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Ecology |
Niwot Ridge Long-Term Ecological Research Program, University of Colorado, Boulder, Colorado 89301 USA
Received for publication December 18, 2002. Accepted for publication March 25, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Colorado Rocky Mountains • demography • germination • plant community • recruitment • survival analysis • tundra
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Harvey et al., 1989
; Silvertown et al., 1992
). Implicit in these schemes is the idea that organisms must allocate limited resources to competing functions with resulting trade-offs between functions. In stressful environments, theory predicts that plants will exhibit a life history that emphasizes stasis of adult stages at the expense of growth and fecundity (Grime, 1977
). Another conservative life history strategy associated with plants living in stressful environments is clonality, which provides for population maintenance in times and places less favorable to seed production and seedling establishment (Bierzychudek, 1985
). A trade-off is also predicted between vegetative and sexual reproduction (Harper, 1967
). Therefore, theory predicts low recruitment in long-lived clonal species living in stressful environments.
Tundra environments are at the stressful end of the habitat spectrum, and tundra species possess a diverse suite of adaptations to deal with climatic and edaphic conditions (Körner, 1999
). Alpine tundra is dominated by unusually long-lived perennials (Billings and Mooney, 1968
). Life spans as long as several centuries have been estimated using demographic modeling (Morris and Doak, 1998
; Forbis, 2002
), while genetic studies of large clones have estimated spans as old as several millenia (Steinger et al., 1996
). The expectation of a negative correlation between adult survival and fecundity (Charnov, 1991
) suggests that alpine plants should have low annual rates of seedling recruitment. Additionally, many alpine species are highly clonal and are therefore expected to have low rates of seedling establishment (Angevine, 1983
; Eriksson, 1989
; Callaghan et al., 1992
). Furthermore, the current alpine environment is often unfavorable for sexual reproduction. Cold temperatures can limit pollination (Totland, 1997
; Bingham and Orthner, 1998
), and developmental processes such as pollen tube growth and seed maturation are limited by short growing seasons (Stephenson, 1981
) and frost damage (Billings, 1987
). Desiccation (Bell and Bliss, 1980
) and needle ice (Hedberg, 1964
) can cause high mortality rates in seedlings. Therefore, by virtue of both their evolved life history strategy and the immediate effects of their environment, alpine plants are predicted to have low levels of establishment from seed.
In fact, during the earliest active period of research on arctic and alpine plants, ecologists often wrote that seedling establishment on the tundra was rare (Billings and Mooney, 1968
; Bliss, 1971
; Billings, 1973
). At that time, there were no data on per capita recruitment or seedling survival rates; rather, it was assumed that these environments were too harsh for successful seed set and seedling survival (Billings, 1987
). Despite a lack of data, the idea that sexual reproduction plays a minimal role in the demography of alpine plants has continued to predominate in the literature (Marchand and Roach, 1980
; Archibold, 1984
; Bauert, 1996
; Totland, 1997
; Gugerli, 1998
).
Some evidence that tundra seedling establishment might occur has come from laboratory studies of seed viability, which have shown that germination rates can be high (Bonde, 1965a
, b
; Chambers et al., 1987
). In studies of clonal diversity in populations of tundra plants, a high proportion of distinguishable genotypes were found within populations (Jonsson et al., 1996
; Gabrielsen, 1998
), suggesting a history of seedling establishment. Some field studies have documented seedling occurrence without quantifying recruitment or survival (Söyrinki, 1938
; Osburn, 1961
). An arctic study (Freedman et al., 1982
) provided evidence of high seedling densities under natural, undisturbed conditions. High seedling densities have also been documented in disturbed alpine (Chambers et al., 1987
; Schlag and Erschbamer, 2000
) and undisturbed arctic-alpine (Welling and Laine, 2000
) sites. Evidence of this kind led Bock (1976)
to suggest that alpine areas may have levels of sexual reproduction similar to those in lower-elevation perennial plant communities, but seedling demography has not been studied in any high-elevation alpine species.
The level of sexual reproduction in populations of alpine plants has important evolutionary and ecological implications. Genetic variation and, therefore, evolutionary potential are determined by current and historic levels of seedling establishment (Ellstrand and Roose, 1987
). Seeds are frequently the only means of long-distance dispersal (Eriksson, 1993
), and dispersal ability is thought to be critical to species' responses to global change (Callaghan et al., 1992
), which is predicted to affect the alpine disproportionately (Arft et al., 1999
). Additionally, in clonal plant communities, species that establish from seed often provide an important component of species diversity (Bullock et al., 1995
; Hobbs and Mooney, 1995
).
The alpine tundra is a particularly good environment in which to test theories about life history patterns. In addition to the untested prediction that rates of seedling establishment should be particularly low, there exists the opportunity to examine life history variation among the different alpine plant communities, which exhibit wide variation in biotic and abiotic conditions including snow accumulation and soil moisture, winter temperatures, biomass, and nutrient levels (May and Webber, 1982
). Here, I address the following hypotheses: (1) that seedling establishment rates for some alpine species are within the range of rates for perennial species from other environments; (2) that recruitment and survival of alpine seedlings correlate with intercommunity and interannual variation in climatic and microclimatic parameters; and (3) that species that rely heavily on seed reproduction are more abundant as seedlings than as adults, while other species that rely more heavily on clonal reproduction are more abundant as adults than as seedlings.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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3500–3700 m) in the Front Range of the Colorado Rocky Mountains (40°03' N, 105°36' W). To allow for comparison of recruitment patterns among vegetation types and microclimatic conditions, plots were located in four major alpine plant communities; fellfield, dry meadow, moist meadow, and wet meadow. Fellfield (FF) sites are the driest, can be snow-free throughout the winter, and have the longest growing season and the greatest fluctuation in soil temperature (May and Webber, 1982
). These exposed sites have higher coverage of bare rock than of plants (Komárková, 1979
). Dominant species in these communities are four forbs with the cushion growth form, Silene acaulis, Minuartia obtusiloba, Trifolium nanum, and Paronychia pulvinata. (Nomenclature follows Weber [1976]
.) Dry meadows (DM) are slightly moister than FF and have slightly shorter growing seasons. The DM sites are dominated by the sedge Kobresia myosuroides, the clubmoss Selaginella densa, and the perennial forb Acomastylis rossii. Moist meadows (MM) have higher soil moisture and shorter growing seasons than FF or DM and are dominated by A. rossii and the grass Deschampsia caespitosa. Wet meadows (WM) have soils that remain saturated throughout much of the growing season, which lasts half as long as that of FF sites on average. Wet meadows are dominated by the forbs Pedicularis groenlandica and Caltha leptosepala and by the sedge Carex scopulorum (May and Webber, 1982
).
Germination
In 1998, 15 permanent plots were created to monitor germination and survival of seedlings in undisturbed examples of the four community types described above (Fig. 1). There were three FF plots of 1 m2 and three WM plots of 0.5 m2. The smaller WM plots had seedling densities that did not differ from 1-m2 plots using a curve for seedling density per area. The sizes of the five DM and four MM plots varied between 0.25 and 1.25 m2 because these plots were scaled to pocket gopher disturbance plots as part of a separate study (Forbis, 2002
). Total plot area in each community exceeded 3 m2.
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Climate data were obtained from the Niwot Ridge LTER Program (methods in Greenland, 1989
). Precipitation and air temperature data from the stations at the east and west ends of the study sites (Saddle and D1, 3525 and 3743 m above sea level, respectively) were averaged for each day. Soil moisture values (measured using time domain reflectometry) from sampling points that occured in each community of interest (DM, FF, MM, WM) were averaged for each date.
Germination data were analyzed at the plot level using values for cumulative, log-transformed growing-season germination density (seedlings per square meter). To test for germination differences among communities and years, a repeated measures ANOVA with year as the repeated factor was conducted (PROC GLM; SAS 8.22; SAS Institute, 2001
). Because the sphericity assumption was violated (Mauchly's sphericity test, P < 0.0001), Greenhouse-Geisser-adjusted P values were used.
To determine the importance of temporal variation in temperature, soil moisture, and precipitation, log-transformed monthly sums of new seedling densities in each community were the response variable in four multiple regressions (by community) with monthly mean, high, and low temperatures, average soil moisture from each community, and total precipitation as predictors. All variables were either determined to have normally distributed error or were transformed to normalize error. Precipitation and high and low temperatures were not found to be predictors of germination density in any community (P > 0.05 in all cases); therefore, reduced analyses that did not include these factors were performed. Type III sums of squares (SS) were used in all analyses.
Survival
To determine whether survival rates differed among communities, whether climatic variables were predictors of survival, and whether species differed in their survival rates, survival analysis, a class of statistical analysis used to study the timing of events, was used. This type of analysis is necessary when data are censored, i.e., the event of interest has not occurred for all individuals at the end of the observation period (Fox, 1993
). A continuous-time proportional hazards model in PROC LOGISTIC in SAS was used. This model allowed for tied observations and inclusion of time-varying climatic variables (Allison, 1995
). An initial model included community and plot nested within community. A priori contrasts comparing all communities to one another were used. To determine whether survival probabilities differed among years, a similar but separate model using year as a time-varying covariate was run. Because summer precipitation is the primary source for soil moisture in FF and DM whereas moisture results from both summer precipitation and snowmelt in MM and WM, the effects of precipitation and soil moisture were determined in separate logistic regressions for each community. For each 2-wk interval, the covariates were soil moisture and square root-transformed precipitation. The soil moisture value from the measurement at the nearest 2-wk cutoff was used for each 2-wk survival observation date, along with a 7-d running average for precipitation for that date.
To determine whether survival rates differed among species, species-level survival comparisons were conducted separately for all seedlings that could be identified (35 species; N = 2067 of 3841). Unidentified seedlings were common because alpine seedlings grow extremely slowly (Wager, 1938
; Billings, 1973
; Chambers et al., 1990
) and are often not identifiable in their first year. Of the unidentified seedlings, 56.4% germinated and died in 1998, the first year of the study. Identification rates improved in subsequent years.
To determine whether seedlings died primarily during the growing season or during the winter, those that were dead at the first census of the year were classified as "winter deaths." The proportions of winter deaths and growing season deaths were compared within levels of community using repeated measures ANOVA with plot as the repeated factor.
Species abundances at seedling and adult stages
Vegetation was sampled in three plots in each community type in July 2001 using a modification of the point-intercept method as described by Barbour et al. (1987)
with 100 sampling points located 20 cm apart within each plot. A pin was inserted vertically into the vegetation, and all species touched were recorded for each point. Species that were not hit but that were present were given a cover value of 0.01%.
To determine whether the species composition of seedling communities reflects the composition of the adult vegetation, ANCOVA was used. Rank abundance of each seedling species was the response variable, with rank abundance of each adult species and community type as predictors. Abundances were ranked so that seedling counts could be directly compared with percent cover of adults. Only seedlings present in 2001 (the year of the vegetation survey) were included. Monocots were lumped into a single group, because monocot seedlings could not be identified to species.
To ask whether communities differed in the proportion of species reproducing by seed, a one-way ANOVA comparing proportions of species with seedlings in the four communities was run. A separate analysis was conducted to identify species that differed significantly between their abundance as seedlings and their abundance as adults. In this analysis, only species with sufficient sample sizes (at least five seedlings or at least 5% cover) were included. Repeated measures ANOVA was used with abundance as the response variable, community and species as predictors, and vegetative vs. seedling as the repeated factor. To look for species-level differences between vegetative and seedling abundances, least-square means were compared using a Bonferroni correction. To determine whether seedling abundances and survival times were correlated within species, a two-way ANOVA of rank-transformed seedling densities on survival times within communities was performed.
Establishment comparison
To determine whether rates of establishment in alpine plants differ from rates in plants from other environments, I compared the data from this study to published demographic data on germination and seedling survival for perennial species including herbs, shrubs, and trees from tropical forest to arctic tundra. I chose demographic studies that included data on adult densities, seedling densities, and seedling survival (while including only species that showed recruitment from seed). From these published data, I used seedling survival probability and per capita recruitment rate (annual number of seedlings per adult ramet), which allowed me to standardize seedling densities from communities with different levels of biomass and clonality. For cases in which more than one age distribution was provided, I used the distribution from the onset of the study. For studies that compared disturbed and undisturbed sites, I used data from the undisturbed site, and for cases in which matrices from multiple transitions or sites were used, I averaged data across the matrices.
For my own data, I concentrated on focal species, the species for which the best data were available. I calculated recruitment and survival rates at the community level so that each rate would be observed in a relatively homogeneous environment. I eliminated species that did not live at least four growing seasons and species-community combinations with fewer than five individuals. Some consider the seedling phase to have ended when the plant becomes autotrophic (Kitajima and Fenner, 2000
), while other workers in the alpine have defined seedling establishment as a period lasting 2 yr (Scherff et al., 1994
). To make the most conservative comparison possible, I used a longer time period—I compared first-season survival from the published studies to cumulative four-season survival for alpine species. Using four growing seasons of survival data allowed me to more realistically compare alpine seedlings, which remain in the seedling stage for many years, to other plants, which pass out of the seedling stage within 1 yr. Because species with higher germination densities do not have higher seedling survival (this study), I calculated an overall establishment value, which is the ratio of seedlings to adults multiplied by the seedling survival probability. This overall measure of seedling establishment allowed me to rank all species using their probability of establishing seedlings.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Survival
Seedlings in all communities had their highest mortality in the first year of life, with mortality ranging from 73.9% in DM to only 48.0% in WM across the 4 yr of study. Mortality was lower in the second year (from 32.1% in MM to only 16.0% in DM) and even lower in the third and fourth years (as little as 1.8% in the DM in year four), suggesting that mortality is concentrated in the first year of life and that alpine seedling survival fits a Deevey type III survival curve (Deevey, 1947
; Fig. 3).
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2 = 1175.6369 and P < 0.0001).
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2 = 61.2378, 33.3073, and 18.4640, respectively; P < 0.0001 in all cases), and precipitation was a predictor of survival in MM and WM (Wald 2 = 48.7715, P < 0.0001 in MM; Wald 2 = 14.7396, P = 0.0001 in WM; Fig. 4). The hazard was low (and therefore survival was high) in FF in 2000, a year with lower than average summer precipitation (total precipitation for June through August = 160 mm vs. 245 mm, the average for 1982 through 2002). This result indicates that survival probabilities of FF seedlings were not at all responsive to soil moisture, in contrast to probabilities in DM, MM, and WM (Fig. 4).
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236,14709 = 131.3952, P < 0.0001). The 10 species with the highest average survival times (and at least five individuals) were Tonestus pygmaeus, Erigeron melanocephalus, Minuartia obtusiloba, Caltha leptosepala, Paronychia pulvinata, Gentianodes algida, Cerastium beeringianum, Trifolium nanum, Artemisia scopulorum, and Silene acaulis. With the exception of C. beeringianum, all of these were most numerous in either the fellfield or the wet meadow. Species that had the highest survival times did not have the highest overall seedling establishment, as indicated by the lack of a significant correlation between seedling abundance and mean survival time (linear regression, F1,36 = 1.54, P = 0.2235). Seedlings died during the growing season more often than during the winter (repeated measures ANOVA F1,33 = 43.55, P < 0.0001), a pattern that differed between communities (ANOVA, community F3,33 = 3.45, P = 0.0485) with WM having the most winter deaths (42.3%), followed by MM (30.1%), FF (22.6%), and DM (12.7%).
Species abundances at seedling and adult stages
Overall, 35 species were identified as seedlings. The proportion of species reproducing by seed varied widely among communities, with FF and WM having higher proportions than DM and MM (one-way ANOVA, F3,12 = 15.80, P = 0.0010; Table 3). Furthermore, the two dominant species in DM were either absent from seedling populations (Kobresia myosuroides) or were more abundant in the vegetation than they were as seedlings (Acomastylis rossii; Fig. 5). In the MM, the two dominant species were both more abundant as adults than as seedlings (A. rossii and Deschampsia caespitosa; Fig. 5). The FF and WM had significantly higher proportions of species reproducing by seed than the other two communities (0.61 and 0.64, respectively; Table 3), and among the species more abundant as seedlings than as adults, four of six were from FF and WM (Arenaria fendleri, Ciminalis prostrata, Erigeron melanocephalus, Tonestus pygmaea; Fig. 5). The four dominant species in FF (Silene acaulis, Minuartia obtusiloba, Paronychia pulvinata, Trifolium nanum) were the four most abundant species among seedling populations. Of the dominant species in WM, (Pedicularis groenlandica, Caltha leptosepala, Carex scopulorum), Caltha leptosepala was the second most abundant perennial in seedling populations, and P. groenlandica was present in small numbers. None of the sedge species was found as a seedling.
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Establishment comparison: alpine vs. other environments
Rates of seedling establishment for alpine species from Niwot Ridge are either above or within the overall range of published rates of seedling establishment for species from tropical to temperate ecosystems (Table 4). In fact, Bistorta bistortoides had higher seedling establishment rates than did Neodypsis decaryi, a tropical palm with the rates of highest establishment in the literature (1.35 vs. 1.13). Sedum lanceolatum, another alpine species, also had a high rate (0.83) and ranked third among seedling establishment rates. Other alpine species fell with the temperate and arctic species at the middle of the list.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Bliss, 1971
), Niwot Ridge had high seedling densities and relatively high seedling survival rates. In fact, seedling establishment values for alpine species fell either above or within the range of values for perennial species from other environments. Interannual and intercommunity variation in recruitment were positively correlated with soil moisture, but even in the driest community (FF), seedling numbers were high in some years. Survival probability differed among species, and species from WM and FF had the highest survival. Seedling abundance did not correlate with average seedling life span, suggesting that fecund species do not have the highest seedling survival. Because some species rely primarily on seedling recruitment for population maintenance and some species rely more on vegetative reproduction, species abundances of adult plants did not correlate with species abundances of seedlings, and several species differed significantly in abundance between seedling and adult stages. Many dominant alpine species (e.g., the DM dominant Kobresia myosuroides and the WM dominant Carex scopulorum) were not represented in seedling populations, evidence for the presence of a continuum from complete reliance on clonal reproduction to complete reliance on sexual reproduction.
Germination
Seedling recruitment rates varied widely across the topographic gradients that define alpine plant communities. The wettest community had the highest germination densities, and the driest community had the lowest, with the other two communities intermediate (Fig. 2). Community-level recruitment differences could be due to differences in either seed or microsite availability. A comparison of outputs of the dominant species in each community would help determine whether seeds or germination sites and/or germination conditions are limiting.
In addition to differing among plant communities, recruitment rates differed dramatically among years. There are at least two possible explanations for interannual variation: variation in seed availability and/or the presence of predictive germination, germination that is directly sensitive to environmental conditions that increase the probability of seedling establishment. Alpine seed output rates (Arft et al., 1999
) and viabilities (Chambers, 1989
) are known to vary from year to year. Interannual differences in seed output could explain the particularly wide variation in germination in the WM (Fig. 2), because late snow-free communities are expected to vary more widely in seed output rates than early snow-free communities (Molau, 1993
).
Another explanation for interannual germination differences could be predictive germination. Between 20 and 40% of alpine species have some form of seed dormancy (Pelton, 1956
; Amen, 1966
; Sayers and Ward, 1966
), and of tundra species reviewed by Baskin and Baskin (1998)
, 89% have physiological dormancy, a dormancy type that requires that the seed be exposed to a particular set of environmental conditions prior to germination. Niwot Ridge plant communities had high germination densities in wet years and low germination densities in dry years, which meant that for most seedlings, the first year of life tended to be a year with conditions conducive to survival. This pattern suggests that seed dormancy may serve an adaptive role in alpine tundra, allowing seeds to sense environmental conditions and germinate in favorable years, assuming that early-season soil moisture correlates with later-season soil moisture. Evidence suggesting predictive germination has been shown for desert annuals (Pake and Venable, 1996
) and desert perennials (Smith et al., 2000
). However, with regard to both my data and the cases mentioned from the literature, it is difficult to determine whether seeds germinated in wet years simply because germination requirements were met or if germination in wet years is tied to increased probability of seedling survival under those conditions. Predictive germination could only be implicated if germination is directly related to seedling survival.
Survival
Survival rates were within the range of those reported for alpine seedlings by other researchers (Wager, 1938
; Bonde, 1968
; Gartner et al., 1983
). The phenomenon of high mortality in the first year of life followed by decreasing mortality over time has been observed at alpine (Osburn, 1961
) and arctic (Gartner et al., 1983
) sites as well as in a wide range of different ecosystems (old fields, De Steven, 1991
; tropical forest, Alvarez-Buylla and Martinez-Ramos, 1992
; temperate forest, Tanouchi et al., 1994
). Therefore, plants with diverse life history strategies may share a common limiting stage: the first year of life.
Seasonal survival patterns indicated that rather than occurring over the winter, mortality was concentrated during the growing season. Similar results have been found for arctic (Gartner et al., 1983
) and other alpine (Urbanksa and Schütz, 1986
) sites. The predominance of this growing-season mortality suggests that alpine seedlings are more vulnerable to summer drought conditions than to winter drought or extreme cold. This pattern was strongest in the wettest (i.e., WM) plant community and weakest in the drier (DM) ones, a result that is counterintuitive given the fact that winter snow depths are much higher in WM and MM than in DM and FF. However, winter snow provides higher temperatures and protection from desiccation (Billings and Bliss, 1959
), which might be predicted to lead to higher winter survival.
Soil moisture is often very low in tundra sites (Oberbauer and Billings, 1981
; Taylor and Seastedt, 1994
) and can be the primary factor limiting seedling establishment (Gold and Bliss, 1995
). However, species from the wettest (WM) and the driest (FF) communities had the highest survival probabilities. It is logical that high soil moisture would result in higher survival in WM, but the conditions in FF (with the lowest soil moisture, lowest winter temperatures, and highest wind speeds of any alpine community) do not seem conducive to seedling survival. Furthermore, survival was not correlated with soil moisture in FF, and FF seedlings had low hazard estimates (and therefore high survival) in the drought year 2000 (Fig. 4). One likely explanation for high FF survival is that, in general, FF species have experienced the driest and coldest conditions available in the alpine and are particularly well adapted to these conditions. The four dominant species in FF have an obvious morphological adaptation—the cushion growth form. It is possible that this growth form, the high root to shoot ratios, and physiological traits such as the accumulation of sugar alcohols that confer resistance to drought and/or freezing mortality (T. N. Rosenstiel, University of Colorado, personal communication) have either acted as exaptations and allowed these species to initially colonize fellfield sites or have evolved in situ by natural selection. The small degree of overlap in species composition between FF and other community types further supports the idea that species traits may be driving survival as much as or more than environmental conditions.
Establishment
Based on their general life history strategy and on the harsh edaphic conditions of the alpine environment, alpine species have often been assumed to have extremely low seedling establishment rates (see Billings and Mooney, 1968
; Bliss, 1971
; Billings, 1973
; Marchand and Roach, 1980
; Archibold, 1984
; Bauert, 1996
; Totland, 1997
; Gugerli, 1998
). However, this idea was based more on its plausibility than on actual data. A comparison between my seedling establishment data and published data on seedling establishment of perennial species from the tropics to the arctic shows that alpine species fall well within, and even above, the overall range of seedling establishment rates available in the literature (Table 4). In fact, an alpine species, Bistorta bistortoides, had higher seedling establishment rates than did species from other ecosystems. The longevity of this alpine species was estimated to be 50–70 yr (Oren Pollack, University of Colorado, unpublished data), much lower than Silene acaulis, which is predicted to live over 300 yr in Alaska (Morris and Doak, 1998
), and Minuartia obtusiloba, which is predicted to live over 200 yr on Niwot Ridge (Forbis, 2002
). However, even these two multi-centenarian plant species had seedling establishment rates within the range of other temperate species (Table 4).
The results of this comparison refute the idea that alpine plant life histories are severely limited in the germination and seedling establishment stages and that the alpine environment is too harsh for frequent seedling establishment. The idea that the alpine environment is suitable for seedling establishment by the species that occur there is further supported by the fact that FF, the driest, most exposed plant community, had the highest proportion of species reproducing by seed (Table 3) and the highest survival after four growing seasons (Fig. 3).
Community-level and functional group dynamics
Species abundances of seedlings differed from the abundances of adult plants; one major factor driving this difference was the predominance of clonal reproduction among many dominant species. Despite the dominance of grasses and sedges in the vegetation, monocots were scarce among seedlings. This difference between monocot and forb life histories is reflected in the differences between communities in the proportion of species reproducing by seed. The FF and WM, which are dominated by forbs, had significantly higher proportions of species reproducing by seed than did the DM and MM, which are dominated by monocots (May and Webber, 1982
). Furthermore, of the dominant species in FF and WM, all but the sedge Carex scopulorum were abundant among seedling populations. Of the dominant species in DM and MM, all were either absent from seedling populations or were significantly more abundant as adults than as seedlings. These results suggest a major difference between the life history strategies of alpine forbs and alpine graminoids. Therefore, this study has shown that seedling establishment is not uncommon among alpine plants and has supported the idea that clonal reproduction is extremely important to the maintenance of alpine plant communities because of the nearly total reliance of many of the dominant graminoids on clonal reproduction.
Of the forbs present, a disproportionate number are species that are known to colonize disturbed sites in DM and MM. Three of the six species that were significantly more abundant as seedlings than as adults (Arenaria fendleri, Sedum lanceolatum, Thlaspi montanum) often occur in gopher-disturbed areas on Niwot Ridge (Forbis, 2002
). This pattern supports previous observations that reliance on seedling establishment rather than clonal reproduction and adaptation to disturbance often occur in the same species (Fenner, 1978
). Additionally, it suggests that species diversity in DM and MM may depend on disturbance, with dominant monocots reproducing clonally and less abundant forbs reproducing by seed when gaps are created.
Conclusion
This study and others (e.g., Söyrinki, 1938
; Freedman, 1982
; Welling and Laine, 2000
) have established that seedlings are present and often abundant in a wide variety of habitats in arctic and alpine tundras. Seedling survival probabilities over 4 yr can be as much as 0.75 for some alpine species, seedlings in the community with the lowest soil moisture and the lowest winter temperatures have the highest survival rates, and overall establishment probabilities fall within or even above the range of those found in other perennial species (this study).
However, high establishment rates for alpine species probably do not reflect higher overall recruitment of adults from seed in these species than in tropical palms or old field herbs. The life history of many alpine plants is fundamentally different from that of shorter-lived perennials. Many alpine species must survive for a decade or more before becoming reproductive adults (Morris and Doak, 1998
; Forbis, 2002
), so that cumulative mortality over the many years required for establishment will be lower than the 3-yr mortality data presented here. Therefore, while the recruitment and early survival stages do not seem to be more limiting to alpine plants, it is likely that mortality during an extended juvenile stage does result in lower net recruitment for alpine plants than for plants in other environments. To determine the ultimate contribution of individuals established from seed to the next generation, a good approach would be a demographic model based on size distributions and/or vital rates.
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FOOTNOTES |
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2 Current address: The Nature Conservancy, P.O. Box 150266, Ely, Nevada 89315 USA (e-mail: tforbis{at}tnc.org)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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