Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. \ a = = z ' i i ‘ ~ Se ay ' ” = ine ‘ = _ . j j a , » = i 7 ra fl ' “ i ' i -_ 2 = 7 i ‘ = ma T fl z re : ' ‘ 1 1 1 or - eatey ee toh E vi is a Tat gr Components of Adaptive Variation in Pinus contorta from the Inland Northwest G. E. Rehfeldt fifiy : THE AUTHOR G. E. REHFELDT is a plant geneticist at the Intermountain Research Station’s Forestry Sciences Laboratory in Moscow, ID. He has been studying the ecological and quantitative genetics of Rocky Mountain conifers since 1968. RESEARCH SUMMARY Genetic variation among 83 populations of Pinus con- torta from the Northern Rocky Mountains of the United States was studied with 7-year-old trees planted in three contrasting environments. Analyses of nine traits reflected adaptation to biotic and abiotic environments and revealed clinal patterns of differentiation that were elevationally steep but geographically gentle. In particular, populations from relatively mild environments had the highest growth potential but suffered the most snow damage when planted at high elevations. Populations from high eleva- tions were most susceptible to needle cast when trans- ferred to low elevation. And populations transferred the greatest geographic distances suffered the most from infestations of mites. Regression models present adaptive landscapes that mirror elevational and geographic gradients in climate. Intermountain Research Station 324 25th Street Ogden, UT 84401 Components of Adaptive Variation in Pinus contorta from the Inland Northwest G. E. Rehfeldt INTRODUCTION Adaptive differentiation is discernible either directly as differential fitness in contrasting environments or in- directly as genetic responses that parallel environmental gradients. In Pinus contorta Dougl., for example, four geographic races are distinguished both morphologically (Critchfield 1957) and enzymatically (Wheeler and Guries 1982). Critchfield (1980) reviewed numerous studies that provide direct and indirect evidence of differential adapta- tion of races to climates as diverse as those of the north- ern Pacific Coast, the Sierra. Nevada, the interior Rocky Mountains, and the Canadian subarctic. In P. contorta spp. latifolia, the subject of this paper, genetic variation among populations is pronounced. Cana- dian populations, native to a region of dissected plateaus, are arranged along gentle clines that follow climatic gradients across 18° of latitude from the Yukon to southern British Columbia (Hagner 1970; Lindgren and - others 1976; Ying and others 1985). In the mountains of Idaho (Rehfeldt 1988a), Utah (Rehfeldt 1985a), and Oregon (Stoneman 1985), population differentiation occurs along steep elevational clines that parallel the environmental changes associated with altitude. Whether geographic or elevational clines predominate, populations from mild en- vironments express a high innate growth potential and low cold hardiness, while those from severe environments display a low growth potential and high hardiness. Adaptive clines result from environmental selection of phenotypes that, for long-lived trees, develop in environ- ments of extreme temporal heterogeneity. Adaptedness, therefore, has many component traits. A consideration of the interrelationships among components inspired Lande (1982) to argue that negative genetic correlations are often obscured phenotypically but commonly set the limits on microevolution. In trees, for example, negative genetic correlations relate growth potential and cold hardiness within families of both Pseudotsuga menziesii (Mirb.) Franco and P. contorta (Rehfeldt 1984). This means that an assessment of adaptive differentiation requires an understanding of component traits and their interrelations. Lande (1982) stressed that this understanding is par- ticularly necessary for fields such as forestry where the traits of agronomic importance—growth and yield—are also major components of fitness. The present study of Pinus contorta assesses genetic variability among populations, relates genetic variability to adaptive differentiation, and presents adaptive landscapes for the Inland Northwest. DISTRIBUTION, ECOLOGY, AND DEMOGRAPHY Adaptive variation must be interpreted according to the spatial environmental heterogeneity within the region of study (fig. 1). The climate of the Inland Northwest varies from the continental in the east to that with a coastal component in the west (Daubenmire and Daubenmire 1968; Pfister and others 1977). This general trend is reflected not only in gradients of temperature and precipi- tation (fig. 2), but also in the composition and distribution of plant communities (Daubenmire and Daubenmire 1968; Pfister and others 1977;.Steele and others 1981). Superim- posed on general climatic gradients are the topographic microclimates associated with mountainous terrain. Ex- treme environmental heterogeneity thus develops from a climatic transition that occurs across a series of rugged mountain ranges. In the Rocky Mountains, P. contorta displays such a broad ecological distribution that it is capable of growing in almost any forest environment (Pfister and Daubenmire 1973). As an early successional species, the pine is com- mon in a variety of plant communities that include associa- tions dominated at maturity by Tsuga heterophylla (Raf.) Sarg. on mesic sites, Pseudotsuga menziesii (Mirb.) Franco on dry sites, and Abies lasiocarpa (Hook.) Nutt. on cold sites (Daubenmire and Daubenmire 1968; Pfister and others 1977; Steele and others 1981). The species forms large continuous populations on subalpine sites at eleva- tions up to 3,000 m and in frost pockets on valley floors as low as 600 m. Natural populations of P. contorta tend to be established in cycles (Lotan and others 1985). Greatly simplified, these cycles involve (1) wildfire, (2) profuse even-aged reproduc- tion of as much as 500,000 seedlings/ha from either open or serotinous cones (Tackle 1959), (8) intense natural thin- ning, which can leave as few as 1,000 trees/ha by age 80 (Tackle 1959; Benson 1982; Vyse and Navratil 1985), and (4) epidemics of the mountain pine beetle (Dendroctonus ponderosae Hopkins) (Amman and others 1973; Shrimpton and Thomson 1983), which supplement the competitive mortality to provide the fuel for (5) wildfire. These demographic cycles have pronounced effects on the genetics of populations. First, adaptive traits will in- clude all traits that either directly or indirectly influence the expression of growth potential and thereby determine which trees are living when fires occur. And second, pop- ulations are frequently established on the same sites on MONTANA Figure 1—Distribution (shading) of Pinus con- : torta (from Little 1971) within the Northern 44° Rocky Mountains, and location of populations sampled (dots). Letters A to E locate the eleva- tional clines of figure 3. 0 = Lost Valley, @ = PREF. Figure 2—Climatic isopleths in relation to the distribution of the species (shading) and location of the Bitterroot Range (symbols) for (A) mean annual number of days with temperatures lower than 0 °C, (B) mean annual days with tempera- tures greater than 30 °C, (C) average annual frost-free period, and (D) mean annual precipita- tion (cm) (U.S. Department of Commerce 1968). which ancestral populations grew, and therefore coadap- tive complexes of traits can readily be perpetuated once such complexes arise. Thus, a broad ecological distribution, a unique cycle of population establishment, and occupancy of an extremely heterogeneous environment make P. contorta an ideal species for studying adaptedness, the degree by which in- dividuals are physiologically attuned to their environment. MATERIALS AND METHODS Genetic variation was studied in seedlings from 83 pop- ulations (fig. 1) that represented the geographic and ecological distribution of P. contorta in the Northern Rocky Mountains. Populations ranged in elevation from 640 to 2,700 m and represented habitat types as diverse as the Tsuga heterophylla/Pachistima myrsinites on moist sites and the Abies lasiocarpa/Vaccinium scoparium in subalpine communities. Because of pronounced genetic heterogeneity within populations (Rehfeldt 1985b; Ying and others 1985), cone collections were conducted in a manner to assure that subsequent seedling populations would represent a large number of parental trees. Thus, about 250 wind-pollinated cones, each containing about 25 viable seeds, were selected from several squirrel caches in each population. Genetic diversity was further assured by selecting a varie- ty of cone morphologies, sizes, and colors from each cache. Seedlings were grown for 6 months in plastic containers (65 em®) in a shadehouse at Moscow, ID (lat. 48.5° N., long. 116.7° W.). In the fall, seedlings were planted at three locations (fig. 1): at 640 m and 1,500 m elevation on the Priest River Experimental Forest (PREF), and at 1,500 m in Lost Valley. The experimental design consisted of a random allocation of linear seedling plots within rows of a rectangular planting. A plot comprised eight seedlings from each population. Nine plots represented each popula- tion at PREF sites, and six plots represented each popula- tion at Lost Valley. Thus, 72 seedlings represented each population at both PREF sites while 48 represented each population at Lost Valley. The spacing of seedlings within and between rows was: 0.5m and 1 m, respectively, at PREF 640; 0.5 m and 0.5 m at PREF 1,500; and 1 m and 1m at Lost Valley. Intensive culture at PREF included control of competing vegetation, gophers, and white grubs. The site at 640 m was irrigated once during years 2 and 3. No control of ex- traneous environmental effects was provided at Lost Valley. Variable cultural regimes and spacing of trees in different physical environments sample a range of condi- tions under which populations exist naturally. Survival was 76 percent at PREF 640, 83 percent at PREF 1,500, and 65 percent at Lost Valley. Periodic measurements or scores provided the following variables: 1. Mean height for each plot after 3 years. 2. Height of individual trees after 7 years. 3. Adjusted height: the 7-year height of individual trees adjusted by regression on 5-year height. 4. Late growth of trees at PREF: the amount of the 7-year, predetermined shoot that elongated after the date when the uppermost leaves of the shortest trees were ap- proximately 2 cm long (June 4 at 640 m; July 3 at 1,500 m). 5. Needle cast: the proportion of 6-year leaves of in- dividual trees at PREF 640 that were infected with Lophodermella, concolor (Dearn.) Darker, scored in July of year 7 with values of 1 to 5, which coded <5 percent, 25 percent, 50 percent, 75 percent, or >95 percent infected leaves. 6. Mites: the presence or absence of Trisetacus camp- nodus Keifer (Hunt 1981) on 7-year shoots of individual trees at PREF 640. 7. Shoot borers: the presence or absence of Hucosma sonomana Kearfott on the 7-year shoots of individual trees at PREF 640. 8. Frost injury: presence or absence of spring frost in- jury to the developing 7-year shoot at PREF 640. 9. Snow damage: scored in the spring of year 7 as the presence or absence of basal injuries of individual trees at PREF 1,500 sufficient to expose the inner bark. Population differentiation was assessed with data from individual trees for all variables except snow damage, frost injury, mites, and shoot borers, for which the propor- tion of injured trees in each plot was analyzed. Allometric traits were transformed to logarithms because variances were proportional to the square of mean values. Scores of needle cast were transformed to (x to normalize distribu- tions. Three-year height was not subjected to rigorous analysis but was used only for correlation. Statistical analyses followed a general model of unweighted means (Steel and Torrie 1960): YVisps = WH 8; + Pj + Sp; + Apazy +°€ where Y;;,, = the performance of the /th tree of the kth plot of the jth population at the 7th planting site, u = the overall mean, s = the effect of the planting site, p = the effect of the population, sp = the interaction of sites and populations, d = the effect of plots within populations and sites, and e = the residual. Multiple regression models were used to relate genetic variation to the elevation and geographic location of the seed source. Independent variables included elevation, latitude, longitude, northwest departure, southwest depar- ture, and their squares. Northwest and southwest depar- tures were derived by rotating the grid of latitude and longitude by 45°. Geographic variables were nested within four geographic regions that had proven useful in previous analyses (Rehfeldt 1980; Rehfeldt and Wykoff 1981): Idaho north or south of the Salmon River, and Montana east or west of the Continental Divide (fig. 1). However, the geo- graphic regions were not represented by dummy variables because such variables force continuous genetic variation to be described discontinuously. Interactions of elevation and geographic variables were not considered because preliminary analyses indicated that the effects of elevation could be described by similar regression coefficients in all geographic regions. Thus, 34 independent variables were screened by a stepwise regression model for maximizing R? (SAS 1982) according to the general model: Y; = Bo + BiH; + BoE? + Dy Xjh ae 2 5j.X7jp jk jk where Y, is the performance of population 7; HE; is the elevation of population 7; X;;,, is geographic variable k for population 7 in geographic region j; and fo, 1, Bo, y;,, and d;, are regression coefficients, 7 = 1...4,k = 1...4. Adequacy of a model was judged according to the goodness of fit (R*), residual variance (s,.,), and patterns displayed by residuals (Draper and Smith 1981). RESULTS Environmentally diverse planting sites strongly influ- enced growth and development of seedlings (table 1). While the average tree at PREF 640 was 106 cm tall, that at PREF 1,500 and Lost Valley was only 76 cm and 62 cm, respectively. Values for adjusted height imply that, even if all trees had been the same height at age 5, by age 7 those at PREF 640 would still have been about 10 em taller than trees on the other sites. Main effects of test environments are also expressed by the occurrence of snow damage only at high elevations and by the relatively high incidence of needle cast, shoot borers, and frost in- juries at low elevation. Environmental effects on late growth also were pronounced; similar stages of shoot elongation occurred about 1 month later at PREF 1,500 than at PREF 640. In these heterogeneous planting environments, mean dif- ferences between populations were statistically significant for all traits except adjusted height (table 1). Pronounced effects of populations were associated with mean differ- ences as large as 55 cm in 7-year height, 5 cm in late growth, 78 percent in leaves infected with needle cast, and 65 percent in number of trees exhibiting snow damage. Weak effects of populations were evident for mites, spring frost damage, and shoot borers largely because of low in- cidence for each variable. Thus, mites occurred on only 10 percent of the trees, but as many as 33 percent of the trees from one population were infested. The proportion of trees damaged from a spring frost was only 2 percent, but mean values for populations ranged up to 11 percent. And the incidence of shoot borers for most populations was in- cidental <5 percent), but 19 percent of the trees were in- jured in one of the British Columbia populations. Interactions of provenances and test environments were also significant for all variables measured on more than one site (table 1). Interactions for late growth and 7-year height obviously represented a scale effect because simple correlations of population means for the same variable at different sites ranged from 0.80 to 0.92. The interaction for adjusted height most likely resulted from various maladaptations that have accumulated between ages 5 and 7 and have differentially affected the height of populations at each planting site. This interaction is clarified by subse- quent analyses that consider adjusted height at each test site as separate traits. Simple correlations (table 2) describe a relationship between the height of populations at ages 3 and 7 that is nearly perfect (r = 0.94). Other extremely strong correla- tions (table 2) show that the tallest populations at ages 3 and 7 also (1) produced the most 7-year elongation after midsummer (late growth) regardless of test site, (2) grew the most from a common height at age 5 when planted at low elevation, (8) suffered little needle cast and few in- juries from spring frost when planted at low elevation, but (4) were the most susceptible to snow damage at high elevations. Populations that were short also had little late growth, suffered the least snow damage, but were most susceptible to needle cast at low elevations. Table 3 illus- trates that the tallest populations suffered the most injury from autumn freezing tests (Rehfeldt 1980) and displayed the greatest growth and longest duration of elongation in studies of the periodicity of shoot elongation involving 2-year-old trees (Rehfeldt and Wykoff 1981). Regression models for relating variation among popula- tions to the elevation and geographic location of the seed source were not only statistically significant for all vari- ables but also accounted for over 80 percent of the genetic variance in five variables (table 4). These models, however, included between 10 and 20 independent variables and were therefore subject to overfitting, the fitting of vari- ables to individual samples rather than to the group (Draper and Smith 1981). Consequently, biological signifi- cance of the results is judged with respect to the least significant difference (Steel and Torrie 1960) at the 95 percent level of probability, lsd (.05), calculated from analyses of variance and presented in figure 3. Table 1—Results of analyses of variance expressed as intraclass correlations. o2, Op; Ope, Obp), aNd of are variance components associated with the ef- fects of environments and populations, interaction, plots in populations, and error variance within plots. o? is the sum of all components Variable o2/o2 o3/o2 o2elo op ylor 07 loz 7-year height 0.39** 0.16** 0.03** Ones 0.30 Adjusted height an — .00 Olin 105 WW Late growth 105F= Ome 205 105 44 Needle cast pore LOiam 41 Mites {0} 95 Shoot borer 105me 195 Spring frost injury .09** OT Snow damage .40** .60 **Statistical significance of F-value at the 1 percent level of probability. Table 2—Matrix of simple correlation coefficients among mean values for 83 populations. Absolute values greater than 0.21 or 0.26 are statistically significant at the 5 and 1 percent levels of probability, respectively Variable Codes HT7 AH1 AH2 AH3 LG NC M SB FI SD 3-year height HT3 0.94 0.82 -0.40 0.14 088 -0.84 -0.29 0.36 -0.43 0.74 7-year height HT7 189 = 32605 129 94 -85 -.26 44 -.41 Whe Adjusted height PREF 640 AH1 -.19 29 92 -93 -.26 41 -.43 .78 PREF 1,500 AH2 +.00 -—.13 31 24 ~=.02 13 -.33 Lost Valley AH3 20: lle 04 14 —-.06 16 Late growth LG -.86 -.20 45 —-.41 .68 Needle cast NC 29 -.41 39 -—.76 Mites M .09 14 -.30 Shoot borer SB -.12 .23 Frost injury Fl — .34 Snow damage SD Table 3—Correlation of 30 population means from field tests with laboratory tests of freezing injury (Rehfeldt 1980) and greenhouse evaluations of the periodicity of shoot elongation (Rehfeldt and Wykoff 1981). Coefficients of absolute value greater than 0.35 and 0.45 are statistically significant at the 5 and 1 percent levels, respectively . Shoot elongation Freezing Variable injury Amount _ Initiation Rate Duration Cessation 3-year height 0.82 0.81 -—0.19 0.67 0.79 0.79 7-year height 81 .80 — .22 63 82 81 Adjusted height PREF 640 215) 62 — .22 .40 81 .80 PREF 1,500 32 —.19 — .30 — .25 — .22 —.27 Lost Valley .09 BWA 25 .16 14 18 Late growth 85 .80 — .26 61 81 .80 Needle cast -.77 —.59 18 — .42 —.76 —.76 Mites -.01 -.14 .29 — .03 -—.11 — .06 Shoot borer -— .43 19 — .20 .06 42 41 Spring frost injury ay f —.55 30 — .48 — .53 — .50 Snow damage 202 59 —.17 47 .69 .69 Table 4—Results of stepwise multiple regression analyses presented as standard errors (s) and coefficients of determination (R*) Independent Dependent variable variables R? s Number 7-year height 10 0.89** 0.0654 Adjusted height PREF 640 13 .86** .0350 PREF 1,500 15 ‘507° 0197 Lost Valley 16 .45** 0273 Late growth 13 One 1878 Needle cast 13 .88* * 1113 Mites 20 ea ieee .0536 Shoot borers 13 VAN A .0258 Spring frost injury 15 .40** .0203 Snow damage 18 780"* .0740 **Statistical significance at the 1 percent level. 7-YR HEIGHT e¢ ADJUSTED HEIGHT, PREF. 640 6.98 A\? oy PERCENT PERCENT 900 1500 2100 2700 900 SNOW DAMAGE 1500 2100 NEEDLE CAST ADJ. HT., LOST VALLEY e _ ce) PERCENT 2100 1500 2700 900 2700 ELEVATION OF SEED SOURCE, M Figure 3—Population means for six variables plotted by elevation of the seed source. Localities A to E are keyed to figure 1. The length of each regression line reflects eleva- tional distributions at each locality. Brackets quantify /sd (0.05). Figure 3 illustrates steep elevational clines for five variables. The higher the elevation of the seed source, the lower the 7-year height, the less the late growth, the greater the needle cast, and the less the snow damage. And, if all trees at PREF 640 had been the same height after 5 years, the height of populations at age 7 still would have been distributed according to elevational clines. Moreover, the model for adjusted height at PREF 1,500 (fig. 3) shows that populations from about the same eleva- tion as the 1,500-m planting site have grown the most from a common height at age 5. Elevational clines for mites, spring frost injury, and ad- justed height at Lost Valley are so gentle that they are biologically insignificant. The model for shoot borers is not presented because it was fit primarily to the population for which 19 percent of the trees were infested. Because elevation and geography are not independent of each other, geographic patterns of variation can be described either as (1) performance at a constant elevation or (2) performance at a base elevation, the lowest eleva- tion that the species occurs at a given locality. Figure 3 shows that populations at the base elevations for localities A and E, for example, differ tremendously for most traits. But most of these differences arise because populations at base elevations occupy much different positions along the elevational cline (fig. 3). Nevertheless, geographic variation at a constant eleva- tion was significant for five of the variables (fig. 3). In all cases, however, the differences barely exceeded lsd (.05). Therefore, geographic clines for a constant elevation can be described by the relatively gentle clines of figure 4. In this figure, geographic patterns are depicted by isopleths NEEDLE CAST 7-YR HEIGHT LOST VALLEY ADJUSTED HEIGHT Figure 4—Geographic patterns that are independent of elevation as predicted by regression models. Patterns are relative to the distribution of the species (shading), the Bitterroot Range, and the Salmon River. Isopleths are positioned relative to the mean value (x) with an interval scaled to Vz/sd (0.05). of relatively equal predicted performance for the mean elevation (1,625 m). Contouring was begun with the mean value, and the interval between isopleths was scaled to Yelsd (.05). Thus, populations separated by two isopleths are expected to differ at about the 95 percent level of probability. The pattern for late growth duplicates that of 7-year height and is not presented. When comparing populations from the same elevation, those from the north were the tallest and had the most late growth; populations from central Idaho were the shortest and exhibited little late growth. The percentage of trees infected with needle cast or infested with mites was directly related to the geographic distance that the seed was transferred to the planting site. Populations of highest adjusted heights at Lost Valley (1,500 m) tended to be from the north where growth potentials were highest. DISCUSSION The results illustrate differentiation of populations for numerous traits which together determine adaptedness, the degree by which populations are physiologically at- tuned to their environment. Genetic differentiation was readily detected experimentally, and patterns of genetic variation were closely associated with the elevation and geographic location of the seed source. Pronounced clines tend to typify genetic differentiation in montane popula- tions of P. contorta spp. latifolia (Ying and others 1985; Rehfeldt 1983a, 1985a; Stoneman 1985). That the clines reflect adaptation to natural environ- ments is demonstrated both directly, as differential fitnes; in contrasting environments, and indirectly, as genetic responses that parallel environmental gradients. Thus, in- direct support is provided by the steep elevational clines that parallel a change of 80 days in the frost-free period across 1,000 m elevation (Baker 1944). Likewise, the geographic patterns of variation illustrated in figure 4 unmistakably parallel geographic climatic patterns of figure 2. As a result, geographic patterns of genetic varia- tion are strongly influenced by the Bitterroot Range and Salmon River Drainage. Thus, populations from mild environments exhibit a high innate growth potential that develops from a long duration of elongation. Those from severe environments exhibit the low growth potential and short duration of elongation ex- pected in populations adapted to a short growing season. For these clines to reflect adaptive differentiation is un- questionable; rejection of such indirect evidence requires acceptance of an untenably improbable alternative: that the systematic patterns have developed randomly. Although direct evidence for adaptive clines ultimately resides with the survival or death of individuals, the demographic cycles of population establishment in P. con- torta endow an adaptive value to all traits that condition growth and development in a particular environment. Thus, direct support of adaptive clines is provided by the differential incidence of snow damage, needle cast, and mites, as well as by differences in growth potential itself. At high elevations, such as PREF 1,500 where snow ac- cumulations commonly exceed 3 m, populations from mild environments suffered considerable damage from the snow. Mean differences calculated within populations showed that the 7-year shoot elongation of trees damaged by the snow was 22 percent less than that of trees not damaged. Consequently, damaged trees had an adjusted height that was 5 percent less than undamaged trees from the same population. Similarly, needle cast is a disease most common in relatively humid valleys (Krebill 1975), and therefore, populations native to either high elevations or arid climates are rarely exposed to the disease. The present data, as well as those of Hoff (1985), show that popula- tions from either high elevations or southern latitudes are much more susceptible to needle cast than populations from low elevations. Mean differences within populations depict a tremendous reduction in 7-year height for diseased trees. For each increase in the damage score of one unit—an increase of approximately 25 percent in infected leaves per tree—7-year height was reduced about 4 percent. This means that reductions in 7-year height between uninfected and severely infected trees from the same population would amount to nearly 17 percent. This reduction accrued largely from a corresponding reduction of 34 percent in shoot elongation during year 7. Mites receive no recognition in pest surveys for Rocky Mountain conifers (Tunnock and others 1984; Gibson and others 1984). But when populations are transferred large geographic distances, infestations by mites increase sub- stantially. Because mites generally deform terminal shoots, growth and development are affected greatly. The components of adaptedness tend to be integrated by values of adjusted height. By representing growth from a common height at age 5, adjusted height was free of most genetic and environmental effects that had accrued up to that age. Consequently, the variable was capable of ex- pressing adaptation of populations to particular environ- ments over a short period. At low elevation, populations from mild environments had larger adjusted heights than populations from severe environments. A part of this dif- ference was due to the high innate growth potential of populations from low elevation, and a part was due to the high susceptibility to needle cast of populations from high elevations. At PREF 1,500, populations from about 1,500 m had the largest adjusted heights; populations from extremely high elevations displayed an innately low- growth potential, while populations from low elevations suffered snow damage. At Lost Valley, however, environ- mental effects sufficient for adaptively differentiating populations evidently are only beginning to be expressed. Although southern populations of low growth potential displayed the smallest adjusted heights, superior perfor- mance of relatively local sources cannot yet be detected (fig. 3). Adaptive differentiation is readily quantified from regression statistics. Table 5 shows that elevational clines are particularly steep for 7-year height, late growth, needle cast, and snow damage. Populations separated by 1,000 m are expected to differ by 16 cm (83 percent of the mean) in 7-year height largely because of a large differ- ence in the amount of late growth. These same popula- tions, when planted at low elevation, are expected to differ by 48 percent in leaves infected with needle cast. When planted at high elevation, they are expected to dif- fer by 28 percent in trees damaged by the snow. Previous studies (Rehfeldt 1980) also predict that populations separated by 1,000 m will differ by 31 percent in trees damaged by a fall frost that causes a mean injury of 50 percent. Elevational clines are so steep that populations separated by merely 224 m tend to be genetically differen- tiated (95 percent level of probability) for late growth while those separated by 500 m are differentiated for numerous traits (table 5). The elevational cline, moreover, is the dominant cline (table 5). For those variables in which differentiation was pronounced (height, late growth, needle cast, and snow damage), the amount of differentiation associated with 1,000 m of elevation varies from half to twice that asso- ciated with 7° latitude at a constant elevation. Conse- quently, differentiation per unit distance is much greater along the elevational cline than along the geographic cline. Nevertheless, both clines arise from environmental selec- tion along climatic gradients. Because the frost-free period decreases by 80 days across an elevational interval of 1,000 m (Baker 1944), an average difference of only 18 days in frost-free period seems sufficient for inducing differentiation of populations for late growth, a variable reflecting tolerance to early fall frosts. Consequently, the steep elevational cline reflects the rapid change in frost- free period associated with elevation. And the gentle geographic cline arises from a gradual geographic gradient in the frost-free period. These results have direct practical application. First, quantitative estimates of differentiation along adaptive clines define the risks involved with seed transfer in artifi- cial reforestation. Reforestation goals involve increasing productivity while maintaining adaptiveness. To accom- plish this, limits of seed transfer must reflect adaptive clines. The steep elevational clines described in this study Table 5—Quantification of the percentage differences expected between populations located along elevational and geographic clines Elevational Difference Difference interval (m) across 1,000 m across 7° associated with Variable elevation latitude Isd (.05)' 7-year height (cm) 16 28 463 Adjusted height PREF 640 (cm) 14 10 544 PREF 1,500 (cm) 6 5 2,461 Lost Valley (cm) 3 12 4,735 Late growth (cm) "A 6 224 Needle cast (%) 48 38 354 Mites (%) 5 14 2,519 Shoot borer (%) 3 10 5,129 Spring frost injury (%) 2 2 2,268 Snow damage (%) 28 17 643 ‘Calculated for linear regressions as the ratio [/sd(.05)]/b and for nonlinear regressions as the solution to the quadratic equation for elevations associated with Y + Y2[/sd(.05)] at the mean geographic intercept. imply that the elevational transfer of seeds should be greatly restricted, but the gentle geographic clines imply that the lateral movement of seeds can be relatively liberal. These conclusions are corroborated by others for the same species in adjacent geographic regions (Rehfeldt 1988a, 1985a; Stoneman 1985). Regardless, there is little doubt that adaptive variation among populations of P. contorta reflects physiological specialization for relatively small segments of the environ- mental gradient. Specialized populations develop from a balance of environmental stimuli that harmoniously adjust the developmental cycle to the physical and biotic environ- ment. By means of this specialized evolutionary mode, coadaptive traits have been designed by natural selection to overcome ecological problems as diverse as variable frost-free periods, insect infestations, and disease epi- demics. Therefore, the large geographic and broad ecological distributions of this species result from the existence. of innumerable populations, each of which is specialized. Perpetuation of specialized populations is readily facilitated by cycles of even-aged reproduction whereby individuals tend to become established on sites that supported their ancestors. As evidenced by substantial genetic variances within populations (Ying and others 1985; Rehfeldt 1985b), specialization has not led to genetic uniformity. Although migration, mutation, and the founder effect undoubtedly contribute to intrapopulation variation, much of this varia- tion likely results from variable selection pressures associated with environmental heterogeneity in time. For long-lived stationary organisms, temporal heterogeneity not only is pronounced but also places a limit on the degree to which specialization can develop. Bryant (1976) noted that, if populations are to survive, the process of specialization cannot deplete the genetic variability re- quired for accommodating environmental heterogeneity in time. Forest trees of the Rocky Mountains have achieved adaptation to heterogeneous environments according to different modes. Pseudotsuga menziesii, like P. contorta, exhibits specialization, but Larix occidentalis Nutt. and Pinus monticola Doug. are generalists: adaptive clines are relatively flat, and populations express a high fitness to a broad range of environments (Rehfeldt 1984). Pinus ponderosa Dougl. ex Laws., moreover, displays an inter- mediate mode. Thus, the specialist mode is characterizing species with broad ecological distributions while the generalist mode is characterizing species with relatively narrow distributions. Indeed, P. contorta and Pseudotsuga menziesii have greater botanical distributions and larger ecological amplitudes than any other western conifer; but the ecological distribution of Pinus monticola and Larix occidentalis is relatively small while that of Pinus ponderosa is intermediate (U.S. Department of Agriculture 1965). An understanding of the ecological genetics of species such as Thuja plicata Donn ex D. Don and Tsuga heterophylla, both of which have narrow ecological ampli- tudes and small geographic distributions, is necessary for assessing this apparent relationship between abundance and the adaptive mode. REFERENCES Amman, G. D.; Baker, B. H.; Stipe, L. E. Lodgepole pine losses to mountain pine beetle related to elevation. Research Note INT-171. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station; 19738. 8 p. Baker, R. S. Mountain climates of the western United States. Ecological Monographs. 14: 223-254; 1944. Benson, R. E. Management consequences of alternative harvesting and residue treatment practices—lodgepole pine. General Technical Report INT-132. Ogden, UT: U.S. Department of Agriculture, Forest Service, Inter- mountain Forest and Range Experiment Station; 1982. 58 p. Bryant, E. H. A comment on the role of environmental variation in maintaining polymorphisms in natural populations. Evolution. 30: 188-190; 1976. Critchfield, W. B. Geographic variation in Pinus contorta. Publication 3. Cambridge, MA: Harvard University, Maria Moors Cabot Foundation; 1957. 118 p. Critchfield, W. B. Genetics of lodgepole pine. Research Paper WO-37. Washington, DC: U.S. Department of Agriculture, Forest Service; 1980. 57 p. Daubenmire, R.; Daubenmire, J. B. Forest vegetation of eastern Washington and northern Idaho. Technical Bulletin 60. Pullman, WA: Washington Agricultural Experiment Station; 1968. 104 p. Draper, N. R.; Smith, H. Applied regression analyses. New York: John Wiley and Sons; 1981. 709 p. Gibson, K. E.; James, R. L.; Livingston, R. L.; Schwandt, J. W.; Beckman, D. P.; Thier, R. W.; Marshall, J. P.; Knapp, K. A. Idaho forest and pest conditions and pro- gram summary 1983. Report 84-1. Missoula, MT: U.S. Department of Agriculture, Forest Service, Northern and Intermountain Regions; 1984. 40 p. Hagner, M. A genecological investigation of the annual rhythm of Pinus contorta Dougl. and a comparison with Pinus sylvestris L. Studalia Forestica Suecica 81. Stock- holm, Sweden: Royal College of Forestry; 1970. 26 p. Hoff, R. J. Susceptibility of lodgepole pine to the needle cast fungus Lophodermella concolor. Research Note INT-349. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experi- ment Station; 1985. 6 p. Hunt, R. S. Trisetacus (Acarina:Eriophyoidea) on Pinus contorta in British Columbia: distribution, symptoms, and provenance effect. Canadian Journal of Forest Research. 11: 651-653; 1981. Krebill, R. G. Lodgepole pine’s fungus-caused diseases and decays. In: Baumgartner, D. M., ed. Management of lodgepole pine ecosystems. Pullman, WA: Washington State University Extension Service; 1975: 377-405. Lande, R. A quantitative genetic theory of life history evolution. Ecology. 63: 607-615; 1982. Lindgren, D.; Krutzch, P.; Twetman, J.; Kiellander, C. L. Survival and early growth of Pinus contorta pro- venances in northern Sweden. General Research Note 20. Stockholm, Sweden: Department of Forestry, Royal College of Forestry; 1976. 42 p. Little, E. L., Jr. Atlas of United States trees. Volume 1. Conifers and important hardwoods. Miscellaneous Publication 1146. Washington, DC: U.S. Department of Agriculture; 1971. 9 p. plus 313 maps. Lotan, J. E.; Brown, J. K.; Neuenschwander, L. F. Role of fire in lodgepole pine forests. In: Baumgartner, David M.; Krebill, Richard G.; Arnott, James T.; Weetman, Gordon F., eds. Lodgepole pine: the species and its management: symposium proceedings; 1984 May 8-10; Spokane, WA; 1984 May 14-16; Vancouver, BC. Pull- man, WA: Washington State University, Office of Con- ferences and Institutes, Cooperative Extension; 1985: 133-152. Pfister, R. D.; Daubenmire, R. Ecology of lodgepole pine, Pinus contorta Doug]. In: Baumgartner, D. M., ed. Management of lodgepole pine ecosystems. Pullman, WA: Washington State University Extension Service; 1975: 27-46. Pfister, R. D.; Kovalchik, B. L.; Arno, S. F.; Presby, R. C. Forest habitat types of Montana. General Technical Report INT-34. Ogden, UT: U.S. Department of Agricul- ture, Forest Service, Intermountain Forest and Range Experiment Station; 1977. 174 p. Rehfeldt, G. E. Cold acclimation in populations of Pinus contorta from the northern Rocky Mountains. Botanical Gazette. 141: 458-463; 1980. Rehfeldt, G. E. Adaptation of Pinus contorta populations to heterogeneous environments in northern Idaho. Cana- dian Journal of Forest Research. 13: 405-411; 1983a. Rehfeldt, G. E. Genetic variation within Douglas-fir populations: implications for tree improvement. Silvae Genetica. 32: 9-14; 1983b. Rehfeldt, G. E. Microevolution of conifers in the northern Rocky Mountains: a view from common gardens. In: Lanner, R. M., ed. Proceedings, eighth North American forest biology workshop; 1984 June 30-August 1; Logan, UT. Logan, UT: Utah State University, College of Natural Resources; 1984: 32-46. Rehfeldt, G. E. Ecological genetics of Pinus contorta from the Wasatch and Uinta Mountains of Utah. Canadian Journal of Forest Research. 15: 524-530; 1985a. Rehfeldt, G. E. Genetic variances and covariances in Pinus contorta: estimates of genetic gains from index selection. Silvae Genetica. 34: 26-33; 1985b. Rehfeldt, G. E.; Wykoff, W. R. Periodicity of shoot elongation among populations of Pinus contorta from the northern Rocky Mountains. Annals of Botany. 48: 371-3877; 1981. SAS Institute. SAS user’s guide. Vol. 2. Carey, NC: SAS Institute; 1982. 548 p. Shrimpton, D. M.; Thomson, A. J. Growth characteristics of lodgepole pine associated with the start of mountain pine beetle outbreaks. Canadian Journal of Forest Research. 13: 187-144; 1983. Steel, R. G. D.; Torrie, J. H. Principles and procedures of statistics. New York: McGraw-Hill; 1960. 481 p. Steele, R.; Pfister, R. D.; Ryker, R. A.; Kittams, J. A. Forest habitat types of central Idaho. General Technical Report INT-144. Ogden, UT: U.S. Department of Agri- culture, Forest Service, Intermountain Forest and Range Experiment Station; 1981. 138 p. Stoneman, C. K. Adaptation of lodgepole pine populations of south-central Oregon; a description of genetic varia- tion based on seedling tests. Moscow, ID: University of Idaho; 1985. 97 p. M.S. thesis. Tackle, D. Silvics of lodgepole pine. Miscellaneous Publica- tion 19. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experi- ment Station; 1959. 24 p. Tunnock, 8.; Dooling, O. J.; Kohler, S. Montana forest pest conditions and program highlights. Report 84-2. Missoula, MT: U.S. Department of Agriculture, Forest Service, Northern Region; 1984. 31 p. U.S. Department of Agriculture, Forest Service. Silvics of forest trees of the United States. Agriculture Handbook 271. Washington, DC; 1985. 762 p. U.S. Department of Commerce. Climatic atlas of the United States. Washington, DC: U.S. Department of Commerce, Environmental Data Service; 1968. 80 p. Vyse, A.; Navratil, S. Advances in lodgepole pine regener- ation. In: Baumgartner, David M.; Krebill, Richard G.; Arnott, James T.; Weetman, Gordon F., eds. Lodgepole pine: the species and its management: symposium pro- ceedings; 1984 May 8-10; Spokane, WA; 1984 May 14-16; Vancouver, BC. Pullman, WA: Washington State University, Office of Conferences and Institutes, Coop- erative Extension; 1985: 173-186. Wheeler, N. C.; Guries, R. P. Biogeography of lodgepole pine. Canadian Journal of Botany. 60: 1805-1814; 1982. Ying, C. D.; Illingworth, K.; Carlson, M. Geographic variation in lodgepole pine and its implications for tree improvement in British Columbia. In: Baumgartner, David M.; Krebill, Richard G.; Arnott, James T.; Weetman, Gordon F., eds. Lodgepole pine: the species and its management: symposium proceedings; 1984 May 8-10; Spokane, WA; 1984 May 14-16; Vancouver, BC. Pullman, WA: Washington State University, Office of Conferences and Institutes, Cooperative Extension; 1985: 45-54. 11 Rehfeldt, G. E. Components of adaptive variation in Pinus contorta from the Inland Northwest. Research Paper INT-375. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station; 1987. 11 p. Genetic variation among 83 populations of Pinus contorta from the Northern Rocky Mountains of the United States was studied with 7-year-old trees planted in three con- trasting environments. Analyses of nine traits reflected adaptation of populations to the biotic and abiotic environments and revealed clinal patterns of differentiation that were elevationally steep but geographically gentle. Regression models present adap- tive landscapes that mirror elevational and geographic gradients in climate. KEYWORDS: genetic variation, population differentiation, microevolution, Pinus contorta INTERMOUNTAIN RESEARCH STATION The Intermountain Research Station provides scientific knowl- edge and technology to improve management, protection, and use of the forests and rangelands of the Intermountain West. Research is designed to meet the needs of National Forest managers, Federal and State agencies, industry, academic institutions, public and private organizations, and individuals. Results of research are made available through publications, symposia, workshops, training sessions, and personal contacts. The Intermountain Research Station territory includes Montana, Idaho, Utah, Nevada, and western Wyoming. Eighty-five percent of the lands in the Station area, about 231 million acres, are classified as forest or rangeland. They include grasslands, deserts, shrub- lands, alpine areas, and forests. They provide fiber for forest in- dustries, minerals and fossil fuels for energy and industrial develop- ment, water for domestic and industrial consumption, forage for livestock and wildlife, and recreation opportunities for millions of visitors. Several Station units conduct research in additional western States, or have missions that are national or international in scope. Station laboratories are located in: Boise, Idaho Bozeman, Montana (in cooperation with Montana State University) Logan, Utah (in cooperation with Utah State University) Missoula, Montana (in cooperation with the University of Montana) Moscow, Idaho (in cooperation with the University of Idaho) Ogden, Utah Provo, Utah (in cooperation with Brigham Young University) Reno, Nevada (in cooperation with the University of Nevada)