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3 Department of Biology, McGill University, 1205 avenue Dr. Penfield, Montréal, Québec, Canada H3A 1B1; and 4 Department of Plant Science, Macdonald Campus, McGill University, 21,111 Lakeshore Road, Ste. Anne de Bellevue, Québec, Canada H9X 3V9
Received for publication August 24, 1999. Accepted for publication January 6, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: ant dispersal • Carex • Cyperaceae • forest herbs • leaf litter • sedges • seed germination • species diversity
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), diversity may be maintained if regeneration requirements vary among species such that each species is either a better competitor in, or more tolerant of, a distinct set of micro-environmental conditions within a heterogeneous habitat (Grubb, 1977
). Alternatively, similar requirements for regeneration may be shared by many species within a given habitat, with diversity maintained by the stochasticity of dispersal and colonization events, and with local community composition in a continuous state of flux, or non-equilibrium (Chesson and Case, 1986
; van der Maarel and Sykes, 1993
; Hubbell et al., 1999
). Given these competing views, it is important to determine the degree to which the local distributions of co-occurring species are limited by their requirements for germination and successful seedling establishment. Comparative studies of closely related species may be particularly informative because potentially confounding evolutionary differences in traits affecting regeneration are minimized (Harvey and Pagel, 1991
; Sultan et al., 1998
).
Patterns of germination and establishment can also be interpreted in relation to the overall characteristics of different habitats. For seeds of herbaceous plants in the understory of temperate deciduous forests, the physical barrier imposed by tree leaf litter often inhibits germination and establishment (Sydes and Grime, 1981a, b
; Beatty and Sholes, 1988
; Facelli and Pickett, 1991
; Eriksson, 1995
). Although leaf litter covers almost all of the forest floor after canopy trees shed their leaves in autumn, redistribution and decomposition create spatial variation in the degree of litter cover in spring and early summer. Seedling establishment is also known to be dependent on seasonally changing light and temperature conditions (Baskin and Baskin, 1998
), on the availability of mineral nutrients (Vasseur and Gagnon, 1994
), and on disturbance events, such as the creation of canopy gaps (Collins, Dunne, and Pickett, 1985
; Whigham, O'Neill, and Cipollini, 1993
; Valerde and Silvertown, 1997
). Seeds of many forest herbs are dormant during summer and require a period of cold temperatures to break dormancy, thus maximizing the likelihood of germinating in spring when conditions for seedling establishment are generally favorable (Baskin and Baskin, 1998
). In addition, a requirement for light and fluctuating temperatures in some species is thought to function in the detection of gaps or areas free of leaf litter (Baskin and Baskin, 1998
; Schütz, 1998
).
In a companion study (Vellend, Lechowicz, and Waterway, 2000
), we demonstrated distinct differences among the environmental distributions of four Carex species in a 1200-ha old-growth northern deciduous forest at Mont St. Hilaire, Québec, Canada (45°32' N, 73°8' W), where >50 Carex species have been recorded (Bell, Lechowicz, and Waterway, 2000
). Three of the species, Carex backii F. Boott., C. communis L.H. Bailey, and C. platyphylla Carey were chosen for study from a group of upland species that frequently co-occur at the study site (Bell, Lechowicz, and Waterway, 2000
). The fourth species, C. plantaginea Lam., was investigated to determine the correlates and causes of its unique distribution among the Carex species at Mont St. Hilaire. It is associated neither with the upland species, nor with a group of lowland species. Rather, we found C. plantaginea most often along the edges of small streams and seepage areas within the upland habitat. Carex plantaginea and C. platyphylla are in the same section of Carex (Careyanae), while C. communis and C. backii are in different sections (Acrocystis and Phyllostachys, respectively) (Starr, Bayer, and Ford, 1999
; Waterway, 1999
). Distinct environmental distributions, particularly among the three upland species, may be interpreted as an indication that local variation in the composition and diversity of Carex species is at equilibrium with environmental gradients. However, C. communis, the only ant-dispersed species, was found in the broadest range of environmental conditions, overlapping considerably with C. backii and C. platyphylla, and we suggested that the distribution of C. communis may be determined more by its ability to disperse to unoccupied microsites than by specific environmental requirements.
In this paper, our primary objective was to experimentally test whether microsites (1 m2) where established individuals of a given species were found ("home" sites) were more favorable to successful seedling establishment than sites where established individuals of one of the other species were found. Greater seedling establishment at home sites would support the hypothesis that species' distributions are in equilibrium with environmental gradients and that the diversity of Carex species at Mont St. Hilaire is maintained, at least in part, by interspecific variation in regeneration requirements combined with environmental heterogeneity. Absence of home-site advantage would indicate that species' distributions are not entirely in equilibrium with environmental gradients and that dispersal and colonization events exert an important control on local diversity. To investigate the possible causes of any advantage at home sites, experiments were conducted to isolate above- and belowground differences among the four plot types. In addition, we tested whether germination and establishment are negatively affected by leaf litter. Due to poor germination of C. backii and C. plantaginea in all experiments, and C. platyphylla in all but one experiment, our discussion is limited largely to C. communis. For C. communis, we consider Handel's (1978a)
hypothesis that ant dispersal in forest-understory Carex species is associated with a suite of characteristics related to regeneration.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), or in a uniform lath-house environment using soil collected from these plots. Focal plants were chosen randomly within 15 sectors (1–30 ha/sector) of Mont St. Hilaire, a 1200-ha UNESCO biosphere reserve in southern Québec, Canada. Sectors in which each species occurred were identified during an intensive survey of the entire reserve in 1996. Field experiments were conducted at 32 C. backii plots (including six that also contained C. communis or C. platyphylla), 28 C. communis plots, 28 C. plantaginea plots (one of which contained C. communis), and 29 C. platyphylla plots (six of which also contained C. backii or C. communis).
Seeds from each species were collected as they matured: C. plantaginea during the first 2 wk of June; C. communis and C. platyphylla during the last 2 wk of June; and C. backii over 3 wk in late June and early July. We combined all seeds of a given species into a single composite batch. Seeds were allowed to air-dry from the time of collection until late October, at which time seeds were either planted in the field (see experiment 1), or stored over the winter in damp sterilized sand, in a sealed plastic container, buried just underneath the soil surface, in a lath house at Mont St. Hilaire (see experiments 2, 3, and 4). Prior to planting, 
10 moist seeds of each species were cut open; all seeds were found to be filled and apparently healthy. When we opened the container in spring 1997, the sand was saturated with water. Therefore, seeds used in experiments 2, 3, and 4 were submerged for an unknown period of time during the winter prior to planting; seed submersion was not deliberate.
Experiments 1 and 2: tests for home-site advantage with respect to aboveground environmental factors and effects of leaf litter on germination and establishment
Figure 1 illustrates the set-up of all field experiments. To ensure accurate relocation of seeds and identification of seedlings, all seeds were planted in bottomless plastic film canisters (3 cm diameter, 5 cm long), which had been pushed into the soil, beneath the leaf litter, leaving 1 cm of each canister above the soil surface. Any leaf litter originally there was carefully replaced after seeds were planted. Though the canisters may have slightly modified the micro-environment where seeds were planted, differences in germination or seedling establishment among plot types are not likely to have been affected.
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Experiment 3: test for home-site advantage with respect to the total environment
In this experiment, seeds were planted in native soil at one of four randomly chosen positions on the perimeter of each focal plot (Fig. 1). The time of planting and the number of seeds per canister were as in experiment 2. Four canisters were planted with seeds of each species at each plot, for a total of 12 seeds of C. backii, 40 seeds of C. communis, 20 seeds of C. plantaginea, and 12 seeds of C. platyphylla.
Censuses of experiments 1, 2, and 3
In all three field experiments, the number of seedlings present in each canister was counted at 2-wk intervals in 1997, starting on 15 June and ending on 7 October. Snow fell for the first time on 22 October and remained on the ground for the winter starting in early November. The final census was on 1 June 1998.
To assess the effect of naturally varying coverage of leaf litter on germination and establishment of seeds planted in the three field experiments, we scored the cover of leaf litter over each group of either four or eight canisters (see Fig. 1) not in a litter-exclusion treatment on 17 July 1997 in two categories: (1) visibility of the four or eight canisters was either completely or partially blocked by litter, or (2) all of the canisters could be seen clearly.
Experiment 4: test for home-site advantage with respect to edaphic conditions
During summer 1996, we collected a total of 2 L of soil at each plot from the same positions used in experiments 1 and 2. Soil samples were frozen at -4°C until 19 May 1997, at which time each 2-L sample was thawed, thoroughly mixed, and divided into two groups of four 250-mL subsamples each. We assigned each group of subsamples to one of two blocks set up in a lath house located at Mont St. Hilaire. In each block, the four subsamples were put into separate square pots (10 cm on a side) and planted with either three seeds of C. backii, ten seeds of C. communis, five seeds of C. plantaginea, or three seeds of C. platyphylla. The four pots of soil from a given plot were kept adjacent to one another, with the position of each group of four pots randomized within its block. We watered the whole experiment on all days without rain. Seedlings in each pot where germination occurred were thinned weekly to one seedling per pot. By early September, some of the plants became pot-bound, and the number of shoots and leaves on each plant were counted, and each plant was harvested between 12 September and 15 September. All samples were dried for 3 d at 70°C, and below- and aboveground plant parts were weighed separately to the nearest milligram.
Statistical analysis of experiments 1, 2, and 3: the field experiments
Of 5616 canisters at the 117 plots used in the field experiments, 18% were lost over the winter of 1996–1997 to either frost heaving or disturbance by animals. Throughout the remainder of the experiments a further 7% were lost, mostly to disturbance by animals. Any canister lost at any time was removed from the data set prior to analysis. For a given canister, we assumed that if no change in seedling number had occurred between censuses, then no germination or seedling mortality had occurred. An increase in seedling number was interpreted as germination; a decrease in seedling number was interpreted as mortality. All statistical analyses were conducted using SAS version 6.12 (SAS Institute, Cary, North Carolina, USA).
Germination responses were sufficient to allow statistical comparisons among treatments for C. communis and C. platyphylla in experiment 1, but only for C. communis in experiments 2 and 3. For experiment 1, we conducted separate analyses for each species. In each analysis we tested for differences among treatments for the following response variables: (1) total germination as of 7 October 1997, (2) total germination as of 1 June 1998, (3) number of seedlings remaining alive on 7 October 1997, and (4) number of seedlings remaining alive on 1 June 1998. Differences in seedling mortality among treatments may be inferred by comparing the results for germination and final seedling number for a given experiment.
For experiments 1 and 2 we tested for effects of plot type and litter exclusion on each response variable using analyses of variance as follows. If either canister in a given pair (litter exclusion canister plus control canister) was not present at the end of the experiments, both were removed from the data set prior to analysis. Data were averaged across replicate canisters. Total germination and number of seedlings were both expressed as proportions of the original number of seeds planted and subsequently arcsine-square-root transformed. For a given variable, we first tested for an effect of plot type on the difference between litter exclusion and control treatments with an intercept included in the model. Thus, the intercept term tests for the main effect of litter exclusion, and the plot-type term tests for an interaction between litter exclusion and plot type. We then tested for an effect of plot type on the sum across litter exclusion and control treatments, thus testing for the main effect of plot type. For experiment 3, we averaged data across replicate canisters at each plot and conducted a one-way ANOVA testing for an effect of plot type on each variable.
To test for effects of naturally varying amounts of leaf litter in mid-summer (17 July) on germination and seedling number in the four data sets described above, we considered each canister not covered by a basket as an independent observation. Though each canister was not strictly independent (there was more than one at each plot), these tests were conducted only to augment the interpretation of the other experiments. Total germination and seedling number in each canister were tabulated as either zero, one, two, or three. For C. communis seeds in experiments 2 and 3, values >3 were assigned a value of 3. We then used categorical models (SAS, PROC CATMOD) to test for an effect of leaf litter cover in mid-summer on each variable in each data set described above.
Statistical analysis of experiment 4: the lath-house experiment
Germination occurred in <5% of the pots for all species except C. communis in this experiment, so statistical analyses were performed only for C. communis. We tested for differences among the four plot types for the following response variables: germination date, total dry mass, number of leaves, number of shoots, and the ratio of root-to-shoot mass. Germination date was the day of the year on which a seedling was first observed in a given pot; all other variables were determined on harvested plants. To improve normality, a square-root transformation was applied to all harvest variables prior to analysis. As there was no effect of block on any variable (paired-sample t test, P > 0.1), we analyzed the mean across blocks for soil from each plot. We conducted a one-way ANOVA to test for an effect of plot type on germination date, with multiple comparisons of plot type means using Ryan's Q procedure (Day and Quinn, 1989
). Since it was apparent that germination date differed among plot types and was negatively correlated with all other variables, we conducted analyses of covariance (ANCOVA) with germination date as a covariate to test for differences among plot types for the remaining variables. Homogeneity of slopes was confirmed before conducting each ANCOVA. Following each ANCOVA, multiple comparisons of least-square means between each pair of plot types were made using Hochberg's (1974)
GT2 test.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Experiments 1 and 2: tests for home-site advantage with respect to aboveground environmental factors, and effects of leaf litter on germination and establishment
For C. communis seeds in experiment 1, there was no significant effect of plot type on either total germination or final seedling number (P > 0.25), but a significant effect of leaf litter exclusion on seedling number both at the end of 1997, and in 1998 (P < 0.05; Table 1, Fig. 2). The number of seedlings in 1998 was greater in the leaf litter exclusion treatment than the control treatment at C. backii, C. communis and C. plantaginea plots, but opposite at C. platyphylla plots (plot type x litter exclusion, P < 0.05; Table 1, Fig. 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Vellend, Lechowicz, and Waterway, 2000
; this study) was to test whether the maintenance of diversity in forest-understory Carex species could be explained by the adaptation of each species to a different set of micro-environmental conditions within a spatially heterogeneous habitat, as proposed by equilibrium community theories (e.g., Pacala and Roughgarden, 1982
; Tilman, 1982
). Relatively distinct environmental distributions of the four species (Vellend, Lechowicz, and Waterway, 2000
) suggested that this could be the case. Adaptations to micro-environmental conditions may be manifested either by competitive superiority or greater tolerance of specific biotic or abiotic stresses. We therefore predicted that, at some stage of the life cycle, each species would have an advantage at its "home sites" compared to sites where its congeners occurred. However, we found no evidence of home-site advantage for seed germination or seedling establishment in this study.
We have shown clearly that the environmental distribution of C. communis is not distinguished from that of its congeners by specific requirements for regeneration from seed or edaphic conditions for establishment. Because the response of C. platyphylla seeds was analyzed for only one experiment planted in potting soil, we cannot rule out the possibility that C. platyphylla seeds may respond to variation among plot types in edaphic conditions not addressed by that one experiment. However, C. platyphylla plots were distinguished from the other plot types mostly by occurring on the steepest slopes, rather than by soil conditions, which varied only minimally among C. backii, C. communis, and C. platyphylla plots (Vellend, Lechowicz, and Waterway, 2000
). Furthermore, soil nutrients are generally of only minor importance for the germination of temperate herbs compared to temperature, light, and soil moisture (Baskin and Baskin, 1988
), which should have been modified little, or not at all in the case of light, by the potting soil. It is likely that specific environmental conditions required for successful seed germination and establishment do not distinguish the distribution of C. platyphylla from that of its congeners. In a separate experiment, we observed the survival and growth of established ramet implants of the four species at all plots, and again found no evidence of home-site advantage, though survival of all species was lowest at C. plantaginea plots (Vellend, 1999
). However, the two species with poor germination in all experiments, C. backii and C. plantaginea, may be more likely to exhibit home-site advantage than C. communis or C. platyphylla, given the evidence that C. plantaginea has a unique distribution among the Carex species at Mont St. Hilaire and that C. backii may depend on fire for successful regeneration.
Our results suggest that the number of Carex species in a small patch of forest floor is limited by the number of species whose seeds disperse there, thus indicating that stochastic dispersal and colonization events play an important role in maintaining the local diversity of Carex species in the forest understory. However, differences in environmental distribution among the four co-occurring congeners (Vellend, Lechowicz, and Waterway, 2000
), as well as among other co-occurring forest herbs (e.g., Bratton, 1976
; Beatty, 1985
; Lechowicz, Schoen, and Bell, 1988
), indicate that adaptations to different micro-environments also contribute to the maintenance of diversity in the forest understory. These results may seem, at first, to be in conflict. However, while the demonstration of different microhabitat preferences among the four species justifies our previous conclusion (Vellend, Lechowicz, and Waterway, 2000
), such data cannot be used to argue for or against the importance of other factors, such as stochastic dispersal and colonization events. In fact, several factors are quite likely to be important in maintaining species diversity within a given system (Huston, 1994
; Palmer, 1994
). Our studies on four Carex species in the forest understory indicate that abiotic or biotic constraints limit the range of micro-environments in which each species occurs, but for at least two species, there are likely to be many microsites within these limits that are unoccupied simply because seeds have not dispersed there.
Our results indicate that the carousel model of van der Maarel and Sykes (1993)
may apply to forest herb communities, as suggested by Fröborg and Eriksson (1997)
. Van der Maarel and Sykes (1993)
observed continual turnover of species through time on small-scale permanent plots in a limestone grassland and suggested that species move about freely within the community, with diversity maintained by the stochasticity of these processes. At Mont St. Hilaire, Holland (1978, 1981)
observed continual immigration and emigration of forest herbs, including C. communis, C. plantaginea, C. platyphylla, and C. pedunculata, among 1-m2 plots over 11 yr. At the end of the study, >20% of the species found in a given plot in the first year had been replaced by a different species, and there was no indication that the rate of replacement was slowing down (Holland, 1981
). In Swedish deciduous forests, Fröborg and Eriksson (1997)
observed a similar situation, and Eriksson and Ehrlén (1992)
found that recruitment in most understory herbs was limited at least as much by dispersal to unoccupied microsites as the availability of suitable microsites.
As predicted, all of our field experiments showed a negative effect of leaf litter on seed germination and/or establishment. Leaf litter may have many interacting effects on the microenvironment at the soil surface (Facelli and Pickett, 1991
). However, the two factors most likely to affect germination in understory Carex species are light (Bond, 1999
; Schütz and Rave, 1999
), which is blocked by litter, and temperature fluctuations (Schütz, 1999
), which are dampened by litter (Facelli and Pickett, 1991
). Both C. communis and C. platyphylla show greater germination in light than in darkness (Bond, 1999
), and most Carex species from deciduous forests in central Europe respond positively to diurnally fluctuating temperatures (Schütz, 1999
). Germination of C. communis and C. platyphylla was likely lower beneath leaf litter due to some combination of reduced light levels, and dampened temperature fluctuations at the soil surface. Reduced light levels may also partly explain the negative effects of litter on seedling establishment. However, it is the physical barrier to emergence that is thought to be the main inhibiting effect of litter on seedling establishment in forest herbs (Facelli and Pickett, 1991
; Eriksson, 1995
), particularly for grasses and sedges because of their growth from basal meristems (Sydes and Grime, 1981b
). The negative effect of leaf litter on seedling establishment in this study was most likely due to the physical barrier to emergence.
The patterns of regeneration for C. communis may be part of an overall strategy (sensu Grime, 1979
) related to its dispersal ecology. Among the four species that we investigated, C. communis is the only one that is dispersed by ants (Handel, 1978b
), the only one that does not require overwinter stratification to germinate (Handel, 1978a
; Bond, 1999
), and the only one with a relative growth rate over two years that was significantly greater than zero (Vellend, Lechowicz, and Waterway, 2000
). Among the studied species, C. communis was found in the widest range of environmental conditions (Vellend, Lechowicz, and Waterway, 2000
), and showed the highest percentage germination both in the laboratory (Bond, 1999
) and in the field (this study). Handel (1978a)
observed a remarkably similar situation in his study of Carex pedunculata, also an ant-dispersed species (Handel, 1976
). Compared to C. plantaginea and C. platyphylla, C. pedunculata is a poorer competitor, occurs in a wider range of local micro-environments, has a greater germination rate, and also does not require overwinter stratification to germinate (Handel, 1978a
). Handel (1978a)
hypothesized that the evolution of ant dispersal in C. pedunculata was a product of the adaptive advantage of dispersal into unoccupied microsites where establishment could occur in the relative absence of competition. Our data do not address the evolution of ant dispersal, but they do suggest an association between ant dispersal and plant strategies in forest understory Carex species. Along an axis of stress tolerance (Grime, 1979
), ant-dispersed carices are less tolerant, relying on greater dispersal, higher germination rates, and faster growth to occupy, perhaps transiently, microsites in a wide range of environmental conditions. Carex communis and C. pedunculata are the two most frequently encountered sedges on Mont St. Hilaire (Bell, Lechowicz, and Waterway, 2000
), and the only two we know to be dispersed by ants, although there may be others. These data are consistent with previous studies indicating that ant dispersal is a particularly successful strategy among herbaceous species in the forest understory (Handel, Fisch, and Schatz, 1981
; Beattie, 1985
).
In summary, we found no evidence for home-site advantage in either C. communis or C. platyphylla, suggesting that stochastic dispersal and colonization events are important in maintaining the diversity of Carex species in upland forests. As expected from previous work on forest herbs, we demonstrated negative effects of leaf litter on germination and establishment. Ant dispersal in C. communis appears to be part of an overall strategy associated with high germination rates and a broad tolerance of environmental conditions.
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FOOTNOTES |
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2 Author for correspondence, current address: Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York 14853-2701 USA (e-mail: mv59{at}cornell.edu
).
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