The effect of a plant growth promoting strain ofBacillus polymyxa was investigated using genotypi... more The effect of a plant growth promoting strain ofBacillus polymyxa was investigated using genotypically-defined mixtures of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.). Addition ofB. polymyxa to a mixture of the species did not induce significant yield effects in perennial ryegrass, but resulted in a 23% (P<0.05) yield increase in the clover component. The clover yield advantage increased further when clones of the legume were inoculated with theB. polymyxa genotypes with which they had previously coexisted in the field from which collections were made. The highest white clover yield was attained when clones of all three organisms (T. repens, L. perenne, andB. polymyxa) that had previously coexisted in the field were grown together in the experimental system.
Genets of Trifolium repens (white clover) were collected from three patches of old permanent past... more Genets of Trifolium repens (white clover) were collected from three patches of old permanent pasture dominated by Agrostis capillaris, Holcus lanatus or Lolium perenne. Plants derived from the genets were grown with plants of one grass species present on one side of each T. repens, and a different grass species on the other side, in all combinations of two of the three grasses. Different modules (a node with its associated internode, leaf, and axillary bud) on the same clover plant responded independently to the microenvironment provided by their own neighbouring grasses. In contrast, all apical meristems on the plant reacted similarly, showing a unified response and integrating the effects of the different microenvironments experienced by the whole clover plant. This is consistent with what is known both physiologically about the nutrition of meristems and modules, and ecologically about the exploratory growth habit of the species. Averaged over all associated grasses, there was no significant variation in the final dry weight of the different clover genets but these differed in their growth habit response to different grasses. In response to Agrostis as a neighbour, each meristem of T. repens rapidly produced many small modules. New modules were produced more slowly and were larger when Holcus or Lolium was the neighbour. The same pattern of differences occurred among clovers sampled from different backgrounds. Either genetic differences paralleled plastic responses, or plastic changes in phenotype that developed in response to different neighbours in the field persisted in the greenhouse. Plants taken from backgrounds of different grass species showed different responses to growing with those grass species. The differences were manifest primarily in a “positive leading diagonal” effect of Holcus or not-Holcus. They were the result primarily of differences in the dry weight per module and the probability of development of the axillary bud into a branch. This confirms earlier results, and implicates the central importance of branching as a means of local response to the microenvironment.
The relationship between morphological variability and biotic environmental heterogeneity was stu... more The relationship between morphological variability and biotic environmental heterogeneity was studied in a pasture population of Trifolium repens L. It had been argued that the unexpectedly high levels of variation in T. repens could be maintained by diversifying selection. The mosaic of neighbours (perennial grasses) with which T. repens co-exists constitutes a prominent element of biotic patchiness that may lead to sorting among T. repens genotypes on the basis of neighbour-specific compatibilities.A variation study was conducted on a set of 400 individuals of T. repens collected on a neighbour-specific basis from a 43–year-old pasture and grown for over 2 years under common garden conditions. Variation in a set of 12 morphological characters was assessed after 4 months and again after 27 months. After 4 months′ growth, a significant proportion of this variation was accounted for by the neighbour with which the individuals of T. repens had been growing in the pasture. The actual amount of variation accounted for, however, was low (6-19%).When the same characters were assessed after 27 months, none of the neighbour-specific differences in morphology were retained. It is concluded that the original results reflected developmental differences carried over from the pasture, and that diversifying selection is not of importance in the maintenance of morphological variation in this population.
Plant functional groups are used to describe patterns of community organization. However, they ar... more Plant functional groups are used to describe patterns of community organization. However, they are defined either by suites of correlated traits or by species groupings, and the responses of these two definitions to changing environmental conditions are unknown. 2. We assessed 14 growth and morphological traits under low-and high-resource conditions of 42 annual plant species from two source communities in Israel that differed in resource availability. As current theory predicts, plants growing in the high-resource treatment were larger, had twofold greater relative growth rate (RGR) and thinner leaves, and allocated less biomass to roots than plants grown in the low-resource treatment. Differences in these traits were less consistent between the two source communities. Instead, taxonomic groups ( grasses, legumes and a group of other forbs), regardless of source, differed in most characteristics. 3. Three general groups of species (functional groups) were identified in both resource treatments using cluster analysis on all 14 traits. In both resource treatments monocots were almost completely separated into one distinct cluster, regardless of source habitat, while the two other, mainly dicot, clusters were partially separated by habitat. However, the species composition and trait characterization of the dicot clusters differed strongly between treatments. Under low-resource conditions the two dicot clusters were separated by size traits and seed mass, but under high-resource conditions, they were separated by above-ground size, morphology and RGR. 4. Principal components analysis demonstrated inconsistency in relationships among traits and species groupings between treatments. The first two principal components emphasized different aspects of growth depending on the treatment; the third axis was defined by growth rates. As with the cluster analysis, plots of species scores revealed relatively little separation of species by habitat. 5. The response of each species varies for different traits and with growing conditions. Variation may differ among species within a functional group, producing different definitions of functional groups under different experimental conditions. Because most functional group analyses are performed on data collected without manipulation of growing conditions, conclusions concerning the response of species or communities to changes in environmental conditions may be problematic. Functional Ecology (2001) 15 , 85-95 1977); 'stress response syndrome' (Chapin 1991); and 'plant ecology strategy scheme' (Westoby 1998) all suggest that suites of morphological and physiological characteristics in plants are correlated with adaptive †To whom correspondence should be addressed. Present address:
Changes in plant community structure after changes in some aspect of the environment such as nutr... more Changes in plant community structure after changes in some aspect of the environment such as nutrients or grazing is often ascribed to changes in competitive relationships among the plants. However, very rarely is competition measured directly in such experiments. To distinguish between the direct effects of environmental treatments and changes in competitive relationships, it is necessary to quantify the influence of competition on community structure and compare the magnitude and direction of this influence between environments. We describe an experimental approach to accomplish this that is based on the classic yield-density experiment of agronomy. The approach is called the community-density experiment and requires experimental establishment of a gradient in total initial community density such that absolute densities of each species increase but initial relative abundances of each species stay constant along the gradient. We define various indices of the magnitude of community-level consequences of increasing density that can be compared among environments such as different fertilizer or grazing treatments. We also discuss various practical ways of achieving the experimental density gradient that are suitable for different kinds of communities. *** DIRECT SUPPORT *** A02DO006 00011
Most studies of density-dependent demography in plants consider the density only of the single fo... more Most studies of density-dependent demography in plants consider the density only of the single focal species being studied. However, density-dependent regulation in plants may frequently occur at the level of the entire community, rather than only within particular species. In addition, because density dependence may differ considerably (even in direction) among demographic parameters, generalizing about patterns of density dependence and extrapolating to lifetime fitness and to population dynamics require comparisons among life history stages, as well as among types of species and physical environments.
1 Invasive species dominate many ecosystems but the competitive strategies underlying this domina... more 1 Invasive species dominate many ecosystems but the competitive strategies underlying this dominance are unclear. Are invasive species generalist competitors, or do they only thrive in certain environments? Do they occur mainly post-disturbance or can they persist throughout succession? 2 We tested the relative importance of resource acquisition (competitive suppression ability) and the ability to tolerate reduced resource levels (competitive tolerance ability) among four C3 perennial grass species in an invaded oak savanna. Two species ( Poa pratensis and Dactylis glomerata ) are exotic invaders and are thought to have replaced the two native species ( Bromus carinatus and Elymus glaucus ) as dominants. 3 Using glasshouse and field experiments we tested whether the two strategies were maintained with changing resource levels and successional conditions, and their relative roles in explaining exotic dominance. 4 The relative importance of suppression-and tolerance-based competition shifted with neighbour density, burning and planting order. Further, the relative importance of particular plant traits changed depending on the imposed conditions, and the exotic dominants were only competitively superior under certain circumstances. 5 Competitive suppression ability was maintained with changing resource levels but was confined to post-disturbance conditions. When planting of neighbours was delayed, the early establishing targets were dominant regardless of species, fertility and neighbour density. 6 Competitive tolerance ability determined long-term patterns of relative abundance and coexistence, but only under the current field conditions of low fertility and limited disturbance. Alteration of these conditions changed the relative abundance of the four grasses, and would probably reconfigure species patterns in the oak savanna community generally. 7 Exotic dominance is presently determined by tolerance-based competitive traits interacting with the long-term absence of disturbance. Dominance is therefore contingent on the interaction of competitive strategies, resource availability and disturbance history rather than any one factor alone. 8 Exotic flora dominate all stages of succession in this savanna because there exists both early (suppression) and late (tolerance) successional specialists. The identity of the dominant changes with succession based on the competitive strategy it employs. This result highlights the importance of examining the historical context of invaded communities and tracking their successional status over time.
Question: Do stressful environments facilitate plant invasion by providing refuges from intense a... more Question: Do stressful environments facilitate plant invasion by providing refuges from intense above-ground competition associated with productive areas, or prevent it by favouring locally adapted native species? Location: An invaded and fragmented oak savanna ecosystem structured along a landscape-level stress gradient associated with soil depth, elevation, and canopy openness. Methods: Vegetation and environmental data were collected from 184 plots in seven savanna remnants along the gradient. Using multivariate (CCA) and post-hoc regression analyses, we determined the relationship between environment and the richness and abundance of invasives. Results: 46 of 119 species were naturalized exotics. CCA indicated the importance of environmental variation (mostly soil depth) for community structure but not for invasion; invasive species richness was similar in all areas. However, the abundance of invasives and their impacts on native diversity appear to increase significantly in less stressful habitats. Deeper soils had lower evenness and significantly fewer native species. This result was associated with dominance by exotic perennial grasses and large increases in vegetation height, suggesting strong above-ground competition. Conclusions: Low-stress environments were not more invasible per se but appear to be more susceptible to invasion by species with strong competitive impacts. The causes of decreasing exotic impact with decreasing soil depth may reflect shifts in competitive intensity or an increased importance of stress tolerance, both of which may favour natives. Alternatively, this ecosystem may simply lack high-impact invaders capable of dominating shallow soils. Conservation challenges are twofold for this endangered plant community: controlling invasives that currently dominate deeper-soils and accounting for a diverse pool of invaders that proliferate when the current dominants are removed.
Few invaded ecosystems are free from habitat loss and disturbance, leading to uncertainty whether... more Few invaded ecosystems are free from habitat loss and disturbance, leading to uncertainty whether dominant invasive species are driving community change or are passengers along for the environmental ride. The ''driver'' model predicts that invaded communities are highly interactive, with subordinate native species being limited or excluded by competition from the exotic dominants. The ''passenger'' model predicts that invaded communities are primarily structured by noninteractive factors (environmental change, dispersal limitation) that are less constraining on the exotics, which thus dominate. We tested these alternative hypotheses in an invaded, fragmented, and fire-suppressed oak savanna. We examined the impact of two invasive dominant perennial grasses on community structure using a reduction (mowing of aboveground biomass) and removal (weeding of above-and belowground biomass) experiment conducted at different seasons and soil depths. We examined the relative importance of competition vs. dispersal limitation with experimental seed additions. Competition by the dominants limits the abundance and reproduction of many native and exotic species based on their increased performance with removals and mowing. The treatments resulted in increased light availability and bare soil; soil moisture and N were unaffected. Although competition was limiting for some, 36 of 79 species did not respond to the treatments or declined in the absence of grass cover. Seed additions revealed that some subordinates are dispersal limited; competition alone was insufficient to explain their rarity even though it does exacerbate dispersal inefficiencies by lowering reproduction. While the net effects of the dominants were negative, their presence restricted woody plants, facilitated seedling survival with moderate disturbance (i.e., treatments applied in the fall), or was not the primary limiting factor for the occurrence of some species. Finally, the species most functionally distinct from the dominants (forbs, woody plants) responded most significantly to the treatments. This suggests that relative abundance is determined more by trade-offs relating to environmental conditions (longterm fire suppression) than to traits relating to resource capture (which should most impact functionally similar species). This points toward the passenger model as the underlying cause of exotic dominance, although their combined effects (suppressive and facilitative) on community structure are substantial.
† Background and Aims Changes in rainfall and temperature brought about through climate change ma... more † Background and Aims Changes in rainfall and temperature brought about through climate change may affect plant species distribution and community composition of grasslands. The primary objective of this study was to test how manipulation of water and temperature would influence the plasticity of stomatal density and leaf area of bluebunch wheatgrass, Pseudoroegneria spicata. It was hypothesized that: (1) an increased water supply will increase biomass and leaf area and decrease stomatal density, while a reduced water supply will cause the opposite effect; (2) an increase in temperature will reduce biomass and leaf area and increase stomatal density; and (3) the combinations of water and temperature treatments can be aligned along a stress gradient and that stomatal density will be highest at high stress. † Methods The three water supply treatments were (1) ambient, (2) increased approx. 30 % more than ambient through weekly watering and (3) decreased approx. 30 % less than ambient by rain shades. The two temperature treatments were (1) ambient and (2) increased approx. 1-3 8C by using open-top chambers. At the end of the second experimental growing season, above-ground biomass was harvested, oven-dried and weighed, tillers from bluebunch wheatgrass plants sampled, and the abaxial stomatal density and leaf area of tillers were measured. † Key Results The first hypothesis was partially supported -reducing water supply increased stomatal density, but increasing water supply reduced leaf area. The second hypothesis was rejected. Finally, the third hypothesis could not be fully supported -rather than a linear response there appears to be a parabolic stomatal density response to stress. † Conclusions Overall, the abaxial stomatal density and leaf area of bluebunch wheatgrass were plastic in their response to water and temperature manipulations. Although bluebunch wheatgrass has the potential to adapt to changing climate, the grass is limited in its ability to respond to a combination of reduced water and increased temperature.
Environmental conditions associated with climate change, such as earlier snowmelt, warmer spring ... more Environmental conditions associated with climate change, such as earlier snowmelt, warmer spring temperatures, and increased soil mineralization, have resulted in shifts in the timing of plant phenological events. We assessed the effects of fertilizer and herbivory on the phenology of 7 plant species from a boreal forest understory using long-term experimental plots in the southwest Yukon. Fertilizer and fencing treatments were initiated in 1990, and discontinued in half of each plot in 2000. There were few effects on phenology of either fertilizer or fencing. In some species, fertilizer affected the final phenological stage reached, but the presence and direction of the effect was species-dependent. Epilobium angustifolium was the only species where the timing of phenological stages responded to fertilization; early phenological stages were advanced with fertilizer. First leaf expansion for Arctostaphylos uva-ursi occurred earlier in fenced plots, although this effect disappeared in plots where the fencing treatment had been discontinued. We conclude that previously observed changes in species abundance with fertilizer treatments are likely not caused by changes in the phenology of these species, and are more likely due to fertilization imposed changes in vegetative growth.
The combustion of fossil fuels and associated warmer temperatures are causing a global increase i... more The combustion of fossil fuels and associated warmer temperatures are causing a global increase in the availability of soil nutrients such as nitrogen. This will have pronounced effects on plants at northern latitudes that are adapted to low nutrient conditions. An experiment in northern Canada set up in 1990 has investigated the effects of long-term nutrient enrichment (fertilizer addition) and mammalian herbivore exclusion (fencing) on an understory plant community. We used this experiment to assess how 22 years of fertilization has affected investment in sexual reproduction in four herbaceous understory species. We measured reproductive output at the plot level (proportion of plants flowering) for four species (Mertensia paniculata, Epilobium angustifolium, Achillea millefolium and Festuca altaica) and at the individual plant level (biomass allocation to flower parts) for M. paniculata and E. angustifolium. Fertilization increased the probability of flowering for A. millefolium and resulted in a higher allocation of biomass to flower parts for E. angustifolium. Sexual reproduction in M. paniculata and F. altaica was largely unaffected by increased nutrient supply, and, as expected, herbivore exclusion had almost no effect on any species. Whereas plants in northern ecosystems currently reproduce mainly through clonal growth, rapidly changing environmental conditions and warmer temperatures will likely result in increased benefits of sexual reproduction. This could give a competitive advantage to species such as A. millefolium and E. angustifolium that increase investment in sexual reproduction when released from nutrient limitation.
Traditional haymaking has created exceptionally high levels of plant species diversity in semi-na... more Traditional haymaking has created exceptionally high levels of plant species diversity in semi-natural grasslands of the Carpathian Mountains (Romania), the maintenance of which is jeopardized by recent abandonment and subsequent vegetation succession. We tested the hypothesis that the different life history strategies of dominant grasses cause different patterns of diversity loss after abandonment of traditional haymaking in two types of meadow. Although diversity loss rate was not significantly different, the mechanism of loss depended on the life history of dominant species. In meadows co-dominated by competitive stress-tolerant ruderals, diversity loss occurred following the suppression of dominant grasses by tall forbs, whereas in meadows dominated by a stress-tolerant competitor, diversity loss resulted from increased abundance and biomass of the dominant grass. We conclude that management for species conservation in abandoned grasslands should manipulate the functional turnover in communities where the dominant species is a weaker competitor, and abundance and biomass of dominant species in communities where the dominant species is the stronger competitor.
Global levels of reactive nitrogen are predicted to rise in the coming decades as a result of inc... more Global levels of reactive nitrogen are predicted to rise in the coming decades as a result of increased deposition from the burning of fossil fuels and the large-scale conversion of nitrogen into a useable form for agriculture. Many plant communities respond strongly to increases in soil nitrogen, particularly in northern ecosystems where nitrogen levels are naturally very low. An experiment in northern Canada that was initiated in 1990 has been investigating the effects of longterm nutrient enrichment (fertilizer added annually) on a boreal forest understory community. We used this experiment to investigate why some species increase in abundance under nutrient enrichment whereas others decline. We focused on four species that differed in their responses to fertilization: Mertensia paniculata and Epilobium angustifolium increased in abundance, Achillea millefolium remained relatively constant and Festuca altaica declined. We hypothesized that the two species that were successful in the new high-nutrient, light-limited environment would be taller, have higher specific leaf area, change phenology by growing earlier in the season and be more morphologically plastic than their less successful counterparts. We compared plant height, specific leaf area, growth spurt date and allocation to leaves in plants grown in control and fertilized plots. We demonstrated that each of the two species that came to dominate fertilized plots has a different combination of traits and responses that likely gave them a competitive advantage; M. paniculata has the highest specific leaf area of the four species whereas E. angustifolium is tallest and exhibits morphological plasticity when fertilized by increasing biomass allocation to leaves. These results indicate that rather than one strategy determining success when nutrients become available, a variety of traits and responses may contribute to a species' ability to persist in a nutrientenriched boreal forest understory.
Across different host plant species, the effects of mycorrhizal colonization on host growth param... more Across different host plant species, the effects of mycorrhizal colonization on host growth parameters can vary, but intraspecific variation in this relationship has rarely been measured. We tested the direction and consistency of the relationship between ectomycorrhizal colonization level and growth responses across seed families of Pinus contorta var. latifolia. Root tips of seedlings from eight full sib seed families varied in levels of ectomycorrhizal fungal colonization from 39% to 100%. We observed positive, negative, or neutral relationships between colonization level and shoot mass, depending on plant family. For the majority of seed families no relationship was observed between colonization level and root mass; however, two seed families showed negative relationships. Shoot height differed only by seed family. Results from our study indicate that the relationship between colonization level and host growth depends on host genotype. We suggest that models of plant intraspecific interactions should consider ectomycorrhizal associations when assessing phenotypic variability.
Ecological theories make divergent predictions about whether extant species inhibit or promote th... more Ecological theories make divergent predictions about whether extant species inhibit or promote the establishment of new species and which aspects of community composition determine these interactions; diversity, individual dominant species, and neutral interactions have all been argued to be most important. We experimentally tested these predictions by removing plant biomass (0%, 7%, 100%) from boreal forest understory communities. The 7% removals were restricted to the numerically dominant species, the second most dominant species, or many low-abundance species, thereby separating the effects of species composition from those of biomass. We tested the effects of all removal treatments on seedling establishment. Competitive effects were driven by one dominant species and were inconsistent with resource complementarity, neutral, or competition-colonization models. Facilitative effects were apparent only following removal of all vegetation, of which the most dominant species comprised more than 80%. Our results indicate that numerically dominant species in a community can influence the establishment of new species more than species diversity, but the direction of interaction can shift from facilitative to competitive as community density increases.
The effect of a plant growth promoting strain ofBacillus polymyxa was investigated using genotypi... more The effect of a plant growth promoting strain ofBacillus polymyxa was investigated using genotypically-defined mixtures of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.). Addition ofB. polymyxa to a mixture of the species did not induce significant yield effects in perennial ryegrass, but resulted in a 23% (P<0.05) yield increase in the clover component. The clover yield advantage increased further when clones of the legume were inoculated with theB. polymyxa genotypes with which they had previously coexisted in the field from which collections were made. The highest white clover yield was attained when clones of all three organisms (T. repens, L. perenne, andB. polymyxa) that had previously coexisted in the field were grown together in the experimental system.
Genets of Trifolium repens (white clover) were collected from three patches of old permanent past... more Genets of Trifolium repens (white clover) were collected from three patches of old permanent pasture dominated by Agrostis capillaris, Holcus lanatus or Lolium perenne. Plants derived from the genets were grown with plants of one grass species present on one side of each T. repens, and a different grass species on the other side, in all combinations of two of the three grasses. Different modules (a node with its associated internode, leaf, and axillary bud) on the same clover plant responded independently to the microenvironment provided by their own neighbouring grasses. In contrast, all apical meristems on the plant reacted similarly, showing a unified response and integrating the effects of the different microenvironments experienced by the whole clover plant. This is consistent with what is known both physiologically about the nutrition of meristems and modules, and ecologically about the exploratory growth habit of the species. Averaged over all associated grasses, there was no significant variation in the final dry weight of the different clover genets but these differed in their growth habit response to different grasses. In response to Agrostis as a neighbour, each meristem of T. repens rapidly produced many small modules. New modules were produced more slowly and were larger when Holcus or Lolium was the neighbour. The same pattern of differences occurred among clovers sampled from different backgrounds. Either genetic differences paralleled plastic responses, or plastic changes in phenotype that developed in response to different neighbours in the field persisted in the greenhouse. Plants taken from backgrounds of different grass species showed different responses to growing with those grass species. The differences were manifest primarily in a “positive leading diagonal” effect of Holcus or not-Holcus. They were the result primarily of differences in the dry weight per module and the probability of development of the axillary bud into a branch. This confirms earlier results, and implicates the central importance of branching as a means of local response to the microenvironment.
The relationship between morphological variability and biotic environmental heterogeneity was stu... more The relationship between morphological variability and biotic environmental heterogeneity was studied in a pasture population of Trifolium repens L. It had been argued that the unexpectedly high levels of variation in T. repens could be maintained by diversifying selection. The mosaic of neighbours (perennial grasses) with which T. repens co-exists constitutes a prominent element of biotic patchiness that may lead to sorting among T. repens genotypes on the basis of neighbour-specific compatibilities.A variation study was conducted on a set of 400 individuals of T. repens collected on a neighbour-specific basis from a 43–year-old pasture and grown for over 2 years under common garden conditions. Variation in a set of 12 morphological characters was assessed after 4 months and again after 27 months. After 4 months′ growth, a significant proportion of this variation was accounted for by the neighbour with which the individuals of T. repens had been growing in the pasture. The actual amount of variation accounted for, however, was low (6-19%).When the same characters were assessed after 27 months, none of the neighbour-specific differences in morphology were retained. It is concluded that the original results reflected developmental differences carried over from the pasture, and that diversifying selection is not of importance in the maintenance of morphological variation in this population.
Plant functional groups are used to describe patterns of community organization. However, they ar... more Plant functional groups are used to describe patterns of community organization. However, they are defined either by suites of correlated traits or by species groupings, and the responses of these two definitions to changing environmental conditions are unknown. 2. We assessed 14 growth and morphological traits under low-and high-resource conditions of 42 annual plant species from two source communities in Israel that differed in resource availability. As current theory predicts, plants growing in the high-resource treatment were larger, had twofold greater relative growth rate (RGR) and thinner leaves, and allocated less biomass to roots than plants grown in the low-resource treatment. Differences in these traits were less consistent between the two source communities. Instead, taxonomic groups ( grasses, legumes and a group of other forbs), regardless of source, differed in most characteristics. 3. Three general groups of species (functional groups) were identified in both resource treatments using cluster analysis on all 14 traits. In both resource treatments monocots were almost completely separated into one distinct cluster, regardless of source habitat, while the two other, mainly dicot, clusters were partially separated by habitat. However, the species composition and trait characterization of the dicot clusters differed strongly between treatments. Under low-resource conditions the two dicot clusters were separated by size traits and seed mass, but under high-resource conditions, they were separated by above-ground size, morphology and RGR. 4. Principal components analysis demonstrated inconsistency in relationships among traits and species groupings between treatments. The first two principal components emphasized different aspects of growth depending on the treatment; the third axis was defined by growth rates. As with the cluster analysis, plots of species scores revealed relatively little separation of species by habitat. 5. The response of each species varies for different traits and with growing conditions. Variation may differ among species within a functional group, producing different definitions of functional groups under different experimental conditions. Because most functional group analyses are performed on data collected without manipulation of growing conditions, conclusions concerning the response of species or communities to changes in environmental conditions may be problematic. Functional Ecology (2001) 15 , 85-95 1977); 'stress response syndrome' (Chapin 1991); and 'plant ecology strategy scheme' (Westoby 1998) all suggest that suites of morphological and physiological characteristics in plants are correlated with adaptive †To whom correspondence should be addressed. Present address:
Changes in plant community structure after changes in some aspect of the environment such as nutr... more Changes in plant community structure after changes in some aspect of the environment such as nutrients or grazing is often ascribed to changes in competitive relationships among the plants. However, very rarely is competition measured directly in such experiments. To distinguish between the direct effects of environmental treatments and changes in competitive relationships, it is necessary to quantify the influence of competition on community structure and compare the magnitude and direction of this influence between environments. We describe an experimental approach to accomplish this that is based on the classic yield-density experiment of agronomy. The approach is called the community-density experiment and requires experimental establishment of a gradient in total initial community density such that absolute densities of each species increase but initial relative abundances of each species stay constant along the gradient. We define various indices of the magnitude of community-level consequences of increasing density that can be compared among environments such as different fertilizer or grazing treatments. We also discuss various practical ways of achieving the experimental density gradient that are suitable for different kinds of communities. *** DIRECT SUPPORT *** A02DO006 00011
Most studies of density-dependent demography in plants consider the density only of the single fo... more Most studies of density-dependent demography in plants consider the density only of the single focal species being studied. However, density-dependent regulation in plants may frequently occur at the level of the entire community, rather than only within particular species. In addition, because density dependence may differ considerably (even in direction) among demographic parameters, generalizing about patterns of density dependence and extrapolating to lifetime fitness and to population dynamics require comparisons among life history stages, as well as among types of species and physical environments.
1 Invasive species dominate many ecosystems but the competitive strategies underlying this domina... more 1 Invasive species dominate many ecosystems but the competitive strategies underlying this dominance are unclear. Are invasive species generalist competitors, or do they only thrive in certain environments? Do they occur mainly post-disturbance or can they persist throughout succession? 2 We tested the relative importance of resource acquisition (competitive suppression ability) and the ability to tolerate reduced resource levels (competitive tolerance ability) among four C3 perennial grass species in an invaded oak savanna. Two species ( Poa pratensis and Dactylis glomerata ) are exotic invaders and are thought to have replaced the two native species ( Bromus carinatus and Elymus glaucus ) as dominants. 3 Using glasshouse and field experiments we tested whether the two strategies were maintained with changing resource levels and successional conditions, and their relative roles in explaining exotic dominance. 4 The relative importance of suppression-and tolerance-based competition shifted with neighbour density, burning and planting order. Further, the relative importance of particular plant traits changed depending on the imposed conditions, and the exotic dominants were only competitively superior under certain circumstances. 5 Competitive suppression ability was maintained with changing resource levels but was confined to post-disturbance conditions. When planting of neighbours was delayed, the early establishing targets were dominant regardless of species, fertility and neighbour density. 6 Competitive tolerance ability determined long-term patterns of relative abundance and coexistence, but only under the current field conditions of low fertility and limited disturbance. Alteration of these conditions changed the relative abundance of the four grasses, and would probably reconfigure species patterns in the oak savanna community generally. 7 Exotic dominance is presently determined by tolerance-based competitive traits interacting with the long-term absence of disturbance. Dominance is therefore contingent on the interaction of competitive strategies, resource availability and disturbance history rather than any one factor alone. 8 Exotic flora dominate all stages of succession in this savanna because there exists both early (suppression) and late (tolerance) successional specialists. The identity of the dominant changes with succession based on the competitive strategy it employs. This result highlights the importance of examining the historical context of invaded communities and tracking their successional status over time.
Question: Do stressful environments facilitate plant invasion by providing refuges from intense a... more Question: Do stressful environments facilitate plant invasion by providing refuges from intense above-ground competition associated with productive areas, or prevent it by favouring locally adapted native species? Location: An invaded and fragmented oak savanna ecosystem structured along a landscape-level stress gradient associated with soil depth, elevation, and canopy openness. Methods: Vegetation and environmental data were collected from 184 plots in seven savanna remnants along the gradient. Using multivariate (CCA) and post-hoc regression analyses, we determined the relationship between environment and the richness and abundance of invasives. Results: 46 of 119 species were naturalized exotics. CCA indicated the importance of environmental variation (mostly soil depth) for community structure but not for invasion; invasive species richness was similar in all areas. However, the abundance of invasives and their impacts on native diversity appear to increase significantly in less stressful habitats. Deeper soils had lower evenness and significantly fewer native species. This result was associated with dominance by exotic perennial grasses and large increases in vegetation height, suggesting strong above-ground competition. Conclusions: Low-stress environments were not more invasible per se but appear to be more susceptible to invasion by species with strong competitive impacts. The causes of decreasing exotic impact with decreasing soil depth may reflect shifts in competitive intensity or an increased importance of stress tolerance, both of which may favour natives. Alternatively, this ecosystem may simply lack high-impact invaders capable of dominating shallow soils. Conservation challenges are twofold for this endangered plant community: controlling invasives that currently dominate deeper-soils and accounting for a diverse pool of invaders that proliferate when the current dominants are removed.
Few invaded ecosystems are free from habitat loss and disturbance, leading to uncertainty whether... more Few invaded ecosystems are free from habitat loss and disturbance, leading to uncertainty whether dominant invasive species are driving community change or are passengers along for the environmental ride. The ''driver'' model predicts that invaded communities are highly interactive, with subordinate native species being limited or excluded by competition from the exotic dominants. The ''passenger'' model predicts that invaded communities are primarily structured by noninteractive factors (environmental change, dispersal limitation) that are less constraining on the exotics, which thus dominate. We tested these alternative hypotheses in an invaded, fragmented, and fire-suppressed oak savanna. We examined the impact of two invasive dominant perennial grasses on community structure using a reduction (mowing of aboveground biomass) and removal (weeding of above-and belowground biomass) experiment conducted at different seasons and soil depths. We examined the relative importance of competition vs. dispersal limitation with experimental seed additions. Competition by the dominants limits the abundance and reproduction of many native and exotic species based on their increased performance with removals and mowing. The treatments resulted in increased light availability and bare soil; soil moisture and N were unaffected. Although competition was limiting for some, 36 of 79 species did not respond to the treatments or declined in the absence of grass cover. Seed additions revealed that some subordinates are dispersal limited; competition alone was insufficient to explain their rarity even though it does exacerbate dispersal inefficiencies by lowering reproduction. While the net effects of the dominants were negative, their presence restricted woody plants, facilitated seedling survival with moderate disturbance (i.e., treatments applied in the fall), or was not the primary limiting factor for the occurrence of some species. Finally, the species most functionally distinct from the dominants (forbs, woody plants) responded most significantly to the treatments. This suggests that relative abundance is determined more by trade-offs relating to environmental conditions (longterm fire suppression) than to traits relating to resource capture (which should most impact functionally similar species). This points toward the passenger model as the underlying cause of exotic dominance, although their combined effects (suppressive and facilitative) on community structure are substantial.
† Background and Aims Changes in rainfall and temperature brought about through climate change ma... more † Background and Aims Changes in rainfall and temperature brought about through climate change may affect plant species distribution and community composition of grasslands. The primary objective of this study was to test how manipulation of water and temperature would influence the plasticity of stomatal density and leaf area of bluebunch wheatgrass, Pseudoroegneria spicata. It was hypothesized that: (1) an increased water supply will increase biomass and leaf area and decrease stomatal density, while a reduced water supply will cause the opposite effect; (2) an increase in temperature will reduce biomass and leaf area and increase stomatal density; and (3) the combinations of water and temperature treatments can be aligned along a stress gradient and that stomatal density will be highest at high stress. † Methods The three water supply treatments were (1) ambient, (2) increased approx. 30 % more than ambient through weekly watering and (3) decreased approx. 30 % less than ambient by rain shades. The two temperature treatments were (1) ambient and (2) increased approx. 1-3 8C by using open-top chambers. At the end of the second experimental growing season, above-ground biomass was harvested, oven-dried and weighed, tillers from bluebunch wheatgrass plants sampled, and the abaxial stomatal density and leaf area of tillers were measured. † Key Results The first hypothesis was partially supported -reducing water supply increased stomatal density, but increasing water supply reduced leaf area. The second hypothesis was rejected. Finally, the third hypothesis could not be fully supported -rather than a linear response there appears to be a parabolic stomatal density response to stress. † Conclusions Overall, the abaxial stomatal density and leaf area of bluebunch wheatgrass were plastic in their response to water and temperature manipulations. Although bluebunch wheatgrass has the potential to adapt to changing climate, the grass is limited in its ability to respond to a combination of reduced water and increased temperature.
Environmental conditions associated with climate change, such as earlier snowmelt, warmer spring ... more Environmental conditions associated with climate change, such as earlier snowmelt, warmer spring temperatures, and increased soil mineralization, have resulted in shifts in the timing of plant phenological events. We assessed the effects of fertilizer and herbivory on the phenology of 7 plant species from a boreal forest understory using long-term experimental plots in the southwest Yukon. Fertilizer and fencing treatments were initiated in 1990, and discontinued in half of each plot in 2000. There were few effects on phenology of either fertilizer or fencing. In some species, fertilizer affected the final phenological stage reached, but the presence and direction of the effect was species-dependent. Epilobium angustifolium was the only species where the timing of phenological stages responded to fertilization; early phenological stages were advanced with fertilizer. First leaf expansion for Arctostaphylos uva-ursi occurred earlier in fenced plots, although this effect disappeared in plots where the fencing treatment had been discontinued. We conclude that previously observed changes in species abundance with fertilizer treatments are likely not caused by changes in the phenology of these species, and are more likely due to fertilization imposed changes in vegetative growth.
The combustion of fossil fuels and associated warmer temperatures are causing a global increase i... more The combustion of fossil fuels and associated warmer temperatures are causing a global increase in the availability of soil nutrients such as nitrogen. This will have pronounced effects on plants at northern latitudes that are adapted to low nutrient conditions. An experiment in northern Canada set up in 1990 has investigated the effects of long-term nutrient enrichment (fertilizer addition) and mammalian herbivore exclusion (fencing) on an understory plant community. We used this experiment to assess how 22 years of fertilization has affected investment in sexual reproduction in four herbaceous understory species. We measured reproductive output at the plot level (proportion of plants flowering) for four species (Mertensia paniculata, Epilobium angustifolium, Achillea millefolium and Festuca altaica) and at the individual plant level (biomass allocation to flower parts) for M. paniculata and E. angustifolium. Fertilization increased the probability of flowering for A. millefolium and resulted in a higher allocation of biomass to flower parts for E. angustifolium. Sexual reproduction in M. paniculata and F. altaica was largely unaffected by increased nutrient supply, and, as expected, herbivore exclusion had almost no effect on any species. Whereas plants in northern ecosystems currently reproduce mainly through clonal growth, rapidly changing environmental conditions and warmer temperatures will likely result in increased benefits of sexual reproduction. This could give a competitive advantage to species such as A. millefolium and E. angustifolium that increase investment in sexual reproduction when released from nutrient limitation.
Traditional haymaking has created exceptionally high levels of plant species diversity in semi-na... more Traditional haymaking has created exceptionally high levels of plant species diversity in semi-natural grasslands of the Carpathian Mountains (Romania), the maintenance of which is jeopardized by recent abandonment and subsequent vegetation succession. We tested the hypothesis that the different life history strategies of dominant grasses cause different patterns of diversity loss after abandonment of traditional haymaking in two types of meadow. Although diversity loss rate was not significantly different, the mechanism of loss depended on the life history of dominant species. In meadows co-dominated by competitive stress-tolerant ruderals, diversity loss occurred following the suppression of dominant grasses by tall forbs, whereas in meadows dominated by a stress-tolerant competitor, diversity loss resulted from increased abundance and biomass of the dominant grass. We conclude that management for species conservation in abandoned grasslands should manipulate the functional turnover in communities where the dominant species is a weaker competitor, and abundance and biomass of dominant species in communities where the dominant species is the stronger competitor.
Global levels of reactive nitrogen are predicted to rise in the coming decades as a result of inc... more Global levels of reactive nitrogen are predicted to rise in the coming decades as a result of increased deposition from the burning of fossil fuels and the large-scale conversion of nitrogen into a useable form for agriculture. Many plant communities respond strongly to increases in soil nitrogen, particularly in northern ecosystems where nitrogen levels are naturally very low. An experiment in northern Canada that was initiated in 1990 has been investigating the effects of longterm nutrient enrichment (fertilizer added annually) on a boreal forest understory community. We used this experiment to investigate why some species increase in abundance under nutrient enrichment whereas others decline. We focused on four species that differed in their responses to fertilization: Mertensia paniculata and Epilobium angustifolium increased in abundance, Achillea millefolium remained relatively constant and Festuca altaica declined. We hypothesized that the two species that were successful in the new high-nutrient, light-limited environment would be taller, have higher specific leaf area, change phenology by growing earlier in the season and be more morphologically plastic than their less successful counterparts. We compared plant height, specific leaf area, growth spurt date and allocation to leaves in plants grown in control and fertilized plots. We demonstrated that each of the two species that came to dominate fertilized plots has a different combination of traits and responses that likely gave them a competitive advantage; M. paniculata has the highest specific leaf area of the four species whereas E. angustifolium is tallest and exhibits morphological plasticity when fertilized by increasing biomass allocation to leaves. These results indicate that rather than one strategy determining success when nutrients become available, a variety of traits and responses may contribute to a species' ability to persist in a nutrientenriched boreal forest understory.
Across different host plant species, the effects of mycorrhizal colonization on host growth param... more Across different host plant species, the effects of mycorrhizal colonization on host growth parameters can vary, but intraspecific variation in this relationship has rarely been measured. We tested the direction and consistency of the relationship between ectomycorrhizal colonization level and growth responses across seed families of Pinus contorta var. latifolia. Root tips of seedlings from eight full sib seed families varied in levels of ectomycorrhizal fungal colonization from 39% to 100%. We observed positive, negative, or neutral relationships between colonization level and shoot mass, depending on plant family. For the majority of seed families no relationship was observed between colonization level and root mass; however, two seed families showed negative relationships. Shoot height differed only by seed family. Results from our study indicate that the relationship between colonization level and host growth depends on host genotype. We suggest that models of plant intraspecific interactions should consider ectomycorrhizal associations when assessing phenotypic variability.
Ecological theories make divergent predictions about whether extant species inhibit or promote th... more Ecological theories make divergent predictions about whether extant species inhibit or promote the establishment of new species and which aspects of community composition determine these interactions; diversity, individual dominant species, and neutral interactions have all been argued to be most important. We experimentally tested these predictions by removing plant biomass (0%, 7%, 100%) from boreal forest understory communities. The 7% removals were restricted to the numerically dominant species, the second most dominant species, or many low-abundance species, thereby separating the effects of species composition from those of biomass. We tested the effects of all removal treatments on seedling establishment. Competitive effects were driven by one dominant species and were inconsistent with resource complementarity, neutral, or competition-colonization models. Facilitative effects were apparent only following removal of all vegetation, of which the most dominant species comprised more than 80%. Our results indicate that numerically dominant species in a community can influence the establishment of new species more than species diversity, but the direction of interaction can shift from facilitative to competitive as community density increases.
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