than -5 bars. A marked decrease in C02 fixation occurred when ^ soil decreased from about -4 bars to about -15 bars. Apparent photosynthesis had ceased when the ^ soil was -38 bars, how- ever; COa production was enhanced by cov- ering the chamber, indicating that some pho- tosynthesis was occurring at that ^ soil even though respiration was at a higher rate than photosynthesis. Some of the visual symptoms of A. dumosa are as follows: plants showed no symptoms of wilt, but the leaves were grayer than the con- trol at a ^ soil of -15 bars. As with A. triden- tata, decrease in metabolic processes such as transpiration and photosynthesis occurred be- fore visual symptoms appeared. Severe chlo- rosis on all but the leaves toward the growing tip occurred when the ^ soil reached -38 bars. At this point there was no apparent photosynthesis, although some transpiration was still occurring. When the ^ soil was -60 bars, the foliage was completely necrotic ex- cept for some color in the axils of leaves. This plant recovered if watered at this stage. Soil in the containers of two separate L. tridentata plants was allowed to dry out. The L. tridentata plants were grown from seed and there was large variation between indi- vidual plants. One plant will be referred to as Larrea 1. Similar to other species, trans- piration tended to decrease at a higher ^ soil than did C02 fixation. However, the other plant (referred to as Larrea 2) showed an op- posite trend that C02 fixation decreased at a higher ^ soil than did transpiration. Differ- ences between the two individual plants, however, go beyond these observations. The Larrea 1 plant responded to drying out by having 50 to 60 percent of the leaves on the plant turn yellow and fall off when the ^ soil was approximately -20 bars. The trend to- ward defoliation and yellowing of leaves con- tinued as the water potential decreased. On the other hand, the Larrea 2 plant had very little yellowing and defoliation under stress. Leaves tended to wilt and turn to a gray- green color. With continued soil drying, the leaves became desiccated. Some yellowing of leaves did occur, but the plant appeared completely different from the Larrea 1 plant. Results for the H. annum plant are also in Table 1. One noticeable difference between H. annuus and the desert shrubs was that a slight decrease in ^ soil down to -1 bar greatly reduced C02 fixation and did not have any significant effect on transpiration. Most of the desert shrubs behaved quite op- positely where transpiration was reduced much more significantly than C02 fixation at low soil. This may be of competitive advan- tage for the desert species. It has been re- ported that halophytes have lower trans- piration rates than nonhalophytes (Schratz 1937), and this may be true also of other plants growing in harsh environments. A ^ soil of -6 bars caused a great reduction in both transpiration and C02 fixation in H. an- nuus plant, but neither process could be monitored at a soil-water potential of -24 bars. When the ^ soil reached -6 bars, the lower leaves were severely wilted and other leaves moderately wilted. When ^ soil reached -20 bars, all leaves were severely wilted and the lower leaves complete necrot- ic. At ^ soil of -33 bars the stem was shriv- eling. The H. annuus experiment was conducted as an indicator of our procedure. Many stud- ies have been conducted on drying soil in which H. annuus plants are growing, and -15 bars has often been quoted as the permanent wilting percentage for H. annuus; our results are in reasonable agreement with that value. Transpiration and photosynthesis measured on H. annuus also appear to be reasonable. This provides some degree of confidence in accepting the results measured on the desert shrubs. The results of a field experiment where both soil-water potential and tissue-water po- tential (^ plant) (Scholander et al. 1965) were measured are shown in Table 2. Gas ex- change measurements on A. dumosa during May and June showed decreasing photo- synthesis and transpiration rates as ^ soil de- creased. Positive carbon dioxide exchange ceased on A. dumosa as ^ plant reached -47 bars. However, L. tridentata, an evergreen desert shrub, continued photosynthesis until ^ plant reached -65 bars. This difference be- tween species is in agreement with green- house studies reported in Table 1 and also with results of Odening et al. (1974). Larrea tridentata has a distinct advantage of being 116 Great Basin Naturalist Memoirs No. 4 Table 2. ^ soil and ^ plant (-bars) as measured on two desert shrubs under field conditions. ° Ambrosia dumoa Larrea tridentata ^soil ^plant ^soil ^ plant May 1 22 39 22 51 9 28 40 28 54 31 42 — 42 56 June 1 48 47 48 63 6 48 dormant 48 _ 12 50 dormant 50 63 12 52 dormant 52 65 "Maximum rates of gas exchange for A. dumosa and L. tridentata were net photosynthesis 35 and 10 mg/g dry wt -h and transpiration 45 and 13 g/dry wt -h, respectively. able to maintain productivity well into the summer months. In both shrub species, max- imum gas exchange activity occurred during the early spring months when favorable mois- ture conditions existed. Acknowledgments This study was supported by Contract EY- 76-C-03-0012 between the U.S. Department of Energy and the University of California. Literature Cited Babalola, O., L. Boersma, and C. T. Youngberg. 1968. Photosynthesis and transpiration of Monterey pine seedlings as a function of soil water suction and soil temperature. Plant Physiol. 43:515-521. Cooper, J. P., ed. 1975. Photosynthesis and productivity in different environments. International Biologi- cal Programme 3. Cambridge University Press, Cambridge, London. Cox, L. M., and L. Boersma. 1967. Transpiration as a function of soil temperature and soil water stress. Plant Physiol. 42:550-556. El-Rahman, A. A. Abd. 1969. Effect of moisture stress on plants. Arid Lands Conf. Abs. Tucson, Ari- zona. Fischer, R. A. 1970. After effect of water stress on stom- atal opening potential. II. Possible causes. J. Expt. Bot. 21:336-404. Fischer, R. A., and N. C. Turner. 1978. Plant produc- tivity in the arid and semiarid zones. Ann. Rev. Plant Physiol. 29:277-317. Heichel, G. H., and R. B. Musgrave. 1966. Photo- synthetic response of corn (Zea mays L.) to leaf water potential. Amer. Soc. Agron. Abs. p. 20. Roller, D. 1970. Determination of fundamental plant parameters controlling carbon assimilation and transpiration by the null-point compensating sys- tem. USAEC Report, UCLA 12-797. Mork, H. M., R. W. Farmer, and K. A. Flygar. 1972. Improved feedback control for the null-point compensating system. Soil Sci. 114:61-68. Odening, W. R., B. R. Strain, and W. C. Oechel. 1974. The effect of decreasing water potential on net CC^ exchange of intact desert shrubs. Ecolo- gy 55:1086-1095. Pallas, J. E., Jr., B. E. Michel, and D. G. Harris. 1967. Photosynthesis, transpiration, leaf temper- ature and stomatal activity of cotton plants under varying water potentials. Plant Physiol. 42:76-88. Schneider, G. W., and N. F. Childers. 1941. Influence of soil moisture on photosynthesis, respiration, and transpiration of apple leaves. Plant Phvsiol. 16:565-583. Scholander, P. F., H. T. Hammel, E. D. Bradstreet, and E. A. Hemingsen. 1965. Sap pressure in vas- cular plants. Science 148:339-346. Schratz, E. 1937. Beitrag zur Biologie der halophyten. IV. Die Transpiration der Strand und Diirnenpflanzen. Jahrb. Wiss. Bot. 84:593-638. Shinn, J. H., and E. R. Lemon. 1968. Photosynthesis un- der field conditions. XI. Soil-plant-water relations during drought stress in corn. Agron. J. 60:337-343. Wieland, P. A. T., E. F. Frolich, and A. Wallace. 1971. Vegetative propagation of woody shrub species from the northern Mojave and southern Great Basin deserts. Madronno 21:149-152. EFFECT OF CERTAIN PLANT PARAMETERS ON PHOTOSYNTHESIS, TRANSPIRATION, AND EFFICIENCY OF WATER USE H. M. Mork1, A. Wallace', and E. M. Romnev Abstract.— Rates of gaseous exchange were measured on selected desert shrubs native to the northern Mojave Desert to determine effects of varying chamber temperature, C02 concentration, relative humidity, and root tem- perature in preliminary studies. Results indicate that changes in these parameters produced differences in the rates of photosynthesis and transpiration. Ceratoides lanata (Pursh) took up CO2 almost equally at 25 and 39 C. Doubling tin' ( ()2 concentration in the below-ambient range roughly doubled photosynthesis rates in C. lanata. Very small Pianges in relative humidity had marked changes in the photosynthesis and transpiration rates of four species stud- ied, with greater effect on transpiration. Photosynthesis and transpiration increased, and water-use efficiency de- creased in two species as soil temperature was increased from 9 to 29 C. The subject of photosynthesis has been dis- cussed and reviewed thoroughly by Sestak et al. (1971), Troughton (1975), and Cooper (1975). These reviews indicate that it is very difficult to predict or explain the photo- synthetic rate of a plant because it is in- fluenced by the simultaneous action of many external and internal factors that affect the rate of photosynthesis. For this reason a study was undertaken with objectives to determine how sensitive the rates of C02 exchange and water loss are to variations in chamber temperature, COa concentration, relative humidity, and the root temperature of the plant (i.e., how much variation can be tolerated in these parame- ters without adversely affecting the validity of gas exchange rates in desert plants). This technique has been described by Koller (1975). Rates of photosynthesis and transpiration were measured using the Null-point Com- pensating System (Koller 1970) with the Im- proved Feedback Mechanism (Mork et al. 1972). The principle of this system is based on maintaining essentially constant condi- tions in the chamber; i.e., the C02 concentra- tion, the relative humidity, the chamber tem- perature, and the bath temperature must be constant to make valid measurements of com- pensation rates. M ATERIALS AND Ml Five species of desert shrubs were tested in February and March 1972: Ephedra neva- densis S. Wats., Larrea tridentata (Sesse & Moc. ex DC.) Cov., and Lycium pallidum Miers in the field, and Atriplex hymenehjtra (Torr.) S. Wats., Ceratoides lanata (Pursh) and L. tridentata in the glasshouse. Field plants were tested at various levels of C02 concentration. Plants in the glasshouse were tested at various chamber temperatures and root temperatures. Initially, a more com- plete and extensive study was envisioned, particularly with regard to the effect of varying the root temperature. All the neces- sary equipment was on hand: the plants were growing in water-jacketed lucite cylinders, so the root temperature could be adjusted by running heated or cooled water through the jacket. In each case one parameter was var- ied, and the rates of photosynthesis and trans- piration were measured using the Null-point Compensating System with the Improved Feedback Mechanism. The light intensity was measured with a portable Weston meter. Air temperature has complex interactions with photosynthetic rate (Bauer et al. 1975, Mooney et al. 1975), but a study of them is not the purpose of this report. The photosynthesis and transpiration rates aboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 90021. 117 118 Great Basin Naturalist Memoirs No. 4 were calculated using the method of Koller (1970) and expressed as mg C02 per g dry tis- sue per h (or as mg C02 per sample per h) and g H20 per g dry tissue per h (or as g H20 per sample per h), respectively. Efficiency of water use is a ratio of photo- synthesis (g C02 fixed per unit time) to trans- piration (g water lost per unit time) X 100 (percent). Results and Discussion Increasing the chamber temperature about 25 C tended to increase the rate of water loss up to 46 C for C. lanata at field capacity soil moisture (Table 1). The rate of photo- synthesis was decreased at 46 C but not at 39 C relative to 25 C. A wide optimum range was indicated for these glasshouse-grown plants. The C02 concentration in the chamber had significant effects on the plant process rates as expected (Table 2). Increasing the C02 concentration (range 90-375 ul/1 air) in- creased the C02 exchange for L. pallidum al- most tenfold while reducing the water loss by about 10 percent. About the same range of increases in C02 increased the C02 exchange about fourfold for L. tridentata, with a reduc- tion in water loss of about 10 percent. Dou- bling the concentration of C02 in these low ranges roughly doubled the rate of C02 ex- change. Decreasing the relative humidity even by very small increments for L. triden- tata and E. nevadensis tended to reduce the C02 uptake (r = +0.68 and +0.99, respec- tively, for the two species) and increase the water loss rate (Table 3). In contrast, the net result of a slight decrease in relative humid- Table 1. Effects of chamber temperature on photosynthesis and transpiration rates and on efficiency of water use of Ceratoides lanta grown in the glasshouse (3-9-72). Light intensity Chamber tL Einsteins Photosynthesis Tr anspiral :ion Efficiency Hour of day temperature C nr2 sec-1 mgCOa/g-h" g H20/g- h° percent 1230-1250 24.8 1206 6.89 0.46 1.50 0953-1013 30.8 446 3.45 1.31 0.26 1128-1148 37.0 1296 7.38 1.23 0.60 1023-1043 38.8 782 8.90 1.30 0.68 1100-1120 46.3 1229 3.10 1.84 0.17 "Measurements made on potted plant, continuing study. Chamber conditions: C02 concentration 320 ^liters/liter air; relative humidity 27.7 percent; root temperature 28 C Table 2. Effect of C02 concentration on photosynthesis and transpiration rates, and on efficiency of water use of Lycium pallidum (3-18-72) and Larrea tridentata (3-10-72) in the field (Rock Valley). Light co2 intensity concentration M ijliter Einsteins Photosynthesis Transpiration Efficient \ Hour of day liter air m-2 sec-1 mgC02/g-h° gH20/g-h° percent Lt/ci \um pallidum 1020-1040 90 2410 4.39 3.34 0.13 1100-1120 132 2460 13.6 3.37 0.60 1200-1220 172 2410 17.8 3.32 0.54 1240- 1300 215 2370 25.2 3.22 0.78 1320-1340 260 2370 28.9 3.16 0.94 1420-1440 375 2280 Urn 38.3 cu tridentata 2.90 1.32 1110-1120 90 2460 6.25 L.19 0.52 1130-1140 132 2410 6.97 1.17 0.60 1200-1140 172 2370 8.00 1.11 0.72 1300-1310 215 2370 9.25 1.10 0.84 1330-1340 368 2370 26.3 1.03 2.55 Chamber conditions; Temperature 25 C; relative humidity 28.0 percent; root temperati 1980 Nevada Desert Ecology 119 ity on C. lanata was an increase both in pho- tosynthesis (r = -0.94) and in water loss (r = -0.99). For L. pallidum the effect on the rates was somewhat the same (r = + 0.26 for photosynthesis and -0.84 for transpiration with decreasing relative humidity). In the experiment with root temperature, the range should have been extended up to perhaps 40 C or higher. The data in Table 4 (for the range 10 to 29 C) showed increases in Table 3. Effect of relative humidity on photosynthesis and transpiration rates, and on efficiency of water use of Lijcium pallidum (3-17-72). Larrea tridentata (3-19-72), Ephedra nevadensis (3-21-72) and Ceratoides lanata (2-3-72) in the glasshouse. Light intensity R.H. Einsteins Photosynthesis Transpiration Efficiency Hour of day percent nr2 sec-1 nig H20/g-h g H2OZg-h percent Lycium pallidum 1413-1433 27.9 2370 28.1 3.55 0.79 1457-1507 26.8 2270 32.2 4.09 0.79 1537-1547 25.5 2010 26.9 Larrea tridentata 4.08 0.66 1530-1540 27.8 2270 14.3 0.90 1.58 1558-1608 26.8 2010 9.25 0.95 0.97 1610-1620 25.0 2010 10.7 1.07 1.00 1630-1640 24.2 2010 9.25 Ephedra nevadensis 1.14 0.81 1550-1600 28.3 1880 4.09 0.12 3.30 1628-1724 27.8 1250 3.32 Ceratoides lanata 0.13 2.57 1330-1350 27.5 693 2.98 0.08° 3.77 1255-1315 26.6 850 3.12° 0.11° 2.96 1220-1240 25.5 890 4.02° 0.17° 2.34 1145-1205 24.0 1200 4.18° 0.20° 2.07 "Values for Ceratoides lanata are nig C02/sample-h and g HoO/sample-h. Chamber conditions: Temperature 25 C; C02 concentration .330 ^liters/liter air. Table 4. Effect of root temperature on photosynthesis and transpiration rates, and on efficiency of water use of Larrea tridentata (3-9-72) and Atriplex hymenelytra (2-29-72) in the glasshouse. Light intensity Einsteins Root Photosynthesis Transpiration Efficiency Hour of day temperature C nr2 sec-1 mgC02/g-h° gHgO/g-h' percent Larrea tridentata 1550-1600 10-11 1030 2.94 0.27 1.09 1530-1540 12-14 1120 4.45 0.54 0.83 1500-1510 16-19 540 3.81 0.73 0.52 1430-1440 23-26 1030 4.83 0.71 0.69 1350-1400 28-29 1030 Atriplex 5.65 hymenelytra 0.70 0.80 1432-1442 9.2 715 11.4 0.52 2.19 1410-1420 10.0 625 1.3.1 0.67 1.96 1315-1325 16.8 1210 21.6 0.93 2.32 1305-1315 20.0 1270 21.7 1.02 2.14 1240-1250 23.1 1340 20.6 1.05 1.97 1225-1235 27.7 985 22.9 1.13 2.02 1125-1135 29.0 1340 26.1 1.14 2.29 'Measurements made on potted plants, continuing study. Chamber conditions: Temperature 25 C; C02 concentration 322 ^liters/liter air; relative humidity 27.7 percent. 120 Great Basin Naturalist Memoirs No. 4 both photosynthesis and transpiration for both L. tridentata and A. hymenelytra grown in the glasshouse. Water-use efficiency de- creased as soil temperature increased, as has been observed elsewhere (Wallace 1970). The C4 plant, A. hymenelytra, had a 4.6-fold greater photosynthetic rate, a 2.9-fold greater water-use efficiency, and a 1.6-fold greater transpiration rate at 29 C root temperature than the C3 plant L. tridentata. At 10 C root temperature the C4 plant had 2.9-fold greater photosynthesis, 2.4-fold greater water use ef- ficiency, and 1.24-fold greater transpiration. The apparent advantages of the C4 character- istics seem to decrease as soil temperature is decreased. Changes in the COa concentration, the chamber temperature, the relative humidity, or the root temperature of the plant may produce tenfold differences in the gas ex- change rates. Therefore, it is important to maintain each of these parameters as con- stant as possible when making observations in experimental tests. Literature Cited Bauer, H., W. Larcher, and R. B. Walker. 1975. In- fluence of temperature stress on C02-gas ex- change. In J. P. Cooper, ed. Photosynthesis and productivity in different environments. Cam- bridge University Press, Cambridge and London. Cooper, J. P., ed. 1975. Photosynthesis and productivity in different environments. International Biologi- cal Programme 3. Cambridge University Press, Cambridge and London. Roller, D. 1970. Determination of fundamental plant parameters controlling carbon assimilation and transpiration by the Null-point Compensating System. Soil Sci. 14: 61-68. 1975. Effects of environmental stress on photo- synthesis-conclusions. Pages 587-589 in ]. P. Cooper, ed. Photosynthesis and productivity in different environments. Cambridge University Press, Cambridge and London. Mooney, H. A., O. Bjorkman, and J. Berry. 1975. Pho- tosynthetic adaptations to high temperature. Pages 138-151 in N. F. Hadley. ed. Environmen- tal physiology of desert organisms. Dowden. Hut- chinson & Ross, Inc., Stroudsburg, Pennsylvania. Mork, H. M., R. W. Farmer, and K. A. Flygab. 1972. Improved feedback control for the Null-point Compensating System. Soil Sci. 114: 61-68. Sestak, Z., J. Catsky, and P. G. Jarvis, eds. 1971. Plant photosvnthetic production: manual of methods. Di-.W.' Junk N.V., The Hauue. Troughton, J. H. 1975. Photosynthetic mechanisms in higher plants. Pages 375-391 in J. P. Cooper, ed] Photosynthesis and productivity in different envi- ronments. Cambridge University Press. Cam- bridge and London. Wallace, A. 1970. Water use in a glasshouse 1>\ Salsola kali grown at different soil temperatures and at limiting soil moisture. Soil Sci. 110:146-149. CARBON FIXED IN LEAVES AND TWIGS OF FIELD LARREA TRIDENT AT A IN TWO-HOUR EXPOSURE TO ^C02 A. Wallace1, E. M. Romney1, and R. B. Hunter1 Abstract.— Six Larrea tridentata (Sesse & Moe. ex DC) Cov. plants were exposed to 14C02 in a field experiment r 2 h. Three of the plants had been irrigated regularly in the preceding year. Ten small twigs from each plant were moved and counted for 14C activity at the end of 2 h. The stem portion of the twigs was of equal dry weight for le two sets of plants, but those irrigated had a greater weight of leaves per twig. The activity of 14C in leaves was jual for the two groups, but was higher in stems for watered plants than for unwatered plants. The results were :>st expressed as ratios. Dry weight of leaves + dry weight of stems was high for watered plants; cpm/g dry weight r leaves ■*■ cpm/g dry weight of stems was higher for unwatered plants. In another experiment in which leaves ere removed before exposing stem portions of twigs to 14C02, small green stems accounted for about '/s the total lotosynthesis for a plant; the coefficient of variation was around 100 percent. Introduction Larrea tridentata (Sesse & Moc. ex DC) !ov. is a perennial well adapted to the hot, ry summers of the Mojave Desert. It is a C-3 lant with a relatively low rate of photo- mthesis (Barbour, 1977). It is an evergreen rith an ability to fix C02 in every month of le year (Bamberg et al. 1973,' 1975). Its nailer stems have chlorophyll, particularly oung stems, and they also are capable of hotosynthesis. The purpose of this report 'as to show the relative importance of leaves nd stems for photosynthesis of this species in le field in the northern Mojave Desert. Part f the plants used in this study were also in- olved in a shoot-root carbon budget study Wallace et al. 1980, this volume) and data 'ere available from which the present results 'ere obtained. Materials and Methods Six L. tridentata were exposed to 14C02 for h on the morning of 14 May 1974 by tech- iques previously used (Bamberg et al. 1973, 974; Wallace et al. 1977). Briefly, 5 ml of a .5 M solution of KH14C03 was mixed with 1C1 inside a plastic bag which was tied at le base of the plant. The 14C activity was 5 Ci/ml. After exposure the bags were re- moved, and ten small twigs containing leaves and stems were removed from each plant and assayed with Q-gas counting for 14C fixed. Each leaf and stem sample was counted in triplicate. Three of the six plants used had been irrigated regularly in the previous year. Since translocation from leaves to twigs was possible during the 2-h test, a second experi- ment was conducted in which leaves were first removed from green stems. These stems were then subjected to the same type of test as twigs previously. Results and Discussion The weight of leaves per twig was higher for the three plants previously irrigated than for those not irrigated (Table 1). The coefficient of variation (C.V.) for within the watered plants was low enough to indicate that that group was a separate population. The weights of stems per twig, however, were similar for both groups of plants. The amount of 14C fixed per g dry weight of both leaves and stems was variable with a C.V. of about 100 percent. However, for leaves the means of each group were essen- tially identical. For stems the watered plants had about 60 percent more 14C than the non- watered plants (Table 1). When the data were considered as ratios with cpm/g dry 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles. California 90024. 121 122 Great Basin Naturalist Memoirs No. 4 weight of leaves -s- the cpm/g dry wt of stems (Table 2) it is apparent that this obser- vation is statistically significant. The C.V. for ratios with unwatered plants was only 7.6 percent and only 5.5 percent for watered plants. When all six plants were grouped to- gether the C.V. was 23.8 percent. The ratio of dry weight of leaves to dry weight of stems was 40 percent larger for the watered plants than for the nonwatered ones. The C.V. of both groups was low (1.7 and 13.2 percent), indicating that they are sepa- rate populations. The previous irrigation then was reflected in a larger growth of leaves on the plants. In the second experiment in which leaves had been removed from green stems before the 14COa was started, it was shown that ap- proximately Vs (the coefficient of variation was around 100 percent) of the photo- synthesis for L. tridentata could be by way of the green stems (Table 3). On the dry weight basis the amount of 14C in green stems was 51 percent that of leaves. Green stems with leaves attached contained more 14C than did green stems with leaves removed, so it can be assumed that there was some translocation from leaves to stems during the 2 h test. There was also some 14C translocated to small branches during the 2 h. Stem photosynthesis is very likely one of the adaptive mechanisms of this drought-tol- erant, heat-resistant desert plant species. Table 1. Dry weight of twigs and 14C in twigs of L. tridentata exposed for 2 h to 14CO^. Dry wt of twigs 14C Leaves Stems Leaves Stems i mg/twig cpm/g dry wt Unwatered (n = 3) Mean 67.1 29.8 67547 41900 S.D. 19.4 8.3 59103 33710 c.v.% 28.9% 27.7% Watered i 87.5% n = 3) 80.5% Mean 85.2 27.1 69753 66380 S.D. 9.4 1.4 72615 6602] C.V.% 11.1% 5.2% Ml plants U)l L% (n = 6) 99 5% Mean 76.2 28.5 68650 54] it) S.D. 16.9 5.5 59227 18763 C.V.% 22.2% 19.3% 86.3% 'to |% Acknowledgments This study was supported in part by the US/IBP Desert Biome, Utah State University, Logan, and the Nevada Applied Ecology Group of the U.S. Department of Energy, Nevada Operations Office. Literature Cited Bamberg, S. A., A. Wallace. G. E. Kleinkopf, A. Vollmer, and B. S. Ausmus. 1973. Plant produc- tivity and nutrient interrelationships of per- ennials in the Mohave Desert. US/IBP Desert Biome Bes. Memo. 73-10. Table 2. Batios of leaf and stem portions of the twigs for dry weight and 14C fixed in L. tridentata. Dry wt of leaves cpm/g dry wt leaves cpm /twig- leaves Dry wt of cpm/g dry cpm/twig- stems wt stems stems Batio Batio Batio Unwatered (n = 3) Mean 2.26 1.56 3.49 S.D. 0.038 0.12 0.21 C.V.% 1.7% 7.6% Watered (n = 3) 6.0% Mean 3.18 1.02 3.19 S.D. 0.42 0.056 0.30 C.V.% 13.2% 5.5% All plants (n=6) 9.3% Mean 2.71 1.29 3.34 S.D. 0.58 0.31 0.28 C.V.% 21.3% 23.8% S 5% Table 3. 14C fixation ot gree from which leaves had been r< stems with leaves attached." stems of /.. tridentata Relative 14G CV Dry wt cpm/g % Relativi cpm g Leaves LOO Green stems (leaves attached) 0.45 Small branches 0.26 Green stems (without leaves attached i 0.45 },127 88.0 1.00 45.233 104.0 0.51 13,773 72,1 0.16 32,910 114.1 0.37 'Rotative photosynthesis for the i;reen stems lor the per plant basis would be 0 15 « 0.37) * I + (0.45 X 0.51) + (0.26 x 0.16) = 0.134 or about 's \ll values in the calculation are in Table 3. Since the CV b a 'I 100V it v value of '* must he considered also .is possibh in error by .is much .is 1(X1"„ 1980 Nevada Desert Ecology 123 1974. Plant productivity and nutrient inter- relationships of perennials in the Mohave Desert. US/IBP Desert Res. Memo. 74-8. Bamberg, S. A., G. E. Kleinkopf, A. Wallace, and A. Vollmer. 1975. Comparative photosynthetic production of Mojave Desert shrubs. Ecology 56:732-736. Barbour, M. G., G. Cunningham, W. C. Oechel, and S. A. Bamberg. 1977. Growth and development, form and function. Pages 48-91 in T. J. Mabry, J. H. Hunziker, D. R. DiFeo, Jr., eds. Creosote bush US/IBP, Synthesis Vol. 6. Dowden, Hutchinson and Ross, Inc., Stroudsburg, Pennsylvania. Wallace, A., S. A. Bamberg, J. W. Cha, and E. M. Romney. 1977. Partitioning of photosynthetically fixed 14C in perennial plants of the northern Mo- jave Desert. In J. K. Marshall, ed. The below- ground ecosystem: a synthesis of plant-associated processes. Range Science Dept., Science Series No. 26, Colorado State University, Fort Collins. Wallace, A., E. M. Romney, and J. W. Cha. 1980. Per- sistence of 14C labeled carbon in Larrea triden- tata up to 40 months after photosynthetic fixation in the northern Mojave Desert. Great Basin Nat. Mem. 4:170-174. THE ROLE OF SHRUBS ON REDISTRIBUTION OF MINERAL NUTRIENTS IN SOIL IN THE MOJAVE DESERT1 E. M. Romney2, A. Wallace", H. Kaaz:, and V. Q. Hale2 Abstract.— Soil profiles underneath shrub clumps and bare desert pavement were examined at 62 study sites lo- cated in both open and closed drainage basins of the northern Mojave Desert. Highly significant differences occurred in the root zone underneath shrub clumps with higher concentrations of the following soil properties: electrical con- ductivity (EC25°), Na, K+ , Ca+ + , Mg++, Ch, N03_, and S04=; exchangeable K+ ; cation exchange capacity: or- ganic C and N; available P, and DTPA-extractable Fe and Mn. These differences reflect differential cycling caused bv different plant species. The decomposition and mineralization of litter deposited underneath the perennial vege- tation can account for these differences in soil properties which, collectively, increase the fertility of the soil under- neath the vegetation canopy. Aboveground biomass of shrubs was measured and the nitrogen and mineral element composition of new photosvnthetic tissue was determined. Estimates from a representative study site indicate that the reservoir of nitrogen and mineral nutrients in new leaf material of shrubs available for litter deposition could contribute 3.64 kg N, 0.31 kg P, 0.57 kg Na, 5.20 kg K, 4.95 kg Ca, 31.82 g Fe. and 4.30 g Mn per hectare. This source probably represents about one-third of the total amount of nutrients involved in annual turnover for the study area during a normal production vear. The remaining contribution would be supplied from the standing dead wood in shrubs and as litter from annual plant species. Efforts to develop the potential benefits of wildland shrubs have increased with man's needs to make arid and semiarid lands more productive and useful. An extensive world lit- erature produced from studies on production and mineral cycling in terrestrial vegetation was summarized in the work of Rodin and Bazilevich (1965), which considers several as- pects of mineral involvement in plant pro- duction between vegetation types represent- ing the broad climatic zones of the world. A review of available literature on the biology and utilization of wildland shrubs in arid and semiarid lands was one of the main objectives of a recent international symposium (McKell et al. 1971). At that symposium Charley (1971) discussed the role of shrubs in nutrient cycling, with emphasis upon the nitrogen conditions encountered in a perennial salt- bush ecosystem. The principles governing im- portant transformation processes involved in shrub production, litter fall, and subsequent decomposition and mineralization in natural ecosystems have been well covered in these and other recent reviews (Rennie 1955, Ovington 1962, 1965, Egunjobi 1969). This paper reports on the influence of shrubs on cycling or redistribution of mineral nutrients in zones near roots in the Mojave Desert. Edaphic factors are important in the distribution of plant species, but plants also are important in determining soil character- I istics. For example, an accumulation of nitro- gen and mineral elements in plant foliage re- sults in the cycling of these elements from litter to the soil underneath the plant canopy (Roberts 1950, Fireman and Haywood 1952, Beadle et al. 1957, Rickard 1965b, Charley and Cowling 1968, Chatterton and McKell 1969, Jessup 1969, Garcia-Moya and McKell 1970, Charley 1971, Sharma and Tongwal 1973, Tiedemann and Klemmedson L973J The extent to which this process occurs un- der northern Mojave Desert conditions was one aspect of concern in studies undertaken in southern Nevada. 'Findings in this paper appeared, with slight modifications, in The belowgronnd ecosystem ,i synthesis .it plant associated processes Pages HI.) 1 10 i« Range Science Department Science Series report No. 26. Colorado State University, Fort ( lollins 1977. We present these findings again for convenience and accessibility to readers interested in the several related papers in this issue 'Laboratory of Nuclear Medicine and Radiation Biology. University ol < ahtoinia. I.os \ngeles, California 9(X)24. 124 1980 Nevada Desert Ecology 125 Description and Methods Investigations were conducted at the USAEC Nevada Test Site to obtain more in- formation on soil and plant relationships in the desert ecosystem to better understand the impact of nuclear testing on the natural envi- ronment. The findings presented herein were synthesized from preliminary raw data re- ported by Romney et al. (1973). The perennial vegetation of the study areas exists as solitary shrubs or as discrete clumps consisting of several different shrub species. Sharp ecotonal demarcation zones are preva- lent among some of the more dominant shrub species. Most of the soils examined have de- veloped on alluvium consisting of limestone or mixed limestone and volcanic material. Except in areas of recent sedimentary deposi- tion, many are now underlaid by layers of re- strictive hardpan formed from the processes of alkaline hydrolysis at depths varying from 30 to 70 cm. Study sites were selected in both open and closed drainage basins. Details of the study areas involved in these in- vestigations have been reported (Wallace and Romney 1972, Romney et al. 1973). At each of 62 study sites a trench was dug with a backhoe extending across a shrub clump and out into the bare desert pavement to a distance of at least 3 m. This was done to permit an examination and sampling of the soil profile underneath both shrub and bare areas in order to investigate the modifying influence of perennial vegetation on the pro- file horizons. The soil profiles were described according to the USDA Soil Conservation Service nomenclature (Soil Survey Staff 1951). Represented among these study sites were soil belonging to several subgroups, in- cluding Typic Torripsamments, Haplic Nadurargids, Entic Durorthids, and Typic and Duric Camborthids. Physical and chemical properties were de- termined on soil samples screened to pass a 2 mm sieve. Sand fractions were measured by mechanical separation on standard testing sieves. Silt and clay fractions were deter- mined by the pipette method described by Day (1965). Available phosphorus was ex- tracted with sodium bicarbonate and deter- mined colorimetrically using the method of Olsen et al. (1954) as described by Chapman and Pratt (1961). Lime content was deter- mined by the inanometric method of Wil- liams (1948). The available micronutrients were extracted with DTPA chelate and de- termined by atomic absorption analysis (Lindsay and Norvell 1978). Organic nitrogen analysis was by the Kjeldahl method (Brem- ner 1965). The analytical methods used to de- termine other physical and chemical proper- ties were those of the USDA Salinity Laboratory Staff (1954). Some of the ecological attributes of the perennial vegetation were determined by nondestructive dimensional measurements (Wallace and Romney 1972:250). Briefly, 2 m X 25 m quadrats were laid out at right an- gles to each other in undisturbed vegetation in the proximity of the soil sampling trench. All shrubs within the quadrats were identi- fied by species and measured for height and width (mean of two dimensions). These mea- surements were used to determine shrub den- sity, frequency, relative dominance, cover, and volume. Biomass estimates were derived from regressions of dry weight on volume in- dexes developed from the destructive sam- pling of shrubs in nearby areas (Romney et al. 1973). Measurements of new photosynthetic production were made for the more promi- nent shrub species by destructive sampling at selected study sites during the peak of sea- sonal leaf flush. Samples of clean foliage were collected in the vicinity of each soil sampling trench for chemical analysis. Oven-dried (70 C) samples were separated into leaf and stem material and finely ground for analysis by optical emission spectrometry (Wallace and Romney 1972:363). Total nitrogen contents were de- termined on leaf tissue using the Coleman Model 29A Nitrogen Analyzer. Results and Discussion Soil Profile Characteristics Most of the soil profiles examined in this study have developed on relatively coarse al- luviums low in clay content under conditions of high temperature and low rainfall. Many profiles clearly indicate an acceleration of the soil-forming processes underneath shrub clumps. Distinct differences also occur in the 126 Great Basin Naturalist Memoirs No. 4 amounts of wind-blown material deposited underneath shrubs and on bare soil. Loess blankets a major portion of the study area (Ekren 1968); volcanic ash falls and wind ac- tion are responsible for its wide distribution. Other prominent characteristics evident un- der shrubs include better developed A hori- zons containing higher concentrations of salt and organic matter, and some decomposition of the underlying hardpan when present. Table 1 contains the profile description for study site No. 5, which is representative of soils with an underlying hardpan developed on alluvium parent material of mixed lime- stone and quartz. Detailed descriptions and properties of other soil profiles are given in Romney et al. (1973). Physical and chemical properties of the soil profile at site No. 5 are listed in Table 2. They reflect the kinds of change generally found between different horizons underneath shrub clumps and bare areas. These proper- ties most notably modified in zones near roots include the salts of sodium, potassium, calcium and magnesium, available phos- phorus, organic carbon and nitrogen, and available iron and manganese. The particle size distribution, water-holding capacity, pH, and lime content essentially remained unal- tered within the depth of the root zone. Elec- trical conductivity (EC25°) of the saturation extract reflected the concentrations of so- luble cations and anions in the profile hori- zons. Highest salt concentrations were found in the A horizons underneath shrubs. No evi- dence was found of an accumulation of so- luble salts in the bare soil areas between shrub clumps as reported by Charley and McGarity (1964) for perennial saltbush com- munities growing on saline soils of the Aus- tralian arid zone. The soils examined here are moderately permeable and subject to leach- ing by rainfall. Except for a few sites located on sediments of closed-drainage basins, most profiles examined were nonsaline-nonalkali within the root zone, i.e, the EC25° was less than 4 mmhos/cm and the exchangeable so- dium percentage was less than 15 (U.S. Sali- nity Laboratory Staff 1954). Several other investigators have described sharp changes in the chemical properties of soil underneath shrub canopies resulting from an accumulation of salts as the result of litter deposition (Roberts 1950, Fireman and Hay- ward 1952, Rickard 1965a, 1965b, Charley and Cowling 1968, Sharma and Tongway 1973). Similarly, significant accumulations of nitrogen and organic matter have occurred as the result of litter decomposition (Garcia- Table 1. Soil profile description at Mercury Valley Study Site No. 5. Area: Mercury Valley, Nye County, Nevada Perennial vegetation: Acamptopappus shockleyi A. Gra Atriplex confertifolia (Torr. & Frem.) Wats., A brosia dumosa (A. Gray) Payne, Ephedra funerea Cov. & Mort. Ephedra nevadensis, Wats., Ceratoides lanata (Pursh) J. T. Howell, Grayia rpinosa (Hook) Moq., Krameria parvifolia Benth., Larrea tridentata (Sesse & Moc. ex DC.) Cov., Lycium andersonii A. Grav., Yucca schidigera Roezl ex Ortgies. Parent material: alluvium from limestone and quartz. Topography: 3 percent southwest slope, smooth relief; well-drained moderate erosion; surface about 80 per- cent rock and gravel; well-developed desert pave- ment; elevation 1096 m. Profile under shrub clump: (C. lanata, E. tridentata, L. andersonii) \\ 0-9 cm 9-13 cm Brown (10YR5/3) loamy fine sand, brown to dark brown (10YR4/3) moist; weak fine sub- angular blocky structure; soft, friable, nonstickv. violently effer- vescent; few micro roots: pH 8.0; abrupt smooth boundary . Ver) pale brown (10YR7/3) sandy loam, yellowish brown (10YR5/4) moist; moderate me- dium platv; slightly hard, friable, slightly sticky; violently efferves- cent; few medium, fine, and mi- cro roots; 20 percent gravel; pH 8.4; abrupt irregular boundary; discontinuous. CI 13-30 cm Very pal brown (10YR7 3) loamy sand, yellowish brown (10YR5/4) moist; weak fine sub- angular blocky structure; soft, triable, nonsticky, violently effer- vescent; lew medium, tine, and micro roots; 20 percent gravel; pH 8.4; clear wavy boundary. C2sicam 36+ cm C Profile under bare area: CI C2sicam 34+ cm Horizon description is the same as ( 1 under shrub. 1980 Nevada Desert Ecology 127 Moya and McKell 1970, Charley 1972, Tiedemann and Klemmedson 1970, Hol- mgren and Brewster 1972, Nishita and Haug 1973). The work of Charley and Cowling (1968) indicates that biologically increased vertical salt gradients in soil seem to become sharper with increased aridity. Perennial Vegetation Characteristics Some of the ecological attributes of shrubs at study site No. 5 are given in Table 3. Beat- ley (1969a) described the vegetation type and association for the area in which this site was located as Larrea-Franseria (Ambrosia). Non- destructive dimensional measurements in- dicate Acamptopappus shockleyi A. Gray and Ambrosia dumosa (A. Gray) Payne were of highest density and frequency. The relative dominance index for basal area was highest for A. dumosa followed closely by Lycium andersonii A. Gray and Krameria parvifolia Benth. Greatest aboveground standing bio- mass was contributed by Yucca schidigera Roezl ex Ortgies (927 kg/ha). Lycium ander- sonii and A. dumosa contributed essentially the same biomass (458 and 456 kg/ha) fol- lowed by Ephedra funerea Cov. and Mort. (228 kg/ha), Larrea tridentata (Sesse & Moc ex DC.) Cov. (162 kg/ha) and K. parvifolia Table 2. Physical and chemical properties of soil profile horizons under shrub and bare areas of site no. 5. Profile horizon properties Shrub clump A2 CI Bare area CI Horizon depth, cm Particle size distribution (% < 2mm) coarse sand (2.0-0.25) fine sand (0.25-0.05) silt (0.05-0.002) clay (< 0.002) Percent moisture retention saturation -0.3 bar -1 bar -15 bar pH (saturated paste) EC (mmhos per cm, 25 C) Saturation extract soluble cations and anions Na, meq/1 K, meq/1 Ca, meq/1 S04, meq/1 B, ug/g Exchangeable cations (NHjOAc-extraction) Na, meq/lOOg Na, % K, meq/lOOg Ca + Mg, meq/lOOg C.E.C., meq/lOOg Percent lime (< 2mm) P, (NaHCCVext.) ug/g Organic carbon, % Organic nitrogen, % DTPA-extractable micronutrients Fe, ug/g Zn, ug/g Cu, ug/g Mn, ug/g 0-9 9-13 13-36 0-34 28.8 26.2 21.0 25.5 53.9 41.3 56.7 50.2 10.9 23.4 16.2 16.9 6.4 9.1 6.1 7.4 44.7 26.6 32.8 26.2 15.7 17.7 19.5 16.5 13.4 14.5 15.3 14.7 9.6 8.7 7.8 8.1 8.0 8.4 8.4 8.3 4.74 1.49 0.55 0.40 2.50 8.32 1.84 0.56 13.20 2.95 1.65 0.63 19.69 5.14 0.56 0.76 1.00 0.14 0.02 0.07 5.10 3.60 0.10 2.90 0.27 0.71 0.42 0.47 1.50 4.10 2.50 3.00 4.06 3.92 3.45 1.78 13.17 12.87 13.01 13.38 17.50 17.50 16.88 15.63 16.00 17.00 17.00 17.00 3.26 0.36 0.04 0.24 2.12 0.48 0.38 0.33 0.211 0.050 0.044 0.035 0.5 0.1 0.2 0.1 0.80 0.80 0.80 0.95 0.20 0.30 0.20 0.25 5.00 1.50 1.15 0.95 128 Great Basin Naturalist Memoirs No. 4 (148 kg/ha). These shrubs accounted for more than 95 percent of the perennial plant biomass. In this particular area, dead wood often accounts for a significant portion of the standing biomass of perennial vegetation. It remains standing for many years and prob- ably contributes about as much mass in an- nual litter-fall as does new leaf material. Leaf/ plant ratios were measured for most shrubs at this site during peak leaf flush in 1968. Yucca schidigera was ignored because of its lack of contribution to mobile leaf litter Table 3. Characteristics of perennial vegetation at study site no. 5. Plant species Density Frequency No/ha % Relative Biomass00 Leaf/ Plant dominance0 kg/ha ratio00 Acamptopappus shockleyi Ambrosia dumosa Atriplex confertifolia Ephedra funerea Ephedra nevadensis Eurotia Janata Grayia spinosa Krameria parvifolia Larrea tridentata Lycium andersonii Yucca schidigera 3589 3274 356 452 561 863 123 1178 561 1000 109 25.6 8.4 26.1 0.137(19) 23.3 23.9 456.5 0.6S9 (38) 2.5 1.4 61.6 0.156i 8) 3.2 7.9 228.6 — 4.0 4.0 63.0 0.010(10) 6.2 2.6 52.5 0.080(22) 0.9 0.6 15.3 0.135(18) 8.4 15.5 148.6 0.188(17) 4.0 7.0 162.7 0.081 (13) 7.1 19.3 458.6 ().054(13i 0.8 5.7 927.1 - ive, dimens ional measurements wa s 24.8 percen . 'Index of basal area occupied by species. Ground cover estimate from nondestruct 'Measurements were made of aboveground parts of shrubs at peak of new leaf flush, 1968 'Number of shrubs from which mean ratio was determined. Table 4. Nitrogen and mineral element composition of perennial vegetation from study site no. 5. Plant species Plant part0 N % P Na % K % Ca % Acamptopappus shockleyi leaf stem 2.98 0.25 0.17 0.161 0.110 4.43 3.32 1.68 1.52 Ambrosia dumosa leaf stem 4.16 0.37 0.24 0.114 0.111 5.4S 4.07 2.98 1.3& Atriplex confertifolia leaf stem 2.96 0.39 0.27 4.414 1.960 6.84 2.21 3.93 2.5i Ephedra funerea shoot 2.32 0.12 0.028 1.17 2.55 Eph edra ne cade n sis shoot 2.94 0.32 0.008 2.37 1.18 Ceratoides lanata leaf stem 3.62 0.22 0.09 0.037 0.005 3.69 3.99 1.42 0.52 Grayia spinosa leaf stem 2.23 0.09 0.08 0.175 0.009 10.13 6.06 4.25 1.23 Krameria parvifolia leaf stem 2.10 0.31 0.24 0.316 0.127 2.13 2.12 1.23 0.S7 Larrea tridentata leaf stem 2.56 0.16 0.07 0.103 0.088 2.13 IIS 1.53 1.10 Lycium andersonii leaf stem 3.26 0.12 0 III 0.013 0.010 5.58 2.12 11.04 2.6EJ "Samples harvested at peak of leaf flush. 1969. 1980 Nevada Desert Ecology 129 due to growth habit. There was no significant increase of new shoots on E. funerea in 1968. It should be noted here that annual photo- synthetic production in this ecosystem differs markedly from year to year, depending upon seasonal rainfall and temperature conditions (Beatley 1969b, Wallace and Romney 1972). New leaf production in 1968 was considered to be about normal for this area. Calculations based upon these biomass and leaf/plant ra- tios indicate that the total contribution of new leaf material available for litter deposi- tion from shrubs was 107.4 kg/ha in 1968. The biomass of annual plants was not mea- sured at this site, but Beatley (1969b) report- ed total winter annual plant biomass values for a nearby studv plot of 60.58, 21.73, and 174.16 kg/ha for 1964, 1965, and 1966, re- spectively. The nitrogen and mineral element compo- sition of perennial vegetation sampled at the peak of leaf flush in 1969 is shown in Table 4. The nitrogen composition of leaf tissues varied among species but fell within the range commonly found in cultivated pasture crops (2.5 to 3.5 percent). Phosphorus con- tents varied within the range of 0.10 to 0.40 percent, and higher levels usually occurred in leaf than in stem tissues. Sodium concentra- tions were relatively low in plant tissues grown at this site; however, A. dumosa, Grayia spinosa (Hook.) Moq., L. andersonii, and, of course, the Atriplex species have the capacity to concentrate much higher levels of sodium than is present in the soil (Wallace and Romney 1972, Romney et al. 1973). Po- tassium is one of the most variable of the nu- trient elements in these desert shrubs; its con- centration in stem tissues often reaches or exceeds that found in leaf tissues. Ambrosia dumosa, Ceratoides lanata (Pursh) J. T. How- ell, and L. andersonii consistently contain rel- atively high levels of potassium, and G. spin- osa usually contains exceptionally high concentrations. High concentrations of cal- cium and strontium are normally found in Table 4 continued. Mg Si Zn Cu Fe Mn B Sr Ba % re re re /*g ^g /^g re re 0.48 0.47 22 8 164 51 46 29 i 0.20 0.04 7 5 48 12 17 43 7 0.54 0.15 28 7 256 2.3 100 56 9 0.54 0.07 13 4 141 19 42 50 14 0.61 0.17 9 5 362 32 73 130 50 0.37 0.07 7 7 237 86 21 131 47 0.34 0.17 7 6 186 68 10 197 52 0.26 0.03 22 2 92 18 24 61 11 0.56 0.05 21 4 110 66 41 45 5 0.23 0.02 6 3 79 28 14 37 6 2.15 0.07 37 5 150 139 65 32 5 0.51 0.01 16 3 20 15 20 25 5 0.37 0.18 17 6 261 43 39 99 17 0.28 0.10 16 5 168 15 27 88 15 0.22 0.45 26 2 585 41 88 44 11 0.21 0.40 16 4 1091 33 28 54 16 1.44 0.05 41 4 162 33 65 648 18 0.24 0.04 9 3 90 5 12 77 11 130 Great Basin Naturalist Memoirs No. 4 leaf tissues of L. andersonii. Stem tissues usu- ally contain less calcium than do leaf tissues of most shrub species. Both G. spinosa and L. andersonii leaves often contain higher con- tents of magnesium than do those of other species from the same location. The micro- nutrients and trace metals vary considerably among the various shrub species, and leaf tis- sues usually contain higher amounts than are concentrated in stem tissues. One striking ex- ception to this was the consistently high iron content of leaf and stem tissues of Larrea tri- dentata, wherever sampled (Romney et al. 1973). Boron contents generally ranged from 10 to 100 ug/g. Modifying Effects of Vegetation on the Soil Properties Near Root Zones Inasmuch as soil properties were charac- terized from existing horizons of varied depths, it was necessary for statistical analysis to normalize all values to assess differences underneath shrubs and bare sites. This was done by computer synthesis to a common depth of 30 cm because most of the active root zone lies within this depth in our study areas. Comparisons were made of the statis- tical significance of differences between the values of properties measured underneath shrubs and bare surfaces at 62 study sites. Bare site values were subtracted from shrub site values, and the means and standard de- viations for each of 22 variables were derived from these differences. For each of these var- iables, the mean differences were divided by the standard deviations and then multiplied by the square root of the sample number to derive a t-value. The null hypothesis that the means are not significantly different from zero was rejected if t was less than -2.000 or greater than 2.000 (p = 0.05). With this test means that were significantly different from each other could be identified and the con- clusion reached that the presence of shrubs modified the soil properties when their mean difference was positive (Table 5). The soil properties which tended to have higher values underneath shrubs, but which were not significantly different, included wa- ter-holding capacity, pH, and exchangeable sodium. The exchangeable calcium and mag- nesium, lime, and DTPA-extractable zinc and copper contents tended to be higher in bare soil, but their differences were not signifi- cant. All the other soil properties tested were significantly higher under shrub clumps in- cluding the saturation extract conductivity (EC25°), the soluble cations and anions, ex- changeable potassium, the cation exchange capacity, organic carbon and nitrogen, avail- able phosphorus, and the DTPA-extractable iron and manganese. The cycling and redistribution of carbon, nitrogen, and mineral elements from the de- composition and mineralization of litter de- posited underneath perennial vegetation can account for these differences in soil proper- ties that, collectively, increase the fertility of the soil underneath the vegetation canopy. These shrub clumps also act as catchments for windblown litter and serve as shelters for most of the annual plant species. The shrub Table 5. A measure of the difference in soil proper- ties underneath shrub and hare areas at 62 studv sites. Mean difference Soil properties (shrub minus bare) t-statistic° Moisture, -0.3 bar 0.48 1.534 pH (paste) 0.08 0.528 EC25 mmhos/cm 1.06 10.435 Saturation extract soli ble cations and anions Na, meq/1 1.64 3.067 K, meq/1 2.96 9.466 Ca, meq/1 8.75 7.921 Mg, meq/1 5.06 8.131 CI, meq/1 3.43 5.090 N03, meq/1 1.98 2.669 S04. meq/1 0.64 4.046 Exchangeable cations NrL^OAc-extraetable): Na, meq/lOOg. 0.09 1.127 K, meq/lOOij. 1.44 S.327 Ca + Mg, meq/ 100 g. -0.21 -0.6.S2 C.E.C, meq/lOOg. 1.21 3.813 Lime, % < 2 mm -0.18 -0.482 Organic C, % 0.46 11.078 Organic Y % 0.041 12.601 P(NaHC03-ext.huu/g 1.17 8.244 DTPA-extractable mi< ronutrients: Fe, ug/g 0.07 4.664 Zn, ug/g -0.07 -1.094 Cu,ug/g -0.01 -1.492 Mn, ug/g 1.42 10.477 °t = (mean difference/standard deviation) X \/N; N = 62; difference is significant (p = 0.05) where t < -2.000 or t > 2.000. 1980 Nevada Desert Ecology 131 clumps that exist in our study areas are very old (Wallace and Romney 1972), so these cy- cling and redistribution processes probably have been underway for many centuries at any given site. Some effects of specific shrub species on the redistribution of mineral nutri- ents in zones near roots are illustrated in the data of Table 6. An estimate of the annual reservoir of ni- trogen and mineral elements in new leaf ma- terial available for litter deposition from study site No. 5 is given in Table 7. If all the litter remained on site, this reservoir could contribute nitrogen, 3.64 kg/ha; phosphorus, 0.312 kg/ha; sodium 0.577 kg /ha; potassium 5.20 kg/ha; calcium 4.95 kg/ha; and iron and manganese 31.82 and 4.30 g/ha, respectively. These values were calculated from new leaf production in 1968 (Table 3) and from chem- ical analysis in 1969 (Table 4). They probably represent about one-third of the total nitro- gen and mineral nutrients involved in the an- nual turnover for the area during a normal year. The remaining contribution of nutrients for cycling would be supplied by litter-fall from the standing dead wood and from the litter of annual plant species. These estimates are based upon a normal production year for this ecosystem. However, two growth seasons have occurred during the past decade (1969 and 1973) in which the new photosynthetic production of many perennial species was from three to five times greater than in the other years (unpubl. data). Conversely, years have also occurred in which new production was less than one-half that of 1968. Beatley Table 6. Soil properties underneath shrub and bare areas at different locations irub species on redistribution of mineral nutrients in zones near roots. lustrating some effects of specific A. canescens A. confertifolia G. spinosa L. tridentata L. ande rsonii Soil properties" Shrub Bare Shrub Bare Shrub Bare Shrub Bare Shrub Bare Moisture, -0.3 bar 13.1 12.1 13.4 14.9 8.8 10.4 18.9 17.1 19.4 15.9 pH (paste) 8.5 8.6 8.6 8.9 8.4 8.7 8.4 8.6 8.4 8.6 EC250 mmhos/cm 3.19 0.34 2.98 0.68 1.36 0.38 2.57 0.64 3.20 0.46 Saturation extract oluble cations and anions: Na, meq/1 7.20 0.55 27.95 4.23 1.03 0.41 2.93 0.23 3.82 0.63 K, meq/1 15.17 1.05 3.98 1.59 13.57 0.72 4.23 1.50 6.58 0.71 Ca, meq/1 32.31 2.42 6.79 1.34 9.68 3.63 30.18 6.09 28.75 3.55 Mg, meq/1 25.54 1.05 4.80 1.22 9.94 1.86 10.99 2.31 12.80 0.43 CI, meq/1 10.87 0.27 12.66 0.82 4.68 2.42 4.29 0.16 10.12 0.53 NO3, meq/1 13.75 0.33 0.68 0.02 - - 36.29 0.05 - - SO4, meq/1 1.52 0.07 0.59 0.13 0.12 0.02 1.19 0.09 0.95 0.02 Exchangeable cations (NH4OA c-extractable): Na, meq/lOOg 0.72 0.48 3.84 1.91 0.41 0.53 0.41 0.30 0.39 0.31 K, meq/lOOg 10.83 6.43 7.75 9.22 5.16 2.64 2.63 2.30 3.59 1.69 Ca + Mg meq/ 100 g 6.99 9.20 5.69 8.11 7.44 7.45 12.55 4.66 10.44 7.32 C.E.C., meq/ 100 g 18.6 16.1 17.3 19.3 13.0 10.6 15.6 13.5 14.5 9.3 Lime, % < 2 mm 5.0 3.0 7.8 7.3 1.0 1.6 13.9 15.1 17.3 25.9 Organic C, % 0.63 0.11 0.40 0.21 0.97 0.12 1.54 0.55 1.18 0.34 Organic N, X 10- 1(Fo 0.84 0.12 0.37 0.22 0.90 0.14 1.31 0.63 1.19 0.31 P (NaHCOj-ext.), ug/g 2.3 0.1 1.2 0.4 5.8 1.2 1.6 0.8 1.1 0.2 DTPA-extractable nicronutrients: Fe, ug/g 0.3 0.2 0.1 0.1 0.6 0.4 0.5 0.4 0.5 0.4 Zn, ug/g 0.57 0.36 0.70 0.38 0.45 0.40 0.70 0.93 0.84 2.21 Cu, ug/g 0.28 0.27 0.17 0.26 0.15 0.10 0.26 0.25 0.13 0.15 Mn, ug/g 1.87 0.49 2.99 2.19 3.83 1.44 3.07 2.68 2.29 1.672 "Values per cm normalized to 30 cm depth under shrub clump and bare areas. 132 Great Basin Naturalist Memoirs No. 4 Table 7. Annual reservoir of nitrogen and mineral elements in new leaves of perennial vegetation available for litter deposition and mineralization at study site no. 5. Plant species N P Na kg/ha ° K Ca Mg Acamptopappus shockleyi 0.11 0.009 0.006 0.19 0.06 0.02 Ambrosia dumosa 1.29 0.115 0.035 1.70 0.93 0.17 Atriplex confetti folia 0.28 0.037 0.424 0.66 0.38 0.06 Ephedra nevadensis 0.02 0.002 0.001 0.02 0.01 0.01 Ceratoides Janata 0.15 0.009 0.002 0.16 0.06 0.02 Grayia spinosa 0.05 0.002 0.004 0.21 0.09 0.04 Krameria parvi folia 0.59 0.087 o.oss 0.60 0.34 0.10 Larrea tridentata 0.34 0.021 0.014 0.28 0.20 0.03 Lydum andersonii 0.81 0.030 0.003 1.38 2.88 0.36 Total 3.64 0.312 0.577 5.20 4.95 0.81 "Calculations based on biomass estimates for 1968 and chemical analyses for 1969; sum of total elements is 15.76 kg/ha. (1969b) reported enormous yearly variations in winter annual production in this ecosys- tem. The nitrogen values in these estimates fall within the range of values for shrubs of a low-fertility desert area reported by Garcia- Moya and McKell (1970) and for a saltbrush community reported by Charley and Cow- ling (1968). These mineral element estimates are in the same range of some values for desert zones reported by Rodin and Bazilev- ich (1965). Acknowledgments This study was supported by contract AT(04-1) GEN 12 between the U.S. Atomic Energy Commission and the University of California. Literature Cited Beadle, N. C. W., R. D. B. Whalley, and J. B. Gibson. 1957. Studies in halophytes. II. Analytical data on the mineral constituents of three spec its of At- riplex and their accompanying soils in Australia. Ecology 38:340-344. Beatley, J. C. 1969a. Vascular plants of the Nevada Test Site, Ncllis Air Force Range, and Asli Mead ows, USAEC Report UCLA 12 705. 122 pp. 1969b. Biomass of desert winter annual plant populations in southern Nevada. Oikos 20:261-273. Bremner, J. M. 1965. Total nitrogen. Pages 1149-1178 in C. A. Black, ed. Methods of soil analysis, Pari 2. Amer. Soc. Agron. Inc., Madison. Wisconsin. Chapman, H. D., and P. F. Pratt. 1001. Methods of analysis for soils, plants and waters. University of California Div. Agric. Sci., pp. 262-264. Charley, J. L. 1971. The role of shrubs in nutrient cy- cling. Pages 182-202 in USDA Forest Service Tech Report INT-1. Charley, J. L., and J. W. McGarity. 1964. High soil nitrate levels in patterned saltbrush communities. Nature 201:1351-1352. Charley. J. L., and S. W. Cowling. 1968. Changes in soil nutrient status resulting from overgrazing and their consequences in plant communities of semi-arid zones. Proc. Ecol. Aust. 3:25-38. Chatterton, N. J., and C. M. McKell. 1969. Atriplex polycarpa: I. Germination and growth as affecte^ by sodium chloride in water cultures. Agron. J. 61 ;448-450. Day, P. R. 1965. Particle fractionation and particle-size analysis. Pages 552-562 in C. A. black, ed. Meth- ods of soil analysis, Part 1. Amer. Soc. ^gron. Inc., Madison, Wisconsin. Egunjobl J. K. 1969. Primary productivit) and nutrient cycling in terrestrial ecosystems. Tuatara 17:49-66. Ekren, E. B. 1908. Geological setting of Nevada Test Site and Nellis Air Force Range. Pages 11 19 in E. B. Eckel, ed. Nevada Test Site. Ceol. Soc. \m. Mem. 110. Fireman, M., and H. E. Hayward. 1952. Indicator siy- nificance of some shrubs in the Escalante Desert. Utah. Bot. Gaz. 114:143 155. Garcia Moya, E., and C. M. M< Kell. 1970. Contribu- tion of shrubs to the nitrogen econom) ot a deserl wash plant community. Ecolog) 51:81 88. Holmgren, R. C., and S. F. Brewster, Jr. 1972. Distri- bution of organic matter reserve in a desert shrub community. USDA Forest Service. Res. Paper [NT 130. 15 pp. |rssi p, R. W. 1909. Soil salinity in saltbrush country of northeastern South Australia. Trans. Roy Soc. §j Aust. 93:09-78. Lindsay, W. I... v\n W. V Norvell. 1978. Devel- opment of a DTPA soil test for zinc. iron, manga] nese, and copper. Soil Sci. Soc. Vmer. J. 12: 121-428. 1980 Nevada Desert Ecology 133 Tabic 7 continued. Si Zn Cu IV g ha° Mn B Sr Ba 0.017 0.08 0.03 0.58 0.18 0.16 0.10 0.01 0.047 0.87 0.22 7.94 0.71 3.10 L.73 0.27 0.016 0.09 0.05 3.47 0.30 0.70 1.21 0.48 0.001 0.01 0.01 0.06 0.01 0.01 0.03 0.01 0.002 0.09 0.02 0.40 0.27 0.17 0.18 0.02 0.001 0.08 0.01 0.31 0.28 0.13 0.06 0.01 0.050 0.47 0.17 7.29 1.20 1.09 2.76 0,17 0.059 0.34 0.03 7.70 0.54 1.10 0.58 0.14 0.012 1.02 0.10 4.01 0.81 1.61 16.04 0.44 0.205 3.05 0.64 31.82 4.30 8.13 22.72 1.85 Mi Kell, C. M., J. P. Blaisdell, and J. R. Goodin, eds. 1971. Wildland shrubs— their biology and utiliza- tion. USDA Forest Service Gen. Tech. Report INT-1.494p. Nimiita. H., and R. M. Haug. 1973. Distribution of dif- ferent forms of nitrogen in some desert soils. Soil Sci. 116:61-58. Olsen, S. R., C. V. Cole, F. S. Watanabe, and L. A. Dean. 1954. Estimation of available phosphorus in soils by extraction with sodium bicarbonate. I'. S. Dept'. Agr. Cir. 939. 19 p. Oyington, J. D. 1962. Quantitative ecology and the woodland ecosvstem concept. Adv. Ecol. Res. 1:103-192. 1965. Organic production, turnover, and mineral cycling in woodlands. Biol. Rev. 40:295-336. Rennie, P. J. 1955. The uptake of nutrients by mature t.iivst growth. Plant and Soil 7:49-95. Rickard, W. H. 1965a. The influence of greasewood on soil-moisture penetration and soil chemistry. Northwest Sci. 39:36-42. 1965b. Sodium and potassium accumulation by greasewood and hopsage leaves. Bot. Gaz. 126:116-119. Roberts, R. C. 1950. Chemical effects of salt-tolerant shrubs on soils. Fourth Internatl. Cong. Soil Sci., Trans. 1:404-406. Rodin, L. E., and N. I. Bazilevich. 1965. Production and mineral cycling in terrestrial vegetation. Scripta Technica Ltd. Transl., Oliver and Boyd, London, 288 p. Romnev, E. M., V. Q. Hale, A. Wallace, O. R. Lunt, J. D. Childress, H. Kaaz, G. V. Alexander, J. E. Kinnear, and T. L. Ackerman. 1973. Some char- acteristics of soil and perennial vegetation in northern Mojave Desert areas of the Nevada Test Site. USAEC Report UCLA 12-916, 340 p. Sharma, M. L., and D. J. Tongway. 1973. Plant induced soil salinity patterns in two saltbrush (Atriplex spp.) communities. J. Range Mgmt. 26:121-125. Soil Survey Staff. 1951. Soil survey manual. U.S. Dept. Agric. Handbook, No. 18, U.S. Govt. Printing Of- fice. 503 p. Tiedemann, A. R., and J. O. Klemmedson. 1970. Nutri- ent availability in desert grassland soils under mesquite (Prosopis juliflora) trees and adjacent open areas. Soil Sci. Am. Proc. 37:107-111. 1973. Effect of mesquite on physical and chem- ical properties of the soil. J. Range Mgmt. 26:27-29. USDA Salinity Laboratory Staff. 1954. Methods for soil characterization. Pages 83-126 in L. A. Rich- ards, ed. Diagnosis and improvement of saline and alkaline soils. U.S. Dept. Agr. Handbook No. 60. Wallace, A., and E. M. Romney. 1972. Radioecology and ecophvsiology of desert plants at the Nevada Test Site. National Technical Information Ser- vices, USAEC Report TID-25954. Williams, D. E. 1948. A rapid manometric method for the determination of carbonate in soils. Soil Sci. Soc. Am. Proc. 13:127-129. ECOTONAL DISTRIBUTION OF SALT-TOLERANT SHRUBS IN THE NORTHERN MOJAVE DESERT E. M. Romney' and A. Wallace' Abstract.— Ecotonal distribution of salt-tolerant shrubs was investigated under different kinds of edaphie condi- tions common to open and closed drainage basins in the northern Mojave Desert. Contributing causal factors in- volved changes in soil salinity, texture, and moisture stress. Varying degrees of halophytism occurred, ranging from plant species that are facultative in their adaptation to salinity to those that require comparatively high salt concen- trations in soil for normal growth and development. The main thrust of recent research on salt sensitivity has centered around osmotic and toxic effects of salts and the interaction be- tween the various salts and ions adversely af- fecting plants (Strogonov 1962, Boyko 1966, Ranwell 1972, Waisel 1972, Reimold and Queen 1974, Poljakoff-Mayber and Gale 1975). Biological adaptation of the halo- phytes to salinity has occurred in different ways. Some halophytes absorb comparatively small amounts of salts as the result of unique biological properties. Others accumulate con- siderable amounts of salts in different plant parts that aid in the regulation of internal os- motic pressure. Some halophytes are capable of regulating their salt balance by mecha- nisms such as excretion of excess salts through special glands or abscission of leaves contain- ing high levels of salt. Due to special biologi- cal features, halophytes can overcome the high osmotic pressure of the soil solution by decreasing their own osmotic potential. In such cases, rates of photosynthesis and trans- piration are little influenced by high salt lev- els (Kleinkopf and Wallace 1974, Gale 1975). In certain plants this osmotic potential devel- ops mainly from an accumulation of organic- substances (Wallace and Kleinkopf 1974). In others it develops due to mineral salts ab- sorbed from the saline soil substrate. Environmental studies conducted in con- junction with nuclear weapons testing pro- grams at the Nevada Test Site provided an opportunity to investigate some ecological characteristics of salt-tolerant shrubs in both open and closed drainage basins in the north- ern Mojave Desert (Wallace and Romney 1972, Romney et al. 1973, Wallace et al. 1973a, 1973b, 1974, Kleinkopf et al. 1975). The bajadas draining into playas of open and closed basins in southern Nevada often have prominent ecotonal demarcation lines below which certain plant species do not grow. Var- ious hypotheses have been presented to ex- plain ecotonal transition zones, including the influence of such factors as low temperature (Beatley 1974), salinity (Shreve 1940, Shantz and Piemeisel 1940), fine-textured soil (Gard- ner 1951, Branson et al. 1967), and excess of water (Fosberg 1940, Shreve and Wiggins 1964). These environmental factors were monitored as part of our environmental stud- ies program and are evaluated in conjunction with the findings reported herein. Methods Readers are referred to earlier reports by Wallace and Romney (1972) and Romney et al. (1973) and those of Beatley (1969, 1974, 1976) for greater details concerning the de- scription of study areas and the results ob- tained from extensive investigations of soil and vegetation in the northern Mojave Desert areas of the Nevada Test Site. We shall present data in this report from three of 'Laboratory of Nuclear Medicine and Radiation Biology. Uni' :>f California, Los Angeles. California 90024. 134 1980 Nevada Desert Ecology 135 several different eeotonal transition zones in- volved in this study. The first eeotonal study site is located in an open drainage basin (Rock Valley) on the east slope of the Amargosa River watershed. The area sampled is approximately 0.5 km2 in extent with a downslope length of 500 m. The elevation is about 1050 m, and the slope is to the northwest with a gradient varying from 1 to 3 percent. The dominant and co- dominant shrub species in this particular eeo- tonal transition area were separated into three generally homogeneous vegetation zones that were oriented in parallel bands of about the same width, perpendicular to the slope. From 25 to 30 quadrats, 2 X 25 m, were sampled in each zone at coordinate lo- cations generated by a computer program to insure random dispersion. All shrubs were identified by nondestructive dimensional measurements (Romney et al. 1973) from which calculations were made to estimate the spatial distribution. Taxonomy of the area was worked out by Beatley (1969, 1976). The second eeotonal transect is located across a sharp La rrea -A triplex demarcation line above the playa on the north side of the Frenchman Flat closed drainage basin. Eleva- tion is about 950 m and the slope is to the south at about 2 percent. A 15 X 500 m transect across the eeotonal zone was divided into 50 m sections within which all shrubs were measured. The third eeotonal transition area studied involved a transect extending down the west- facing bajada onto the Frenchman Lake playa. Five sampling stations were estab- lished along the transect, about 1 km apart, at which shrub measurements were made in 2 X 50 m quadrats. Elevation changed from 1040 to 940 m, with the slope varying from 1 to 3 percent at the three sampling sites on the bajada. Soil sampling pits along each of these transects were dug either by hand or by back-hoe to permit an examination and sam- pling of the soil profile underneath both shrub and bare areas. The depth of each pit was to the caliche hardpan or, if no restric- ting layer existed, to a depth well into the C horizon. Profile horizons were described and samples were collected for physical and chemical analysis by the methods of the USDA Salinity Laboratory (1954). Vegetation samples were collected from each of the sampling sites for subsequent chemical analysis by methods previously de- scribed (Romney et al. 1973). Results and Discussion Distribution of shrubs within the three vegetation zones investigated in the Rock Valley open drainage basin is listed in Table 1. The zones are numbered 1, 2, and 3, repre- senting upper, intermediate, and lower posi- tions downslope along the transect, respec- tively. Edaphic conditions within this eeotonal transition area are typical of the open- drained bajadas we have investigated in southern Nevada. The soil is derived from heterogeneous, highly calcareous alluvium, composed primarily of Cambrian limestones with some tuff and basalt. The surface is a well-developed desert pavement with a mas- sive and strongly cemented caliche layer at depths ranging from 30 to 70 cm. The soil profile horizons are young and often poorly developed. Salt concentrations in the upper profile are relatively low and consist mainly of calcium and magnesium salts. Sodium salts Table 1. Distribution of shrubs in eeotonal zone seg- ments in an open-drainage area of Rock Valley- (Direc- tion of slope -) Number of plan ts per hectare Species Zone 1 Zone 2 Zone 3 Acamptopappus shockleyi Gray 46 0 28 Ambrosia dumosa (Gray) Payne 1844 2474 2504 Atriplex confertifolia (Torr. & Frem.) Wats. 0 0 276 Ephedra nevadensis Wats. 849 845 275 Ceratoides lanata (Pursh) J. T. Howell 155 951 443 Grayia spinosa (Hook) Moq. 203 702 2202 Krameria parvifolia Benth. 1957 951 773 Larrea tridentata (Sesse & Moc. ex DC.) Cov. 1122 907 1004 Lijcium andersonii A. Gray 1136 452 83 Lijcium pallidum Miers 88 706 815 Total 7400 7988 8403 136 Great Basin Naturalist Memoirs No. 4 tend to accumulate at lower depths in the soil profile, especially in local areas where terrain features have restricted drainage. In the case of this particular study area, the main difference in edaphic features was an increase in the silt content of the soil profile, accompanied by an accumulation of sodium salts at depths below 30 cm, proceeding downslope along the transect. Each of the shrub species present in this transition zone exhibit some degree of facul- tative adaptation for salt tolerance, but none is known to have obligate requirements (Wal- lace and Romney 1972, Wallace et al. 1973a, 1973b, 1974). Those species which increase in density in zones progressing downslope are the ones that commonly are more salt toler- ant in nature. Notably so are Ambrosia du- mosa (A. Gray) Payne, Atriplex confertifolia (Torr. & Frem.) Wats., Graijia spinosa (Hook) Moq., and Lycium pallidum Miers. There also was an interaction with soil texture in this area. Ambrosia dumosa, Ceratoides la- nata (Pursh) J. T. Howell, and L. pallidum are better adapted to finer textured soils than Larrea tridentata (Sesse & Moc. ex DC.) Cov. or Lycium andersonii A. Gray, and these spe- cies responded accordingly at this study site. The second transect is across a sharp eco- tonal demarcation line between L. tridentata and Atriplex canescens (Pursh.) Nutt. commu- nities situated between the bajada and playa of the Frenchman Flat closed drainage basin. Plant distribution along the transect is shown in Table 2. The analyses disclosed an ecoton- al demarcation zone among several other species at this study site that is not so appar- ent from visual observations. The transition across the ecotone was equally sharp among several plant species. The distribution pat- terns for A. dumosa, C. lanata and L. triden- tata were much more alike than under the situation given in Table 1. Different edaphic factors, therefore, could be involved. Since C. lanata distribution paralleled L. tridentata, one might conclude that soil salinity is not a simple causal factor at this particular eco- tone, inasmuch as C. lanata can grow in moderately saline soil (Vest 1962) but L. tri- dentata does not. An alternative explanation could be that ecotypes are differentially sen- sitive to salt. We hasten to point out, there- fore, that the distribution of C. lanata extend- ed well beyond the Larrea demarcation line onto the playa at some of the other transects investigated elsewhere around the ecotone. It should be taken into account that sensi- tivity of seeds and young seedlings to soil salinity and to pH could be limiting causal factors at this ecotone, either at the present time or earlier when the population initially became established. The soil profiles along this transect showed surface pH values rang- ing from 8.0 to 9.0. The soluble salts at the surface were moderate (EC25 varied from 2.07 to 3.06 mmhos/cm). Salt accumulations, including soluble boron and nitrate, generally increased at greater depths in the soil profile within the playa. In some earlier work, Bar- bour (1968) observed that L. tridentata seed germination was not affected by high soil pH but that subsequent seedling development was markedly decreased, especially above pH 8.0. The lack of any young seedlings in the ecotonal area suggests that seedling survival is a rare event that may be partially regu- lated by the high soil pH levels now present. Size stratification occurs within the shrub population, however, which indicates that fa- vorable moisture conditions for seedling sur- Table 2. Distribution of plant species along a 500 m transect across the Larrea-Atriplex ecotone lin Frenchman Flat. (Direction of slope ■*) in north Species Number of plants in 15 X 50 in segments Ambrosia dumosa (Gray) Payne 263 168 96 16 17 29 23 14 0 0 Atriplex canescens (Pursh.) Nutt. 54 17 74 100 149 246 L83 192 240 202 Ceratoides lanata (Pursh) J. T. Howell 99 SI 52 10 25 b' 4 2 I 1 Larrea tridentata (Sesse & Moc. ex DC.) Cov. 55 75 40 54 28 3 0 1 0 0 Lycium andersonii A. Gray 2 5 3 1 0 0 1 0 0 0 Salsola iberica Sennen & Pan 1 0 0 0 18 54 42 56 44 56 Sphaeraleea ambigua Gray 37 61 50 106 49 121 126 129 256 203 1980 Nevada Desert Ecology 137 vival have occurred periodically, presumably many decades apart. Another factor thought to be involved in regulating shrub distribution at this ecotone is periodic flooding of the basin floor beyond the boundaries of the dry lake bed. Shrubs that are sensitive to poor root aeration or standing water on foliage can be damaged when inundated with flood water. A case in point was observed recently near Baker, Cali- fornia (Wallace and Romney 1972). Larrea tridentata is known to require good aeration of the root zone and well-drained soils (Shreve and Wiggins 1964). Complete in- undation need not be effected to keep L. tri- dentata from populating a site; a higher than normal water table could be just as detri- mental to some sensitive species. Monitoring of soil and air temperatures from May 1967 to January 1973 gave very little indication that a temperature differen- tial across this ecotone was an important causal factor compared to the edaphic fea- tures. The mineral element composition of shrubs did not differ significantly across the ecotonal transition zone. Data in Table 3 for shrubs sampled near the middle of the transect are representative of those sampled elsewhere along the ecotone. The uniformity of mineral composition (i.e., cation and anion balance) in these salt-tolerant shrubs, irrespective of location and edaphic conditions (Romney et al. 1973), attests to their adaptive capacity to regulate their own salt balance. The distribution of shrub species along the 5 km ecotonal transect across the bajada and playa in east Frenchman Flat is shown in Table 4. Species diversity became less com- plex progressing downslope onto the bajada. These data are a good example of how the more sensitive species give way to the more salt-tolerant ones under changing edaphic conditions. It would not be prudent in this in- stance, however, to assume that increasing salinity was the only causal factor of commu- nity or species distribution. Factors of mois- ture stress and soil texture also must be taken Table 3. Mineral composition of leaves of shrubs from the north Frenchman Flat ecotonal zone. N CI S P Na K Ca Mg B Soccics Percent ol dry weight ug/g A. dumosa 3.68 0.85 0.71 0.44 0.08 5.63 2.28 0.53 130 A. canescens 2.44 1.65 1.80 0.15 1.05 8.02 3.80 1.14 60 C. Janata 2.79 0.41 0.34 0.19 0.10 3.38 1.67 0.43 49 L. tridentata 2.37 0.68 0.12 0.29 0.03 2.53 1.64 0.19 76 L. andersonii 2.32 1.00 0.85 0.24 0.03 6.30 9.92 0.91 75 S. ambigua 2.12 0.31 0.26 0.72 0.13 3.35 2.98 0.53 160 Table 4. Shrub distribution across a 5 km ecotonal transect n east Frenchman Flat. (Increasing salt gradient ■*) Percent frequency at >ampling sites' Species 1 2 3 4 5 Acamptopappus shockleyi Gray 2.1 9.2 - - - Ambrosia dumosa (Gray) Payne 2.4 59.2 13.5 — — Atriplex canescens (Pursh) Nutt. — — 5.0 90.2 — Atriplcx confetti folia (Torr. & Frem.) Wats. 6.4 - 43.2 9.8 100 Psorothamnus fremontii (Torr.) Barneby 19.1 — — — — Epliedra nevadensis Wats. 4.3 — — — — Ceratoides lanata (Pursh) J.T. Howell 6.5 6.6 5.4 — — Hymenoclea salsola T & G. 4.3 - 5.4 - - Krameria parvifolia Benth. 8.5 1.3 — — — Larrea tridentata Ses 40.4 13.2 24.3 — — Sphaeralcea ambigua Gray 2.1 - 2.7 - - Thamnosma montana Torr. & Frem. 2.1 — — — — Yucca hrcvifolia Engelm. in Wats. 2.1 6.6 - - — "Shrubs contributing less than 1 percent are unlisted. 138 Great Basin Naturalist Memoirs No. 4 into account because marked changes oc- curred, progressing downslope onto the play a. Some soil properties at the different sam- pling sites across the East Frenchman Flat transect are listed in Table 5. Increased soil salinity occurred, especially at the two sites along the play a where only the two Atriplex species grew in abundance. Calcium domi- nated the soluble cation pattern in the up- land area of the transect, and sodium became more dominant on the playa, as one might expect in a closed drainage basin. Chloride and nitrate concentrations increased in the soil profile of the dry lake playa. The east Frenchman Flat transect is a good example relating salinity to community or species distribution. On the other hand, this relationship was not so clear-cut at several other transects across the ecotonal zone, sur- rounding the Frenchman Dry Lake playa (Romney et al. 1973), where better correla- tion occurred with the edaphic features of soil moisture stress and texture. This has been the experience of other investigators where attempts to relate salinity alone to commu- nity or species distribution gave inconsistent results (Gates et al. 1956, Branson et al. 1967). Branson et al. (1970) speculate that upland halophytes dominate certain areas be- cause of tolerance to high osmotic stress or high physical moisture stress, or a com- bination of both. We find indications of this occurring at our study areas in southern Ne- vada. We also see much evidence, as ex- pressed by Unger (1966), "that the most salt- tolerant species have the widest salinity tol- erance and can survive under low as well as high salinities. The less tolerant species are limited in their distribution to low and non- saline areas." A summary of the bibliography of the veg- etation and soils of Nevada was compiled by Tuelleretal. (1971). Acknowledgment This study was supported by Contract EY- 76-C-03-0012 between the U.S. Department of Energy and the University of California. Literature Cited Barbour, M. G. 1968. Germination requirements of the desert shrub Larrea divaricata. Ecology 49:915-923. Beatley, J. C. 1969. Vascular plants of the Nevada Test Site, Nellis Air Force Range and Ash Meadows. USAEC Report, UCLA 12-705. 1974. Effects of rainfall and temperature on the distribution and behavior of Larrea tridentatd (Creosote-Bush) in the Mojave Desert of Nevada! Ecology 55:245-261. 1976. Vascular plants of the Nevada Test Site and central-southern Nevada: ecologic and geograph- ic distributions. National Technical Service] USERDA Report TID-16881. Bovko, H., ed. 1966. Salinity and aridity: new ap- proaches to old problems. Dr. W. Junk Publ. The Hague. Table 5. Soil properties (30 cm depth) across the 5 km transect in east Frenchman Flat. (Direction ol slope • Soil properties Site 1 Site 2 Site 3 Site 4 Site 5 pH, sat. extract EC25, mmhos/cm Saturation extract ions Na (meq. /liter) K (meq. /liter) Ca (meq. /liter) Mg (meq./liter) CI (meq./liter) NO3 (meq./liter) SO4 (meq./liter) 8.7 2.15 3.57 6.13 22.09 8.97 1.26 6.62 0.52 S.7 1 .89 1.37 1.48 17.39 5.85 L.40 6.16 0,41 8.8 1.00 2.47 2.48 6.93 3.65 2.28 5.00 0.40 8.8 2,40 L5.45 5.25 L1.78 12.4.i 6.37 17.50 0.54 8.9 2.98 27.95 3.98 6.79 4.80 12.66 23.10 0.59 Exch. Na, percent 2.0 2.2 22.1 1980 Nevada Desert Ecology 139 Branson, F. A., R. F. Miller, and I. S. McQueen. 1967. Geographic distribution and factors affecting the distribution of salt desert shrubs in the United States. J. Range Management 20: 287-296. 1970. Plant communities and associated soil and water factors on shale-derived soils in north- eastern Montana. Ecology 51: 391-407. Fosberg, F. R. 1940. The aestival flora of the Mesilla Vallev Region, New Mexico. Anier. Midi. Nat. 23: 573-593. Gale, J. 1975. The combined effect of environmental factors and salinity on plant growth. Pages 186-192 in A. Poljakoff-Mayber and J. Gale, eds. Plants in saline environments. Springer-Verlag, New York, Heidelberg, Berlin. Gardner, J. L. 1951. Vegetation of the creosote bush area of the Rio Grande Valley in New Mexico. Ecol. Monographs 21: 379-403.' Gates, D. H., L. A. Stoddart, and C. W. Cook. 1956. Soil as a factor influencing plant distribution on salt-deserts of Utah. Ecol. Monographs 26: 155-175. Kleinkopf, G. E., and A. Wallace. 1974. Physiological basis for salt tolerance in Tamarix ramosissima. Plant Sci. Letters 3:157-163. Kleinkopf, G. E., A. Wallace, and J. W. Cha. 1975. Sodium relations in desert plants: 4. Some phys- iological responses of Atriplex confertifolia to dif- ferent levels of sodium chloride. Soil Sci. 120: 45-48. Poljakoff-Mayber, A., and J. Gale. 1975. Plants in sa- line environments. Springer-Verlag, New York, Heidelberg, Berlin. Ranwell, D. S. 1972. Ecology of salt marshes and sand dunes. Chapman and Hall, London. Reimold, R. J., and W. H. Queen (eds.). 1974. Ecology of halophytes. Academic Press, New York and London. Romney, E. M., V. Q. Hale, A. Wallace, O. R. Lunt, J. D. Childress, H. Kaaz, G. V. Alexander, J. E. Kinnear, and T. L. Ackerman. 1973. Some char- acteristics of soil and perennial vegetation in northern Mojave Desert area of the Nevada Test Site. UCLA # 12-916, USAEC Report. Shantz, H. L., and R. L. Piemeisel. 1940. Types of veg- etation in Escalante Valley, Utah, as indicators of soil conditions. USDA Tech. Bull. 713. Shreve, F. 1940. The edge of the desert. Yearbook As- soc. Pacific Geographers 6: 6-11. Shreve, F., and I. L. Wicgins. 1964. Vegetation and flora of the Sonoran Desert. Stanford University Press, Stanford, California. Strogonov, B. P. 1962. Physiological basis of salt toler- ance of plants (translated from Russian). Israel Program for Scientific Translations, Jerusalem. Tueller, P. T., J. H. Robertson, and B. Zamora. 1971. The vegetation of Nevada: a bibliography. Nev. Agr. Exp. Sta. Bull. R78. Unger, I. A. 1966. Salt tolerance of plants in saline areas of Kansas and Oklahoma. Ecology 47: 154-155. U.S. Salinity Laboratory Staff. 1954. Methods for soil characterization. In L. A. Richards, ed. Diagnosis and improvement of saline and alkaline soils. USDA Handbook 60. Vest, E. D. 1962. Biotic communities in the Great Salt Lake Desert. Inst. Environ. Biol. Res., Ecol. and Epizool. Ser. 73, University of Utah. Waisel, Y. 1972. Biology of halophytes. Academic Press, New York and London. Wallace, A., and G. E. Kleinkopf. 1974. Contribution of salts to the water potential of woody plants. Plant Sci. Letters 3: 251-257. Wallace, A., R. T. Mueller, and E. M. Romney. 1973b. Sodium relations in desert plants: 2. Dis- tribution of cations in plant parts of three differ- ent species of Atriplex. Soil Sci. 115:390-394. Wallace, A., and E. M. Romney. 1972. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Technical Information Ser- vices, Springfield, Virginia, TID 25-954. Wallace, A., E. M. Romney, J. W. Cha, and G. V. Alexander. 1974. Sodium relations in desert plants: 3. Cation-anion relationships in three spe- cies which accumulate high levels of cations in leaves. Soil Sci. 118:397-401. Wallace, A., E. M. Romney, and V. Q. Hale. 1973a. Sodium relations in desert plants: 1. Cation con- tents of some plant species from the Mojave and Great Basin deserts. Soil Sci. 115: 284-287. PARENT MATERIAL WHICH PRODUCES SALINE OUTCROPS AS A FACTOR IN DIFFERENTIAL DISTRIBUTION OF PERENNIAL PLANTS IN THE NORTHERN MOJAVE DESERT A. Wallace1, E. M. Romney1, R. A. Wood1, A. A. El-Ghonemy:, and S. A. Bamberg' Abstract.— An area of 0.46 km2 divided into six zones in the northern Mojave Desert transitional with the Great Basin Desert has been studied. Diversity is high among the perennial plant species within the 0.46 km2 area. Com- mon species for the two deserts that are present in the area studied are Atriplex confertifolia (Ton. & Frem.) S. Wats., Ceratoides lanata (Pursh) J. T. Howell, Grayia spinosa (Hook.) Moq., Ephedra nevadensis S. Wats. Some other species present include Lycium andersonii A. Gray, Lijcium pallidum Miers, Ambrosia dumosa (A. Gray) Payne., Larrea tridentata (Sesse & Moc. ex DC) Cov., Acamptopappus shockleyi A. Gray, and Krameria parvifolia, Benth. Some of the species are relatively salt tolerant and some are relatively salt sensitive. A total of 4282 individual plants were measured. There was considerable variation in distribution of the 10 dominant species present, apparently due to zonal variations of salinity dispersed within the study area. Correlation coefficients among pairs of the species for different zones illustrate interrelationships among the salt-tolerant and salt-sensitive species. Observations on an ad- jacent hillside with rock outcroppings indicate that the saline differences in this area are partly due to outcroppings of parent volcanic rock materials that yield Na salts upon weathering. A vegetational map of a 0.46 km2 area in Rock Valley of the northern Mojave Desert was presented elsewhere (El-Ghonemy et al. 1980, this volume). This is the Rock Valley Desert Biome validation site used in the In- ternational Biological Program (Turner 1973, 1975, 1976, Turner and McBrayer, 1974). The purpose of this report is to further ex- plore the differences in plant species distribu- tion on that site as influenced by zonal varia- tions in salinity. The information involved also has relationships with the ecotonal lines studied elsewhere at the Nevada Test Site (Romney and Wallace 1980, this volume). Materials and Methods Data collected for the IBP validation site (Turner 1973, 1975, 1976, Turner and McBrayer 1974) and used in the development of a vegetational map and other findings (El- Ghonemy et al. 1980a and 1980b, this vol- ume) were also used in this report. Sampling and data calculation procedures were de- scribed in those reports. An additional 4 X 100m belt transect was established on a hillside further upslope from the main study plot. It was selected because of rock outcroppings that gave vegetational patterns somewhat similar to the differences observed within the large study plot. All plants were identified, counted, and mea- sured by dimension analysis (Wallace and Romney 1972), and leaf tissue samples were taken for chemical analysis. Soil samples were taken at 10 m intervals along the tran- sect. They were subjected to determination of EC and pH. For convenience of presenting results, the transect was divided into four plots each 25 m long. The rock outcrop was near the top of the transect. Mineral element contents of plants were determined by emission spectrography; nitro- gen was determined by Kjeldahl analysis; CI was determined by titration. Results and Discussion The number of plants per hectare in vari- ous zones of the 0.46 km2 plot are shown in Table 1. The zone numbers were designated in earlier IBP reports (Turner 1973, 1976, Turner and McBrayer 1974). Results serve to illustrate the differential distribution encoun- 'l.aboratory of Nuclear Medicine and liadiatu 'University of Tanta, Tanta, Egypt. Biology, Universits of t alifomia, Los \ngeles, California 90024, 140 1980 Nevada Desert Ecology 141 tered because of the soil differences. Zones 24 and 25 were the only ones having Atriplex confertifolia (Torr. & Frem.) S. Wats. This species is highly tolerant of salt (Wallace et al. 1973a). Graijia spinosa (Hook.) Moq. was present in Zones 20 and 21 in small numbers only, but was very prominent in Zones 23, 24, and 25. Lyciiim andersonii A. Gray was present in exactly the opposite manner, whereas, Lycium pallidum was distributed as was G. spinosa. Lycium pallidum Miers is much more tolerant of salt than is L. ander- sonii (Ashcroft and Wallace 1976, Wallace et al. 1973b, Beatley 1976). Correlation coefficients were calculated for the species pairs for the data in Table 1 to further show relationships between the spe- cies according to differences in the soil in- volved (Table 2). Atriplex confertifolia was not included in these correlations because of its absence in four of the zones. Of salt-toler- ant plants, Lycium pallidum and G. spinosa were positively correlated. Of salt-non- tolerant species, L. tridentata, Krameria par- Table 1. Number of plants per hectare in the 0.46 km2 study plot. Species Zone 20 21 22 23 24 25 46 _ 28 45 28 101 - - - 276 192 849 845 1177 1438 275 785 155 951 122 2179 443 773 1844 2474 2998 3877 2504 3402 203 702 252 1830 2202 3388 1957 951 2103 1394 773 1166 1122 907 1205 1046 1004 1043 1136 452 981 713 83 314 88 706 224 697 815 1020 7400 7988 9090 13219 8403 12184 Acamptopappus slwckleiji A. Gray Atriplex confertifolia (Torr. & Frem.) S. Wats Ephedra nevadensis S. Wats Ceratoides lanata (Pursh) J. T. Howell Ambrosia dumosa (A. Gray) Payne Grayia spinosa (Hook.) Moq. Krameria parvifolia Benth. Larrea tridentata (Sesse & Moc ex DC.) Cov. Lycium andersonii A. Gray Lycium pallidum Miers Total Table 2. Correlation coefficients between number of plants/ha for the various species among zones in Rock Val- ley (± 0.700 needed for P = 0.05). Ephedra Ceratoides Ambrosia Grayia Krameria Larrea Lycium Lycium nevadensis lanata dumosa spinosa parvifolia tridentata andersonii pallidum Ephedra nevadensis S. Wats. 0.534 +0.570 -0.287 +0.571 +0.373 +0.633 -0.303 Ceratoides lanata (Pursh) J.T. Howell Ambrosia dumosa (A. Gray) Payne 0.534 • 0.570 + 0.724 + 0.724 +0.352 -0.332 -0.415 -0.179 -0.470 -0.555 -0.105 +0.033 -0.197 +0.517 Grayia spinosa (Hook.) Moq. -0.287 + 0.352 -0.555 -0.623 -0.338 -0.748 +0, K rameria parvifolia Benth + 0.571 -0.332 -0.105 -0.623 •0.890 +0.941 -0.858 Larrea tridentata (Sesse Moc. ex DC.) Cov. + 0.373 -0.415 +0.033 -0.338 +0.: + 0.700 -0.674 Lycium andersonii A. Gray + 0.633 -0.179 -0.197 -0.748 +0.941 +0.700 -0.900 Lycium pallidum Miers -0.303 +0.470 +0.517 +0.888 -0.858 -0.674 -0.900 142 Great Basin Naturalist Memoirs No. 4 vifolia Benth., and L. andersonii were posi- tively correlated. The individuals of the two groups were highly negatively correlated with one other. Mineral analyses of leaves of plants from the various zones (Table 3) indicate little dif- ference that can explain the results. The CI concentration in leaves may be slightly high- er from Zones 24 and 25. The frequency of plant species in the four sections of the hillside transect (Table 4) showed characteristics similar to the large plot. Visual study of the transect area in- dicated that the salt-tolerant shrubs were more prevalent on sites containing outcrops of parent material. The average pH of the soil (0-15 cm) at the four intervals along the transect from bottom to top was 8.78, 8.90, 8.85, and 9.09. There were few differences except that the soil around the parent rock outcrop was slightly more alkaline. The EC (mmho/cm) values of the four soil samples beginning at the bottom were 2.43, 2.53, 2.07, and 2.75. None were really excessively Table 3. Mineral element composition of leaf samples from the various zones. Samples taken in May 1973. N CI P K Ca Mg Na B Zone Percent of dry weight ug/g Grayia spinosa (Hook.) Moq. 20 3.48 0.90 0.24 3.01 1.82 — 1,635 52 20E 3.65 1.10 0.24 2.67 2.20 1.41 1,261 48 21 1.56 0.57 0.14 3.45 2.20 1.07 99 58 23 2.38 0.42 0.24 3.50 2.69 1.35 352 58 24 1.73 1.19 0.14 2.92 2.63 1.37 341 69 25 2.27 1.04 0.17 Lychin 2.99 andersonii 2.38 A. Gray 1.18 719 66 20 3.17 3.29 0.16 2.03 4.90 0.76 2,133 28 20E 3.33 4.18 0.17 1.65 5.06 0.88 2,739 35 21 3.21 4.93 0.15 2.09 5.71 0.88 3,000 30 22 3.15 4.57 0.19 1.83 4.90 0.87 2,453 33 23 3.14 4.14 0.22 2.14 5.96 0.97 3,268 37 24 2.86 4.95 0.14 2.34 6.73 0.89 3,276 35 25 3.24 6.46 0.18 1.54 5.21 0.68 2,228 31 Lycium pallhhin Miers 20 4.19 1.98 0.29 2.15 3.74 1.08 12,600 56 21 4.08 3.47 0.20 1.68 3.96 1.18 10,600 20 22 2.85 2.61 0.18 1.49 3.35 1.15 2,100 34 23 3.26 2.59 0.16 2.11 3.84 1.26 2,700 40 24 3.18 3.83 0.17 1.28 4.81 1.34 2,200 47 25 ' 3.26 4.02 0.15 3.04 5.54 1.22 3,000 20 Larrea tridentata (Sesse & Moc. ex DC.) Cov. 20 2.18 0.30 0.18 1.53 1.20 0.13 279 56 20E 2.18 0.24 0.21 1.90 0.90 0.21 328 48 21 2.37 0.16 0.26 2.06 1.34 0.18 .343 72 22 1.97 0.12 0.27 1.72 1.67 0.26 377 56 23 1.95 0.17 0.34 1.68 1.45 0.23 811 67 24 2.12 0.30 0.24 2.40 1.08 0.18 473 74 25 2.04 0.33 0.21 At rip lex conferti 1.74 folia (Torr. 1.31 & Frem.) S. 0.20 Wats 867 77 21 3.73 4.58 0.33 2.65 1.79 0.61 3.04 35 24 3.98 5.29 0.28 2.07 2.52 0.56 2.58 36 Coleogy ie ramosissima Torr. 20 1.79 0.07 0.22 1.47 3.83 0.49 180 25 20E 2.06 0.01 21 2.34 0.02 0.25 2.03 1.53 0.27 210 33 25 2.10 0.01 0.27 1.17 2.92 0.41 76 21 1980 Table 3 continued. Nevada Desert Ecology 143 N CI P K Ca Mg Na B Zone Percent of dry weight ug/g Ceratoides lanata (Pursh) J.T. Howell 21 3.55 0.16 0.29 2.66 0.93 0.34 194 20 23 3.73 0.37 0.32 2.45 1.55 0.54 103 35 24 3.40 0.24 0.29 2.61 1.53 0.39 90 27 25 3.07 0.32 0.29 2.58 1.19 0.33 74 26 Ephedra nevadensis S. Wats. 20 3.90 0.39 0.40 3.31 0.75 0.22 66 32 20E 4.05 0.34 0.90 5.43 2.39 0.66 228 57 21 4.00 0.40 0.41 3.05 0.73 0.19 68 33 24 3.52 0.35 0.36 3.20 0.57 0.18 135 26 25 4.24 0.51 0.37 3.55 0.70 0.18 96 27 Ambrosia dumosa (A. Gray) Payne 20 4.56 0.88 0.31 2.70 3.05 0.43 241 43 20E 4.18 0.72 0.37 2.94 1.47 0.42 237 88 22 4.57 1.03 0.39 2.83 1.93 0.52 483 59 24 4.28 1.23 0.32 2.70 3.11 0.48 261 89 25 4.25 1.33 0.32 3.94 2.75 0.46 563 82 saline within the first 15 cm of the soil pro- file. Nevertheless, the outcrops of exposed rock were high in sodium salts. The salt re- sulting from the weathering processes in the rock probably leaches away rapidly because of the slope. Table 5 shows some other vegetational characteristics of this hillside transect. More detailed studies of these sites should elucidate some of the subtle ways that soil properties can determine the nature of vegetation in this desert. Acknowledgments This study was supported by Contract EY- 76-C-03-0012 between the U.S. Department of Energy and the University of California and by the U.S. IBP Desert Biome Program. Table 4. Frequency of plant species in the four sections of the hillside transect (top is the rock outcrop with saline characteristics). Species Transect on saline outcrop Base lA Va V* Percent frequency Top V* Atriplex confertifolia (Torr. & Frem.) S. Wats Psorothamnus fremontii (Torr.) Barneby Ceratoides lanata (Pursh) J.T. Howell Grayia spinosa (Hook.) Moq. Lijcium pallidum Miers Larrea tridentata (Sesse Moc. ex DC.) Cov. Lycium andersonii A. Gray Ambrosia dumosa (A. Gray) Payne Krameria parvifolia Benth Machaeranthcra tortifolia (A. Gray) Cronq. & Keck Ephedra nevadensis S. Wats. Lepidium fremontii S. Wats. Sphaeralcea ambigua A. Gray Oryzopsis hijmenoides (Roem. & Schult.) Ricker Encelia virginensis A. Nels. 0.0 0.0 1.7 16.3 0.0 0.0 8.5 4.1 9.7 9.7 20.3 6.1 0.0 0.0 5.1 0.0 29.0 19.4 3.4 20.4 12.9 6.5 1.7 0.0 9.7 9.7 11.9 6.1 3.2 9.7 13.6 18.4 29.0 25.8 5.1 8.2 0.0 6.5 3.4 12.2 3.2 9.7 15.3 4.1 0.0 0.0 6.8 0.0 3.2 0.0 3.4 0.0 0.0 3.2 0.0 0.0 0.0 0.0 0.0 4.1 144 Great Basin Naturalist Memoirs No. 4 Table 5. Vegetation characteristics of the hillside transect (divided into one-qnarter segments for comparisons). Plant Plant Plant Cover Rel.Dom. area volume biomass Species percent percent m2/ha m3/ha kg/ha Ephedra nevadensis Ceratoides lanata Ambrosia duinosa Krameria parvifolia Larrea tridentata Lycium andersonii Lycium pallidum Sphaeralcea ambigua Basal lA segment 1.4 1.7 8.4 16.2 19.1 17.7 35.2 0.1 15.58 22.1 26.7 130.6 253.1 297.6 176.5 548.7 3.1 Total 9.3 15.6 74.5 58.8 192.5 168.5 336.1 0.9 856.2 13.4 47.1 165.7 127.2 304.6 309.1 274.4 0.4 1242.0 Lower Ephedra nevadensis Ceratoides lanata Ambrosia dumosa Krameria parvifolia Larrea tridentata Lycium andersonii Lycium pallidum Oryzopsis hymenoides Machaeranthera tortifolia segment 1.0 2.4 4.6 21.1 18.4 22.1 29.1 0.1 1.3 13.10 13.4 30.8 60.2 275.8 241.7 289.8 380.6 0.8 16.7 Total 4.3 12.5 19.4 62.7 228.8 203.1 229.9 0.1 3.3 764.1 6.3 37.7 43.2 135.5 250.3 342.1 176.2 0.9 5.3 997.5 Atriplex confertifolia Psorothamnus fremontii Ephedra nevadensis Ambrosia dumosa Ceratoides lanata Grayia spinosa Krameria parvifolia Larrea tridentata Lycium andersonii Lycium pallidum Sphaeralcea ambigua Lepidium fremontii Machaeranthera tortifolia Upper segment 1.7 14.6 20.9 11.1 11.6 6.8 5.3 0.9 20.6 4.8 0.2 0.5 1.0 13.69 23.7 199.8 188.7 152.3 158.4 93.6 72.0 12.6 281.6 65.3 2.6 7.1 13.3 Total 10.7 76.7 134.3 58.8 51.1 45.4 13.9 3.8 158.2 31.1 0.2 1.1 54.8 189.5 195.1 176.9 113.7 90.5 30.1 22.7 361.0 40.7 0.1 3.4 1284.9 Atriplex confertifolia Psorothamnus fremontii Ephedra nevadensis Ceratoides lanata Ambrosia dumosa Krameria parvifolia Lycium andersonii Lycium pallidum Encelia virginensis Machaeranthera tortifolia Top V* segment (rock outcrops) 17.5 144.4 59.6 305.8 3.1 25.5 7.1 17.6 0.8 6.3 1.1 1.6 0.8 6.9 1.4 4.1 23.9 196.5 56.2 125.2 16.9 139.0 27.8 60.1 3.6 29.6 8.2 43.2 27.8 229.2 85.0 135.5 1.5 12.6 3.1 0.0 4.1 33.6 8.2 13.2 .24 Total 706.4 1980 Nevada Desert Ecology 145 Literature Cited Ashcroft, R. T., and A. Wallace. 1976. Sodium rela- tions in desert plants: 5. Cation balance when grown in solution culture and in the field in three species of Lycium from the northern Mojave Desert. Soil Sci. 122:48-51. Beatley, J. C. 1976. Vascular plants of the Nevada Test Site and central-southern Nevada. Tech. Informa- tion Center, Office of Technical Information, ERDA Report TID- 16881. El-Ghonemy, A. A., A. Wallace, and E. M. Romney. 1980a. Frequency distribution of numbers of per- ennial shrubs in the northern Mojave Desert. Great Basin Nat. Mem. 4:32-36. El-Ghonemy, A. A., A. Wallace, E. M. Romney, and W. Valentine. 1980b. A phytosoeiological study of a small desert area in Rock Valley, Nevada. Great Basin Nat. Mem. 4:57-70. Romney, E. M., and A. Wallace. 1980. Ecotonal distri- bution of salt-tolerant shrubs in the northern Mo- jave Desert. Great Basin Nat. Mem. 4:132-137. Romney, E. M., A. Wallace, H. Kaaz, V. Q. Hale, and J. D. Childress. 1977. Effect of shrubs on redis- tribution of mineral nutrients in zones near roots in the Mojave Desert. Pages 303-310 in J. K. Marshall, ed. The belowground ecosystem: a syn- thesis of plant-associated processes. Colorado State University, Fort Collins, Colorado. Turner, F. B., ed. 1973. Rock Valley Validation Site re- port, 1972. US/IBP Desert Biome Res. Memo 73- 2. 1975. Rock Valley Validation Site report. US/IBP Desert Biome Res. Memo 75-2. 1976. Rock Valley Validation Site report. US/IBP Desert Biome Res. Memo 76-2. Turner, F. B., and J. F. McBrayer, eds. 1974. Rock Valley Validation Site report 1973. US/IBP Desert Biome Res. Memo 74-2. Wallace, A., R. T. Mueller, and E. M. Romney. 1973a. Sodium relations in desert plants. 2. Dis- tribution of cations in plants of three different species of Atriplex. Soil Sci. 115:390-394. Wallace, A., and E. M. Romney. 1972. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Technical Information Ser- vice, USAEC Report TID-25954. Wallace, A., E. M. Romney, and V. Q. Hale. 1973b. Sodium relations in desert plants. 1. Cation con- tents of some plant species from the Mojave and Great Basin deserts. Soil Sci. 115:284-287. FREQUENCY DISTRIBUTION AND CORRELATION AMONG MINERAL ELEMENTS IN LYCIUM ANDERSONII FROM THE NORTHERN MOJAVE DESERT A. Wallace1, E. M. Romney1, G. V. Alexander1, and J. E. Kinnear1 Abstract.— Two hundred samples of leaves of Lycium andersonii A. Gray, each representing one plant and di- vided among six different locations, were assayed by emission spectrography. Information for 12 different elements is reported in terms of concentrations, frequency distribution, correlations, and some soil characteristics. The objective was to ascertain the nature of variability for mineral elements within a species. Composition varied significantly for all 12 elements among locations, all within about 20 km. At least part of the variation was due to soil characteristics. Samples from Rock Valley were highest in K, Na, and Li, which effect is associated with volcanic outcrop. Samples from Mercury Valley were highest in P, Mg, Ba, and B. At least Mg is related to the soil composition. Correlation coefficients between element pairs were often very different for all 200 samples versus those obtained for individual locations. Some of the values for all 200 samples together proved to be artifacts. The highest correlation was for Ca X Sr (positive) and next was Ca X Mg (also positive). Most correlations were slightly or strongly positive (24 of 32). Only P X Ca, Ca X Na, Ca X B, and Sr X P seemed to be significantly negative of the 32 correlations examined. Frequency distribution patterns where common populations were grouped were often normally distributed. Li, as previously reported, and Na, Cu, Mn, and B and Ba at some locations were not normally distributed. Wide varia- tions in the concentrations of individual elements in leaves of these species were encountered. Mineral composition of the plants in any ecosystem is one of its distinguishing charac- teristics. The essential nature of at least 13 mineral elements for plants, with their abun- dance in soil, in many cases helps to deter- mine the nature of the vegetational pattern. The same also can be said for some nones- sential elements. In fact, excesses of both es- sential and nonessential elements largely de- termine vegetational characteristics under many conditions, and this occurrence is very common in desert ecosystems where young, poorly leached soils are usually involved (Fuller 1975, Romney et al. 1973). The purpose of this report is to explain in some detail the mineral compositon of leaves of one plant species occurring with fair abun- dance in the northern Mojave Desert. The species, Lycium andersonii A. Gray, accumu- lates relatively high levels of Ca and Li and characteristically avoids salinity (Romney et al. 1973, Wallace et al. 1973, Ashcroft and Wallace 1976). Such data also would help to indicate the presence of ecotypes. Data for Li in these plants were reported previously (Romney et al. 1977). Materials and Methods Lycium andersonii samples were collected in May 1976 from six different areas in the southern portion of the Nevada Test Site (northern Mojave Desert). The areas were Mercury Valley, west Mercury Valley, Rock Valley, base of Skull Mountain in Rock Val- ley (near 410 road), Frenchman Flat, and southwest Frenchman Flat. Each sample con- sisted of about 2 g of dry leaves that involved about 2000 individual leaves for each sample. There were 33 or 34 samples from each loca- tion and 200 total samples for all the loca- tions. Each sample represented a single plant. Samples were collected just after a series of rains and otherwise were not washed (Al and Ti analysis indicated minimum con- tamination by soil). The samples were dried, weighed, ground in a plastic mill, and other- wise prepared for analysis by emission spec- trography. The soils characteristics from these areas are detailed in the report of Romney et al. (1973). 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 90024. 146 1980 Nevada Desert Ecology 147 Results and Discussion The mineral composition of leaves from all six locations differed for all 12 elements in- cluded in this report (Table 1). The samples from Mercury Valley were highest in P, Mg, B, and Ba. Rock Valley, which is partly over- lain with volcanic material and igneous out- crops (Beatley 1976) had leaves with the highest Na, K, and Li. The Rock Valley samples were also lowest in P, Fe, and Mn, and the southwest French- man Flat location was lowest in Cu, Sr, Ba, and Li. The variability in composition from loca- tion to location was largely due to variations Phosphorus 18 .30 .42 15 10 i I Magnesium \ \ .8 1.0 1.2 1.4 Potassium i ' i ' i ' i i i i i i — i i r 230 260 290 320 350 /ig/g dry wt. Fig. 1. Frequency distribution of K, Ca, P, Mg, and Fe in indicated groupings of locations for L. andersonii leaves in which groupings are not statistically different according to analysis of variance. (K = WM, F, M, 410; Ca = WM, SWF, 410; P = F, SWF, 410; Mg = WM, SWF, 410; Fe = RV, WM, M). See Table 1 for meanings of locations. 148 Great Basin Naturalist Memoirs No. 4 Table 1. Mineral composition of May 1976 leaves of Lycium andersonii from six different locations in the north- ern Mojave Desert. Mercury French nan SW Frenchman W. M 3rcury Mean S.D. Mean S.D. Mean S.D. Mean S.D. P, Mg/g 2030a 884 1385b 795 1579b 897 1015c 372 Na, % 0.044 0.070 0.024b 0.016 0.025b 0.004 0.148b 0.177 K, % 3.42ab 0.972 3.32b 0.751 2.70c 0.900 2.98bc 0.893 Ca, % 6.72d 1.83 9.47b 1.43 10.74a 1.64 10.51a 1.77 Mg, % 1.78a 0.24 1.06b 0.18 0.94c 0.12 0.88v 0.34 Cu, jig/g 2.58c 1.11 2.97bc 1.28 1.85d 0.84 4.38a 2.34 Fe, /xg/g 292cd 23 313b 42 364a 31 287d 38 Mn, [ig/g 47b 27 128a 50 87a 27 47b 28 B> Mg/g 41.5a 18.5 28.4c 5.8 36.5ab 7.6 35.9ab 10.2 Sr, Mg/g 628b 106 550c 101 429d 70 737a 137 Ra, /xg/g 56a 17.9 40bc 20.7 34d 4.6 34c 7.0 Li, Mg/g 38.5ab 31.0 22.3cd 20.4 14.6d 14.3 44.8abc 36.5 Cation sum me/100 g 574 647 686 682 "Values for each element followed bv a common letter are not statistical different at the 0.05 level. Table 2. Correlation coefficients for pairs of elements of Lycium andersonii leaves from different locations in the northern Mojave Desert. Pairs P X Ca P X Cu P X B P X Fe K X Na K X Ca K X Sr K X Ba K X Li Ca X Na Ca X Mg Ca X Fe Ca X Mn Ca X B Ca X Sr Ca X Ba Ca X Li Na X P Na X Sr Na X Li Mg X Sr Mg X Ba Mg X Li Fe X Mg Fe X Mn Fe X Sr Fe X Ba Fe X Li Sr X Ba Sr X P Sr X Li Ba X Li *r = 0.14 sig at 0.05 for all locations; 0.33 for individual locations. Mean All for 6 SW locations locations Mercury Frenchman Frenchman 194-200 194-200 33 33 34 -0.28 -0.29 -0.40 -0.12 -0.51 + 0.16 0.40 0.11 0.68 0.77 + 0.23 0.21 0.04 0.06 0.38 + 0.14 0.00 -0.06 + 0.48 -0.03 0.30 0.22 0.22 -0.04 0.65 -0.20 -0.07 0.17 -0.27 -0.19 0.25 0.08 -0.13 -0.15 0.09 0.15 0.01 -0.08 0.05 0.10 0.29 0.20 0.37 0.29 0.06 -0.26 0.16 0.05 -0.29 -0.43 -0.37 0.46 0.04 0.60 0.57 0.34 0.22 0.16 0.22 0.17 0.21 0.17 -0.01 0.32 0.21 -0.13 -0.17 -0.19 -0.26 -0.10 0.09 + 0.50 0.58 0.47 0.60 -0.25 0.23 0.46 -0.26 0.01 -0.08 0.05 -0.11 0.27 -0.11 -0.25 0.13 0.01 -0.09 0.64 0.33 -0.04 0.20 0.04 -0.22 0.32 0.14 0.15 -0.26 0.33 0.14 + 0.32 0.20 0.48 0.24 0.48 0.07 0.04 0.1 1 0.01 0.12 0.32 0.18 0.71 0.17 -0.09 0.32 0.20 0.48 0.24 0.37 0.19 0.07 0.33 0.36 -0.20 0.42 0.61 0.39 0.1:5 -0.11 0.31 0.26 0.31 0.13 -0.15 0.11 0.11 0.33 -0.01 0.27 0.21 0.20 -0.12 0.41 -0.26 -0.18 -0.38 0.14 -0.36 0.19 0.02 -0.19 0.28 -0.06 0.10 0.11 -0.01 -0.02 0.36 1980 Nevada Desert Ecology 149 Table 1 continued. Rock Valley Highway 410 F ProbF Mean S.D. C.V. LSD Mean S.D. Mean S.D. ratio extended of all of all of all 0.05 957c 407 1525b 463 11.1 0.0000 1423 761 53.5 327 0.951a 0.525 0.0.33b 0.039 87.1 0.0000 0.198 0.399 201.5 0.108 3.91a 1.356 3.45ab 1.107 5.6 0.0001 329 1.07 32.5 0.49 7.63c 1.73 10.77a 1.54 36.3 0.0000 9.34 2.29 24.5 0.80 1.13b 0.20 0.90c 0. 13 105.8 0.0000 1.19 0.36 30.3 0.09 2.72bc 0.99 3.37b 1.04 1.3.1 0.0000 2.98 1.56 52.3 0.66 282d 26 305bc 36 27.1 0.0000 307 43 40.2 16.1 41b 16 46b 19 44.8 0.0000 66 43 34.6 14.4 32.5bc 11.3 38.8ab 12.6 5.21 0.0002 35.6 12.3 34.6 5.63 756a 109 655b 160 36.2 0.0000 625 161 25.8 56.7 39bc 7.2 41b 10.6 29.2 0.0000 39 15.1 38.7 5.6 54.5a 45.4 30.9bcd 32.8 4.9 0.0004 36.3 35.3 97.2 16.1 703 lC5i Table 2 continued. West Rock Hwy No. Mercurv Valley 410 loc. Sig. of Location sig. at all 32 34 34 + - 0.05 loc. -0.28 -0.32 -0.08 0 6 2 0.01 0.37 0.37 0.10 6 0 4 0.05 0.10 0.20 0.48 6 0 2 0.01 + 0.09 -0.28 -0.18 2 4 1 0.05 0.23 0.30 -0.05 4 2 1 0.01 0.16 0.01 0.0.3 3 3 0 0.01 0.01 0.17 0.47 4 2 1 0.01 -0.21 0.14 0.05 4 2 0 0.05 -0.04 0.54 -0.01 4 2 2 0.01 -0.15 0.03 -0.16 2 4 1 0.01 0.56 0.55 0.44 6 0 5 0.01 0.05 0.33 0.39 6 0 2 0.01 0.21 0.16 0.13 5 1 0 0.01 0.19 -0.63 0.04 2 4 1 0.10 0.38 0.55 0.44 6 0 6 NS 0.22 0.60 0.37 5 1 3 0.01 -0.24 0.27 0.21 3 3 0 NS 0.00 0.04 0.20 4 1 1 0.01 -0.16 -0.05 -0.07 2 4 0 0.01 0.57 0.15 -0.10 4 2 2 0.01 0.11 0.58 0.33 6 0 3 0.05 -0.11 0.36 0.01 5 1 1 0.01 0.16 0.31 0.41 6 0 2 NS 0.11 0.58 0.33 6 0 3 NS 0.12 0.13 0.12 6 0 2 0.01 0.50 0.54 0.32 6 0 4 0.01 0.23 0.52 0.41 6 0 2 NS -0.08 -0.02 0.31 3 3 1 0.01 0.40 0.57 0.18 5 1 3 0.01 -0.08 -0.41 0.03 2 4 3 0.01 -0.19 0.27 -0.21 2 4 0 0.01 0.03 0.16 0.16 4 2 1 NS 150 Great Basin Naturalist Memoirs No. 4 in the edaphic characteristics (Romney et al. 1973). The area with high Mg in leaves (and low Ca) had high available Mg in soil (Rom- ney et al. 1973). The relationship between Ca and Mg was not always simple. For each of the locations except one, the correlation coefficient ob- tained when Ca and Mg were correlated was strongly positive (Table 2). The one not sig- nificant was at the location having highest Mg and the lowest Ca (r = + 0.04), so even then the relationship was not inverse. When all 200 samples were included in a common correlation, however, the r was -0.37 com- pared with a mean of +0.46 for the six loca- tions determined individually. The overall r then must be considered as an artifact and in- dicates possible erroneous conclusions that can be made when correlation coefficients are obtained for large variable populations. Most of the 32 correlation coefficients in Table 2 were positive (24 of them as the av- erage of the 6 locations). This generally con- forms to the report of Garten (1976) for data elsewhere. Consistent and important negative 35 30 25 20 15- {2 io H i 5 _i Q. \f / Manganese L UJ CD 5 25 Z 20 15 10 30 50 70 90 110 130 150 /xg/g 10 i- VTti \ In Manganese \ k^ 3.0 3.4 3.8 4.2 4.6 5.0 In /xg/g Fig. 2. Frequency distribution of Mn in L. andersonii leaves (RV, 410, M, and WM for arithmetic and the same grouping for In of Mn concentration). See Figure 1 for further explanation. correlations were obtained for P X Ca, Ca X Na, Ca X B, and Sr X P. There are known physiological bases for some of these. In addition to Ca X Mg, other strong posi- tive correlations existed between P and Cu (r = +0.40), Ca X Sr (r = +0.50), Mg X Sr (r = +0.32), and Fe X Sr (r = +0.42). Frequency distribution patterns of the ele- ments were obtained for groups of locations where analysis of variance data indicated that no differences existed between or among the particular locations. This permitted the use of as many as all samples (200) and, at least, about one-half of them in a frequency distribution determination. Where normal distribution was not apparent, data were also plotted as logarithm-normal. The histograms (Fig. 1) for Ca, Mg, P, K, and Fe with n of around 90 showed normal distribution (Table 3). Manganese did not show a normal distri- bution (Fig. 2), but it did on the In normal basis (Fig. 2 and Table 3). Two of three B groupings gave a normal distribution (Fig. 3 and Table 3); a third grouping gave a In nor- mal distribution (Fig. 3, Table 3). The Cu concentrations of these L. ander- sonii plants were low in comparison to most plant species. The values were lower than those found for L. andersonii collected over a wider area (Wallace and Romney 1972). The Cu values were not normally distributed (Fig. 4) but skewed toward the smaller values (Table 3). When all six sites were combined, a In normal distribution was obtained even though there were four distinct populations (Table 1). Part of this Cu variation in distri- bution could be analytical. Two Ba groupings gave a normal distribu- tion and one did not (Fig. 5, Table 3). Again the In normal gave a distribution which could not be rejected as normal (Fig. 5, Table 3). Two Sr groupings gave normal distribu- tion (Fig. 6, Table 3). In the former study of Li where distribu- tion was neither normal nor In normal (Rom- ney et al. 1977), differential distribution of Li in soil was given as an explanation. In one of the present data groupings, however, Li did give a In normal distribution (Fig. 7, Table 3). It would appear that this species tends to- ward a normal distribution of metals, but that soil variation shifts to other types of distribu- tion. 1980 Nevada Desert Ecology 151 Table 3. Evaluation(l) of normality of frequency distribution histograms (Figs. 1-7) Mean Chi2 goodness of fit Element percent Locations n Test of normality Skewness Kurtosis(2) P 0.150 FR,SWF,410 98 Cannot reject 1.2407° ° 0.7023° ° Na 0.0550 All except RV 168 Reject 4.2458° • Not tested K 3.291 WM,F,M,410 134 Cannot reject 0.9592 0.8031 Ca 10.673 WM,SWF,410 102 Cannot reject -0.2688 0.8214 Mg 0.903 WM,SWF,410 102 Cannot reject 0.5005°° 0.7933 Cui 2.8 MRV.FM 98 Reject 0.7430°° 0.7932 Cu2 3.0 410,RV,FM 99 Reject 0.5837°° 0.8135 Cu3 2.9 410,RV,FM,M 132 Reject 0.5866°° 0.8178 C114 3.0 ALL 199 Reject 1.4078°° 0.7494°° Fe 287 RV,WM,M 99 Cannot reject -0.1201 0.7719 Mn 44.2 RV,WM,M,410 133 Reject 1.5509°° 0.7475°° B, 36.0 RV,WM,410,SWF 134 Cannot reject 1.6832°° 0.7069°° B2 37.5 WM,M,410,SWF 134 Reject 1.8032°° 0.7008°° &3 36.5 WM,RV,M,410,SWF 166 Cannot reject 1.5353°° 0.8875°° Sri 637 M,410 67 Cannot reject 0.9195°° 0.7754 Sr2 746 WM.RV 66 Cannot reject -0.2714 0.8198 Baj 364 WM,RV,F 99 Reject 0.2980 0.8069 Baa .38.8 RV,F,410 99 Cannot reject 0.5407°° 0.7726 Ba3 37.7 WM,RV,F,410 133 Cannot reject 0.5526 0.7837 Li, 23.9 F,410,SWF 71 Reject 2.3171°° 0.7028°° Li2 31.6 F,410,M 78 Reject 1.5474°° 0.7631 Li3 37.9 WM,M,410 90 Reject 1.1725°° 0.7918 Li4 46.4 RV,WM,M 88 Reject 0.9633°° 0.8054 In 1 44 3.391 RV(WM,M 88 Cannot reject -0.6095° ° 0.8127 (1) Statistical significance level for all tests is 5 percent. ° "Indicates significance (2) Alternate lcurtosis index proposed for N < 200 by R. C. 28, 295 (1936) (Index is about 0.80, depending on sample size. Geary. See Snedecor & Cochran Statistical Methods, 6th ed., Table of probability points is in reference). and R. C. Geary, Biometrika Boron 2 48- 4?- t -\ In Boron 2 36- 30- 24- f \ 18- 12- 6- J V. 100 2.6 3.0 3.4 3.8 4.2 4.6 ln>i.g/g Fig. 3. Frequency distribution of B in leaves of L. andersonii (Boron 1 = RV, WM, SWF, 410; Boron 2 = WM, SWF, 410, M; Boron 3 = RV, WM, SWF, 410, M). See Figure 1 for further explanation. 152 Great Basin Naturalist Memoirs No. 4 A cluster tree for 21 elements in all sam- ples of L. andersonii leaves is shown in Fig- ure 8. Calcium, the dominant mineral ele- ment in L. andersonii leaves, clusters with Cr. These in turn cluster closely with the so- called dust elements Fe, Ti, Al, Si, and in this case also Mn. The trace metal Li that is prominent in L. andersonii (the species is an accumulator of Li) clusters with another monovalent metal, Na, which also is in L. an- dersonii in trace quantities only. These two elements are joined by the monovalent K, which is present in leaves of this species at levels of from about 2 to 5 percent. These three elements later join with Cu, V, and Sr. Mg and Ba are clustered and these join with 25 20 15 10 5 /- Copper I Copper 2 1.0 2.0 3.0 4.0 5.0 6.0 i rn i rn i in i ti i in .9 1.8 2.7 3.6 4.5 5.4 6.3 Copper 4 all sites .9 1.8 2.7 3.6 4.5 5.4 6.3 ^g/g n ii ii ii i i i i 1.0 3.0 5.0 7.0 9.0 Mg/g 2 5-1 In Copper 4 20 n. ,arrl Zn, Mn, Fe, and ( u in 50 samples of Atriplex hymenelytra leaves. 1980 Nevada Desert Ecology 159 Sodium 6- 4- 2- n- / / / \ Strontium 2.5 4.0 5.5 7.0 8.5 10.0 11.5 13.0 % dry weight Barium 200 400 600 800 /ig/g dry weight 0 5 10 15 20 25 30 /ig/g dry weight Fig. 3. Frequency distribution of B, Na, Sr, and Ba in 50 samples oiAtriplex hymenelytra leaves. Silicon 0 15 30 45 60 75 90 105 0 1.0 2.0 3.0 4.0 5.0 6.0 /^g/g dry weight h-Q^Q dry weight Fig. 4. Frequency distribution of Al, Si, Ti, and V in 50 samples oiAtriplex hymenelytra leaves. 160 Great Basin Naturalist Memoirs No. 4 Table 2. Correlation matrix for the pairs of elements indicated in the analysis of Atriplex hymenelytra" . Na Ca Mg Zn Cu Fe Mn Na -0.188 K -0.049 0.175 Ca -0.376 -0.361 0.172 Mg -0.326 -0.163 0.175 0.492 Zn 0.417 0.039 0.206 -0.042 -0.372 Cu 0.521 -0.092 0.289 -0.058 -0.228 0.462 Fe 0.155 -0.075 -0.153 -0.081 0.257 -0.147 0.002 Mn 0.423 -0.153 0.083 -0.125 -0.074 0.214 0.473 -0.126 B 0.120 -0.127 -0.378 -0.131 -0.110 -0.192 -0.303 0.405 -0.232 Al 0.168 -0.104 -0.202 -0.017 0.294 -0.161 -0.020 0.920 -0.118 0.391 Si 0.260 -0.133 -0.204 -0.131 0.209 -0.148 0.060 0.862 -0.102 0.361 Ti 0.122 0.038 -0.124 -0.135 0.228 0.212 -0.065 0.861 -0.180 0.376 V -0.398 0.415 0.218 0.263 0.293 -0.295 -0.278 -0.033 -0.287 -0.119 Mo 0.3807 0.077 0.338 -0.091 0.078 0.075 0.577 -0.177 0.487 -0.418 Sr -0.255 -0.126 -0.111 0.185 0.213 -0.277 -0.225 0.056 -0.103 -0.100 Ba 0.061 0.040 -0.074 -0.114 0.408 -0.256 0.013 0.719 0.135 0.138 Li 0.076 -0.064 -0.040 -0.079 -0.146 -0.168 -0.205 -0.294 0.330 0.108 Pb 0.073 0.412 0.165 -0.287 -0.012 0.163 0.180 0.068 -0.026 -0.128 Al Si Ti V Mo Sr Ba Li Si 0.920 Ti 0.861 0.805 V -0.007 -0.124 0.051 Mo -0.240 -0.198 -0.161 -0.128 Sr 0.070 -0.037 0.118 0.206 0.129 Ba 0.718 0.696 0.677 0.062 0.033 0.118 Li -0.277 -0.195 -0.249 0.002 0.022 -0.245 -0.306 Pb 0.024 0.062 0.044 0.194 -0.007 -0.142 0.177 -0.061 'A value of ± 0.168 needed for significan ce at the 0.05 level. 2- Molybdenum l6-i 14- Lithium o ■ 12- 8 ■ 6- io- 8- / <* \ 4- 2- o- n ,n n 6- 4- 2- / \ \ V en \- < _l 1 rH-4- 1 r— 1 1 Q- t -i— O or 0 1.5 3.0 4.5 6.0 75 9.0 10.5 0 10 20 30 40 50 H- g/q dry weight 8 12 16 20 ^.g/g dry weight Fig. 5. Frequency distribution of Mo, Li, and Pb in 50 samples ol Atriplex hymenelytra leaves. 1980 Nevada Desert Ecology 161 TREE PWIMTEU OVER CORRELATION MATRIX (SCALED 0-100). CLUSTERING Br AVERAGE DISTANCE METHOD. VARIABLE NAME NO. . / P (?) 76 68 71/70/53/40 30 53 47/31 33 37 57 58 63 56 53 56/ / / / / / / / / / / CU ( 8) 78/73/73/39/45 36 58 64/47 38 38 50 49 53 46 50 34/ / / / / / / / / / / / / MO ( l6)/74/53/51/46 43 49 66/45 53 56 41 37 40 41 51 29/ / / / / / / / / / / MN ( 101/60/66/42 35 48 54/<»3 46 4*. 43 44 44 41 43 38/ / / / / / / / / ZN ( 7)/41/52 35 58 60/47 31 36 42 41 42 39 37 40/ / / / / / / LI ( 19) /46 50 46 43/46 42 37 35 36 40 37 34 55/ / / / / NA ( 3) 70/70/58/31 41 43 46 44 43 51 52 43/ / / / / / / / / V ( 15)/59/60/63 64 60 48 49 43 52 53 44/ / / / / / / PB ( 20)/58/35 49 42 46 51 53 52 58 43/ / / / / K ( 4)/58 58 44 42 39 39 43 46 31/ / / CA ( 5) 74/59/45 49 43 43 44 43/ / / / / / / MO ( 6)/60/62 64 60 61 70 44/ / / / / SR ( 17J/52 53 48 55 55 45/ / / FE ( 9) 95 S#3/93/85/70/ / / / / / / / / AL ( 12) 95/93/85/69/ / / / / / / / / SI ( 13)/90/64/68/ / / / / / / TI ( l4)/83/68/ / / / / BA ( 18>/56/ / / B ( 11)/ Fig. 6. Cluster analyses tree described from the correlation matrix. The values in this tree have been scaled 0 to 100 according to the following: Value above 0, correlation -1.000; value above 5, correlation -0.900; value above 10, correlation -0.800; value above 15, correlation -0.700; value above 20, correlation -0.600; value above 25, correlation -0.500; value above 30, correlation -0.400; value above 35, correlation -0.300; value above 40, correlation -0.200; value above 45, correlation -0.100; value above 50, correlation 0.000; value above 55, correlation 0.100; value above 60, correlation 0.200; value above 65, correlation 0.300; value above 70, correlation 0.400; value above 75, correlation 0.500; value above 80, correlation 0.600; value above 85, correlation 0.700; value above 90, correlation 0.800; value above 95, correlation 0.900. 162 Great Basin Naturalist Memoirs No. 4 the normality of each of the histograms is presented in Table 1. Even though the sam- ples were collected over a range of about 150 km, normality could not be rejected for sev- eral of the elements. Included were P, Ca, and Mg (Fig. 1), Zn and Mn (Fig. 2), Na, Sr, and Ba (Fig. 3), and V (Fig. 4). The mean Na concentration was 8.79 per- cent. The C.V. of this value was 24.6 percent, which was, except for V, lowest of the ele- ments. Only 20 percent of this 24.6 percent was due to analytical variance. The frequen- cy distribution for Na gave a normal curve (Fig. 3). It is of interest that all the samples from the collection covering about 150 km resulted in a uniform population for Na. It must be recognized that part of the Na would be on the leaf surface due to salt glands (Jones and Hodgkinson 1970). The cluster analysis (Fig. 6) showed a marked relationship among the "dust" ele- ments Fe, Al, Si, and Ti. An explanation of the variable clustering process as shown in the diagram (Fig. 6) follows: the process be- gins with the cluster consisting of variable Cu (8), the second variable listed in the diagram. This cluster joins with the cluster below it consisting of the variable Mo (16). The new cluster is indicated on the figure by the inter- section of the dashes beginning above vari- able Cu (8), with the slashes starting next to the variable Mo (16). This cluster joins with the cluster below it consisting of the variable Mn (10). The new cluster is indicated on the tree by the inter- section, of the dashes beginning above vari- able Cu (8), with the slashes starting next to variable Mn (10). This cluster joins with the cluster above it consisting of the variable P (2). The new clus- ter is indicated on the tree by the inter- section of the dashes beginning above vari- able P (2) with the slashes starting next to variable Mn (10). This cluster joins with the cluster below it consisting of the variable Zn (7). The new cluster is indicated on the tree by the intersection of the dashes beginning above variable P (2), with the slashes starting next to variable Li (19). This cluster joins with the cluster below it consisting of the variables Na (3) down to K (4). The new cluster is indicated on the tree by the intersection of the dashes beginning above variable P (2), with the slashes starting next to variable K (4). The process continues until each variable is joined to at least one other variable. Twenty-seven significant negative correla- tion coefficients were observed among pairs of elements (Table 2). This is a greater pro- portion than observed by Gartner (1976) for East Coast vegetation. Acknowledgments This study was supported in part by Con- tract EY-76-C-03-0012 between the U.S. De- partment of Energy and the University of California. Literature Cited Gartner, C. T., Jr. 1976. Correlations between concen- trations of elements in plants. Nature 26:686-688. Hunt, C. B. 1966. Plant ecology of Death Valley, Cali- fornia. U.S. Geol. Surv. Prof. Paper 509. Jones, R., and K. C. Hodgkinson. 1970. Root growth of rangeland chenopods: Morphology and produc- tion of Atriplex nummularia and Atriplex fes- icaria. Pages 77-85 in R. Jones, ed. Studies of the Australian Arid Zone— the biology of Atriplex. Div. Plant Ind. Commonwealth Sci. Ind. Res. Or- gan, Canberra, Australia. Romney, E. M., V. Q. Hale, A. Wallace. O. R. Lint, J. D. Childress, H. Kaaz, G. V. Alexander, J. E. Kinnear, and T. L. Ackerman. 1973. Some char- acteristics of soil and perennial vegetation in northern Mojave Desert areas of the Nevada Test Site. UCLA #12-916. Wallace, A., and E. M. Romney. 1972a. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Technical Information Ser- vice, USAEC Report TID-25954. 1972b. Characteristics of Atriplex species (salt bushes). Pages 168-187 in A. Wallace and E. M. Romney, eds. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Technical Information Service. USAEC Report TID-25954. Wallace, A., E. M. Romney, G. V. Alexander, and J. E. Kinnear. 1980. Frequency distribution and correlation among mineral elements in Lycium andersonii from the northern Mojave Desert. Great Basin Nat. Mem. 4:144-153. Wallace, A., E. M. Romney, and V. Q. Hale. 1973a. Sodium relations in desert plants: 1. Cation con- tents of some plant species from the Mojave and Great Basin deserts. Soil Sci. 115:284-287. Wallace, A., R. T. Mueller, and E. M. Romney. 1973b. Sodium relations in desert plants. 2. Dis- tribution of cations in plant parts of three differ- ent species of Atriplex. Soil Sci. 115:390-394. FIELD STUDIES OF MINERAL NUTRITION OF LARREA TRIDENTATA: IMPORTANCE OF N, pH, AND Fe R. B. Hunter1, A. Wallace1, and E. M. Romney1 .Abstract.— Multivariate analysis of soil and plant data from the northern Mojave Desert was used to investigate aspects of the mineral nutrition of Larrea tridentata (Sesse & Moc. ex DC.) Cov. Larrea tridentata biomass was signif- icantlv correlated with soil NO5 and pH and leaf Fe content. Leaf cation accumulation was negatively correlated with leaf Fe concentration. There are several hypotheses for the often strikingly discontinuous distribution of Larrea tridentata (Sesse & Moc ex DC.) Cov. in southwestern U.S. deserts. Beatley (1974) sug- gested that absence of L. tridentata from playas of the Nevada Test Site in the north- ern Mojave Desert is due to limiting cold during winter temperature inversions. Elimi- nation from playas by occasional flooding (Wallace and Romney 1972) would be re- lated to root oxygen deprivation, which has been studied by Lunt et al. (1973). Hallmark and Allen (1975) studied 11 west Texas soil variables and found weak correlations of L. tridentata distribution with lime and gravel content. Barbour (1970) found no significant effects of pH and salinity changes across L. tridentata ecotone lines, although germina- tion of L. tridentata was related to salinity. Romney et al. (1973) published a volume of soil, plant, and meteorological data ex- haustively describing 78 sites in the Mojave Desert and Mojave-Great Basin transition zones of the Nevada Test Site. Fifty of these sites support a L. tridentata population. For this study we used these data to investigate edaphological factors involved in L. triden- tata mineral nutrition and plant size. Methods Programs for multivariate statistical analy- ses-correlation matrices, multiple linear re- gression, and principal component analysis were prepared by Dixon (1971). The analyses were run for 49 of the 50 sites, because one site that lacked biomass data for L. tridentata was deleted. Sum of cations and cations minus N were the sums in me/ 100 g of leaf K, Na, Mg, and Ca, with me N/100 g subtracted in the latter case. "Dust" contamination of several elements was calculated using a simple linear regres- sion line of leaf concentration of the element versus either Si or Al, whichever correlated most strongly. The residual of the equation was assumed to represent "metabolic" con- tent (abbreviated "meta"), and the slope times the Si or Al concentration was consid- ered contamination. (The terms dust and metabolic express one of several possible in- terpretations of these factors.) Soil depth was considered either the deep- est point recorded or the depth to a caliche hardpan. Results Table 1 lists means and standard deviations of the variables on 49 L. tridentata-inhabited sites used in the subsequent analyses. The soils are very gravelly, high in lime, and in some cases underlain by a caliche hardpan. The pH fluctuates narrowly near 8.3. Above- ground biomass ranged from 932 to 3726 kg/ha, and L. tridentata biomass ranged from 9 to 1664 kg/ha. Leaf sum of cations aver- aged 158 me/ 100 g, and cations minus N av- eraged -10 me/ 100 g, indicating approx- 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 90024. 163 164 Great Basin Naturalist Memoirs No. 4 Table 1. Averages for variables analyzed in this study. Measurements were made at 49 sites in the northern Mo- jave Desert. Variable Avg ± sd Unit Variable Avg ± Unit Community parameters Soil variables* Total biomass 1859 ± 770 kg/ha Clay Total density 10 ± 5 thousands/ha Silt Larrea biomass 714 ± 349 kg/ha Organic C Larrea size 757 ± 300 g/plant Organic Larrea leaf minerals Saturation extract N 169 ± 20 me/lOOg pHAj P 0.21 ± 0.06 % pHC, Na 0.05 ± 0.03 % EC25 Si 0.3 ± 0.1 % K 56 ± 18 me/100 g CEC Ca 80 ± 21 me/lOOg Na Mg 19 ± 6 me/ 100 g K Sum of cations 158 ± 34 me/ 100 g Ca Dust corrected sum of cations 152 ± 34 me/lOOg so4= Cations - N -10 ± 34 me/100 N03 -N Dust Mg 4 ± 2 «g/g Dust Na 1.1 ± me/lOOg Zn 25 ± 7 ug/g Highest Na Cu 3.2 ± 1.9 ug7g Fe 384 ± 171 ug/g Highest NO3 Meta Fe 183 ± 125 ug/g Highest NO3 below A] Dust Fe 204 ± 98 ug/g Mn 40 ± 11 ug/g DTPA extract B 79 ± 20 "g/g Cu Al 537 ± 261 "g/g Fe NaHC03 extract P 4 ± 3 % 7 ±4 % 0.3 ± 0.2 % 0.04 ± 0.02 % 8.4 ± 0.4 8.6 ± 0.3 1.0 ± 0.9 mmho/cm 12 ± 4 me/ 100 g 3 ± 4 me/1 3 ± 2 me/1 7 ± 9 me/1 1 ± 2 me/1 13 ± 32 "g/g 14 ± 20 67 ± 88 50 ± 91 ug/g "g/g ug/g 0.13 ± 0.06 ug/g 0.22 ± 0.17 ug/g 1 ± 1 "g/g aAll soil variables are for the Cj horizon except pH A and those "highest" values from which the greatest value measured at the site was used. imate equality between nitrogen and cation milliequivalents. Variables correlating significantly (p = <0.05) with L. tridentata biomass and sum of cations are presented in Table 2. These inde- pendent variables "explain" generally less than 20 percent of the variability in biomass. Total leaf Fe, meta Fe, and leaf P and Zn correlated unusually strongly with the sum of cations. Of the three soil variables studied (Table 2), C,NC>3 correlated most strongly with plant size. The Ci refers to soil horizon. Table 3 shows variables that correlated sig- nificantly (p = <0.05) with leaf Fe frac- tions. Of the soil variables, only depth and silt content correlated significantly with meta Fe, clay with dust Fe, and soil depth with overall leaf Fe. Soil Fe, as extracted by DTPA, did not correlate significantly with any leaf Fe variable. Table 4 presents the results of multiple lin- ear regression of independent variables versus L. tridentata biomass per plant and sum of cations. The analyses were run in such a way that no variable had an F-to-enter < 4.0. With three variables entered, 44 percent of the plant size variability is explained, and metabolic Fe variations explained 36 percent of the sum of cations. The first 2 of 20 principal components of Table 2. Correlation coefficients (r) of selected varia- bles correlating significantly (p = <0.()5t with L. tri- dentata biomass and leaf cations. L. tridentata Leaf Variables Biomass/plant Sum of cations C.pH -0.30 0.29 QNO^g/g 0.45 ns Soil EC25° 0.39 ns Cj Ca 0.43 ns Soil depth cm 0.39 ns C, Zn -0.36 ns Leaf Fe -0.36 -0.40 Meta Fe -0.37 -0.56 Leaf P ns 0.47 Leaf Zn ns 0.50 1980 Nevada Desert Ecology 165 Table 3. Correlation coefficients of variables significantly (p = <0.05) correlated with leaf Fe fractions and DTPA extractable soil Fe. Leaf Fe Dust Fe Meta Fe Sum of leaf cations Leaf Na Leaf K Leaf Ca Leaf Si Leaf Al Leaf Mn Leaf P Leaf Zn Dust Fe Meta Fe Larrec-Biomass/plant Overall Biomass/ha Depth Cj Clay C] Silt Organic C Organic N Water holding capacity -0.40 ns -0.56 ns 0.39 0.54 ns ns -0.50 ns -0.56 ns -0.31 ns -0.47 ns 0.70 0.87 ns ns 0.70 1.00 ns ns 0.48 0.59 ns ns ns ns -0.43 ns ns ns -0.46 ns 0.70 1.00 ns ns 0.82 ns 1.00 ns -0.36 ns -0.37 ns -0.30 ns -0.32 ns -0.32 ns -0.35 ns ns 0.31 ns 0.40 ns ns 0.30 0.47 ns ns ns 0.55 ns ns ns 0.53 0.29 ns ns 0.47 ition ac- Discussion L. tridcntata leaf mineral composition ac- counted for 46 percent of the total variance. Variables scoring highest on the first com- ponent were leaf Fe and sum of cations. Those scoring highest on the second com- ponent were cations minus N and leaf Al. Analyses of community and soil variables showed a diffuse distribution of variance among the factors studied. Soil pH correlated significantly with per- cent clay (r = +0.30) and saturation extract Mg (r = -0.40), besides the correlations with L. tridentata size and sum of cations (Table 2). Both saturation extract and paste pH were measured. Paste pH did not correlate signifi- cantly with sum of cations and was not con- sidered for the bulk of this study. There are a multitude of variables affect- ing size of L. tridentata plants in the field. Several important factors not considered here include rainfall, plant age, incidence of graz- ing, and competition with neighboring shrubs. Because these data are from the field, no variable was controlled. We thus feel jus- tified in imputing significance to variables that can explain just 10 to 20 percent of the variability in plant size. Of the three factors correlating strongest with plant size, only the first, QNO3 concen- tration, is easily explained. The correlation implies that NO3 levels, measured at single points, limit plant growth, and that they are Table 4. Multiple linear regressions of independent variables affecting Larrea tridentata biomass0 per plant and leaf sum of cations.00 Step Variable added Coefficient Multiple r2 Sum of leaf cations, me/100 g = 181.5 - 0.16 (Meta Fe) 1. MetaFe -0.16 0.36 Constant A. Biomass per plant (g) = 3.47 (C,N03) - 1.13 (Meta Fe) - 376.8 (pH) - 4139 1. C,N03 3.47 0.20 2. MetaFe -1.13 0.33 3. C,pH -376.8 0.44 688 865 4139 No other independent variables had significant F value to enter. 'Larrea tridentata biomass per hectare was deleted. "Deleted variables were eation-N, leaf K, leaf Ca, and leaf Mg. 166 Great Basin Naturalist Memoirs No. 4 representative of' those over the whole 100 m2 transect used to determine plant size. The data are also consistent with the distribution of shrub roots that are primarily in the Band C horizons. It is somewhat surprising that the small variations in pH should correlate with L. tri- dentata size. The hydrogen ion concentration ranges from 10-8 to 10-9 M, though other ca- tions are present at 10~3 M (Table 1). Because the correlation was negative, it is possible the higher soil pH values tend to inhibit L. tri- dentata growth. No attempt was made to measure rhizos- phere pH, though data of Turner (1972) sug- gest that in desert soils rhizosphere pH is re- duced even in these heavily calcareous soils. Smiley (1974) found that lime buffered soil against pH changes caused by nitrogen up- take, but Stark (1973) and Hanawalt and Whittaker (1977) found that an acid soil ex- tract represented plant-available nutrients better than neutral extracts. Van Egmond and Aktas (1977) reported that Fe-efficient soybeans excrete more H + into the medium than do Fe-inefficient varie- ties. However, we found no correlation be- tween soil pH and leaf Fe variables. Larrea tridentata should certainly be con- sidered an Fe-efficient species. The negative correlation between leaf Fe and plant size may be explained in several ways, but it is not consistent with suggestions of Fe defi- ciency affecting size. Indeed, the correlation reflected a cause-effect relationship, and Fe toxicity would be indicated. Iron uptake, translocation, and physiology have been extensively studied, but may still be characterized as poorly understood (Thorne and Wallace 1944, Brown 1956, Khadr and Wallace 1964, Brown and Ambler 1974, Jones 1976). A frequent observation has been an association of Fe with K uptake (Thorne and Wallace 1944, Brown 1956, Hernando and Sanfuentes 1976). In this study we found correlation of leaf Fe variables with both leaf K and L. tridentata biomass, but not between biomass and leaf K. The strong negative association of meta Fe with both leaf K and sum of cations are consistent with the hypothesis that lime-induced chlo- rosis is related to cation-anion balance and internal leaf pH (Wallace et al. 1976). The positive correlation of leaf Zn with sum of cations (r = +0.50), and the negative corre- lation of meta Fe (r = -0.56) suggest an Fe- Zn interaction. One implication of these findings is that the Fe nutrition of L. tridentata growing on calcareous soils is similar to, but different in degree from, that of species exhibiting lime- induced chlorosis. I Acknowledgments This study was supported by Contract EY- 76-C-03-0012 between the U.S. Department of Energy and the University of California. Literature Cited Barbour, M. G. 1970. Age and space distribution of the desert shrub Larrea divaricata. Ecology 50:679-685. Beatley, J. C. 1974. Effects of rainfall and temperature on the distribution and behavior of Larrea triden- tata (creosote bush) in the Mojave Desert of Ne- vada. Ecology 55:245-261. Brown, J. C. 1956. Iron chlorosis. Ann. Bev. Plant Phys- iol. 7:171-190. Brown, J. C. and J. E. Ambler. 1974. Iron-stress re- sponse in tomato (Lycopersicon esculentum): sites of Fe reduction, absorption and transport. Phvs- iol. Plant 31:221-224. Dixon, W. J., ed. 1971. Biomedical computer programs. 2d ed. University of California Press, Los Angel- es. Hallmark, C. T., and B. L. Allen. 1975. The distribu- tion of creosote bush in west Texas and eastern New Mexico as affected by selected soil proper- ties. Soil Sci. Soc. Amer. Proc. 39:120-124. Hanawalt, R. B., and R. H. Whittaker. 1977. Altitu- dinal patterns of Na, K, Ca, and Mg in soils and plants in the San Jacinto Mountains, California. Soil Sci. 123:25-36. Hernando. V., and J. R. Sanfuentes. 1976. Effect of K in absorption and translation of Fe by beans (var. borriol) using Fe-59 in presence of sodium bi- carbonate. Agrochimica 20:264-274. Jones, J. B., Jr. 1976. Iron deficiency and its correction. Commun. Soil Sci. Plant Anal. 7:i. Khadr, A., and A. Wallace. 1964. Uptake and trans- location of radioactive iron and zinc by trifoliate orange and rough lemon. Proc. Am. Soc. Hort. Sci. 85:189-200/ Lunt, O. R., J. Letey, and S. B. Clark. 1973. Oxygen requirements for root growth in three species of desert shrubs. Ecology .54:1356-1362. Romney, E. M„ V. Q. Hale, A. Wallace, O. R. Lunt, J. D. Childress, H. Kaaz, G. V. Alexander, J. E. Kinnear, and T. L. Ackerman. 1973. Some char- acteristics of soil and perennial vegetation in northern Mojave Desert areas of the Nevada Test Site. UCLA Report 12-916. 1980 Nevada Desert Ecology 167 Smiley, R. W. 1974. Rhizosphere pH as influenced by US/IBP Desert Biome reports of 1971 progress, plants, soils and nitrogen fertilizers. Soil Sci. Soc. Vol. Ill, Utah State University. Am. Proe. 38:795-799. Van Egmond, F., and M. Aktas. 1977. Iron-nutritional x, ,™-™ t^- t-n . j i- c j aspects of the ionic balance of plants. Plant Soil Stark, N. 19/3. Distillation-condensation of water and c y r . . i 4o:oo5-7U3. nutrient movement in a desert ecosvstem. irv_n ,. . ,,.,,„„ „ .. _„ .. ITl , c. . T'T . Wallace, A., and fc>. M. Romney. 1972. Radioecologv US/IBP Res. Memo. 73-44. Utah State Univer- ' ' . . , °7 and ecophysiology of desert plants at the Nevada Test Site. National Technical Information Ser- Thorne. D. W., and A. Wallace. 1944. Some factors viceS) USAEC Report TID-25954. affecting chlorosis on high-lime soils. I. Ferrous Wallace, A., R. A. Wood, and S. M. Soufi. 1976. Ca- and ferric iron. Soil Sci. 57:299-312. tion-anion balance in lime-induced chlorosis. Turner, F. B. 1972. Rock Valley Validation Site report. Commun. Soil Sci. Plant Anal. 7:15-26. RETRANSLOCATION OF TAGGED CARBON IN AMBROSIA DUMOSA A. Wallace1, J. W. Cha1, R. T. Mueller', and E. M. Romney1 Abstract.— Ambrosia dumosa (A. Gray) Payne cuttings grown in solution culture were exposed to 14COa to mea- sure the distribution of labeled photosynthate among leaves, stems, and roots after 4, 24, and 48 h. For all sampling periods, the highest levels of 14C were found in leaves and the lowest in roots; however, considerable 14C had moved to roots in 48 h. In a 12-week study of A. dumosa in solution culture, plants increased in size more than 17 times and flowered and produced seeds. The plants had received 14COa in photosynthesis at the start. The gradual loss of 14C from the plants in the 12 weeks averaged 3.5 percent per week (coefficient of variation = 58 percent). This repre- sents an average respiration rate of 0.21 mg C g dry weight-1 h"1. This compares favorably with other means for determining respiration rate. The percentage of 14C in the root portion of the plant varied little over 6 sampling periods, indicating that essentially none of the initially fixed 14C left the roots during the 12 weeks of test. The 14C entering fruits and seeds came from leaves only. The biomass of fruit parts resulted more from new photosvnthate than from retranslocation from leaves. In a study in which A. dumosa plants were defoliated, little 14C moved from roots to new shoot growth. The United States International Biological Program Desert Biome has concentrated con- siderable research effort in studies of the car- bon cycle. Certain questions could not be an- swered easily by conventional procedures, but tagging of plants with 14C in photo- synthesis was one means of obtaining answers for some questions (Wallace et al. 1979, Vol- lmer et al. 1975, 1976). Among the questions of concern were the following: Does carbon move from leaves to roots continuously, or as a pulse from that which has been newly fixed? Does carbon in roots contribute to new shoot growth? Does carbon in leaves or stems and/ or roots contribute to fruit growth? What is the rate of carbon loss due to respira- tion? These questions could be approached with the 14C technique under controlled con- ditions. Materials and Methods Ambrosia dumosa (A. Gray) Payne cuttings were grown for 30 days in solution culture in a glasshouse, at which time the shoots were about 15 cm tall. The shoots were then ex- posed to 14C02 (about 5 uCi/ plant) in plastic- bags for 2 h. Two plants each were separated into leaves, stems, and roots after 4, 24, and 48 h. The methods generally were like those previously used (Bamberg et al. 1975). Two-month-old A. dumosa cuttings grow- ing in 3700 ml nutrient solutions in a glass- house were exposed to 14C02 by the general procedures described above. Three plants were separated into plant parts for 14C deter- mination after 24 h, 1 week, 2 weeks, 4 weeks, 8 weeks, and 12 weeks to determine changes in distribution with time. To ascertain movement of previously fixed 14C from crown and root materials to shoots, an experiment was conducted in which four A. dumosa plants, each growing in 1600 g soil, were exposed to 14C02 as above. Leaves of the plants were sampled at 2 h and 24 h. After 48 h the shoots of the plants were cut off. The shoots were allowed to regrow and at 78 days the plants were removed from the soil and separated into parts, including fine roots separated by salt-flotation with MgS04. All plant parts were counted for 14C by Q-gas counting. Results and Discussion Table 1 shows the distribution of 14C in leaves, stems, and roots of A. dumosa plants at 4, 24, and 48 h after labeling. Most of the 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 90024. 168 1980 Nevada Desert Ecology 169 label was confined to the leaves and stems, with only 4.7-7.4 percent going to the roots, even though they comprised 15-19 percent of the biomass. Changes with time in the per- centage of 14C in the different plant parts were not readily apparent, although the pro- portion of root 14C might have increased slightly. As the experiment progressed, the amount of 14C per unit weight decreased due to both dilution by new growth and respira- tory loss. Roots maintained a relatively con- stant 14C:weight ratio, but that of leaves and stems dropped sharply. This seems to in- dicate that most of the gains and losses of carbon during this 48-hour period occurred in the latter two structures or that dilution was involved. Redistribution or reallocation of carbon in A. dumosa was studied over a 12-week peri- od in a solution culture experiment (Table 2). Changes were followed over six different sampling times. The plants flowered and fniited during the test, which permitted a measure of mobility of the carbon from the initially fixed 14C. Three plants were har- vested at each time period for the measure- ment. The plants increased in size over 17 times during the course of the 12-week experiment. The respiratory loss of the 14C was relatively small. The estimates were about 9 percent at one week, 4 percent for 2 weeks, 14 percent for 4 weeks, 33 percent for 8 weeks, and 22 percent for 12 weeks. The irregularity of the values indicate variability. A normalized val- ue for all five values results in 3.5 percent loss per week as an average. The standard de- viation for the 3.5 percent value is 2.07 per- cent, with a coefficient of variation of 58 percent. If this value (3.5 percent per week) repre- sents a respiration rate, it would be 2.1 X 10-4 mg C mg dry weightm1 or 0.21 mgCg dry weight-m-1 at any point in the history of these plants. This compares fairly well for ac- tual respiration measurements. It represents the respiration rate for the active growing stages and not for dormancy for this species (Vollmer et al. 1976). The percentage of 14C in the root portion Table m thesis 1. Distribution of 14C in Ambrosia dumosa grown in solution culture after tagging with 14COa in photo- Hours after labeling Leaf Stem Root 642 Drv weight, mg 409 252 1025 429 254 1215 616 376 49.3 Percent plant parts bv weight 31.4 19.3 60.0 25.1 14.9 55.1 27.9 17.0 424 cpm /plant part (X 1000) 159 25.2 437 145 35.3 366 147 51.0 69.7 Percent of 14C in plant parts 26.2 4.1 70.8 23.5 5.7 66.1 26.5 7.4 660 cpm/g(X 1000) 389 100 427 339 139 301 240 109 Whole plant 1303 1708 2207 100 100 100 617 554 100 100 100 467 362 251 170 Great Basin Naturalist Memoirs No. 4 of the plants varied little for the six sampling periods, even when seeds were produced. It was about the same at 24 h (5.1 percent) as at 12 weeks (5.8 percent). We may conclude, therefore, that the 14C moved to roots only on the day of fixation. None left the roots thereafter during the 12 weeks of test. More dry matter than 14C was moved to the fruit- ing parts and seeds, implying that most of the photosynthate used for fruiting was new. The 14C that was translocated to seeds seemed to come from leaves only. Redistribution of carbon in A. damosa was further studied with plants grown in soil. Four plants exposed to 14C02 were defoliated after 2 days, and a portion of the stem was also removed. Any 14C thereafter found in leaves and new stems had to be translocated from old parts. After 78 days following defo- liation 8 percent of the 14C was in the leaves, and 0.5 percent was in new stems with more than 57 percent in roots (Table 3). This in- dicates as in the other tests that 14C is not readily moved from roots after initial fix- ation. The small amount of 14C that did move to the leaves probably was mobilized when the leaves were initiated. At 78 days, 24 per- cent of the plant biomass was leaves with 8 percent of the 14C. Thirty percent of the plant biomass was roots with 57.5 percent of the 14C. Table 2. Dry weight and distribution of dry weight and 14C in plant parts at different times following exposure of Ambrosia dumosa to 14CC>2. Plant part 2h 1 week 2 weeks 4 weeks 8 weeks 12 weeks Leaf 992 Dry 1.191' weight, mg/plant 3,350 3,520 7.655 18,361 Stem 700 1,454 1,758 3,226 6,752 10.150 Transition 58 94 133 212 224 1,078 Root 190 397 576 890 2,304 2,555 Seed - - - 2,023 2,673 1,816 Total 1,940 3,856 5,817 9,871 19,608 33,960 Leaf 553,267 274,933 cpm/g 163,347 109,760 41,950 18,990 Stem 338,980 124,540 109,673 61,447 20,240 17,910 Transition 72,695 62,120 91,013 34,860 26,020 16,780 Root 222,033 112,987 78,693 54,513 17,630 14,830 Seed - - - 38,920 18,910 36,360 Leaf 548,841 525,395 cpm/plant 547,212 386,355 321,127 348,675 Stem' 237,286 181,081 192,806 198,228 136,660 181,787 Transition 4,216 5,839 12,105 7,390 5,828 18,089 Root 42,186 44,856 45,327 48,516 40.620 37,891 Seed - - - 78,735 .50,546 66,030 Leaf 51.1 Dry weight/total plant weight (percent) 49.6 57.6 35.7 39.0 54.0 Stem 36.1 37.7 30.2 32.7 34.5 29.9 Root 9.8 10.3 9.9 9.0 11.8 7.5 Transition 3.0 2.4 2.3 2.1 1.1 3.2 Seed 0.0 0.0 0.0 20.5 13.6 5.4 Total 100.0 100.0 100.0 100.0 100.0 100.0 Leaf 65.9 cpm/plant (percent) 69.4 68.6 53.7 57.9 53.4 Stem 28.5 23.9 24.2 27.6 24.6 27.9 Transition 0.5 0.8 1.5 1.0 1.1 2.8 Root 5.1 5.9 5.7 6.8 7.3 5.8 Seed 0.0 ().() 0.0 10.9 9.1 10.1 Total 100.0 100.0 100.0 100.0 100.0 100.0 1980 Nevada Desert Ecology 171 Table 3. Distribution of 14C in A. dumosa plants 78 days aftt removal of all the leaves and the stems from the plants. ° exposure of the shoots to 14C02 and 76 days afte CVof SDof percent Plant Dry wt cpm/ Percent percent dist. part g/plant cpm/g plant dist. dist. percent. Leaves 2.02 New stems 0.53 Old stems 2.53 Crown 0.82 Big root 0.37 Small root 0.33 Fine root 1.82 Totals or means 8.42 610 135 1520 1725 2580 3525 3730 1838 1232 72 3846 1415 955 1163 6789 15472 8.0 0.5 24.9 9.1 6.2 7.5 43.8 100.0 2.50 0.07 7.31 6.03 3.18 1.47 11.78 "Leaf concentration of 14C at 2, 24, and 48 h from exposure to 14C02 were 82,135, 39,670, and 37,230 cpm/g dry weight, respectively. Leaf concentrations of 14C at 2 h, 24 h, and 48 h from exposure to i4C02 (82,135, 39,630, and 37,230 cpm/g) indicated that either there was considerable loss due to dark respi- ration in this C-3 plant or that this period was the time in which translocation to roots primarily occurred. It can be argued that these experiments under partially controlled conditions may not represent field conditions adequately. In the companion study with Larrea tridentata (Sesse & Moc ex DC.) Cov. in the field (Wal- lace et al. 1980), 14C persisted in plants, espe- cially in the roots, for more than three years after time of fixation. Acknowledgments This study was supported by an IBP Desert Biome subcontract with Utah State Univer- sity, Logan, Utah, and Contract EY-76-C-03- 0012 between the U.S. Department of Energy and the University of California. Literature Cited Bamberg, S. A., G. E. Kleinkopf, A. Wallace, and A. Vollmer. 1975. Comparative photosynthetic production of Mojave Desert shrubs. Ecology 56:732-736. Vollmer, A. T., S. A. Bamberg, A. Wallace, and J. W. Cha. 1975. Plant productivity and nutrient inter- relationships of perennials in the Mohave Desert. US/IBP Desert Biome Res. Memo 75-7. Utah State University, Logan, Utah. Vollmer, A. T., A. Wallace, J. W. Cha, and T. Hartsock. 1976. Plant productivity and nutrient interrelationships of perennials in the Mohave Desert. US/ IBP Desert Biome Res. Memo. 76-6. Utah State University, Logan, Utah. Wai lace, A., S. A. Bamberg, and J. W. Cha. 1977. Par- titioning of photosynthetically fixed 14C in per- ennial plants of the northern Mohave Desert. Pages 144-148 in IBP/Interbiome Symposium: the belowground ecosystem: a synthesis of plant- associated processes, Range Sci. Series No. 16, Colorado State University, Fort Collins, Colo- rado. Wallace, A., E. M. Romney, and J. W. Cha. 1980. Per- sistence of 14C labeled carbon in Larrea triden- tata up to 40 months after photosynthetic fixation in the northern Mojave Desert. Great Basin Nat. Mem. 4:170-174. PERSISTENCE OF 14C LABELED CARBON IN LARREA TRIDENT AT A UP TO 40 MONTHS AFTER PHOTOSYNTHETIC FIXATION IN THE NORTHERN MOJAVE DESERT A. Wallace', E. M. Romney1, and J. W. Cha1 Abstract.— Larreo tridentata (Sesse Moc. ex DC) Cov. exposed to 14C02 retained about 20 percent of its 14C after 16 and also after 26 months. In leaves, however, a lower specific activity was present at 26 months than at 16 months, and a smaller percentage of 14C in the plant occurred in leaves at 26 months than at 16 months (3 percent vs 10 percent). This indicates some, but little, reuse of carbon from the structural components of the plants. The strong tendency of the species to retain this carbon may be related to a survival mechanism. After 40 months the results were more erratic, with 11 percent of the 14C remaining in plants and only 2 percent of the total remaining in the leaves. The specific activity of 14C in the organic debris fraction obtained with saturated salt flotation of roots after small and fine roots had been physically removed indicated that from 27 to 35 percent of the organic debris had the same specific activity as roots and probably could be considered as roots. This compares with the 45 percent value determined previously by a different technique. The below-ground to aboveground ratio for biomass of these plants was about 2.5:1. The below-ground to above-ground ratio for the 14C was about 0.5 at 16 months, 1.3 at 26 months, and 2.5 at 40 months. The estimates obtained in this study were used to correct our previous data for below- ground biomass. Accordingly, somewhere between 3000 and 5000 kg/ha roots are present in the Rock Valley area. An increase with time of the below-ground to aboveground 14C ratio probably indicates loss of 14C from above- ground parts rather than additional transport to roots. One difficult aspect of plant studies in deserts is that of estimating below-ground biomass. Our previous studies have empha- sized the magnitude of this problem (Bam- berg et al. 1973, 1974, Vollmer et al. 1975, 1976, and Wallace, Bamberg, and Cha 1974), and it is further emphasized by the wide dif- ferences in below-ground biomass reported for the same area by different workers within our own group using different techniques of measurement. Some approximations for root: shoot ratios in our studies for the northern Mojave Desert are near 1:1, but others approach 4:1. In the Great Basin Desert root:shoot ratios were re- ported that varied from around 8:1 to more than 12:1 (Caldwell and Camp 1974, Cald- well et al. 1974, Caldwell et al. 1976). The purpose of this study was to determine the persistence of 14C labeled carbon in Larrea tridentata (Sesse & Moc. ex DC) Cov. and to further assess the problem of root biomass of this desert species. Materials Methods Plants in Mercury Valley, Nevada, were exposed to 14CO, with techniques previously used (Bamberg et al. 1973, 1974, Wallace et al. 1974). Six naturally growing L. tridentata were exposed to 14C02 for 2 h on the morn- ing of 14 May 1974. Each plant was exposed to 125 uCi 14C02. Twigs were sampled at the end of this 2-h period for use in estimating the total 14C02 fixed by the plants. Two of these plants were excavated 16 months later on 17 September 1975 (Vollmer et al. 1975). Samples of all parts were then counted for 14C by Q-gas technique and corrected for self absorption by methods reported previously (Bamberg et al. 1973). Two other plants were excavated on 16 July 1976, and the last two plants were excavated for analysis on 21 Sep- tember 1977. Soil from within a radius of 2.5 times the radius of the plant canopy was sampled for use in fine root biomass determinations. Soil 'Laboratory of Nuclear Medicine and Radiation Biolog} Universit) .>! I lalifomia, Los Angeles, < lalifoi 172 1980 Nevada Desert Ecology 173 samples (1 liter) were added to a saturated NaCl solution. Soil organic matter was sepa- rated by flotation and hand-sorted to obtain fine roots. The organic debris which could not be identified as roots was also separated. These samples were prepared and counted for 14C. The roots were dried and ashed and found to contain 60-75 percent non- combustible ash. The high ash content was due to soil and salt contaminants adhering to roots. Root weights were normalized to 25 percent ash. The amount of roots in the soil surrounding the plants was estimated by ex- trapolating from small soil samples to the to- tal volume of soils within a radius of 2.5 times the radius of the plant canopy to a depth of 30 cm. This method is similar to that employed by Bamberg et al. (1974) and Vollmer et al. (1975). Results and Discussion Larrea tridentata that had been exposed to 14C02 16 months previously at the Nevada Test Site retained about 20 percent of the 14C that was originally fixed (Table 1). This amount was essentially unchanged at 26 months (Table 2). At 26 months, however, a lower specific activity was found in leaves than at 16 months, and a smaller percentage of the remaining 14C was in the leaves at 26 months (3 percent) compared with 16 months (about 10 percent). This is indicative of low respiratory turnover and low remobilization of carbon from the structural components of this desert shrub. Even though L. tridentata is evergreen, it does have turnover of leaves close to an- nually (Wallace and Romney 1972), so there would be a regular loss of 14C in an experi- ment such as this. The loss would not be as great as the 14C content of leaves, however, because of the retranslocation of around 50 percent of the carbon from the leaves to the shrub before leaf abscission. Except for leaves, we were unable to distinguish be- tween the 14C contents of the two sets of plants collected at 16 months and at 26 months. Because three annual phenological cycles are involved for the pair of plants har- vested at 26 months and because somewhere around 20 percent of the original fixed 14C was still in the plants similar to that at 16 months, a survival mechanism may be in- Table 1. Plant biomass and 14C content within a radius of 2.5 times the canopy radius on 17 September 1975 for two previously tagged (16 months) Larrea tridentata. Plant No. 1 Plant No. 4 Biomass (dry weight) Grams Percent Grams Percent 30.3 8 38.2 8 118.5 20 90.9 19 52.1 — 80.1 — 109.0 — 164.0 — 43.4 _ 52.4 — 204.5 52 296.5 61 245.4 62 355.8 73 Leaves Stems Small roots (<1 mm) Medium roots (1 to 3 mm) Other roots Total roots Total roots" Total 394.2 100 100 Plant No. 1 14C Content cpm/g Plant No. 4 cpm/g cpm Percent dry wt cpm Percent dry wt May 1974 1,583,000 - - 3,200,000 - - September 1976, total plant 283,300 100 751 481,700 100 1054 Leaves 23,100 8 762 72,100 15 1887 Stems 150,500 53 1270 211,400 44 2326 Roots 91,500 32 447 165,100 34 557 Total roots corrected for organic debris" 109,700 39 447 198,200 41 557 "Organic debris corrected to specific activity of roots. See Table 2. 174 Great Basin Naturalist Memoirs No. 4 volved. A high degree of conservation of car- bon occurred. The third pair of plants originally exposed to 14C02 in May of 1974 was sampled at ap- proximately 40 months. Data are in Table 3. About 2 percent of the remaining 14C was in leaves at this date. About 11 percent of the original 14C fixed remained in the plants. The information in Tables 1 to 3 has bear- ing on the below-ground to aboveground ra- tio of biomass and the below-ground annual productivity of shrubs. Workers from the Great Basin desert have found larger propor- tions of roots than we have for the Mojave Desert (Caldwell and Camp 1974, Caldwell et al. 1974, Caldwell et al. 1976). Our origi- nal estimate of below-ground to aboveground ratios were low (around 1) (Wallace, Bam- berg, and Cha 1974). More recent estimates for the Mojave Desert were around 2 or 3 (Vollmer et al. 1976, Bamberg et al. 1974). The biomass root /stem ratios for the two plants in Table 2 were 4.9 and 3.1; for 14C the ratios were 1.9 and 1.1, respectively. The same values for the plant in Table 1 (16 months after labeling) were 2.1 and 3.9 for biomass and 0.7 and 0.9 for 14C. The biomass root/stem ratios for the two plants sampled at 40 months were 6.0 and 7.6, and the 14C ratios were 1.6 and 5.1. It appeared that the biomass ratio was slightly higher at 26 or 40 months than at 16 months and that the 14C ratio generally increased as time passed by. Part of the difference, however, could be due to technique, and part may be due to loss of 14C from aboveground parts with age. The data in Table 2 further resolve the problem of whether or not organic debris floated from the soil samples with saturated solutions of salts should be considered as roots. Three factors relate to the problem. Such material is very high in ash because of the saturated salt and the soil contamination. Correction values are necessary. Not all sub- Table 2. Plant biomass and 14C content within a radius of 2.5 times the canopy radius on 16 Jnlv 1976 of two Larrea tridentata plants exposed to 14C02 26 months previously at Mercury, Nevada (roots normalized to 25 percent ash). Plant No. 2 Plant No 6 Biomass (dry weight) Grams Percent Grams Percent Flowers 5.5 1.0 5.1 0.8 Leaves 37.7 6.7 49.8 8.0 Stems 88.1 15.7 138.8 22.2 Roots 333.3 59.4 320. 1 51.1 Organic debris" 355.8 — 275.9 _ Corrected value of O.D. ° ° 96.7 17.2 111.8 17.9 Total roots 430.0 76.6 431.9 69.0 Total 561.3 100.0 625.6 100 Plant No. 2 Plant No. 6 14C Content cpm/g cpm/g cpm Percent dry wt cpm Percent dry wt 14 May 1974 2,793,000 _ _ 3,775,000 _ _ 16 July 1976, total plant 595,200 100.0 120 701,500 100.0 1.121 Flowers 665 0.1 110 510 0.1 100 Leaves 15,000 2.5 398 22,900 3.3 460 Stems 201,500 33.9 2,287 325,300 46.3 2,344 Roots 293.000 49.2 879 261.500 37.3 817 Organic debris" 85,000 - 239 91,300 - 331 Corrected value ofO.D."" 85,000 14.3 879 91,300 13.0 817 Total roots 378,000 63.5 879 352,800 563 817 "Organic material floated from soil with concentrated NaCl + normalized to 25 percent ash. Some of it may be a very fine fraction of roots. • "Organic debris corrected to specific activity of roots. 1980 Nevada Desert Ecology 175 samples from soil about the 14C-treated plants had 14C in the organic debris floated from the soil samples, and such probably should not be considered as root material. In about 90 per- cent of the cases the fine roots contained 14C, but only 10 percent of the organic debris samples contained the isotope. The specific activity of the 14C in the organic debris is lower than that for roots (see Tables 1 and 2). Nonroot material then is involved to the ex- tent of correction of weights to a constant specific activity as was done in Tables 1, 2, and 3. This, of course, could be erroneous be- cause the 14C could arise from dead, partially decayed roots. The proportion of the organic- debris not considered as roots then was 73 percent and 60 percent for the two shrubs in Table 2. Vollmer et al. (1975) had deter- mined that 45 percent of the organic debris was roots and the results of the two studies do not differ greatly. The ratio of weight of roots to above- ground parts in Tables 2 and 3 varied from about 1.6 to over 3 with the corrected values for roots. The average of all 4 cases was 2.5. The ratio of the root-shoot distribution of 14C in the plants after the 26 months is also inter- esting (Table 1). The average ratio for the corrected root 14C ratio was about 1.4. Nei- ther the 2.5 nor the 1.4 ratio approach those found for Great Basin shrubs (Caldwell and Camp 1974). They do, however, indicate the presence of greater biomass below-ground than aboveground. The root /stem ratio of 1.4 for 14C from Table 2 is of further interest. After 26 months, more of the 14C in the plants was be- low ground than above ground, which corre- sponds with the root weights. In our earlier studies (Wallace and Romney 1980, this vol- ume) the 14C ratio for root/stem was around 0.2 for the relatively short-time basis. The shift may be related to loss of 14C-containing materials from shoots rather than to transport of more of it below ground. This would in- dicate that, over a period of years, there is a greater loss of aboveground parts than below- ground parts. Table 3. Plant biomass and 14C content within a radius of 2.5 times the canopy radius on 21 September 1976 of two Lama tridentata plants exposed to 14C02 40 months previously at Mercury, Nevada (roots normalized to 25 percent ash). Plant No. 3 Plant No. 5 Biomass (dry weight) Crams Percent Grams Percent Leaves 59.3 9.0 52.1 11.6 Stems 86.2 13.1 46.1 10.0 Roots 247.3 37.4 206.3 45.9 Organic debris0 864.7° °° - 428.2 — Corrected value of O.D. ° ° 267.0 40.5 145.2 32.3 Total root 514.3° °° 77.9 351.5 78.2 Total 659.8 100.0 449.7 100.0 Plant No. 3 14C Con cpm/g ent Plant No. 5 cpm/g cpm Percent dry wt cpm Percent dry wt 14 May 1974 3,049,000 _ _ 5,985,000 - - 21 Sept. 1977, total plant 220,300 100.0 333.9 825,600 100.0 Leaves 6,000 2.7 100.2 13,600 1.7 261.0 Stems 82,000 37.2 951.3 133,600 16.2 2898.0 Roots 63,600 28.9 257.5 506,600 61.4 2455.6 Organic debris0 68,700 _ 79.5 171,800 — 832.7 Corrected value of O.D. ° ° 68,700 31.2 257.5 171,800 20.8 2455.6 Total roots 132.300 60.1 257.5 678,400 82.2 2455.6 'Organic material floated from soil with concentrated NaCl + normalized to 25 percent ash. Some of it may be a v ° "Organic debris corrected to specific activity of roots. ""Value is abnormally high because of a broken irrigation sprinkler nearby which resulted in much grass in the area. ■ fine fraction of roots. 176 Great Basin Naturalist Memoirs No. 4 In 1974, we made some estimates of root biomass for the northern Mojave Desert (Table 2 of Wallace et al. 1974). It would ap- pear from the data reported here that the root values of the earlier study probably should be further corrected to include the root portion in the organic debris portion. An estimated correction factor is the corrected versus the uncorrected values in Table 1. These are 1.29 (430/333) and 1.35 (432-320) for weights of the two plants, and the values in Table 2 of Wallace et al. (1974) should be corrected by that amount (mean = 1.32). An interspace correction should also be made in that the 1974 samples were extended into the interspace soil only as far as we found roots. The development of a correction factor of 1.23 for interspace is given elsewhere, and the values for the earlier data are calculated in Table 4. The twice-corrected values for root /stem in Table 4 is 1.73 and for root /root + stem is 0.63. These values are, of course, subject to errors, but they are still lower than com- parable data from the Great Basin desert. Acknowledgments This study was supported by Contract EY- 76-C-03-0012 between the U.S. Department of Energy and the University of California. Literature Cited Bamberg, S. A., A. Wallace, G. E. Kleinkopf, A. Vollmer, and B. S. Ausmus. 1973. Plant produc- tivity and nutrient interrelationships of per- ennials in the Mohave Desert. US/IBP Desert Biome Bes. Memo. 73-10. Utah State University, Logan. 1974. Plant productivity and nutrient inter- relationships of perennials in the Mohave Desert. US/IBP Desert Biome Bes. Memo. 74-8. Utah State University, Logan. Caldwell, M. M., and L. B. Camp. 1974. Below-ground productivity of two cool desert communities. Oeeologia 17:123-130. Caldwell, M. M., L. B. Camp, B. S. Holthausen, and H. Neuber. 1976. Gas exchange translocation, root growth, and soil respiration of Great Basin plants. US/IBP Desert Biome Bes. Memo 76-7. Utah State University, Logan. Caldwell, M. M., E. J. DePuit, O. A. Fernandez, H. H. Wiebe, and L. B. Camp. 1974. Gas exchange, translocation, root growth, and soil respiration of Great Basin plants" US/ IBP Desert Biome Bes. Memo. 74-9. Utah State University, Logan. Vollmer, A. T., S. A. Bamberg, A. Wallace, and J. W. Cha. 1975. Plant productivity and nutrient inter- relationships of perennials in the Mohave Desert. US/IBP Desert Biome Bes. Memo 75-7. Utah State University, Logan. Vollmer, A. T., A. Wallace, J. W. Cha, and T. Hartsock. 1976. Plant productivity and nutrient interrelationships of perennials in the Mohave Desert. US/ IBP Desert Biome Bes. Memo. 76-6. Wallace, A., S. A. Bamberg, and J. W. Cha. 1974. Quantitative studies of roots of perennial plants in the Mojave Desert. Ecology 55:1160-1162. Table 4. Estimates of standing stem and root w< irom Wallace et al. 1974.) ights for plot at Bock Valley (from a 0.46 km2 plot). (Bevised Boot Boot corrected corrected Calc. Standard for for stem deviation debris interspace No. of dry wt of stem wt Stem Boot (1.32) ,1.23) Species plants/ha g/plant g/plant kg/ha kg/ha kg/ha kg ha Acamptopappus shockleyi 47 68.3 82.8 3.2 1.8 2.4 2.9 Atriplex confertifolia 75 33.7 32.8 2.5 1.1 1.5 1.8 Ephedra nevadensis 783 119.1 202.9 93.3 77.9 102.8 126.5 Eurotia Janata 478 62.7 96.9 30.0 27.0 35.0 43.8 Ambrosia dumosa 2394 108.7 111.0 200.2 301.0 397.3 488.7 Grayia spinosa 1196 74.3 85.8 88.9 04.0 84.5 103.9 Krameria parvifolia 14S2 136.4 96.9 202. 1 159.1 210.4 258.8 Larrea tridentata 1040 137.9 454. 1 458.0 566.7 748.0 920.1 Larrea andersonii 710 171.2 244.3 263.7 220.1 290.5 357.4 Lydum pallidum 459 264.4 216.4 121.3 199.7 263.6 324.3 Total 8670 - - 1523.3 1618.7 2136.6 2628.2 "C DISTRIBUTION IN ROOTS FOLLOWING PHOTOSYNTHESIS OF THE LABEL IN PERENNIAL PLANTS IN THE NORTHERN MOJAVE DESERT A. Wallace', R. T. Mueller1, J. W. Cha', and E. M. Romney' Abstract.— In April and May of 1973, 24 individual plants were exposed to 14CC>2 with techniques used in our other studies in the field. Seven to 8 months later, part of the plants were excavated and counted by plant part for 14C. The remainder of the plants were excavated at 13 months. The results indicated that from 3 to 20 percent of the carbon for leaves in the next year came from stems and roots of Grayia spinosa (Hook) Moq., Ceratoides lanata (Pursh) J. T. Howell, Atriplex confertifolia (Torr. & Frem.) S. Wats., Lycium pallidum Miers, Ambrosia durnosa (A. Gray) Payne, and Acamptopappus shockleyi A. Gray. Nearly all of the root segments were labeled at sampling time; however, some of the roots were labeled at higher amounts than others. Some roots had very little 14C, and these are assumed to be very new roots rather than dead roots because of their small size. The roots with high levels of 14C are assumed to be formed near the time of labeling, and those with low levels to be formed after the time of labeling. From 17 to 65 percent of the 14C fixed was recovered after 7 to 13 months. Introduction 14C techniques have been used in studies of carbon allocation in desert plants and of root growth and distribution (Caldwell et al. 1974, 1975, 1976, Bamberg et al. 1973, 1974). One of the questions which arose in those studies is the nature of root labeling when a single labeled pulse is fixed in photosynthesis. Much can be deduced from the nature of carbon al- location according to the manner in which a single pulse of 14C is distributed within the plant. The major purpose of this study was to determine the distribution of 14C in individ- ual roots and in segments of those roots fol- lowing a single exposure to 14C02 of the shoots of plants growing in the desert. Anoth- er purpose was to ascertain the proportion of new growth in the springtime that arises from retranslocation from old parts of winter deciduous plants. The specific activity of 14C in the new shoot growth compared with that in the old parts could result in an estimate of the portion of the new growth that is para- sitic on the old parts versus the fraction which comes from new photosynthesis. It is recognized that this approach could only in- dicate a minimum of the fraction coming from old parts. The 14C then would under- estimate because it is not uniformly mixed with all the labile pool carbon. Another pur- pose in this study was to ascertain if new roots could be identified by absence in them of labeled 14C in the year after its appli- cation. Materials and Methods In May 1973, 24 perennial plants in Rock Valley and Mercury Valley, Nevada, were ex- posed to 14C02 with the technique previously used in these studies (Bamberg et al. 1973, 1974, Wallace et al. 1974). Briefly, at about 0900, four Ambrosia dwnosa (A. Gray) Payne plants were covered with transparent plastic bags of 2 mil thickness, and 125 uCi 14C02 were released into each bag. Considerable water vapor condensed on the inside of the bags. Two hours later the bags were re- moved, and leaf and stem samples were taken from each plant for determination by Q-gas counting of the amount of 14C fixed, using the technique of Hendler (1959). All values were corrected to sample size of 50 mg. Counting efficiency with the procedure is of the order of 10 percent. Counting accuracy was made to a confidence level of 95 percent. The sub- samples of leaves and twigs represented be- tween 5 percent and 10 percent of all those on the plant, but for each subsample a pre- cise number of leaves was collected and an accurate estimate of those remaining on the 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 90024. 177 178 Great Basin Naturalist Memoirs No. 4 plant was made so that a reasonably accurate assessment of the total 14C fixed by the plant could be determined. Part of these plants (ap- proximately half) were removed from the soil and separated into individual roots, stems, and leaves in December 1973 and January 1974 (during the dormant season). The rest of the plants were removed in June 1974 and treated similarly. This sampling was after the spring new growth period. Activity of 14C and weights of plant parts were obtained for all plants. Table 1. 14C status of plants from Mercury Valley, exposed to 14CC>2 in May 1973. Species Initial total 14C fixed X 103 % of 14C remaining cpm Roots Stems Leaves Total 2529 10.3 14.7 12.8 37.8 3340 3.0 18.9 18.4 40.3 2095 5.2 7.1 4.3 16.6 622 14.1 51.7 0.0 65.8 2051 2.9 21.8 11.2 35.9 1621 9.7 53.9 0.7 64.3 1343 7.6 45.8 0.0 53.4 2690 11.8 16.1 8.3 36.2 Larrea tridentata Atriplex confertifolia Ambrosia dumosa Krameria parvifolia Atriplex confertifolia Ambrosia dumosa Acamptopappus shockleyi Larrea tridentata Means 8.1 28.8 5.8 44.9 Larrea tridentata Atriplex confertifolia Ambrosia dumosa Ambrosia dumosa Ambrosia dumosa Acamptopappus shockleyi Means 835 7.3 7.6 4.9 19.8 2600 3.5 20.5 5.7° 29.7 914 5.2 41.8 8.9° 55.8 2039 8.7 25.6 3.4° 37.7 2022 ND 46.2 8.0° ND 1277 3.9 23.7 6.5° 34.0 5.7 35.3 "These values represent retranslocation from old stems and roots to new growth. ND is not determined. + leaves were dead. Table 2. 14C status of plants from Rock Valley, Nevada, exposed to 14CC>2 in April 1973. Species Initial total 14C xlO3 % 14C fixed remaining cpm Roots Stems Leaves Total 960 40.4 31.9 72.3 999 26.9 34.2 - 61.1 2397 10.5 21.5 3.1 35.1 2092 7.9 12.7 10.7 42.3 909 29.4 28.3 - 57.7 L. andersonii G. spinosa C. lanata A. confertifolia L. pallidum Mean 53.7 L. andersonii G. spinosa C. lanata A. confertifolia L. pallidum Mean 988 26.9 29.5 1.3° 57.7 1226 13.3 39.4 7.3° 60.0 1959 13.6 44.6 4.9° 63.1 2193 4.6 12.8 2.9° 20.3 792 19.8 16.4 1.0° 37.2 28.5 47.7 "These values represent retranslocation from old stems and roots to new growth following dormancy 1980 Nevada Desert Ecology 179 Results and Discussion The amount of 14C02 fixed in the 14 pe- rennial plants exposed to 14C02 in Mercury Valley in May 1973 and the 10 in Rock Val- ley in April 1973, together with the distribu- tion among plant parts in either December 1973 or May or June 1974, are in Tables 1 and 2. From 17 to 65 percent of the 14C re- mained in the plants at sampling time, de- pending on time and location. This was the range for both 7 and 13 months at each of Table 1 continued. g new Root leaves that could have root come from % relat ive distribution of hc Dry wt g/plant + stem roots or Roots Stems Leaves Roots Stems Leaves stems Excavated in December 1973 27.2 38.9 33.9 21.0 21.2 8.8 49.8 — 7.4 46.9 45.7 20.2 44.2 45.4 31.4 - 31.3 42.8 25.9 45.9 69.9 10.0+ 39.6 - 21.4 78.6 0.0 120.4 95.5 0.0 55.8 _ 8.1 60.7 31.2 12.1 26.0 12.1 31.8 - 15.1 83.8 1.1 29.2 26.0 4.3 52.9 _ 14.2 85.8 ().() 15.6 28. 0 0.0 35.8 _ 32.6 44.6 22.8 85.0 92.0 16.0 41.0 - 19.7 60.3 20.0 - - - 46.6 - Excavated in June 1974 36.9 38.4 24.7 128.8 122.7 62.0 51.2 15.3 11.7 69.0 19.2° 87.6 137.0 123.0 39.0 23.6° 9.3 74.9 15.9° 47.0 65.1 16.1 41.7 2.6° 23. 1 67.9 9.0° 75.6 76.0 43.0 49.9 3.9° ND ND ND ND 70.0 37.9 ND ND 11.5 69.7 19.1° 25.1 35.2 22.2 41.7 4.2° 18.5 64.0 17.5° - - - 44.7 - Table 2 continued. g new Root leaves that itive distribution of 14C Dry wt g/plant could have % rek root + stem % come from roots or Roots Stems Leaves Roots Stems Leaves stems Excavated in December 1973 55.9 44.1 _ 108.9 77.1 _ 57.4 - 44.0 56.0 _ 76.4 58.5 _ 56.6 — 29.9 61.3 8.8 20.6 17.0 14.9 54.3 — 18.7 56.0 25.3 26.1 56.0 20.0 31.8 - 51.0 49.0 - 59.9 18.6 - 76.3 - 39.9 53.3 6.8 - - - 55.4 - Excavated in Mav 1974 46.6 51.1 2.3° 87.9 99.4 15.7 46.9 0.4° 22.2 65.0 12.2° 51.2 77.1 23.5 39.9 2.9° 21.6 70.7 7.8° 112.2 114.3 21.8 49.6 1.7 21.7 63.1 14.3° 40.4 58.7 25.3 40.8 3.6° 53.2 44.1 2.7° 147.6 36.3 5.7 80.3 0.2° 33.3 58.8 7.9° - - - 51.5 - 180 Great Basin Naturalist Memoirs No. 4 the areas studied. These values are of inter- Small quantities (3 to 20 percent) of the est. In a companion study with Larrea triden- 14C remaining in the plants were present in tata (Sesse & Moc. ex DC.) Cov., about 10 the sPring leaves of deciduous plants that had percent of the >C label remained in the become defoliated in the fall and winter. This plants 40 months after labeling. Losses each ™anS ** flf°m 3 \? 20 ?e\cent at ]east °f . , . ... , the new leat growth was derived from C year would come through respiration, ab- coming frQm oH stems and roQts The re seised leaves, and fruit production. mainder came from new C02 fixation. Be- Table 3. Distribution of 14C in roots of plants from Mercury Valley, Nevada, excavated seven months following exposure of leaves to 14C02. Larrea tridcntata-Mercury Valley-December 1973 (2,690,000 cpm 14C fixed) Depth Length of root, cm Drv from weight surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 100-110 of roots Root cm cpm/g dry weight 18.16 5620 2.99 4.61 0.33 2920 3.12 0.54 1.62 4.11 1.65 3840 3740 3600 3540 8.01 2.09 1.97 3500 3360 12.88 3.13 4280 0.82 Miscellaneous and fine roots 1140 cpm/g dry wt (7.8 g); litter 4640 (23.4 g); leaves 13.920 (16.0 g); stems 4720 (92.0 g)- Main - 4580 8280 2740 2480 2080 1960 A 2 2640 2480 3220 3760 3800 3840 4080 B 3 2500 2420 2740 2860 4300 4020 3780 C 2 760 920 1320 1300 1300 D 6 2720 2860 3140 3220 3340 3020 2840 E 7 4000 3820 3780 3920 4600 F 2 2760 2460 2080 2100 2180 2020 1840 G 10 2220 2180 2120 2300 3040 2520 1960 H 3 3940 6050 8020 9040 I 1 8 11 2980 2280 2900 2080 3120 2240 3300 3240 3080 3680 K 9 6160 6380 6200 6680 L 3 4660 4520 2220 4040 4260 3640 3600 M 5 6680 6960 6560 6220 5520 5180 N 6 5340 4540 4420 4460 4280 4120 4180 A trip lex eo n fe rtifo Ha - -Mercurv Vallev- December 1973 (3,340,000 cpm 14C fixed Depth Length of root , cm Dry from weight surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 of roots Root cm cpm/g dry weight g Main _ 3440 2440 2460 2920 3880 4060 10.77 A 4 2400 2200 0.31 B 8 11020 13280 12100 13000 12560 13280 12980 12020 12050 0.19 C 8 6980 7000 8200 10580 10360 10220 11060 10200 0.15 D 6 22820 16800 14200 15420 16200 17000 15210 15280 19200 1.03 E 4 3080 3000 2980 0.30 F 22 1100 1220 0.27 G 20 5980 5820 5700 5600 5460 0.33 H 30 2100 2160 2170 2220 0.24 I offD 720 780 720 340 220 0.34 I 27 2340 2200 2080 2610 2740 2.11 K 29 2040 1760 L780 0.11 Fine roots 4820 cpm/g dry wt (0.2 g); crown pieces 2100 (3.9 g); dead stump 200 (14.9g); miscellaneous roots 3000 (1.3 g); litter 9540 (30 g); leaves 13520 (45.4 g); stem 15420 (40.3 g). 1980 Nevada Desert Ecology 181 Table 3 continued. Root Ambrosia dumosa-Mercury Valley-December 1973 (1,621,000 cpm 14C fixed) Depth Length of root, cm Dry from wei§ht surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 of roots cm cpm/g dry weight g Main _ 5320 6340 6500 7820 4920 A 9 5840 6080 6160 B 9 8780 8080 7700 7280 5500 4820 C 9 5640 6800 5500 5580 6000 6440 5200 4860 D 11 4520 4960 5520 5500 5660 4320 4860 5620 E 18 5860 5960 5780 6200 6520 F 16 3820 4200 4620 G 19 1216 H 25 7140 7500 8460 5940 6050 7640 10.70 0.19 0.99 4.51 2.56 1.13 0.71 0.19 9.80 Miscellaneous and fine roots 2020 (3.6 g); litter 16540 (4.9 g); no leaves; stem 33600 (26.0 g). Atriplex confertifolia-Mercury Valley-December 1973 (2,051,000 cpm 14C fixed) Depth Length of root, cm from surface 0-10 10-20 20-30 30-40 40-50 cm cpm/g dry weight 3900 9000 Main 0 6720 4500 4960 5100 A 4 8540 9480 17800 B 9 4240 6020 7433 7680 C 11 4180 4020 3800 3720 D 25 3960 4020 4160 Dry weight of roots g 6.91 0.11 0.33 0.87 0.30 Miscellaneous and fine roots 2700 cpm/g dry wt (1.9 g); litter 13,680 (10.3 g); leaves 19,000 (12.1 g); stems 17,100 (26.0 g). Ambrosia dumosa-Mercury Valley— December 1973 (2,095,000 cpm 14C fixed) Depth Length of root, cm from surface 0-10 10-20 20-30 30-40 Dry weight of Root cpm/g dry weight Main A B C D E F G H 1 J K L M N O P — 1120 840 1220 6 2460 10 640 760 660 10 940 1080 1200 10 1800 1600 1580 10 1760 1800 2720 11 1140 1220 1300 13 1740 2440 2020 10 1640 1760 2020 15 3100 3840 4940 15 1900 1680 2220 17 860 20 200 120 100 20 1680 1960 1800 25 100 20 25 160 100 27 .500 398 20 25.28 0.04 2.11 1.26 0.78 0.63 0.60 0.85 0.99 1.09 0.58 0.13 1.24 0.81 0.24 0.20 0.47 Crown 1260 cpm/g dry wt (7.4 g); shoots 2880 (79.9 g); fine roots 1180 (2.3 g) 182 Great Basin Naturalist Memoirs No. 4 Table 3 continued. Larrea tridentata— Mercury Valley— December 1973 (2,529,000 cpm 14C fixed) Depth Length of root, cm from surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 Root cm cpm/g dry weight Main _ 14600 9900 3580 A 1/2 67380 6360 2440 A, 1/10 9360 500 2020 Aa 1/10 5940 740 B 2/10 13540 6900 5240 C 1/2 3660 2100 D 1 21040 20620 22320 15280 E 1/2 - - (not counted) F 3 500 G 3 27660 23940 25120 23500 22580 23360 21560 H 13 8620 8020 9120 9000 10400 I 20 9560 9240 9860 10720 9000 8420 Depth Length of root, cm Dry from weight surface 70-80 80-90 90-100 100-110 110-120 120-130 of roots Root cm cpm/g dry weight g Main _ 7.16 A 1/10 0.67 A, 1/10 0.10 A2 1/2 0.17 B 2/10 0.26 C 1/2 0.16 D 1 0.72 E 1/2 0.36 F 3 0.09 G 3 21000 20280 21020 216(H) 21780 21760 3.50 H I 13 20 0.72 1.87 Crown cpm/g dry wt 10,000 (2.3 g); leaf 36,860 (8.8 g) stem 18640 (21.2 g); tun roots 2740 (2.5 g). Acamptopapp us shocklcyi -Mercury Valley-December 1973 (1,343,000 cpm 14C fixed) Depth Length of root, cm Dry from weight surface 0-10 10-20 20-30 30- -40 40-50 of roots Root cm cpm/g dry weight g Main 4200 8660 3660 5.21 A 3 (not counted) 0.03 B 3 (not counted) 0.04 C 3 (not counted) 0.04 D 2 (not counted) 0.02 E 3 (not counted) 0.05 F 5 (not counted) 0.06 G 8 6220 5620 5420 5880 5600 1.37 H 7 14520 15220 18760 0.37 1 10 6660 8040 10060 0.73 J 10 6500 10900 8800 0.97 K 11 1560 1040 300 0.45 L 13 7880 8940 0.80 M 16 6660 9880 9860 23360 0.65 \ 15 4600 0.25 Miscellaneous and fine roots 6540 (2.0 g); Litter 5760 (2.3 g); no leaves; stem 21980 (28.0 g). 1980 Nevada Desert Ecology 183 Table 3 continued. Krameria parvifolia-Mercury Valley-December 1973 (622,000 cpm 14C fixed) Depth Length of root, cm from surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 Root cm cpm/g dr y weight Main _ 320 540 1360 A 7 260 220 160 160 160 300 240 80 80 100 B 4 8280 8000 C 3 12640 D 10 980 820 680 780 1540 2010 3560 5920 E 13 1100 620 400 .340 120 40 F 6 500 4S0 460 500 540 720 780 G 14 1320 980 760 920 880 1080 H 4 2560 2000 1460 1400 1480 I 12 360 340 380 340 660 620 520 400 320 280 J 9 L260 1300 1720 1610 880 780 720 780 760 1160 K 12 700 620 540 600 700 760 900 2020 L 10 740 740 780 700 720 680 720 960 1040 1020 M 7 340 280 420 680 220 300 180 Depth Length of root cm Dry from weight surface 100-110 110-120 120-130 130-140 160-170 170-180 180-190 200-250 250-300 of roots Root cm cpm/'j; dry we ight g Main _ 56.76 A 7 3.98 B 4 0.32 C 3 0.19 D 10 2.15 E 13 1.31 F 6 1.96 G 14 1.40 II 4 0.53 I 12 6.72 J 9 1160 1160 17.13 K 12 2.54 L 10 960 4.80 M 7 220 2(H) 180 240 360 500 300 380 32.25 Litter 7060 cpm /g dry weight (18.8 g); miscellaneous and fine roots 660 (1.0 g) no leaves ; stem 5980 (95.5 g) tween December and May and June there seemed to be greater loss of the 14C from roots than from leaves, but this may not be related to the transfer to the new leaves. The distribution of the 14C in segments of individual roots at sampling time, 7 to 13 months after exposure to 14C02, is in Tables 3 to 6. Although most roots were reasonably uniformly labeled for a given plant, some roots had higher activities than the majority and others were less labeled. The high specif- ic activity of roots may represent roots being formed at the time of labeling, and the low specific activity roots may be those formed after the time of labeling. Usually, each root was uniformly labeled along its length with little indication of a pulse point. It seems that old roots were labeled to some degree or other. Our expectation of finding new unlabeled roots was not fulfilled. New roots, like new leaves, seemed to have a fraction of their carbon coming from old plant parts so that they were labeled also. Old carbon then is at least in part labile in both stems and roots among the winter de- ciduous perennials. It may be less so in the evergreen L. tridentata, which seems to con- serve carbon and use very little of it in new 8 920 920 920 920 920 1000 1120 1320 1000 10 440 440 ,500 660 620 940 680 1160 1200 30 500 520 420 440 .30 240 200 160 33 160 160 33 200 200 320 480 500 640 700 720 680 184 Great Basin Naturalist Memoirs No. 4 Table 4. Distribution of 14C in roots of plants from Mercury Valley, Nevada, excavated one year after exposure of leaves to 14C02. Larrea tridentata No. 4-Mercury Valley-June 1974 (835,000 cpm 14C fixed) Depth Length of root, cm from surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 Root cm cpm /g dry weight Main 340 160 280 260 200 160 180 A B C D E F F 39 560 520 360 140 Depth Length of root, cm Drv from weight surface 90-100 100-110 110-120 120-130 120-140 140-150 150-160 170-180 of roots Root cm cpm/g dry weight g Main 56.48 A 8 700 860 1040 920 800 940 28.0.3 B 10 1160 1100 1040 1000 860 800 800 680 7.00 C 30 1.14 D 30 0.97 E 33 0.39 F 33 640 .500 280 13.00 F 39 0.4.5 Miscellaneous roots cpm/g dry wt 260 (5.0 g); leaves 660 (62.0 g); small stem 1000 (26.7 g); large stem 380 (96.0 g). Atriplex confertifolia-Mercmy Valley-June 1974 (2,600,000 cpm 14C fixed) Depth Length of root, cm Dry from weight surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 of roots Root cm cpm/g dry weight g 740 700 30.09 0.05 0.06 3.78 24(H) 0.40 0.55 920 4.17 2.50 0.42 0.06 0.09 0.32 1060 920 940 2.02 0.15 2.15 Miscellaneous roots 1980 (3.9 g); leaves 1200 (123.0 g); stem 3520 (90.0 g); stem crown 20 (37.0 g); litter 2620 (83.0 g). Ambrosia dumosa- Mercury Valley-June 1974 (2,039,000 cpm 14C fixed) Depth Length of root, cm Dry from weight surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 of roots Root cm cpm/g dry weight g Main - 1220 1980 1620 680 A 6 100 120 B 7 200 C 14 1760 1260 1520 1340 D 16 3080 2460 2900 2300 E 16 680 500 510 120 F 18 1000 1780 1040 880 G 22 3800 3400 4200 3000 H 24 1200 860 I 22 1200 440 J 27 720 4450 K 27 1100 1940 1.340 1120 L 21 1440 1140 1140 1020 M 29 1100 1325 N 19 1200 800 660 720 1980 Nevada Desert Ecology 185 Table 4 continued. Main - 2300 2360 2700 1840 1440 1460 1160 32.57 A 8 5520 6080 7320 1.11 B 9 3280 4.340 4960 .5040 0.74 c: 10 .3400 4160 0.26 D 8 2500 2120 1180 2180 1400 4.14 E 12 2340 2240 2400 3600 1.69 F 12 4160 3900 2680 4700 4380 4700 4380 3.81 G 14 2.380 2760 2820 2640 2320 2120 2460 3300 5.59 H 12 5380 5320 4980 4580 5300 4860 4540 5240 4460 3.35 I 13 680 520 540 700 1.77 f 14 1920 1540 0.45 K 18 2640 .3000 3.360 3340 3700 3200 2100 5.42 L 20 3520 2260 3160 1.41 M 25 1100 1240 1240 1060 1140 1.16 N 26 1480 1120 700 680 720 1360 1278 2.52 Miscellaneous roots cpm/g drv wt 1180 (1.0 g); leaves 1600 (43.0 g); small stem 9680 (27.6 g); large stem 5120 (48.4 g); litter 800 (5.3 g). Acamptopappu.s shockleiji— Mercury Valley— June 1974 (1,277,000 cpm 14C fixed) Depth Length of root, cm Dry from weight surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 of roots Root cm cpm/g dry weight 2120 2340 2360 14.94 0.05 0.04 0.08 0.07 0.08 0.04 2780 0.70 0.48 0.30 2320 3700 5720 5300 0.81 0.42 2820 3500 1.02 Miscellaneous roots cpm/g dry wt 2360 (1.7 g); miscellaneous roots 446 (0.7 g); leaves 3740 (22.2 g); small stems 9940 (15.0 g); large stems 7580 (20.1 g). Ambrosia dumosa— Mercury Valley No. 7— June 1974 (914,000 cpm 14C fixed) Depth Length of root, cm Dry from weight surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 of Main 0 2020 1820 1720 A 3 3930 3750 2000 B 4 2560 21000 C 5 980 D 6 1380 4000 E 5 4420 F 9 2520 G 20 1360 1300 1860 H 20 2460 2680 3520 I 20 1000 960 J 20 1860 1640 2060 K 23 2120 2400 2400 L 26 2120 2740 2540 roots Root cm cpm/g dry weight Main - 1420 1500 600 1020 16.68 1.36 0.75 0.21 0.08 0..54 1.52 2.42 4.45 660 2.65 2.65 1500 2260 2.66 A 10 480 880 900 B 9 1840 2060 2020 C 12 120 42 D 10 6091 E 13 1640 700 540 F 17 0 0 0 0 G 16 0 0 0 0 H 16 780 1000 760 420 240 I 18 860 900 800 560 720 J 16 1540 1480 1480 1120 K 16 1440 1.300 1220 1220 1400 0.24 Leaves 5040 cpm/g dry wt (16.1 g); small stems 22000 (15.9 g); large stems 4360 (49.2 g). 186 Great Basin Naturalist Memoirs No. 4 Table 5. Distribution of 14C in roots of plants from Rock Valley, Nevada, excavated eight months following expo- sure to 14C02. Ceratoides lanata-Rock Valley-December 1973 (2,397,000 cpm 14C fixed) Depth Length of roots, cm Dry from weight surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 of roots Root cm cpm/g dry weight g Main - 14280 12300 10940 7920 7960 A 3 30100 26400 27200 25800 B 5 17960 17200 18800 C 9 15820 17600 18200 20740 16800 D 10 19020 19600 18100 15210 15300 E 9 16980 18900 23100 27600 22800 F 11 10180 14600 14100 33800 30000 G 14 11040 11460 10980 12100 10160 H 15 12280 11160 10060 9920 8060 10800 12320 5260 5200 7600 7480 7300 8.08 0.09 0.12 0.23 1.19 0.26 0.11 0.82 1.85 Miscellaneous and fine roots 11000 cpm/g dry wt (5.2 g); stems 30380 (17.0 g); leaves 22420 (14.9 g); litter 4220 (14.1 g)- Atriplex confertifolia-Rock Valley-December 1973 (2,092,000 cpm 14C fixed) Depth Length of root, cm from surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 Root cpm/g dry weight Dry weight of roots g Main - 8200 4740 A 9 3660 3620 4260 4320 B 5 15260 9820 4860 4700 4720 C 13 1180 1300 1100 1200 D 17 2060 2125 1820 1700 E 18 40 20 F 20 6720 6000 G 18 8980 7210 6620 6240 6000 5920 H 18 4480 4162 3200 2080 3602 3200 4060 I 14 2760 3000 3700 3280 3620 2810 2300 5620 2500 10.1.3 0.42 0.38 0.17 0.46 0.11 0.35 1.09 2.46 3.35 Litter 12720 cpm/g dry wt (13.3 g); miscellaneous and fine roots 4240 (3.8 g); stem 8860 (56.0 g); leaves 11220 (20.0 g)- Lycium pallidum-Rock Valley-December 1973 (909,000 cpm 14C fixed) Depth Length of root, cm from surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 Root cpm/g dry weight Main _ 3460 3780 4016 3910 3880 4520 4620 A 3 1660 1580 B 6 1400 1400 C 8 4120 2620 2420 1700 1680 1640 D 8 2740 2700 1720 1700 420 302 E 10 2810 2740 F 12 6860 6600 G 18 4440 4000 43(K) 4820 5060 4340 2800 H 33 1360 1400 1800 I 40 4920 5420 7860 6040 3540 4040 4200 J 45 5560 4980 K 55 5420 5210 5200 5000 4880 L 36 7860 7520 7660 7840 7900 9500 9400 1980 Nevada Desert Ecology 187 Table 5 continued. Lycium pallidum continued. Depth Length of root, cm Dry from weight surface 70-80 80-90 90-100 100-110 110-120 of roots Root cm cpm/g dry weight g Main 5120 5000 4500 3020 28.50 A 3 0.09 B 6 0.23 C 8 0.65 D 8 0.45 E 10 0.08 F 12 0.12 G 18 2700 2.61 H 33 0.57 I 40 4460 4700 4640 3820 3700 7.63 J 45 0.17 K 55 1.19 L 36 9020 8300 1.56 Dead crown material 20 (68.4 g); live crown material 5400 (5.1 g); live roots 2800 (0.4 g); stems 4280 (13.5 g); litter 6180 (4.7 g); miscellaneous roots 2560 (7.9 g). Grayia spinosa- Rock Valley-December 1973 (999,000 cpm 14C fixed) Depth Length of root, cm Dry from weight surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 of roots Root cm cpm/g dry weight g Main 4240 5140 3640 2560 2660 17.99 A 5 960 40 0.13 B 5 5120 3620 3500 3460 3480 3420 .3460 .3410 5.03 C 4 12280 11680 12100 11920 13400 16320 0.84 D 5 9840 8120 6600 5580 4440 4310 4014 3.86 E 13 9260 9410 9600 9980 9620 8240 9210 10460 2.26 F 5 23140 22160 21000 19800 19820 0.76 G 10 2660 2600 2560 2300 2160 3000 3500 0.41 H 15 60 60 0 0 0.20 I 19 .5400 5480 5180 6900 7800 1.75 J 30 2720 2600 2580 2260 2020 1.41 K 40 2080 1960 0.25 L 38 2580 0.25 M 35 2280 2200 2420 2580 2620 3.17 Miscellaneous and fine roots cpm/g dry wt 1040 (2.8 g); litter 9580 (12.9 g); stem 5780 (55.8 g). Lye ium ande •sonii-Rock Vallev- December 1973 (960,000 cpm UC fix ed) Depth Length of root, cm from surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 Hoot cm cpm/g dr ■ weight Main _ 1020 900 720 700 700 880 7.50 600 A 6 1 1320 11000 1 1900 12000 13320 B 6 220 180 40 50 60 C 16 3000 3(XX) 3200 3100 D 15 2140 2240 2060 2100 2300 2500 3060 2980 E 13 1260 1420 1740 1920 2320 33(H) 4260 F 3 10280 8850 8170 6080 6280 G 24 360 240 180 40 60 40 11 4 2700 2680 2660 2640 2540 24(H) I 8 1820 1780 2060 2180 24(H) 2200 2160 J 8 2860 2.380 2000 2240 19(H) 3660 K 3 5980 54(H) 4260 4820 5560 79(H) 8020 8140 L 3 5320 5100 6140 6320 7220 7260 89(H) 11020 M 10 11080 7820 64(H) 4840 4.520 \ 3 60 80 80 140 240 180 320 180 O 20 160 100 100 80 0 40 2(H) 360 P 5 2080 .3000 4720 47(H) 4680 38(H) 3680 37(H) 188 Great Basin Naturalist Memoirs Table 5 continued. Lycium andersonii continued. Length of root, cm 3_90 90-100 100-110 110-120 120-130 130-140 140-150 cpm/g dry weight No. 4 Depth from surface Root cm Main _ A 6 B 6 C 16 D 15 E 13 F 3 G 24 H 4 I 8 J 8 K 3 L 3 M 10 N 3 O 20 P 5 Dry weight of roots g 750 700 620 190 600 280 2620 9060 9960 13240 8000 100 39.31 1.05 1.02 1.16 2.63 5.57 0.48 4.14 1.05 5.45 1.27 7.37 2.62 0.24 10.73 2.47 3.05 Stem crown materials 2120 (34.3 g); dead main root 5020 (35.5 g); litter 1700; stem 4820 (42.8 g). growth . A summary of the numbers of roots found with low medium and high rates of labeling is given in Table 7. There were a few roots with high amounts of label at the tip or high label near the point of attachment to the main root. This is indicative of some pulse ef- fect. The fact that 14C is uniformly distributed among different roots implies exchange and equilibrium. Table 6. Distribution of 14C in roots of plant leaves to 14CQ2. from Rock Valley, Nevada, excavated one year after exposure of Atriplex confertifolki- -Rock Vallev— Mav 1974 (2,193 .000 cpm 14C fixed) Depth Length of root, cm Drv from weight surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 of roots Root cm cpm/g di ry weight g Main _ 2660 3200 2140 1560 1440 1380 24.10 A 7 1009 0.08 B 10 1480 1840 1610 0.24 C 9 4370 6770 6200 6100 6000 0.19 D 9 8700 7670 10520 9370 17435 21200 0.26 E 8 1020 1230 1650 1920 2570 0.23 G 18 1620 3760 4840 8430 8080 0.96 H 7 1900 1740 1280 1200 1120 1020 1020 700 2.48 I 18 1240 1140 900 920 1180 1060 660 1.13 J 15 2560 2340 2100 0.49 K 13 6860 7060 8720 8500 6920 6700 6040 1.05 L 16 2460 880 0.33 Miscellaneous roots cpm/g dry wt 1340 (3.6 g); large stem 5340 (25.9 g); small stem 4380 (32.8 g); leaves 2540 (25.3 g); litter 10820 (13.8 g). 1980 Nevada Desert Ecology 189 Table 6 continued. Grayia spinosa-Rock Valley-May 1974 (1,226,000 cpm 14C fixed) Depth Length of root, cm Dry from weight surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 of roots Root cm cpm/g dry weight g Main _ 2580 2260 1500 1660 1320 1300 1360 1480 1740 2320 20.98 A 5 3720 3520 3400 3460 3080 2545 1.17 B 4 2920 1740 5962 0.24 C 5 7830 3043 0.09 D 6 2310 1130 0.11 E 5 2840 1882 643 93 0.23 F 7 3820 3100 2920 2200 2400 2280 2300 9.94 G 5 8960 7960 5940 4980 3980 4620 4.60 H 11 3220 2840 3540 5820 0.93 I 9 4160 4500 2900 1.23 J 12 2680 2680 2340 1.20 K 13 3780 3040 2050 0.26 L 17 1700 1680 2060 0.42 M 21 1980 1300 0.20 Miscellaneous roots cpm/g dry wt 4320 (2.9 g); small stem 4200 (28.2 g); large stem 7440 (48.9 g); leaves 3780 (23.5 g); litter 160 (41.3 g). Ceratoides lanata-Rock Valley-May 1974 (1,959,000 cpm 14C fixed) Depth Length of root, cm Dry from weight surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 of roots Root cm cpm/g dry weight 26.28 0.16 0.31 0.34 0.77 0.73 3000 4200 3080 1.24 0.19 0.46 2.86 0.25 0.46 0.48 1.21 1.15 1.34 0.95 0.34 0.27 0.83 0.85 0.59 0.60 0.70 0.25 1.30 Other roots 1360 cpm/g dry wt (43.2 g); leaves 4440 (21.8 g); miscellaneous roots 2940 (9.6 g); small stem 10560 (59.9 g); large stem 4440 (54.4 g); litter 12,880 (1.1 g). Main - 3500 2000 A 7 4000 4710 18040 4800 2080 1160 B C 6 6 5350 18740 8750 19100 11120 13300 2730 32900 1761 D 10 2960 2660 1580 1420 1800 3020 E 10 2740 1240 1640 860 860 F 8 7220 6840 7760 7060 4900 4200 2900 G 10 4890 6040 8640 25120 5590 H 9 5340 4720 6320 4740 4580 4460 3590 I 9 3320 3220 2020 2120 2420 1820 1840 J 8 18430 16400 11560 7150 K 10 12520 9060 8960 6580 6060 L 15 3880 2600 2680 3520 M 6 4980 3980 2080 1560 1820 940 N 8 3420 2200 2640 2640 1400 0 10 1760 2780 2740 3060 P 18 1380 1200 1100 880 o 11 10120 8100 5540 4520 R 8 240 250 143 357 294 S 12 1960 1740 1920 1560 1560 1420 T 8 5440 4520 4780 4360 3520 3260 U 11 6180 4840 4080 3240 2280 2330 V 8 4340 3940 3100 4320 w 9 4960 4880 5380 6220 6580 6260 5480 X 15 Y 8 1260 1160 1480 1210 Z 10 420 40 40 190 Great Basin Naturalist Memoirs No. 4 Table 6 continued. Lycium pallidnm-RocV Valley-May 1974 (792,000 cpm 14C fixed) Depth Length of root, cm from surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 Root cm cpm/g dry weight Main _ 1400 620 300 600 700 700 720 720 A 10 1420 980 .540 540 580 620 620 640 B 17 220 240 60 160 140 160 C 2 280 200 160 220 220 D 22 480 380 540 956 957 1675 E 22 360 400 400 160 160 F 20 200 375 160 100 120 191 G 29 580 560 560 560 600 660 540 540 H 23 160 106 300 I 25 220 180 180 120 J 25 540 .560 640 680 560 620 740 800 K 25 200 186 L 26 240 240 260 260 M 6 (large) 3680 2720 3740 4260 3580 3660 3340 3700 N 9 (off M) 1040 400 595 538 O 4 (off M) 1880 1460 1120 1220 Depth Length of root, cm Dry from weight surface 80-90 90-100 100 -110 110 -120 120-130 130-140 of roots Root cm cpi n/g dry we: ight g Main _ 800 840 620 520 620 620 80.97 A 10 740 720 2.24 B 17 4.13 C 2 0.72 D 22 0.97 E 22 1.88 F 20 0.55 G 29 580 580 11.26 H I 23 25 0.19 0.79 J 25 900 620 3.56 K 25 0.15 L 26 1.05 M 6 (large) 3800 14.52 N 9 (off M) 0.33 O 4 (off M) 0.77 Miscellaneous roots 140 epnr i/g dry wt (4.2 g); dead crown 60 (62.7 g); small stem 4060 (5.4 g ); large stem 3520 (30.9 g); leaves 1440 (5.7 g); litter 1420 (16.2 g). Acknowledgments This study was supported in part by U.S. IBP Desert Biome Program and by Contract EY-76-C-03-0012 between the U.S. Depart- ment of Energy and the University of Cali- fornia. Literature Cited Bamberg, S. A., A. Wallace, G. E. Kleinkopf, and A. Vollmer. 1973. Plant productivity and nutrient interrelationships of perennials in the Mojave Desert. US/IBP Desert Biome Res. Memo 73-10. Utah State University, Logan. 1980 Nevada Desert Ecology 191 Table 7. Summary of labeling patterns of the roots from Table 5 and 6. Species Larrea tridentata Atriplex confertifolia Ambrosia dumosa Krameria parvifolia Atriplex confertifolia Ambrosia dumosa Acamptopappus shockleyi Larrea tridentata Larrea tridentata A triplex confertifolia Ambrosia dumosa Ambrosia dumosa Acamptopappus shockehji Lycium andcrsonii Grayia spinosa Ceratoides lanata A trip lex con ferti folia Lycium pallidum Lycium andcrsonii Grayia spinosa Ceratoides lanata Atriplex confertifolia Lycium pallidum No. of No. of No. of No. of medium Date high S. A.00 low S.A.00 nonlabeled S.A.°° sampled roots roots roots roots Mercury Vallev Dec. 1973 3 5 0 7 " 3 6 0 3 " 0 1 0 8 " 2 9 1 2 " 2 0 0 3 " 1 3 2 11 " 4 1 0 4 " 5 1 0 4 June 1974 0 5 3 0 0 11 2 2 " 0 2 3 8 " 1 3 0 11 " 0 3 0 10 Rock Vallev Dec. 1973 5 2 4 5 " 2 3 0 9 8 1 0 0 2 0 1 " 2 5 0 7 May 1974 0 4 1 11 " 0 3 0 11 5 3 1 " 2 4 0 6 " 0 5 3 8 'Some roots appeared to have a pulse or a demarcation in the distribution of the C. °S.A. is specific activity of 14C. Bamberg, S. A., A. Wallace, G. E. Kleinkopf, A. Vollmer, and B.S. Ausmus. 1974. Plant produc- tivity and nutrient interrelationships of pe- rennials in the Mojave Desert. US/IBP Desert Biome Res. Memo 74-8. Utah State University, Logan. Caldwell, M. M., E. J. DePuit, O. A. Fernandez, H. H. Wiebe, and L. B. Camp. 1974. Gas exchange, translocation, root growth and soil respiration of Great Basin plants" US/IBP Desert Biome Res. Memo 74-9. Utah State University, Logan. Caldwell, M. M., E. J. DePuit, O. A. Fernandez, H. H. Wiebe, L. B. Camp, R. S. Holthausen, and H. Neuber. 1975. Gas exchange, translocation. root growth and soil respiration of Great Basin plants. US/ IBP Desert Biome Res. Memo. 75-8. Utah State University, Logan. Caldwell, M. M., L. B. Camp, R. S. Holthausen, and H. Neuber. 1976. Gas exchange, translocation, root growth and soil respiration of Great Basin plants. US/ IBP Desert Biome, Res. Memo 76-7. Utah State University Logan. Hendler, R. W. 1959. Self-absorption correction for car- bon-14. Science 130:772-777. Wallace, A., S. A. Bamberg, and J. W. Cha. 1974. Quantitative studies of roots of perennial plants in the Mojave Desert. Ecology 55:1160-1162. DISTRIBUTION OF PHOTOSYNTHETICALLY FIXED »C IN PERENNIAL PLANT SPECIES OF THE NORTHERN MOJAVE DESERT1 A. Wallace2, J. W. Cha:, and E. M. Romney2 Abstract.— The distribution of photosynthate among plant parts subsequent to its production is needed to fullv understand behavior of vegetation in any ecosystem. The present study, undertaken primarily to obtain information on transport of assimilates into roots of desert vegetation, was conducted in the northern Mojave Desert, where the mean annual rainfall is about 10 cm. Shoots of Ambrosia dumosa (A. Gray) Payne plants were exposed to 14C02 in 1971, and the distribution of 14C in roots, stems, and leaves was subsequently measured at 1 week, 2 months, and 5 months. Only about 12 percent of the 14C photosynthate was stored in the root. Much of that stored in stems was available for new leaf growth. Photosynthate was labeled with 14C for 24 plants representing eight species in 1972. Results showed that after 127 days the mean percentage of 14C in roots as compared with the estimate of that origi- nally fixed was 11.8; the percentage in stems was 43.8. The mean ratio of root to root plus stem for 14C was 0.212, but this value was only half that of the ratio for actual weights of these parts of field plants. The correlation coefficient for (14C in roots)/(14C in root + stem) X (dry wt of root)/ (dry wt of root + stem) was +0.89. Small stems were the major storage organ for the 14C. To check the validity of the 14C data, root growth of eight perennial desert plants grown in the glasshouse was followed as plants increased in size. The mean percent of the whole plant that was root for eight species was 17.7 percent. The mean proportion of the increase in plant weights that went below ground for the eight species was 19.5 percent. This value is higher than the fraction of 14C found below ground, and therefore the 14C technique underestimates the movement of C to roots. Results of an experiment de- signed to test the value of the 14C-pulse technique for determining current root growth for some perennial species from the desert indicated that the transition part of roots where root growth continued after exposure to 14C was highly labeled. Old growth contained less 14C than new growth. The distribution of the products of photo- synthesis among leaves, stems, roots, and re- productive parts must be understood if the dynamics of any plant community are to be known. Some data of this nature are available in the literature for desert plant species, but very little is quantitative (Cannon 1870, Dit- tmer 1964, Markle 1917). Jones and Hodg- kinson (1970) give values for root and shoot weights of two Atriplex species. It is, of course, recognized that shoot-root ratios of plants and assimilate distribution vary with environmental conditions (Harris 1914, Koch- enderfer 1973, Moore and West 1973, Ward- law 1969). The new photosynthate or assimilate in plants is subject to distribution among vari- ous plant parts, depending upon the phenological state. Such distribution is most likely under control of growth regulators (Richmond and Lange 1957). As phenological events change, some of the assimilate will be- come redistributed (Schmer and Knievel 1972, Moser 1977). The sinks for such redis- tribution are often known (fruiting, leaf, stem, or root growth), but the sources are of- ten obscure (leaf, stem, root, or other). In the case of redistribution prior to leaf abscission, the source is known, but the sinks are more obscure. The sources of assimilate for new growth following dormancy or defoliation from mechanical means (grazing, wind, har- vest, etc.) are also obscure. Even when the plant part that constitutes the source is known, there remains the question of what really involves available carbohydrates or other assimilates. Several 14C techniques have been devel- oped recently to study assimilate distribution in plants. The pulse technique (Caldwell et 'Findings in this paper appeared, with several modifications, in The belowground ecosystem: a synthesis of plant-associated processes. Pages 303-310 in Range Science Department Science Series Report NA 26. Colorado State University, Fort Collins, 1977. We present these findings again for conve and accessibility to readers interested in the several related papers in this volume. 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 90024. 192 1980 Nevada Desert Ecology 193 al. 1972, Wardlaw 1969), in which one or more foliar applications (pulses) of 14C02 are made and then two points of 14C concentra- tions are looked for in the root systems, was partially evaluated under glasshouse condi- tions in the present study. The technique of foliar application of 14C02, in which gross distribution among leaves, stems, roots, and reproductive parts was measured (Gej 1972, Warembourg and Paul 1973, 1977), was also used. Techniques used elsewhere include fo- liar application of 14C-urea (Clifford et al. 1973) and 14C02 labeling of specific leaves (Morooka and Kasai 1972). In the present study 14C02 was used to gain information on distribution of photosynthate and provide an estimate of annual primary productivity go- ing below ground in perennial plants of a desert ecosystem. Materials and Methods Photosynthate Distribution (14C) in Ambrosia dumosa in 1971 in the Field On 11 June 1971 about 0900, four Am- brosia dumosa (A. Gray) Payne plants in the northern Mojave Desert (1971 rainfall, 14.7 cm; 1972 rainfall, 11.8 cm) were covered with transparent plastic bags of 2 mil thick- ness, and 125 uCi 14C02 was released into each bag. Considerable water vapor con- densed on the inside of the bags. Two hours later the bags were removed and leaf and stem samples were taken from each for deter- mination by Q-gas counting of the amount of 14C fixed. The technique for 14C counting was that of Hendler (1959). All values were cor- rected to a sample size of 50 mg. Counting accuracy was made to a confidence level of 95 percent. The subsample of leaves and twigs was taken to represent between 5 and 10 percent of all those on the plant. A pre- cise number of leaves was collected in each case, and an accurate estimate of those re- maining on the plant was made so that a rea- sonably accurate assessment of the total 14C fixed by the plants could be determined. Plants were excavated after 1 week, 2 months, and 5 months. Total 14C present in small roots, large roots, small stems, large stems, and leaves was determined. Photosynthate Distribution (14C) in Eight Plant Species in 1972 in the Field On 21 March 1972 and 27 March 1972, 24 individual plants representing eight species, A. dumosa, Atriplex confertifolia (Torr. & Frem.) Wats., Lijcium andersonii A. Gray, Larrea tridentata (Sesse & Moc. ex DC.) Cov., Atriplex canescens (Pursh) Nutt., Eph- edra nevadensis Wats., Lycium pallidum Miers, and Ceratoides lanata (Pursh) J. T. Howell, were exposed to 14C02 each for 2 h in the field as in 1971. Ten mCi 14C in NaH- C03 were present in 200 ml solution and 5 ml was used for each plant. The 14C02 (250 uCi) was released inside the plastic bag by pouring HC1 into the NaHC03. After 126 to 127 days the plants were excavated and sepa- rated as before. At this time most of the leaves had abscised on the species which un- dergo summer dormancy. Photosynthate Distribution Determined by Separation of Plants into Parts for Eight Plant Species Grown in a Glasshouse Eight species of desert plants were propa- gated in the glasshouse in 1971, some by seedlings and others by cuttings, and planted individually into containers of Yolo loam soil (3.7 kg dry wt.). Nitrogen fertilizer (50 ug N/g as NH4NO3 monthly on dry weight of soil basis) was added, and the soil moisture tension was kept at around minus one-third bar during the study. The species employed were A. canescens, cuttings; A. confertifolia, cuttings; Atriplex hymenelytra (Torr.) Wats., cuttings; E. nevadensis, cuttings; A. dumosa, seedlings; L. tridentata, seedlings; L. ander- sonii, cuttings; Lycium pallidum, seedlings. After about two months, individual plants were separated into leaves, stems, and roots at approximately two-week intervals to give a series of plants of different increasing sizes. Dry weights were determined, and the sam- ples were counted for 14C contents. The num- ber of plants per species varied from six to eight replicates. 194 Great Basin Naturalist Memoirs No. 4 Evaluation of Pulse Technique for Measuring Root Growth Glasshouse studies were undertaken in 1973 to evaluate the pulse technique (Ward- law 1969) for measuring root growth follow- ing foliar fixation of 14C02. The idea is that a pulse of 14C assimilate will be transported to the growing point of the roots and that this deposited 14C mostly will not interchange with new assimilate being later transported to roots. Supposedly then, it would be pos- sible to measure root growth extension from a given point in time by identifying that point with 14C label. Also, it is believed pos- sible that growth between two time intervals could be determined by using two separate pulses. If roots produced annual growth rings, or if new assimilate exchanged with old mate- rials, the technique would be of little value. To assess the utility of the pulse technique, plants were grown in solution culture with Hoagland nutrient solution. Eight species were grown in duplicate. These were E. nevadensis, A. hymenelytra, Coleogyne ramo- sissima Torr., Atriplex cuneata A. Nels., Jun- cus mexicanus Willd., L. tridentata, L. palli- dum, and A. dumosa. Plastic bags were placed over the foliage and 5 uCi 14C02 was released into each. The roots were marked with black iron powder so that old and new root growth could be separated. After three weeks the amount of 14C in new root growth in two increments as well as in other parts of the plants was determined by the procedures given above. Results and Discussion Photosynthate Distribution (14C) in A. dumosa in 1971 in the Field In the 1971 14C-fixation study, the A. du- mosa plants fixed about 4 percent of the 14C supplied. Between the time of fixation and sampling dates, little of the 14C seemed to have been lost to respiration because recov- ery after two months was around 90 percent of that originally fixed (Table 1). An inter- esting aspect of the data was the relatively low levels transferred to the roots (9.4 per- cent at one week; 12.3 percent at two months; 10.0 percent at five months). This contrasts with 80 percent found for grass- lands by Dahlman (1968), Singh and Coleman (1977), and Warembourg and Paul (1973, 1977). The very low level with A. dumosa may indicate that the newly fixed 14C is en- tering a carbohydrate pool before transport to roots, under which conditions the label would underestimate the amount of trans- location of photosynthate to roots because of dilution in the pool. The leaves of A. dumosa seemed to serve as a storage sink for some time, but the major storage sinks were twigs and stems (Table 1). Ambrosia dumosa is a deciduous plant, so that photosynthate remaining in leaves is lost to the plant at the time of leaf abscission. Stored reserves in the stems become mobi- lized and are used in early development of new growth when environmental conditions become favorable. In A. dumosa the time of new growth development depends mainly on adequate soil water and is somewhat inde- pendent of temperature (Wallace and Rom- ney 1972). The transport of about 10 percent of the 14C label below ground in fieldgrown plants contrasts with the 16.3 percent of new growth of the glasshouse plants cora- partmented in roots (see below). For solution culture (see also below) the root/root + stem for 14C was 8.2 percent and for dry weight 28 percent. In the plant sampled 5 months after label- ing with 14C, 56 percent of the estimated 14C02 fixed was still present in the plant (Table 1). In addition to respiration losses and losses from abscised leaves, there were losses due to flowering and fruiting and possibly also to consumption by herbivores. A portion (13.5 percent of the 56 percent) was present in new leaves that had grown in response to a late summer rain. At this point the root to root plus stem ratio for the 14C was 20.7 per- cent, which is considerably less than for the weights of field plants (53.6 percent) (Wal- lace et al. 1974). One possible indication is that biomass losses from stems (animal, weather) are greater than losses from roots. Root to root and stem dry weight ratios of old plants then would be higher than the same ratio for 14C measured after a short pe- 1980 Nevada Desert Ecology 195 riod. As mentioned previously, the mixing of 14C in a carbohydrate pool before trans- location is another possibility. Photosynthate Distribution (i4C) in Eight Plant Species in 1972 in the Field The 1972 data confirm the trend indicated by A. dumosa in the 1971 study (Table 2). In comparison with the estimated amount of 14C originally fixed, the mean 14C in roots for the eight species was 11.8 percent. It ranged from a low of 3.9 percent with A. confer- tifolia to a high of 22.3 percent for L. palli- dum. These are the same species with low and high transport values for the glasshouse study (Table 3) and for root to root plus stem dry weight ratios from a field study (Wallace et al. 1974). The correlation coefficients for the root to root and stem ratios for 14C and the ratios of weights for field plants (Wallace et al. 1974) was +0.89. Again the ratios for 14C are much below those for weight. The hypotheses mentioned above for A. dumosa presumably apply to all the other species studied. That is, in the field biomass loss is greater for stems than for roots so that the measured ratio is greater than the ratio of new photosynthate distributed between stems and roots. Also there may be some exchange between the labeled assimilate and older car- bohydrates due to presence of pools, particu- larly for A. confetti folia, although a large proportion of this species is leaves and seed or flowers. The transfer to roots of 14C was especially low in those species which retained a high proportion of leaves at time of sampling. This was pronounced for A. confertifolia, A. ca- Table 1. Distribution of 14C label of photosynthate in plant parts of A. dumosa (1971). 1 week 2 months 5 months cpm fixed (2 h)° per plant 2,600,000 2,400,000 2,700,000 % remaining 98 90 g dry wt/plant 56 Leaves 18.77 ' 9.54 3.96°° Small stem 37.87 12.16 19.43 Large stem 27.99 42.97 25.12 Large roots 36.72 46.82 27.09 Small roots 6.33 8.02 6.38 % distribution at ampling times of 14C remaining in plant Leaves 57.0 22.2 ± 7.31 13.5° ° Small stem 25.7 35.4 ± 8.37 43.3 Large stem 7.7 28.8 ± 14.31 25.3 Large roots 8.5 10.4 ± 1.77 15.6 Small roots 1.1 3.2 ± 0.15 2.3 Total 100.0 100.0 100.0 % of origi nal fixed 14C in stems and roots at sampling times Small stem 25.3 31.9 ± 7.5 24.2 Large stem 7.6 25.9 ± 12.9 14.2 Large roots 8.3 9.4 ± 1.6 8.7 Small roots 1.1 2.9 ± 0.14 1.3 Total roots 9.4 12.3 10.0 3.50 14C in stems/14C in roots (ratio) 4.90 22.2 'C root/root + stem (%) 17.5 20.' ± is plus or minus the standard deviation. °cpm fixed at 50 mg counting wt ° "Original leaves had abscised and a new flush of leaves had grown in response to late summer rain, but some of these leaves had abscised also. 96 Great Basin Naturalist Memoirs No. 4 xescens, C. lanata, and L. tridentata, in vhich the mean was 7.1 percent. The mean or 14C found in roots for the other four spe- cies was 16.5 percent. The latter may be the nore accurate estimate for distribution of lew photosynthate below ground for the ime period involved. Perhaps a longer time vould be needed to evaluate root transport or the evergreen or near evergreen species. Mso, temperature and soil water may change ransport of assimilates (Schmer and Knievel L972, Wardlaw 1969). Phenology conditions may also induce transport to roots at later dates. From the various studies made, it seems possible that only 10 to 20 percent (some- times less) of the annual photosynthate goes into the root systems. Considering, however, that there are aboveground losses to respira- tion, herbivores, leaf abscission, wind, flower- ing, and fruiting, the estimates may be realis- tic. Net standing biomass of aboveground and below-ground structural plant parts is close to 2.5:1 (Wallace et al. 1980, this volume) in Table 2. Distribution of 14C label of photosynthate in parts of field-grown plants (1972) (after 126 or 127 days). Ambrosia dwnosa Atriplex confertifolia Lycium pallidum Species Lycium Larrea andersonii tridentata Atriplex canescens Ceratoides lanata Ephedra nevadensis Number of plants 4 14C fixed in 2 h (10s Cpm /plant) 3.637 4 4.411 4 4.127 2 3.297 2 1.046 2 4.558 3 4.488 3 1.643 % remain- ing at sampling 63.5 86.6 66.2 66.5 77.6 78.5 78.2 90.4 Leaves 10.9 30.1 g dry wt/plant at 1.6 0.0 sampling time 6.4 39.4 13.4 Large stems 48.5 15.8 18.7 22.3 3.2 39.7 11.8 - Small stems 59.8 16.4 16.1 31.5 3.0 38.7 22.9 — Roots 118.4 14.7 62.2 25.0 4.2 29.5 9.7 82.4 Leaves 0.50 2.41 14C in plant parts at san 0.11 0.0 pling (106 epm/plant) 0.36 1.79 0.79 Stems 1.24 1.24 1.70 1.76 0.34 1.39 2.48 1.24 Roots 0.56 0.173 0.92 0.43 0.11 0.40 0.24 0.25 Final distribution of 14C in stems and roots 14C fixed in 2 h (10s Cpm /plant) 20.3 Small stem 48.7 Large root 19.7 Small root 11.3 Total 100.0 14C roots0 14C root + stem 0.310 19.7 68.0 7.2 5.1 100.0 0.122 23.3 41.6 23.3 11.8 100.0 0.351 17.4 63.0 11.0 8.6 100.0 0.196 26.5 48.7 12.2 12.6 100.0 0.244 38.9 38.8 13.4 8.9 100.0 0.224 15.3 75.9 4.8 4.0 100.0 0.08" 16.2 67.1 9.0 100.0 0.168 Stem Roots Percentage of original fixed 14C in roots or stem at harvest time 34.2 28.1 ^ 41.2 53.4 32.5 30.5 55.3 75.5 15.4 3.9 22.3 13.0 10.5 8.8 5.3 15.2 "«: 'Correlation coefficient of "r root + stem g root + g stem field 1980 Nevada Desert Ecology 197 spite of the low percentage of new assimilate going below ground. Small stems (twigs) may constitute the major storage site for carbon in these desert plants. The 1971 and 1972 years resulted in relatively little biomass produc- tion because of limited rainfall. This may af- fect the proportion of photosynthate being transported and stored in various plant or- gans and that lost through reproductive pro- portions being stored below ground. Photosynthate Distribution Determined by Separation of Plants into Parts for Eight Plant Species Grown in a Glasshouse In the glasshouse study on roots, the per- centage increase in dry root weight com- pared with the percentage increase in total weight as plants increased in size indicated a mean percentage of new growth going below ground of 19.5 percent (Table 3). Highest value was for L. pallidum (33.7 percent) and lowest was for A. confertifolia (4.7 percent). In a companion field study with eight spe- cies, the highest root to root plus stem ratio was for L. pallidum (62.2 percent), and the lowest was for A. confertifolia (29.9 percent) (Wallace et al. 1974). The correlation coefficient between the ratios in Table 3 and the root to root plus stem ratio for the field (last column in Table 3) was + 0.98. Evaluation of Pulse Technique for Measuring Root Growth The results of the glasshouse 14C studies of plants in solution culture revealed that the pulse technique (Wardlaw 1969) may have some value in providing an estimate of cur- rent root growth (Table 4). There was a hot spot at the transition zone where growth con- tinued after the date of exposure to 14C. The 14C, at least for the three weeks of the study after exposure to 14C02, continued to be transported to the new roots. It is possible that this would not continue indefinitely, but even so there definitely would not be a com- pletely sharp demarcation between roots grown before and after the date of labeling because both old and new growth contained 14C. Carbohydrate pools, exchange among carbohydrates, as well as root developmental biology must be better understood to eval- uate such techniques. Caldwell et al. (1972) came to similar conclusions. The root/root + shoot ratio was generally much higher for dry matter than for 14C (Table 4). This indicates that 14C02 may not be an accurate means of determining below-ground biomass. Conclusions The studies indicate that, as an average, somewhere around 10 to 20 percent of the carbon fixed by the perennial shrubs in the northern Mojave Desert was subsequently found below ground after a few months. Two different techniques gave close to the same results, although actual weighing of parts of plants grown under semicontrolled conditions gave higher values for transport to roots than did the 14C procedure. Possible reasons for 14C to underestimate the amount of below- ground transport may be the mixing of 14C pools of carbon, so that the amount trans- ported would be diluted in its content of 14C and also the loss of roots in sampling. The closeness of the two methods indicates that the proportion of the carbon fixed in photosynthesis in these woody plants that is transported below ground is much less than 50 percent, although for the standing biomass of these species 50 percent or more of it is below ground (Wallace et al. 1974). The dif- ferences, however, are not difficult to recon- cile because the processes of respiration, flowering, fruiting, leaf abscission, harvesting by herbivores, etc., are constantly causing losses of carbon. In the Great Basin desert, Bjerregaard (1971) found much higher values for below-ground standing biomass than those found by us for the northern Mojave Desert. Grazing has occurred recently in that desert, however, but there has been no graz- ing in our study site for over three decades, and this may be an important factor in the differences. Best answers for the questions of partition- ing of photosynthate to below ground or to root, perhaps, can be obtained from field studies in which new seedlings are monitored for gas exchange (Koller 1970) for some years and for losses of carbon due to phenological 198 Great Basin Naturalist Memoirs Table 3. Root, stem and leaf relationships for the plants grown in the glasshouse. No. 4 No. Root Stem Leaf of plants % distribution A. dumosa 9 20.5 44.4 35.1 E. nevadensis 10 14.5 85.5 — A. hymenelytra 6 19.3 27.7 53.0 A. confertifolia 8 4.3 30.9 64.8 A. canescens 7 12.0 41.8 46.2 L. pallidum 9 26.3 59.3 14.4 L. andersonii 7 21.9 73.0 5.1 L. tridentata 7 23.5 39.6 36.9 Means 17.7 "Calculated by using the smallest plant as the base, root "C.V. is coefficient of variation of — — root + stem •Reference (Wallace et al. 1974). events or harvesting. After several years the plants could be excavated and below-ground parts measured. If root losses due to soil fauna were minimal, then below-ground transport could be estimated fairly accurately for field conditions. Such experiments would be costly, however, and would require sever- al years. We did a study slightly like this for 40 months (Wallace et al. 1980, this volume). The present estimates, although crude, per- haps would serve most purposes. Acknowledgments This study was supported in part by Con- tract EY-76-C-03-0012 between the U.S. De- partment of Energy and the University of California and by the U.S. IBP Desert Biome Program. Literature Cited Bjerregaard, R. S. 1971. The nitrogen budget of two salt desert shrub plant communities of western Utah. Ph.D. dissertation. Utah State University, Logan. Caldwell, M. M., R. T. Moore, R. S. White, and E. J. Depuit. 1972. Gas exchange of Great Basin shrubs. USIBP Desert Biome RM 72-20. Utah State University, Logan. Cannon, W. A. 1870. The root habits of desert plants. Carnegie Inst, of Wash. Pub. No. 131. Clifford, P. E., C. Marshall, and G. R. Sager. 1973. Value of carbon- 14-labeled-urea as a source of carbon- 14 dioxide for studies of assimilate distri- bution. Ann. Bot. (London) 37: 37-44. Dittmer, H. J. 1964. Certain characteristics of the roots of desert plants. Amer. J. Bot 51: 673. Dahlman, R. C. 1968. Tagging native grassland vegeta- tion with 14C. Ecology 49: 1*99-1203. Gej, B. 1972. Leaf uptake of carbon-14 dioxide and translocation of carbon-14 assimilates in bean plants grown under conditions of phosphorus-, calcium-, or potassium-deficiency. Bull. Acad. Pol. Sci. Ser. Sci. Biol. 20:803-808. Harris, F. S. 1914. The effect of soil moisture, plant food, and age on the ratio of tops to roots in plants. J. Amer. Soc. Agron. 22: 65-75. Hendler, R. W. 1959. Self-absorption correction for car- bon-14. Science 130: 772-777. Jones, R., and K. C. Hodgkinson. 1970. Root growth of rangeland chenopods: morphology and produc- tion of Atriplex nummularia and Atriplex vesi- caria. Pages 77-85 in R. Jones, ed. The biology of Atriplex. Commonwealth Scientific and Industri- al Res. Org., Canberra, Australia. Kochenderfer, J. W. 1973. Root distribution under some forest types native to West Virginia. Ecolo- gy 54: 445-448. Koller, D. 1970. Determination of fundamental plant parameters controlling carbon assimilation and transpiration by the null-point compensating sys- tem. USAEC Report UCLA # 12-797. Markle, M. S. 1917. Root systems of certain desert plants. Bot. Gaz. 64: 177-205. Moore, R. T., and N. E. West. 1973. Distribution of galleta roots and rhizomes at two Utah sites. J. Range Management 26: 34-36. 1980 Nevada Desert Ecology 199 Table 3 continued. Mean Root Stem Root C.V.°° Root000 increase roo + stem root + stem drv wt (field data) in roots" % in root g g % SEM % % % in whole plant 26.4 5.33 10.43 33.8 3.01 26.7 53.6 13.2 1.03 6.0S 14.5 0.68 14.8 45.5 19.5 4.52 6.47 41.1 2.57 15.3 — 4.7 1.19 8.59 12.2 1.66 38.4 29.9 12.7 3.15 11.77 21.1 2.01 25.2 40.2 33.7 5.74 12.91 30.8 1.96 19.1 62.2 20.9 6.56 19.20 25.5 2.72 28.1 45.5 25.0 4.78 8.07 37.2 2.94 20.9 55.3 19.5 Table 4. Distribution of foliar absorbed 14C from plants grown in solution culture after new roots had developed three weeks after the 14C02 application. Root Leaf Stem Root Root Root root + old transition0 new stem Species i pin 14C/gdry wt ratio Ephedra nevadensis 'ISM I _ 380 640 600 0.013 Atriplex hymenelytra 69120 9920 .580 2780 1300 0.063 Coleogyne ramosissima 65520 8580 680 2280 800 0.073 Atriplex cuneata 30360 28940 lis JO 34720 29000 0.352 Junius mexicanus r>7oo°° 7040 36940 18240 0.451 Larrea tridentata 28180 10240 2700 .3040 2080 0.043 Lycium pallidum 13260 6440 1760 34400 34160 0.092 Ambrosia dumosa 20860 gdrv 13140 wt plan I960 t part 14820 7980 0.082 Ephedra nevadensis 17. 12°° — 2.06 1.26 0.916 0.21 Atriplex hymenelytra 1.61 1.10 1 .01 0.039 0.030 0.50 Coleogyne ramosissima 0.72 1.04 1.09 0.025 0.022 0.52 Atriplex cuneata 1.79 0.63 0.56 0.031 0.037 0.50 Junius mexicanus 9.02°° — 6.29 0.201 0.029 0.42 Larrea tridentata 3.29 7.95 1.06 0.202 0.089 0.15 Lycium pallidum 1 . 15 11.51 2.63 0.038 0.046 0.19 Ambrosia dumosa 4.33 5.28 1.85 0.132 0.081 0.28 'New root growth adjacent to the old. °° Mostly stem tissue. Morooka, M., and Z. Kasai. 1972. Translocation of car- bon-14 assimilated by a flag leaf of rice plants in- fluenced bv nitrate-nitrogen or ammonium-nitro- gen. Nippon Dojo-Hiryogaku Zasshi 43(10): 380-382. Moser, L. E. 1977. Carbohydrate translocation in range plants. Pages 47-71 in R. E. Sosebee, ed. Range- land plant phvsiology. Range Science Series No. 4. Richmond, A., and A. Lange. 1957. Effect of kinetin on detached protein content and survival of Xan- thium leaves. Science 125: 650-651. Schmer, D. A., and D. P. Knievel. 1972. Carbohydrate translocation in blue grama, buffalo grass and western wheat grass as influenced by temper- ature and growth stage. USIBP Grassland Biome Tech. Report No. 160. Colorado State University, Fort Collins. Singh, J., and D. C. Coleman. 1977. Evaluation of func- tional root biomass and translocation of photo- assimilated carbon- 14 in a shortgrass prairie eco- system. Pages 123-136 in J. K. Marshall, ed. The below-ground ecosystem. Range Science Series No. 26, Colorado State University, Fort Collins. 200 Great Basin Naturalist Memoirs No. 4 Wallace, A., and E. M. Romney. 1972. Characteristics of Franseria dumosa (burro bush or burr sage). Pages 151-161 in A. Wallace and E. M. Romney. eds. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Techni- cal Information Services, USAEC Report TID- 25954. Wallace, A., E. M. Romney, and J. W. Cha. 1980. Per- sistence of 14C labeled carbon in Larrea triden- tata up to 40 months after photosynthetic fixation in the northern Mojave Desert. Great Basin Nat. Mem. 4:170-174. Wardlaw, I. S. 1969. The effects of water stress on translocation in relation to photosynthesis and growth effect during leaf development in Lolium temulentum L. Aust. J. Biol. Sci. 22: 1-16. Warembourg, F. R., and E. A. Paul. 1973. The use of 14CC^ canopy techniques for measuring carbon transfer through the plant-soil svstem. Plant Soil 38:331-335. 1977. Seasonal transfers of assimilated 14C in grassland. Plant production and turnover, trans- location and respiration. Pages 133-141 in J. K. Marshall, ed. The below-ground ecosystem. Range Science Series No. 26, Colorado State Uni- versity, Fort Collins. DEPTH DISTRIBUTION OF ROOTS OF SOME PERENNIAL PLANTS IN THE NEVADA TEST SITE AREA OF THE NORTHERN MOJAVE DESERT A. Wallace1. E. M. Romney', and J. W. Cha1 Abstract.— The root systems of 48 perennial plants, representing nine species from the Rock Valley area within the northern Mojave Desert, were excavated by 10 cm depth increments to determine, by depth of soil, the distribu- tion of roots larger than about V2 mm diameter. The depth of the root zone of all species was relatively shallow and abviously limited bv depth of penetration of precipitation (about 10 cm mean annual rainfall). There were species differences, however, in distribution of roots. Even though a sizeable proportion of the root systems was in the first 10 cm of soil, this portion consisted largely of multiple woody tap roots with relatively few small roots. In all cases except one (Krameria parvifolia Benth), more small roots were in the second 10 cm than in the first. From 50 to more than 80 percent of the total root systems were in the first 20 cm. In most cases the majority of small roots was found between 10 and 30 cm in depth. Very fine roots were sampled separately by depth *nd zone without regard for species because they could not be differentiated by species. Relative depth distribution af very fine roots at Rock Valley for 0-10, 10-20, and 20-30 cm, was about 17, 42, and 41 percent, respectively. The total for the first 20 cm was 59 percent. On a 22 April date, there were 225 kg/ha roots from winter annuals in the Rock Vallev area; 19 percent of them were in the first 5 cm of soil in contrast to 8 percent in 10 cm of soil for perennials. On Pahute Mesa located in the southern Great Basin desert area of the Nevada Test Site in Artemisia tridentata Nutt. var. tridentata, 8 percent of the roots was in the first 5 cm, indicating more shallow rooting com- pared with the northern Mojave Desert. Any understanding of the role of soil on desert ecosystems requires that the distribu- tion of plant roots in soil profiles be known. This investigation was to obtain some of this information. Rooting habits of desert plants in the western United States have been stud- ied with conclusions that they generally are not deep-rooted unless they are in places where rain water accumulates (Cannon 1870, Dittmer 1964, Markle 1917, Waterman 1923). These workers recognized that depth of rooting was often limited by caliche layers near the soil surface or by unfavorable soil chemistry or soil physics. None of them, how- ever, reported quantitative information on the amounts of roots at different depths. Con- sequently, the distribution with depth of roots of several major perennial plants in the Rock Valley area of the northern Mojave Desert was obtained. Materials and Methods Root systems of 48 individual plants repre- senting nine species were excavated during the spring and summer of 1972. The species, with numbers of individuals sampled, were: Atriplex canescens (Pursh) Nutt. (four-wing salt bush) (6), Acamptopappiis shockleyi Gray (3), Atriplex confertifolia (Torr. & Frem.) Wats, (shadscale) (7), Larrea tridentata (Sesse & Moc. ex DC.) Cov. (creosote bush) (3), Ephedra nevadensis Wats. (Mormon tea) (7), Lijcium andersonii A. Gray (wolfberry or desert thorn) (5), Lycium pallidum Miers (wolfberry or desert thorn) (6), Krameria par- vifolia Benth. (3), Ambrosia dumosa (A. Gray) Payne (burro bush) (8). The numbers in parentheses refer to the number of plants ex- cavated for each species. Nomenclature of the species follows Beatley (1976). These col- lections were made in connection with other studies that involve the shoot:root relation- ship of perennial desert plants in the field. The excavations were made by hand shovel and roots were separated by 10 cm depth in- crements. The soil was carefully excavated for each plant and often 1 to 3 m3 of soil was re- moved. The soil was not screened to remove 'Laboratory of Nuclear Medicine and Radiation Bioloijv, University of California, l.os Angeles. California 9(K)24. 201 202 Great Basin Naturalist Memoirs No. 4 Table 1. Distribution by depth of roots from nine perennial plant species collected from Rock Valley to northern Mojave Desert (values are percent of total root system). A. shockleyi L. tridentata L. ancle rsonii L. pallidum Depth cm (3) (3) (5) (6) Large roots 0-10 45.7 ± 9.4 24.4 ± 0.8 25.9 ± 5.4 27.5 ± 4.5 10-20 25.3 ± 7.5 25.4 ± 1.0 15.5 ± 3.6 28.3 ± 4.8 20-30 5.2 ± 2.9 12.6 ± 1.9 15.1 ± 2.6 9.8 ± 2.0 30-40 0.8 ± 0.8 7.0 ± 1.6 9.2 ± 2.0 5.4 ± 0.4 40-50 0.0 3.1 ± 2.2 8.7 ± 3.8 3.5 ± 1.6 Over 50 0.0 0.0 0.0 0.0 Small roots 0-10 5.9 ± 1.6 2.1 ± 0.7 2.2 ± 0.8 2.9 ± 1.1 10-20 8.5 ± 6.1 8.3 ± 2.6 8.2 ± 2.0 10.5 ± 3.4 20-30 7.1 ± 6.4 7.2 ± 1.7 7.3 ± 1.7 6.5 ± 2.5 30-40 1.5 ± 1.5 4.1 ± 1.5 4.6 ± 1.2 3.2 ± 0.9 40-50 0.0 2.9 ± 1.8 3.1 ± 0.7 2.6 ± 1.1 Over 50 0.0 0.0 0.0 0.0 Percent of Total 23.0 24.6 25.5 25.7 ± is standard error of mean. Numbers in parentheses under species are number of plants in sample. fine roots (smaller than about Vi mm) but suf- ficient soil was removed with each plant to obtain the large majority of the root system. We estimate that no more than 30 percent of the root system was missed and this mostly because of very fine roots that were handled separately. Plants were selected to give min- imum interference to adjoining shrubs. The fine-root problem was handled as follows: In April and September 1976 at the Nevada Test Site, a series of soil samples 1 liter in volume each were collected on the patterns for samples used by Bamberg et al. (1974). The purpose was to estimate the fine roots and organic debris floated with conventional salts. Only a portion (35 percent) of the or- ganic debris was considered as roots because that was the maximum possible according to our 14C labeling techniques (Wallace et al. 1980, this volume). Results and Discussion The mean weight of root systems of field plants, together with percentage distribution by depth with standard errors for each in- crement, are given in Table 1. Virtually all the root systems were distributed in the first 50 cm of soil. Most of the biomass was at depths more shallow than that. Means for the nine species showed 39 percent in the first 10 cm, 70 percent in the first 20 cm, 86 percent in the first 30 cm, and 95 percent in the first 40 cm. This shallow rooting is related to the sparcity of precipitation [mean annual is about 10 cm (Beatley 1967, Wallace and Romney 1972)] and with the presence of a caliche layer at 30 to 50 cm. Phenology of the species concerned over a four-year period has been reported (Wallace and Romney 1972) as has the behavior of winter annuals in the area (Beatley 1967). The portion of the root system in the first 10 cm of soil, though relatively large, was mostly in the form of multiple taproots. Fur- ther evidence of this was the small propor- tion of small roots to total roots in this zone (mean was 3.2 percent for eight of the nine species compared with 8.7 percent for the second 10 cm). Most of the small roots were in the 10 to 30 cm zone. It can be expected that high temperatures of soil surfaces, to- gether with the fact that soil surfaces are drier than lower horizons, are responsible for this behavior. These two factors would ac- count for the sparsity of small roots in the first 10 cm of soil. There were species differences in root dis- tribution. Acamptopappus shockleyi and K. 1980 Nevada Desert Ecology 203 Table 1 continued. E. nevadensis A. dumosa K. parvi folia A. canescens A. confertifolia (7) (8) (8) (6) (7) (above 2 mm) 38.4 ± 5.3 24.8 ± 2.4 39.9 ± 3.3 39.8 ± 5.0 39.7 ± 6.1 19.7 ± 3.6 25.7 ± 2.2 20.1 ± 5.2 14.9 ± 2.2 16.1 ± 1.5 11.2 ± 2.0 10.4 ± 1.7 2.1 ± 2.1 10.6 ± 2.5 6.7 ± 1.5 5.2 ± 1.8 4.0 ± 1.6 2.0 ± 2.0 4.9 ± 1.8 2.8 ± 0.6 1.0 ± 0.4 1.7 ± 1.2 0.0 6.0 ± 2.0 1.4 ± 0.6 0.0 0.0 0.0 1.4 ± 1.4 1.2 ± 1.2 (2 mm or less) 1.6 ± 1.0 2.9 ± 0.7 16.3 ± 7.7 3.4 ± 1.1 6.1 ± 1.1 5.7 ± 1.1 9.9 ± 1.9 14.3 ± 3.3 10.7 ± 2.6 10.0 ± 3.0 10.6 ± 3.3 6.4 ± 1.6 2.9 ± 2.9 8.4 ± 1.4 7.4 ± 1.6 5.4 ± 2.2 4.4 ± 0.8 2.5 ± 2.5 4.9 ± 1.4 5.6 ± 1.5 1.1 ± 0.5 1.4 ± 0.9 0.0 4.5 ± 1.2 2.4 ± 0.9 0.0 0.0 0.0 1.1 ± 0.7 0.6 ± 0.6 24.6 24.4 36.0 33.0 32.1 Table 2. Root sampling in Rock Valley, 22 April 1976, in typical Lycium pallidum dominated area, using the pattern of Bamberg et al. (1974a). Inter- space (80%) In canopy (13.3%) Under plant (6.7%) Total (100%) Large roots ° Small roots Fine roots Fine roots in organic debris00 Large roots" Small roots Fine roots Fine roots in organic debris00 Large roots" Small roots Fine roots Fine roots in organic debris" Totals 0-10 cm 10-20 cm 20-30 cm kg/ha 0- Id cm 10 3 3 16 26 7 11 44 kg/ha 10 -20 cm 105 - 32 137 54 _ 3 57 30 11 11 52 68 7 22 97 kg /ha 20-30 cm 143 14 7 164 38 7 5 50 30 4 9 43 75 9 15 99 579 62 118 759 kg/ha totals by depth 36 10 14 60 257 18 68 343 286 34 36 356 "The large tap and main branching roots were not included in the sample. "The maximum amount of the organic debris obtained with salt flotation that would be considered i labeling (Wallace et al. 1980); value reported here takes that into account. 35 percent, a value determined with I4C 204 Great Basin Naturalist Memoirs No. 4 Table 3. Root sampling in Frenchman Flat, 22 April 1976, in typical Ambrosia dumosa dominated area, using the pattern of Bamberg et al. (1974a) (large roots not sampled). Inter- In space canopy (80%) (13.3%) Large roots (3 mm)° Small roots (1 to 3 mm) Fine roots (< 1mm) Fine roots in organic debris' Under Total plant (6.7%) (100%) Large roots" Small roots Fine roots Fine roots in organic debris0 Large roots" Small roots Fine roots Fine roots in organic debris0 Totals kg/ha 0-10 cm - - - 15 3 32 51 kg/ha 10-20 cm _ 13 5 18 — 4 1 5 42 8 11 61 kg/ha 20-30 cm - 25 14 39 - 5 2 6 18 17 1 36 17 15 3 35 251 "See Table 2. "See Table 2. parvifolia were more shallow rooted than other species. More than 85 percent of the root systems for these two species was in the first 20 cm. Lower stems of K. parvifolia were usually covered with about 10 cm of blow sand because of the catchment nature of the shrub, so that roots actually were not as close to the surface as indicated. Lyciiim andersonii roots were more uniformly dis- tributed throughout the root zone than most other species, although L. pallidum was somewhat similar. The two species that re- Table 4. Root sampling in Mercury, 22 April 1976, in typical Lycium andersonii dominated area, using the pat- tern of Bamberg et al. (1974a). Liter- In space canopy (13.3%) Under plant (6.7%) Total (100%) Large roots" Small roots Fine roots Fine roots in organic debris0 Large roots" Small roots Fine roots Fine roots in organic debris Large roots" Small roots Fine roots Fine roots in organic debris0 Totals 'See Table 2. "See Table 2. kg/ha 0-10 cm — — 21 21 1 — — 4 4 S 17 - 4 21 i 18 12 87 117 \ kg/ha 10-20 cm — _ 9 9 : 1 — _ 7 7 > 51 2 7 60 84 5 48 137 kg/ha 20-30 cm - 25 457 482 — 5 60 65 27 17 22 66 53 15 75 143 250 81 801 1132 1980 Nevada Desert Ecology 205 main photosynthetically active longer in the season than others (L. tridentata and K. par- vifolia) were not too much unlike other plants, except for the shallow nature of K. parvifolia mentioned above. Krameria parvi- folio had a greater proportion of small roots than did other species. Depth distribution of the very fine roots for Rock Valley was in kg/ha, 60, 149, and 142 for 0-10, 10-20, and 10-30 cm, respec- tively (Table 2). This was not different from roots in general. The surface soils of the northern Mojave Desert are low in both large and fine roots, and this probably is related to high soil surface temperatures and low soil moisture of the summer months. This condi- tion (few perennial roots in the surface 10 cm) does support a relatively large number of winter annuals after normal winter rainfall (Turner and McBrayer 1974). Soil samples were also taken to measure primarily fine roots in Frenchman Flat and Mercury Valley by the procedures of Bam- berg et al. (1974). These were not designed to collect the large and intermediate roots, al- though some appear in the samples (Tables 3 and 4). Indicated were 251 kg/ha for small and fine roots in the site in Frenchman Flat Table 5. Depth distribution of roots from an ed 22 April 1976). plants from different locations on the Nevada Test Site (collect- 100% of area, kg/ha 20% of area, kg/ha French- man Mercun Flat Rock Valley Mercury French- man Rock Flat Valley Littei 0-5 cm deptli 870 367 Large roots0 Small roots Fine roots Fine roots in organic debris Large roots" Small roots Fine roots Fine roots in organic debris' Large roots" Small roots Fine roots Fine roots in organic debris0 Large roots0 Small roots Fine roots Fine roots in organic deb Totals 0-5 cm 5-10 cm 10-20 cm 20-30 cm "See Table 2. °°SeeTable2. — - — _ _ 38 - 25 - - 5 — — 24 _ _ 5 570 181 87 114 36 18 5-10 cm depth _ _ 20 _ _ 4 145 5 27 29 1 5 212 31 76 38 5 15 10-20 cm depth 183 — LSI 37 — 36 L96 109 62 29 22 12 227 53 42 45 11 8 444 82 193 99 16 39 20-30 cm depth 146' - - 29 - - 185 — — 37 — 15 163 25 77 33 5 9 511 104 228 101 21 46 3013 590 1278 602 118 255 Totals h\ depth 570 181 325 114 36 66 387 36 123 77 7 24 444 82 193 99 16 39 1006 129 352 201 26 70 206 Great Basin Naturalist Memoirs No. 4 Table 6. Roots in soil samples collected in Larrea-Ambrosia communities on 24 September 1976. Values norma- lized 17 percent ash and corrected (organic debris corrected to 35 percent). Inter- In space canopy (80%) (13.3%) Under plant (6.7%) Total (100%) Large roots" Small roots Fine roots Fine roots in organic debris" Large roots ° Small roots Fine roots Fine roots in organic debris' Large roots" Small roots Fine roots Fine roots in organic debris' kg/ha 0-10 cm _ _ 19 19 25 19 8 52 19 18 81 118 kg/ha 10-20 cm _ 137 48 185 46 34 26 106 16 8 43 67 kg /ha 20-30 cm — 23 42 65 _ 25 46 71 25 12 12 49 15 5 45 65 Totals 281 370 797 "See Table 2. "See Table 2. Table 7. Root distribution in Artemisia tridentata on Pahute Mesa at the Nevada Test Site (organic debris cor- rected to 35 percent). Distribution Depth cm Roots, kg/ha for 5 cm increments Under Canopy Interspace Total Big roots (over 3 mm) 0-5 — — — — 0.0 5-10 93 56 260 409 13.7 10-20 590 260 Small roots (1-3 mm) 850 14.3 0-5 2 10 12 0.4 5-10 17 23 140 180 6.1 10-20 70 140 140 Fine roots (under 1 mm) 350 5.9 0-5 21 10 31 1.0 5-10 32 47 77 156 5.2 10-20 78 77 98 Fine roots in organic debris" 253 4.3 0-5 151 34 5 190 6.4 5-10 58 36 100 194 6.5 10-20 69 100 181 Totals 350 5.9 0-5 174 54 5 233 7.8 5-10 200 162 577 939 31.6 10-20 807 577 419 1803 30.3 Total 1181 793 1001 2975 100.0 1980 Nevada Desert Ecology 207 and 1132 kg/ha for the site in Mercury Val- ley. These samples were taken in spring, so they should have shown a component of fine roots due to phenological stage (Caldwell and Fernandez 1975). Roots associated with winter annual plants are shown in Table 5. Two sets of values are shown. One is based on the assumption that the biomass is uniform and the other (realis- tic) is that the winter annuals occupy 20 per- cent of the land area. On this basis the esti- mated biomass in kg/ha for winter annual roots was 602, 118, and 255 for Mercury Val- ley, Frenchman Flat, and Rock Valley, re- spectively (22 April 1976). The depth distribution of the annual roots was more shallow than for perennial plants, as expected. The first 5 cm of soil had 19, 31, and 26 percent, respectively, for Mercury Valley, Frenchman Flat, and Rock Valley. In the first 10 cm of soil from Rock Valley, only 8 percent of the perennial roots (mostly fine roots) (Table 2) were present. In Frenchman Flat and Mercury Valley they were 20 and 14 percent, respectively, but these values are for 10 cm and those for annuals were for 5 cm. Another set of soil samples by the same procedure was taken on 24 September 1976 in a L. tridentata-A. dumosa community (Table 6). The total root biomass in kg/ha was 797 at this September date, which is es- sentially the same as the April date in Table 2 (759 kg/ha). To compare the root patterns of the first 5 cm of soil of the Great Rasin desert (an Arte- misia community) with the northern Mojave Desert, a root sampling procedure as above was used in Pahute Mesa (Table 7) of the Ne- vada Test Site. The percentage of the roots in the first 5 cm was 7.8 percent, which is about the same as in the first 10 cm of the northern Mojave Desert. In the first 10 cm at Pahute Mesa, 39 percent of the roots were present. Solid rock existing below 20 cm at the site sampled prevented root distribution at lower depths. The root sample of 2975 kg/ha com- pares with an estimated aboveground bio- mass of about 3000 kg/ha (Wallace and Rom- ney 1972). Acknowledgments This study was supported by U.S. Inter- national Desert Riome and Contract EY-76- C-03-0012 between the U.S. Department of Energy and the University of California. Literature Cited Bamberg, S. A., T. Ackerman, H. O. Hill, H. W. Kaaz, and A. Vollmer. 1974. Perennial plant popu- lations. Pages 30-35 in F. B. Turner and J. F. McBraver, eds. Rock Valley validation site re- port. Beatley, J. C. 1967. Survival of winter annuals in the northern Mojave Desert. Ecology 48:745-750. 1976. Vascular plants of the Nevada Test Site and central-southern Nevada. Tech. Information Cen- ter, Office of Tech. Infor., ERDA Report TID- 16881. Caldwell, M. M., and O. A. Fernandez. 1975. Dyna- mics of Great Basin shrub root systems. Pages 38-51 in N. F. Hadley, ed. Environmental physi- ology of desert organisms. Dowden, Hutchinson & Ross, Inc. Stroudsburg, Pennsylvania. Cannon, W. A. 1870. The root habits of desert plants. Carnegie Inst, of Wash., Pub. No. 131. Dittman, H. J. 1964. Certain characteristics of the roots of desert plants. Amer. J. Bot. 51:673. Markle, M. S. 1917. Root systems of certain desert plants. Bot. Gaz. 64:177-205. Turner, F. B., and J. F. McBrayer. 1974. Rock Valley validation site. US/IBP Desert Biome Res. Memo. 75-7. Utah State University, Logan. Wallace, A., and E. M. Romney. 1972. Radioecology and ecophysiology of desert plants at the Nevada Test Site. National Technical Information Ser- vices, USAEC Report TID-25954. Wallace, A., E. M. Romney, and J. W. Cha. 1980. Per- sistence of 14C labeled carbon in Larrea triden- tata up to 40 months after photosynthetic fixation in the northern Mojave Desert. Great Basin Nat. Mem. 4:170-174. Waterman, W. C. 1923. Development of root systems under dune conditions. Bot. Gaz. 68:22-58. RODENT-DENUDED AREAS OF THE NORTHERN MOJAVE DESERT R. B. Hunter', E. M. Romney', and A. Wallace' Abstract.- Populations of pocket gophers and rabbits regulate or control the perennial vegetation on relatively w sites in the northern Moiave Desert. Aboveground shoots are pruned and whole plants are killed by complete rge sites in the northern Mojave Desert. Above itting of main root In western Mercury Valley and Frenchman Figure 1 is an aerial photograph of several 'lat on the Nevada Test Site are several such areas in Mercury Valley. The largest reas lacking the normal desert shrub cover. shown covers approximately 60 ha. mill iH'iW i "■in iriiw t Fig. 1. Aerial view of rodent-denuded areas in west Mercury Valley, Nevada Test Site. Largest site (arrow) covers about 60 hectares. Highway transects northeast corner of photo. 'Laboratory of Nuclear Medicine and Radiation Biology, Universit) oi California, Los Angeles, California 90024. 208 1980 Nevada Desert Ecology 209 Fig. 2. Grazing rabbits severely prune foliage of transplanted shrubs and newly developing seedlings. Inexpensive ire enclosures offer protection and help ensure survival. 210 Great Basin Naturalist Memoirs No. 4 Fig. 3. Example of newly killed Larrea tridentata shrubs destroyed by gophers (Thomomys bottae). 1980 Nevada Desert Ecology 211 We have concluded from observations of these areas that they are caused by the activi- ties of burrowing pocket gophers and grazing rabbits. These observations are: 1. The soil surface of the denuded areas is densely pitted with burrow entrances and fresh gopher mounds; the soil is soft, as if freshly plowed; and the surface rocks are uniformly small and retain carbonate de- posits, indicating short residence on the surface. 2. Shrubs transplanted onto these areas have been destroyed by severe grazing pressure when left unfenced. Some fenced shrubs also appear to have been killed by bur- rowing pocket gophers, and nearly all have been pruned to the fences by grazing rabbits (Wallace et al. 1976) (Fig.' 2). 3. Dying and recently killed Larrea triden- tata (Sesse & Moc. ex DC.) Cov. shrubs on the edge of one such area were uprooted, exposing evidence of severe root pruning. The sharp, oblique tooth cuts in healthy wood by pocket gophers (Thomomys bot- tae) were clearly distinguishable from in- sect damage and decay (Fig. 3). Further characteristics of these areas are a relatively high population of winter annuals; a gradual transition zone from scattered L. tridentata shrubs to a normal shrub commu- nity occurring over approximately 20 to 50 meters; and the presence of Stanleya pinnata (Pursh) Britt., a small, pithy-stemmed shrub. A few remnant L. tridentata shrubs occur within the denuded area, but the absence of standing dead wood indicates the areas have been bare for at least several decades. Many new shrub seedlings are seen, but survival of seedlings to young, well-established shrubs is extremely rare in these areas. Although rodent population dynamics have been well characterized in the adjacent shrub communities (O'Farrell and Emery 1976), no studies of rodents have been performed in conjunction with these disturbed areas. Nevertheless, their existence, along with vis- ible evidence, indicates that burrowing and grazing animals play a significant role in plant distribution and soil disturbance in the Mojave Desert. The high density of annuals occurring on these areas may be important to maintenance of the desert rodent populations in dry years (Beatley 1969). Acknowledgments This study was supported by Contract EY- 76-C-03-0012 between the U.S. Department of Energy and the University of California. Literature Cited Beatley, J. C. 1969. Dependence of desert rodents on winter annuals and precipitation. Ecology 50:721-724. O'Farrell, T. P., and L. A. Emery. 1976. Ecology of the Nevada Test Site: a narrative summary and annotated bibliography. U.S. ERDA, Report NVO-167. Wallace, A., E. M. Romney, and R. B. Hunter. 1977. The challenge of a desert: revegetation of dis- turbed desert lands. Pages 17-40 in Transuranics in desert ecosystems. U.S. DOE Report NVO-181. FENCING ENHANCES SHRUB SURVIVAL AND GROWTH FOR MOJAVE DESERT REVEGETATION R. B. Hunter1, A. Wallace', and E. M. Romney1 Abstract.— Fourteen species of native shrubs were transplanted to bare areas of the northern Mojave Desert in 1972 and 1973. By 1978 plants surrounded by small fences were larger (0.26 vs 0.11 m3 overall average for several species) and survived better (42 percent versus 23 percent) than unfenced plants. These effects are primarily due to reduced grazing of shoots. Loss of shrubs to pocket gophers or other burrowing rodents was not prevented by fenc- ing. Natural revegetation of disturbed desert lands is a very slow process (Shreve 1917, Shreve and Hinckley 1937, Wells 1961, Shields and Wells 1963, Wallace et al. 1977). Seeding and transplanting of shrubs have of- ten failed as the result of problems such as poor germination, poor growing conditions, grazing by rodents, and inadequate soil prep- aration (Graves 1976). Transplanting Atriplex canescens (Pursh) Nutt. onto desert lands has been successful in several instances (Springfield 1970, Aldon 1972, Cable 1972, Nemati 1977). Much effort has been put into timing for maximum avail- ability of soil moisture. Our experience with transplants watered through the first summer of growth showed more persistent problems related to grazing and pruning by rabbits and smaller rodents than to drought conditions. The grazing problem has been noted by others working with A. canescens (Springfield 1970, Cable 1972, Graves 1976, Shetron and Carroll 1977). were watered monthly through the following summer (10 to 20 liters of water per plant per month). On 16 February 1973, 62 additional plants representing seven of the species listed in Table 1 were transplanted to a nearby site similarly devoid of shrubs. Three of nine rows were fenced, and the transplanted shrubs were watered through the summer of 1973 in amounts indicated above. Surviving plants were recorded 31 May 1973, 15 February 1977, and were counted and measured 7 June 1978. Plant volumes were calculated from the height and the av- erage of two width measurements, assuming a cylindrical shape. A further planting of 381 plants of assorted species in 127 groups of three was made 7 May 1977 in a nearby area where much of the surface had been removed for gravel. All plants were fenced and watered every 4 to 6 weeks through August 1977. Materials and Methods On 16 February 1972, 100 shrubs of 14 species (Table 1) were transplanted from a glasshouse to a 29 ha bare area in a natural Mojave Desert shrub community on the Ne- vada Test Site (Frenchman Flat). Each plant in three of eight rows was enclosed within a fence of about 0.5 m diameter of 2.5 cm mesh chicken wire (Fig. 1). Each fence was supported by three lath stakes. All plants Results and Discussion Table 1 reports survival of shrubs at the three census periods for the 1972 and 1973 plantings. By 1978 only four unfenced species survived [Ambrosia dumosa (A. Gray) Payne, A. canescens, Larrea tridentata (Sesse & Mod ex DC) Cov., and Lycium andersonii (A. Gray)]. In contrast, nine fenced species sur- vived. Overall survival rates were marginally improved by fencing. 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, ( alifornia 90024. 212 : h 1980 Nevada Desert Ecology 213 Prevention of grazing resulted in greater size of fenced plants (Table 2, Fig. 1). When calculated as percent of average size for each species, the fenced shrubs were significantly larger (P < 0.05) than unfenced shrubs. Failure of shrubs to survive seems to be re- lated primarily to rodent browsing and prun- ing activity, although a few species may have succumbed to weather and transplant shock [Salvia sonomensis (Kell.), Salazaria mexi- cana Torr. and Stephanomeria pauciflora (Torr.) Nutt.] Rodents in this area include pocket gophers (Thomomys bottae), rabbits (Lepus californicus and Sylvilagus audu- bonii), kangaroo rats (Dipodomys merriami), and mice (Onychomys torridus and Per- omyscus spp.) (O'Farrell and Emery 1976). Unfenced palatable shrubs (Ceratoides la- nata (Pursh) J. T. Howell, Yucca spp., Arte- misia tridentata Nutt.) were killed by grazing of shoots until only stubs were left. Fenced plants, however, also were killed by bur- rowing rodents, particularly pocket gophers (Fig. 2). Losses continued through 1978 for fenced shrubs. Plantings made on the gravel excavation site in 1977 survived and grew very well through the first year after transplanting. Only two plants were lost, one of which ap- peared to be dying within a month of trans- planting. Only Atriplex species were grazed heavily. The rocky, sandy soil appears to have discouraged burrowing rodents at that site. Grazing of shoots appeared sporadic and heaviest when most shrubs were dormant and annual plant species were absent (summer and fall). The fencing technique used is rapid, in- expensive, and effective against non- burrowing rodents. The particular sites plan- ted here appear to harbor an unusual density of burrowing species (Hunter et al. 1980, this volume) seriously reducing the effectiveness of the fences. Seed production occurred in most surviv- ing species in 1978 (except Coleogyne ramo- sissima Torr. and Yucca spp.). Although natural seedling establishment normally may be severely inhibited by grazing animals, we believe that revegetation of sites disturbed by human activities would be enhanced by tak- ing steps to protect newly developing seed- lings through the use of inexpensive, fenced enclosures. Acknowledgments This study was supported by Contract EY- 76-C-03-0012 between the U.S. Department of Energy and the University of California. Table 1. Numbers of surviving plants transplanted to two disturbed areas in Frenchman Flat in February 1972 and February 1973. Survivors were counted May 1973, February 1977, and June 1978. Fenced Unfenced Species" Original 1973 1977 1978 Original 1973 1977 1978 Ambrosia dumosa 9 8 3 3 18 15 10 8 Atriplex canescens 6 5 4 4 10 9 5 5 A rtem isia triden ta ta 3 3 3 1 6 4 1 0 Ceratoides lanata 5 3 2 2 9 1 0 0 Coleogyne ramosissima 1 1 1 1 1 0 0 0 Grayia spinosa 2 1 0 0 4 1 0 0 Larrea tridentata 10 8 6 5 16 12 11 11 Lycium andersonii 3 2 2 2 5 3 1 1 Lycium pallidum 3 1 0 0 5 0 0 0 Salvia sonomensis 3 0 0 0 6 0 0 0 Salazaria mexicana 1 0 0 0 3 1 0 0 Stephanomeria pauei flora 1 0 0 0 2 0 0 0 Yucca brevifolia 2 2 1 1 4 4 0 0 Yucca schidigera 10 10 6 6 17 16 1 0 Total survival 58 44 18 25 104 66 29 25 Percent survival 76 47 42 62 27 23 "Full names of the species will be found in text except for Grayia spinosa (Hook.) Moq., Lycium pallidum Miers, Yucca brevifolia Engelm. in Wats., and Yticca schidigera Roezl ex Ortgies. 214 Great Basin Naturalist Memoirs No. 4 ,.. Ife-xJSu '*■..-.',. Fig. 1. Inexpensive wire enclosures protect transplanted shrubs from grazing rabbits. 1980 Nevada Desert Ecology 215 Literature Cited 1 moisture levels for field bush. |. Range Manage- Aldon, E. F. 1972. Critical planting four-wing s ment 25:311-312. Cable, D. R. 1972. Fourwing saltbush revegetation trials in southern Arizona. J. Range Management 25:150-153. Graves, W. 1976. Revegetation of disturbed sites with native shrub species in the western Mojave Desert. Pages 11-35 in B. L. Kay, ed. Test of seeds of Mojave Desert shrubs. Progress report BLM Contract 53500-(T4-21N). Department of Agronomy and Range Science, University of Cali- fornia, Davis. Hunter, R. B., E. M. Romney, and A. Wallace. 1980. Rodent-denuded areas of the northern Mojave Desert. Great Basin Nat. Mem. 4:206-209. Nemati, N. 1977. Shrub transplanting for range im- provement. J. Range Management 20:148-150. O'Farrell, T. P., and L. A. Emery. 1976. Ecology of the Nevada Test Site, a narrative summary and annotated bibliography. Report NV0-167 U.S. Department of Energy. NTIS, U.S. Dept. of Commerce, 5285 Port Royal Rd., Springfield. Vir- ginia 22151. Shetron, S. G., and D. A. Carroll. 1977. Performance of trees and shrubs on metallic mine mill wastes. J. Soil and Water Conservation 32:222-225. Shields, L. M., and P. V. Wells. 1963. Recovery of vegetation on atomic target areas at Nevada Test Site. Pages 207-310 in Vincent Schultz and Al- fred W. Klement. Jr., eds. Radioecology. Rein- hold, New York. Shreve, F. 1917. The establishment of desert perennials. J. Ecol. 5:210-216. Shreve, F., and A. L. Hinckeley. 1937. Thirty years of change in desert vegetation. Ecology 18:463-478. Springfield, H. W. 1970. Germination and estab- lishment of fourwing saltbush in the southwest. U.S. Forest Service, Rocky Mountain Forest Range Experiment Station Research Paper RM 55, Fort Collins, Colorado. Wallace, A., E. M. Romney, and R. B. Hunter. 1980. The challenge of a desert: revegetation of dis- turbed lands. Great Basin Nat. Mem. 4:214-223. Wells, P. V. 1961. Succession in desert vegetation on streets of a Nevada ghost town. Science 134:670-671. Table 2. Average sizes of fenced and unfenced sur- viving transplants, June 1978 (Vol, m3 ± SEM). Species Fenced Unfenced Ambrosia dumosa 0.093 ± 0.017 0.050 ± 0.010 Atriplex canescens 1.064 ± 0.616 0.263 ± 0.025 Artemisia tridentata 0.108 Ceratoides Janata 0.171 ± 0.019 Colcogyne ramo.sis.sima 0.026 Larrea tridentata 0.239 ± 0.087 0.089 ±0.014 Lyciwn andersonii 0.022 ± 0.001 0.031 Yucca brevifolia 0.009 Yucca schidigera 0.040 ± 0.011 0.260 ±0.115 Overall average" 0.109 ± 0.018 "Average of all surviving plants, numbers of each species are given Table 1. Fig. 2. Pocket gophers (Thomomy.s bottae) destroyed some transplanted shrubs initially protected with wire enclo- sures. THE CHALLENGE OF A DESERT: REVEGETATION OF DISTURBED DESERT LANDS1 A. Wallace2, E. M. Romney2, and R. B. Hunter2 Abstract.— The revegetation of disturbed, arid lands is one of the great challenges of a desert. An attempt to encourage it is not an impossible task, however, if the natural and the man-made resources available are utilized and managed. Where rainfall and temperature conditions approach or exceed those of the Great Basin desert, restoration of disturbed land will occur through natural revegetation processes within a reasonable period of time. This is not generally the case in the more arid Mojave Desert areas where the moisture and temperature conditions are less favorable for germination and seedling survival. Restoration of vegetation by natural reseeding can, however, occur within local sites where moisture has concentrated as the result of terrain features forming catchment basins. Other- wise, the natural revegetation processes in the Mojave Desert areas require much longer periods of time (possibly decades or centuries) than are practical for meeting environmental protection standards imposed by current legisla- tion. Through better understanding of the processes governing revegetation and the ability to control them, it is pos- sible for man to more rapidly restore disturbed desert lands. Terrain manipulation to form moisture catchment ba- sins, selection of seed from pioneering shrub species, preservation of existing shrub clump "fertile islands'" in the soil, supplemental fertilization, irrigation, organic amendments, and transplanting vigorous shrub species are some of the important things that can be done to help restore disturbed desert land. With current stress on maintenance of the quality of the environment and with respon- sibility for its status placed upon those using a given area, it is increasingly important that we who are involved understand many as- pects of the ecosystem in which we work. The facts that deserts are very fragile and that efforts to restore them after disturbance can lead to frustration and failure are well known. We are proceeding in our work with the assumption that our ability to control the factors related to restoration of deserts, whenever the need arises, is proportional to how well we understand the processes gov- erning germination and survival of desert plants. In this report we describe some as- pects of natural processes that are of great importance to revegetation problems in the northern Mojave and southern Great Basin deserts (Beatley 1965, Wallace and Romney 1972, 1974, 1976). Synopsis and Discussion of Experimental Findings A listing and description of some of the more important behavioral aspects of the deserts in which we work are given below. Details of experiments from which some of the findings were obtained have been pre- viously published (Wallace and Romney 1972, Romney et al. 1973, Romney et al. 1974, Hunter et al. 1975, 1975a, Romney et al. 1977a). The work of Beatley contributes considerably to an understanding of plants under the desert conditions involved in these studies (Beatley 1965, 1965a, 1965b, 1965c, 1966, 1967, 1973, 1974, 1974a, 1975, 1976). 1. Water is the most important parameter governing biological responses in the desert ecosystem; however, equal quantities of wa- ter via natural precipitation do not always result in equal responses. Some of the factors 'Modified from DOE Report NVO-181. 1978. 'Laboratory of Nuclear Medicine and Radiation Biology, University of California, Los Angeles, California 9 1976 Artemisia tridentata (80)* Ceratoides lanata Ephedra nevadensis Grayia spinosa Lycium andcr.sonii Mixed grasses Artemisia tridentata (0) Ceratoides lanata Chrysothamnus nauseosus Ephedra nevadensis Grayia spinosa Mixed grasses Salsola iberica Tetradymia axillaris Partially killed plot (100 X 100 m) 35 5 60 20 8 400 Totally killed plot (100 X 100 m) 111 2 1 1 15 500 < 7,000 390 5 63 28 8 5,000 2 4 3 0 25 5,000 2,000 4 1,170 5 60 55 5 < 5,000 21 0 34 < 5,000 < 500 1 Artemisia tridentata (1,000) Ephedra nevadensis Grayia spinosa Mixed grasses Undisturbed control of plot (100 X 100 m) 6 85 14 100 65 85 14 120 "Values in parenthesis indicate number of original shrubs still living in plot as of September 1967. Each of the plots had about 1000 shrubs before disturb- ance, based upon dead shrub count. 224 Great Basin Naturalist Memoirs No. 4 Acknowledgments Support for this study was provided by Ne- vada Applied Ecology Group, Department of Energy/Nevada Operations Office, IBP/Desert Biome, and Contract EY-76-C- 03-0012 between the U.S. Department of Energy and the University of California. Literature Cited Beatley, J. C. 1965. Ecology of the Nevada Test Site. II. Status of introduced species. USAEC Report UCLA 12-554. 1965a. Ecology of the Nevada Test Site. III. Sur- vival of winter annuals, 1963-1964. USAEC Re- port UCLA 12-555. 1965b. Ecology of the Nevada Test Site. IV. Ef- fects of the Sedan detonation on desert shrub vegetation in northeastern Yucca Flat, 1962-1965. USAEC Report UCLA 12-1571. 1965c. Effects of radioactive and nonradioactive dust upon Larrea divaricate Gov., Nevada Test Site. Health Phys. 11:1621-1625. 1966. Ecological status of introduced brome grasses (Bromiis spp.) in desert vegetation of southern Nevada. Ecology 47:548-554. 1967. Survival of winter annuals in the northern Mojave Desert. Ecology 48:745-750. 1973. Russian-thistle (Salsola) species in western United States. J. Range Manage. 26:225-226. 1974. Effects of rainfall and temperature on the distribution and behavior of Larrea tridentata (Creosote-bush) in the Mojave Desert of Nevada. Ecology 55:245-261. 1974a. Phenological events and their environ- mental triggers in Mojave Desert ecosystems. Ecology 55:856-863. 1975. Climates and vegetation pattern across the Mojave/Great Basin desert transition of southern Nevada. Amer. Midi. Natur. 93:53-70. 1976. Vascular plants of the Nevada Test Site and central-southern Nevada. Tech. Information Cen- ter, Office of Tech. Information, ERDA Report TID-16881. Charley, J. L. 1972. The role of shnibs in nutrient cy- cling. Pages 182-203 in C. M. McKell, J. P. Blai's- dell, and J. R. Goodin, eds. Wildland Shrubs- their biology and utilization. U.S. Forest Service Technical Report INT-1. Charley, J. L., and S. W. Cowling. 1968. Changes in soil nutrient status resulting from overgrazing and their consequences in plant communities in semi-arid zones. Proc. Ecol. Soc. Aust. 3:25-38. Charley, J. L., and N. E. West. 1975. Plant-induced soil chemical patterns in some shrub-dominated semi-desert ecosystems of Utah. J. Ecol. 63:945-963. Day, J. M., D. Harris, P. J. Dart, and P. VanBerkum. 1975. The Broadbalks experiment: an in- vestigation of nitrogen gains from nonsymbiotic nitrogen fixation. In W. D. P. Stewart, ed. Nitro- gen fixation by free-living microorganisms. Cam- bridge University Press, Cambridge. Dick, J. L., and T. P. Baker, Jr. 1967. Monitoring and decontamination techniques for plutonium fall- out on large-scale surfaces. Operation Plumbbob. Report WT1512. El-Ghonemy, A. A., A. Wallace, and E. M. Romney. 1980a. Frequency distribution of numbers of per- ennial shrubs in the northern Mohave Desert. Great Basin Nat. Mem. 4:32-36. 1980b. Socioecological and soil-plant studies of the natural vegetation in the northern Mojave Desert-Great Basin interface. Great Basin Nat. Mem. 4:71-86. Evanari, M., L. Shanon, and N. Tadmor. 1971. The Negev, the challenge of a desert. Harvard Uni- versity Press, Cambridge, Massachusetts. Garcia-Moya, E., and C. M. McKell. 1970. Contribu- tion of shrubs to the nitrogen economv of a desert-wash plant community. Ecology 51:81-88. Hunter, R. B., E. M. Romney, and A. Wallace. 1980. Rodent-denuded areas of the northern Mojave Desert. Great Basin Nat. Mem. 4:206-209. Hunter, R. B., E. M. Romney, A. Wallace, J. D. Childress, and J. E. Kinnear. 1975a. Responses and interactions in desert plants as influenced by irrigation and nitrogen applications. US/IBP Desert Biome, Res. Memo. 75-13. Hunter, R. B., A. Wallace, E. M. Romney, and P. A. T. Wieland. 1975b. Nitrogen transformations in Rock Vallev and adjacent areas of the Mohave Desert. US/IBP Desert Biome Res. Memo. 75-35. Jenny, H., S. P. Gessel, and F. T. Bingham. 1949. Com- parative studv of decomposition ratio of organic matter in temperate and tropical regions. Soil Sci. 58:419-432. Paulsen, H. A. 1953. A comparison of surface soil prop- erties under mesquite and perennial grass. Eco- logy 34:727-732. Plummer, A. P., D. R. Christenson, and S. B. Monsen. 1968. Restoring big game range in Utah. Utah Div. Fish and Game, Pub. 68-3. Plummer, A. P., A. C. Hill. Jr.. G. Stewart, and J. H. Robertson. 1955. Seeding rangelands in Utah. Nevada, southern Idaho, and western Wyoming. USD A Handbook 71. Rhoads, W. A., R. B. Platt, and R. A. Harvey. 1969. Radiosensitivity of certain perennial shrub spe- cies based on a study of the nuclear excavation experiment, Palanquin, with other observations of effects on vegetation. USAEC Report CEX- 68.4 CETO. Rickard, W. H. 1965. The influence of greasewood on soil moisture and soil chemistry. Northwest Sci. 39:36-42. Roberts, R. C. 1950. Chemical effects of salt-tolerant shrubs on soils. Fourth Int. Congr. Soil Sci. 1:404-406. Romney, E. M.. V. O. Hale. A. Wallace, O. H. Lint, J. D. Childress, H. Kaaz, G. V. Alexander, J. E. Kinnear, and T. L. Ackerman. 1973. Some char- acteristics of soil and perennial vegetation in northern Mojave Desert areas of the Nevada Test Site. USAEC Report UCLA 12-916. 1980 Nevada Desert Ecology 225 Romney, E. M., A. Wallace, and J. D. Childress. 1971. Revegetation problems following nuclear testing activities at the Nevada Test Site. Pages 1015-1022 in Proc. Third Natl. Symp. on Radio- ecology, Oak Ridge, Tennessee. Romney, E. M., A. Wallace, J. D. Childress, J. E. Kinnear, H. Kaaz, P. A. T. Wieland, M. Lee, and T. L. Ackerman. 1974. Response and inter- actions in desert plants as influenced by irriga- tion and nitrogen applications. US/IRP Desert Riome Res. Memo. 74-17. Romney, E. 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Dowden, Hutchinson & Ross, Inc., Stroudsburg, Pennsylvania. 1980. Regulative effect of dodder (Cuscuta neva- densis Jtn.) on the vegetation of the northern Mo- jave Desert. Great Basin Nat. Mem. 4:96-97. Wieland, P. A. T., E. F. Frolich, and A. Wallace. 1971. Vegetative propagation of woody shrub species from the northern Mojave and southern Great Basin deserts. Madrono 21:149-152. INDEX A phytosociological study of a small desert area in Rock Valley, Nevada, p. 57. Ackerman, T. L., article by, p. 4. Alexander, G. V., articles by, pp. 144, 154. Bamberg, S. A., article by, pp. 37, 138. 14C distribution on roots following photo- synthesis of the label in perennial plants in the northern Mojave Desert, p. 175. Carbon fixed in leaves and twigs of field Lar- rea tridentata in two-hour exposure to i4CO„p. 119. Cha, J. W., articles by, pp. 166, 170, 175, 190, 199. Clark, S. B., article by, p. 108. Depth distribution of roots of some perennial plants in the Nevada Test Site area of the northern Mojave Desert, p. 199. Distribution ot photosynthetically fixed 14C in perennial plant species of the north- ern Mojave Desert, p. 190. Ecotonal distribution of salt-tolerant shrubs in the northern Mojave Desert, p. 132. Effect of certain plant parameters on photo- synthesis, transpiration, and efficiency of water use, p. 115. El-Ghonemy, A. A., articles by, pp. 32, 40, 57, 71, 138. Fencing enhances shrub survival and growth for Mojave Desert revegetation, p. 210. Field studies of mineral nutrition of Larrea tridentata: importance of N, pH, and Fe, p. 161. Frequency distribution and correlation among mineral elements in Lijcium an- dersonii, from the northern Mojave Desert, p. 144. Frequency distribution of numbers of per- ennial shrubs in the northern Mojave Desert, p. 32. Frequency distribution of three perennial plant species to nearest neighbor of the same species in the northern Mojave Desert, p. 87. Further attributes of the perennial vegetation in the Rock Valley area of the northern Mojave Desert, p. 37. Hale, V. C\, article by, p. 122. Hartsock, T. L., article by, p. 98. Hunter, R. B., articles by, pp. 22, 26, 37, 92, 96, 119, 154, 161, 206, 210, 214. Kaaz, H., article by, p. 122. Kinnear, J. E., articles by, pp. 4, 22, 87, 144, 154. Kleinkopf, G. E., article by, pp. 98, 108. Letey, J., Jr., article by, p. 108. Lunt, O. R., article by, p. 108. Mineral composition of Atriplex hymenehjtra growing in the northern Mojave Desert, p. 154. Mork, H. M., article by, p. 115. Mueller, R. T., article by, pp. 166, 175. Multivariate analysis of the vegetation in a two-desert interface, p. 40. Parent material which produces saline out- crops as a factor in differential distribu- tion of perennial plants in the northern Mojave Desert, p. 138. Persistence of 14C labeled carbon in Larrea tridentata up to 40 months after photo- synthetic fixation in the northern Mo- jave Desert, p. 170. Phenology of desert shrubs in southern Nye County, Nevada, p. 4. Photosynthetic strategies of two Mojave Desert shrubs, p. 98. Preface, p. 1. Regulative effect of dodder (Ciiseuta neva- densis Jtn.) on the vegetation of the northern Mojave Desert, p. 96. Relationship of small washes to the distribu- tion of Lyeium andersonii and Larrea tridentata at a site in the northern Mo- jave Desert, p. 92. Residual effects of supplemental moisture on the plant populations of plots in the northern Mojave Desert, p. 22. Retranslocation of tagged carbon in Ami brosia dumosa, p. 166. Rodent-denuded areas of the northern Mo- jave Desert, p. 206. 226 1980 Nevada Desert Ec 227 Romney, E. M., articles bv, pp. 4, 22, 26, 29, 32, 37, 40, 57, 71, 87, 92, 96, 98, 108, 115, 119, 122, 132, 138, 144, 154, 161, 166, 170, 175, 190, 199, 206, 210, 214. Socioecological and soil-plant studies of the natural vegetation in the northern Mo- jave Desert-Great Basin desert inter- face, p. 71. The challenge of a desert: revegetation of disturbed desert lands, p. 214. The pulse hypothesis in the establishment of Artemisia seedlings at Pahute Mesa, Nevada, p. 26. The role of pioneer species in revegetation of disturbed desert areas, p. 29. The role of shrubs on redistribution of miner- al nutrients in soil in the Mojave Desert, p. 122. Transpiration and C02 fixation of selected desert shrubs as related to soil-water potential, p. 108. Valentine, W., article by, p. 57. Wallace, A., articles by, pp. 1, 4, 22, 26, 29, 32, 37, 40, 57, 71, 87, 92, 96, 98, 108, 115, 119, 122, 132, 138, 144, 154, 161, 166, 170, 175, 190, 199, 206, 210, 214. Wood, R. A., article by, p. 138. 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