B 583. 12J Hi leiwc 19U1 Lesicai Peter The ettect of the introduced weed? Centaurea maculosa on Arabis lecundat a threatened Montana STATE DOCUMENTS COLLECTION SEP C 2 1992 M0NIA5?5A!T&EavBERARY helena, montana 59620 THE EFFECT OF THE INTRODUCED WEED, CENTAUREA MACULOSA ON ARABIS FECUNDA . A THREATENED MONTANA ENDEMIC Prepared for: The Montana Natural Heritage Program State Library 1515 East Sixth Avenue Helena, Montana 59620 Prepared by: Peter Lesica P.O. Box 8944 Missoula, Montana 59807 and J. Stephen Shelly Montana Natural Heritage Program State Library 1515 East Sixth Avenue Helena, Montana 59620 January, 1991 ! kJ k-. RE1 Nl leiwc l.'.il STATE DOCUMENTS COLLECTION SEP C 2 1992 MONTANA STATE LIBRARY 1515 E. 6th AVE. HELENA, MONTANA 59620 THE EFFECT OF THE INTRODUCED WEED, CENTAUREA MACULOSA ON ARABIS FECUNDA . A THREATENED MONTANA ENDEMIC Prepared for: The Montana Natural Heritage Program State Library 1515 East Sixth Avenue Helena, Montana 59620 Prepared by: Peter Lesica P.O. Box 8944 Missoula, Montana 59807 and J. Stephen Shelly Montana Natural Heritage Program State Library 1515 East Sixth Avenue Helena, Montana 59620 January, 1991 i )i ii Ktii 1 Introduction Competition is a difficult phenomenon to study effectively (Harper 1977, Keddy 1989). Much of the literature on competition is rife with circular reasoning and expositions of "the ghost of competition past" (Connell 1980) . Experiments are often phenomenological and non-mechanistic, measuring what happens without examining how it happens (Tilman 1987). Tilman (1987, 1988) has urged a more mechanistic approach to the study of interspecific competition. He argues that such studies more often result in testable hypotheses, ultimately leading to more useful predictive generalizations. Werner (1979) divided competitive interactions into two mechanistic categories: (1) preemptive i.e., competition for space and (2) interactive i.e., competition for water or nutrients. Similarly, Tilman (1988) divided resources into two broad groups: (1) space and light, and (2) nutrients and water. Many researchers have recognized the importance of regeneration in competitive interactions (Grubb 1977, Harper 1977, Grime 1979, Werner 1979) . Nonetheless, most studies of plant interspecific interference have focused on competition for light, nutrients, and water during later life stages and have ignored the effects of competition for space on seedling establishment (Schoener 1983) . Another failing of many competition experiments is their short duration. Competition may be occurring, but relatively long periods of time may be necessary to observe it (MacArthur 1972) . Furthermore, the effects of competition may change with time, thus a short-duration study can yield misleading results with regard to long-term community dynamics (Tilman 1988) . Arabis fecunda Rollins is a short-lived, rosette- and taproot-forming perennial in the Mustard Family (Brassicaceae) . This recently described species (Rollins 1984) is endemic to highly calcareous soils in the foothills of the Sapphire Range in Ravalli County and in the Pioneer Range in Beaverhead and Silver Bow counties in southwest Montana (Lesica 1985, Schassberger 1988) . Plants bloom in early spring and disperse seed during early to mid-summer. The small seeds germinate readily without stratification (Lesica, unpublished data) . Presumably most germination occurs following late summer and autumn rains, but a second bout of germination may occur in the spring. Mortality of seedlings and first year plants is high (Lesica and Shelly, unpublished data) . Plants generally grow for at least one year before blooming and then produce one or more inflorescences with 3-many flowers. Individuals may branch at the root crown to form multi-rosette plants at any time during the life cycle. Rosettes are generally 1-4 cm in diameter. 3 Arabis fecunda is threatened by livestock trampling (Lesica and Shelly 1991a) and encroachment of habitat by the aggressive exotic Centaurea maculosa (spotted knapweed) at the Ravalli County sites. It also occurs in areas of historic mining activity in Beaverhead and Sliver Bow counties (Schassberger 1988) . Arabis fecunda is a candidate for listing as a threatened or endangered species by the U.S. Fish and Wildlife Service (USDI-FWS 1990) and is considered threatened in Montana (Lesica and Shelly 1991b) . Centaurea maculosa Lam. (Asteraceae) is a native of Eurasia, widely introduced in North America, and has become a serious pest of semi-arid grasslands in the Pacific Northwest and intermountain valleys of the Northern Rocky Mountains (Watson and Renney 1974, French and Lacey 1983). Its ability to invade and replace native plants is well-documented (Morris and Bedunah 1984, Harris and Cranston 1979, Tyser and Key 1989). In addition, leachates of Centaurea maculosa inhibit germination of grass and conifer seedlings (Kelsey and Locken 1987) ; however, concentrations high enough to inhibit germination would seldom be found in natural environments (Kelsey and Bedunah 1989) . Centaurea maculosa was introduced into western Montana in the 1920 's and has since come to dominate large areas of rangeland (French and Lacey 1983). Like A^ fecunda. it is a rosette- forming, taprooted perennial with an average life-span of 3-5 years (Watson and Renney 1974, Boggs and Story 1987). 4 Germination occurs in the fall or the following spring. Fall emerging plants overwinter as a rosette and may flower during their first summer. Spring-germinating plants generally bloom their second summer (Watson and Renney 1974) . In Ravalli County nearly all Arabis fecunda habitat has been invaded by Centaurea maculosa. Classical competition theory predicts that species with similar ecological reguirements should compete most strongly (Hardin 1960) . The morphological similarity between these two species suggests that C^_ maculosa may be interfering with A^. fecunda at these sites. The purpose of our study was to examine the effects of Centaurea maculosa on populations of Arabis fecunda over a period of three years. We employed technigues of demographic monitoring (Menges 1986) in order to determine which life stages are being most affected and which mechanisms are most important. Study Areas We conducted our study at Birch Creek and Charley's Gulch, in Ravalli County. Both sites are on gentle southwest-facing slopes, the Birch Creek site at 14 3 0 m and the Charley's Gulch site at 1525 m. At Hamilton, ca. 8 km southwest and 300 m lower, mean temperatures for July and January are 19.4° and -3.8° C respectively, and mean annual precipitation is 32 cm. Soils are sandy loams derived from calcareous parent material (Lesica 5 1985) . Zonal vegetation surrounding the sites is foothills Aaropvron-Festuca grasslands (Weaver 1980) with scattered Pinus ponderosa and Pseudotsuga menziesii. The effects of trampling and grazing by livestock are noticeable at both sites. At both sites Aj. fecunda occurs in communities with a relatively low density of vascular plants. Methods In 1987 we established a permanent belt transect consisting of 10 adjacent 1-m2 plots at each of the study sites following methods outlined in Lesica (1987). Transects were placed in areas with moderate to high cover of Centaurea maculosa and Arabis fecunda. All Cj. maculosa plants were removed from five randomly selected plots in each transect in late May of each year by cutting the plants below the root crown using a sharp knife with a hooked blade. We were careful to disturb the soil as little as possible. Individual A_;_ fecunda plants were mapped and recorded using the following system: S = plant with a single rosette < 2 cm diameter R = plant with a single rosette > 2 cm diameter S or R = a plant with x rosettes 6 In addition, for each reproductive plant we recorded the number of inflorescences and the number of fruits matured. We classified plants into four life-stage classes: Seedling = S, Single Rosette = R1 Multiple Rosette = S>1 or R^ Reproductive A plants *s demographic properties are often more closely correlated with life-stage (Werner and Caswell 1977, Caswell 1989) , although both age and size may be important in predicting an individual's fate (Young 1985). We chose these classes because they correspond to age as well as size (Table 1; Lesica and Shelly, unpublished data) . These categories also represent a reasonable compromise between having many categories with too few observations each and few categories with many observations (Vandermeer 1978) . We censused Arabis fecunda in the permanent plots in late May, 1987-1990. We chose late May because A^. fecunda fruits are mature or nearly so, but dispersal has not yet occurred. Plants smaller than 0.5 cm in diameter were not recorded because they could not be reliably distinguished from other species. The 7 experiment at Charley's Gulch had to be terminated after 1989 because plots were trampled by livestock. We used stage-based transition matrix models to compare the performance of A,, fecunda populations between the plots from which CL. maculosa was removed (treatment) and the controls. Matrix projections assume fixed life-stage structures and simulate changes in a population through time (Lefkovitch 1965, Menges 1990) . One-year transition probabilities were estimated as the number of plants in life-stage class i moving into class j over the course of one year, divided by the number of plants in stage i at the beginning of the year. This method assumes that an individual's transition one year depends only on its life- stage class at the beginning of the period, and is independent of its transition the previous year. Transition probabilities for the multi-year periods were made by summing the 1-year frequency tables for the period (Caswell 1989, p. 81). The "mean" matrix summarizes the behavior of the population over 2-3 years and integrates the effects of fluctuating environment during this period to give a more realistic estimate of the effect of the treatment on population growth. This method of constructing a mean matrix tends to minimize the effects of year-to-year differences in demographic patterns, but the bias is usually small (Huenneke and Marks 1987). The equilibrium growth rate (lambda) is the dominant eigenvalue of the transition probability matrix (Lefkovitch 1965, Caswell 1989). We estimated lambda and calculated confidence intervals using a bootstrap resampling procedure with a sample size of 1000 following methods outlined in Caswell (1989, p. 190). A lambda > 1.0 indicates population increase, while lambda < 1.0 indicates decrease. We used 2X2 contingency table analysis to determine whether there were differences in performance of plants in particular life-stages between the treatment and control plots. At each site we compared the number of plants in a life-stage class that had moved to a smaller class (or died) , with the number that had stayed in the same class or moved to a larger class between the treatment and control. Transitions from each of the three yearly intervals were pooled. All plants recorded during the study were either present at the beginning of the study or recruited during its course. The ratio of the latter to the former is a measure of recruitment over the three-year period, and the difference between treatment and control plots was also assessed in a 2 X 2 contingency table. Simple population growth rate was calculated as the increase in number of individuals at the end of a time period divided by the number of individuals present at the beginning of the time period. A positive rate indicates that population size has increased. The effect of treatment on fecundity (fruits/ reproductive plant) was analyzed using analysis of variance (ANOVA) on log-transformed data. Statistical 9 calculations were performed on a microcomputer using SYSTAT (Wilkinson 1986) . Eigenvalues and confidence intervals were obtained using GAUSS (Anon. 1988) . Results Before the start of the experiment, cover of Centaurea maculosa was ca. 30% at Birch Creek and 25% at Charley's Gulch, and the difference between treatment and control plots was not significant (Table 2) . After one year, C^ maculosa cover was significantly reduced relative to controls at Charley's Gulch, but not at Birch Creek (Table 2). The resiliency of the C^ maculosa population during the first year was due mainly to recruitment from the seed bank. In spite of this recruitment, removal of C_j_ maculosa probably reduced competitive effects on A^. fecunda. After two years, C\_ maculosa cover was significantly reduced at both sites (Table 2) . Summary demographic statistics for both sites are presented in Table 3 . At Birch Creek there were fewer plants in the treatment plots compared to controls at the beginning of the experiment, but more than controls after one year. There were nearly equal numbers of fruiting plants in treatment and control plots during the first year of the experiment, but more in treatment plots in the last two years. Treatment plots had a higher recruitment rate during the first two years, but often » also a higher mortality rate. Simple population growth was much higher in treatment plots the first year, but lower than controls the following two years. Nonetheless, simple population growth for the three years combined was greater for treatment plots (1.16) than controls (0.39). At Charley's Gulch there were also fewer A-. fecunda plants in the treatment plots compared to control plots at the beginning of the experiment, but more than in the control plots after one year. Treatment plots had more fruiting plants than controls at both the beginning and end of the experiment. Treatment plots had higher recruitment rates, and lower mortality, and higher fc simple population growth than controls. Over the two-year period, simple population growth for A^. fecunda was greater for treatment plots (0.56) than controls (-0.03). Fecundity, as measured by number of fruits per plant, was higher in treatment plots throughout the study period at both sites. Results of ANOVA indicate that treatment had a significant effect on the fecundity of Arabis fecunda (Table 4). However, a near-significant difference was present before the beginning of the experiment (ANOVA; N=135, F=3.16, p=0.078). Recruitment rates at both sites were significantly higher in treatment plots than controls (Table 5). Otherwise, there were P no significant differences between the performance of any life- 11 stage classes between treatment and control plots at either site (Table 6). However, at Charley's Gulch, there was a trend for plants in both S, and R, classes to become larger in the treatment plots. Lack of a significant difference is probably due to the small sample size. If these two classes are combined, the difference between treatment and control is significant (X2=3.957, p<0.05) . At both Birch Creek and Charley's Gulch, the equilibrium growth rate (lambda) of Arabis fecunda for the first two years of the study was significantly larger in plots with Centaurea maculosa removed compared to controls (Table 7) . At Birch Creek the equilibrium growth rate is larger for the first three years as well, but the difference is not significant (Table 7). Over the study period, equilibrium growth rates of Aj_ fecunda in control plots were not significantly different from 1.0 (p<0.05). Discussion The results of our study indicate that Centaurea maculosa is competing with Arabis fecunda and that this competition is being effected primarily by a reduction in recruitment (preemptive competition sensu Werner 1979) rather than interference at later stages. During the first two years following removal of C^. maculosa, both equilibrium qrowth rate calculated from projection matrices and simple qrowth rate of At fecunda populations were 12 higher in treatment plots. Reproductive rates as measured by the number of new plants per reproductive individual were also higher in treatment plots at both sites for the first two years after removal of C^_ maculosa, and recruitment rate over the period of the study was significantly higher at both sites. With the exception of the slight tendency for small plants to survive better in treatment plots at Charley's Gulch, there is no evidence that plants in any life-stage class performed better in plots from which C^. maculosa had been removed. The presence of Centaurea maculosa could adversely affect Arabis fecunda recruitment by (1) reducing fecundity and/or (2) curtailing establishment of seedlings. We detected only a small increase in A_j_ fecunda fecundity in the treatment plots during the first two years following removal of Cj. maculosa. Although this difference could be important, it is more likely that the observed increase in A^. fecunda recruitment is due to enhanced seedling establishment. Establishment of A^. fecunda seedlings could have been inhibited by competition with C_j_ maculosa, but this seems unlikely since C\_ maculosa did not interfere with larger plants. Allelopathy could also explain our results, and extracts of cnicin from Centaurea maculosa do inhibit germination of grasses and conifers (Kelsey and Locken 1987) . However, Kelsey and Bedunah (1989) concluded that concentrations of cnicin high enough to cause interference are rarely present in natural situations. Thus, it seems most likely that C\. maculosa reduces 13 A. fecunda recruitment by usurping space and light from germinating seedlings. Gurevitch (1986) found that inhibition of seedling establishment was an important effect of interspecific competition on the grass, Stipa neomexicana, although she also found that competition limited the growth and flower production of mature plants. At Birch Creek, removal of Centaurea maculosa resulted in increased recruitment and population growth for the first two years, but by the third year the difference between treatment and control plots had disappeared. This pattern suggests that A^_ fecunda underwent a two-year period of competitive release following removal of C^. maculosa but was approaching a new, higher eguilibrium after two years. Put simply, removal of Centaurea maculosa increased the carrying capacity of the habitat for Arabis fecunda. The results of our study indicate that the main mechanism by which Centaurea maculosa interferes with Arabis fecunda is reduction of space available for seedling establishment. This hypothesis suggests that competitive displacement of A_;_ fecunda will not occur if density of C\_ maculosa stays near current levels. The fact that eguilibrium growth rates were near 1.0 for the study period lends support to this conclusion. If C. maculosa increases in these habitats, the size of A^_ fecunda 14 populations will decrease and the risk of chance extinction events will become more likely (Goodman 1987) . Herbicide application is currently the most commonly employed method for controlling Centaurea maculosa infestations (Harris and Cranston 1979, McKone et al. 1989); however, little is known about specific effects on Arabis fecunda, and correct rates of application are difficult to determine for novel situations, such as the azonal A^. fecunda sites (Peter Rice, pers. com.) . Removing C^ maculosa by hand would result in serious damage to the fragile slope habitat. To date, biological control vectors have been unable to reduce infestations of Centaurea maculosa. Although introduced seed-head gall flies (Urophora af finis and Ul guadrifasciata) can reduce seed production from 50-75% (Story 1989), they have been unable to reduce the density of C^ maculosa (Muller and Schroeder 1989) . However, with the recent introduction of a root-boring moth (Aqapeta zoeqana) , control of C^. maculosa should be possible (Story 1989, Muller and Schroeder 1989). Since biological control causes minimal disturbance and promises to have the ability to halt population growth of C^. maculosa, it appears to be the best option for protecting populations of Arabis fecunda from extinction due to encroachment by Centaurea maculosa. 15 Acknowledgements We are grateful to John Robbins and George Frost for permission to conduct this study on their land. Brian Steele and Dave Patterson helped with statistics. Keith Boggs and Peter Rice advised on the ecology of Centaurea maculosa. 16 Literature Cited Anon. 1988. GAUSS. Aptech Systems Inc., Kent, Washington. Boggs, K. W. and J. M. Story. 1987. The population age structure of spotted knapweed (Centaurea maculosa) in Montana. Weed Science 35: 194-198. Caswell, H. 1989. Matrix population models. Sinauer Associates, Sunderland, Massachusetts. 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Ecology 66: 1879-1883. Table 1. Percent of Arabis fecunda plants in each life-stage class that are i RF>0 + Rec. Si .00 .00 .00 .00 + .53 Ri .20 .23 .03 .30 + 1.36 S,R>1 .00 .07 .25 .06 + .28 RF>0 .00 .14 .13 .13 + .10 To Si "i S.R>i RF>0 + Rec Si .00 .01 .00 .00 + 1.00 "i .18 .25 .00 .30 + 2.42 S,R>1 .00 .05 .09 .14 + .46 RF>0 .09 .13 .27 .14 ♦ .35 Control: lambda=0.811 Treatment: lambda=1 .161 t=3.395, p<0.001 Birch Creek 1987-1990 ro Si Rl S,R>i RF>0 + Rec »i .04 .01 .00 .00 + .85 h .16 .26 .02 .25 + 1.39 >,R>1 .04 .07 .25 .05 + .24 0 .04 .18 .25 .23 + .08 Si Ri S.R>i RF>0 + Rec. .00 .01 .00 .00 + 1.18 .14 .24 .05 .18 + 1.63 .00 .04 .10 .09 + .27 .07 .17 .34 .23 + .19 Control: lambda=0.977 Treatment: lambda=1 .070 t=1.348, p=0.889 Char ey's Gulch 1987-89 To Si From Ri S.R>i RF>0 + Rec- To Si From Ri S,R> 1 RF>0 + Rec Si .25 .08 .03 .00 + .38 Si .00 .04 .07 .10 + .80 Rl .13 .46 .07 .39 + .46 Ri .58 .48 .07 .65 + .90 S,R>1 .00 .05 .43 .31 + .23 S,R>1 .00 .05 .40 .03 + .07 RF>0 .00 .20 .33 .11 + .04 RF>0 .08 .30 .33 .06 + .10 Control lambda =0.962 Treatment: ambda=1 199 t=4.210, p <0.001 UW