Trait convergence and trait divergence in herbaceous plant communities: Mechanisms and consequences

In landscapes subject to intensive agriculture, both soil fertility and vegetation disturbance are capable of impacting strongly, evenly and simultaneously on the herbaceous plant cover and each tends to impose uniformity on the traits of constituent species. In more natural and ancient grasslands greater spatial and temporal variation in both productivity and disturbance occurs and both factors have been implicated in the maintenance of species‐richness in herbaceous communities. However, empirical data suggest that disturbance is the more potent driver of trait differentiation and species co‐existence at a local scale. This may arise from the great diversity in opportunities for establishment, growth or reproduction that arise when the intensity of competition is reduced by damage to the vegetation. In contrast to the diversifying effects of local disturbances, productivity‐related plant traits (growth rate, leaf longevity, leaf chemistry, leaf toughness, decomposition rate) appear to be less variable on a local scale. This difference in the effects of the productivity and disturbance filters arises from the relative constancy of productivity within the community and the diversity in agency and in spatial and temporal scales exhibited by disturbance events. Also, evolutionary responses to disturbances involve minor adaptive shifts in phenological and regenerative traits and are more likely to occur as micro‐evolutionary steps than the shifts in linked traits in the core physiology associated with the capacity to exploit productive and unproductive habitats. During the assembly of a community and over its subsequent lifespan filters with diversifying and convergent effects may operate simultaneously on recruitment from the local species pool and impose contrasted effects on the similarity of the trait values exhibited by co‐existing species. Moreover, as a consequence of the frequent association of productivity with the convergence filter, an additional difference is predicted in terms of the effects of the two filters on ecosystem functioning. Convergence in traits selected by the productivity filter will exert effects on both the plant community and the ecosystem while divergent effects of the disturbance filter will be restricted to the plant community.     

Species divergence and trait convergence in experimental plant community assembly

Despite decades of research, it remains controversial whether ecological communities converge towards a common structure determined by environmental conditions irrespective of assembly history. Here, we show experimentally that the answer depends on the level of community organization considered. In a 9‐year grassland experiment, we manipulated initial plant composition on abandoned arable land and subsequently allowed natural colonization. Initial compositional variation caused plant communities to remain divergent in species identities, even though these same communities converged strongly in species traits. This contrast between species divergence and trait convergence could not be explained by dispersal limitation or community neutrality alone. Our results show that the simultaneous operation of trait‐based assembly rules and species‐level priority effects drives community assembly, making it both deterministic and historically contingent, but at different levels of community organization.     

Concurrent changes in plant weight and soil water regimes in herbaceous communities in Central Australia

Two studies of arid zone plant communities are described. In the first, the perennial grass Eragrostis eriopoda was sampled over 8 months for dry weight and water stress of tops, and water content and tension of soil to a depth of 150 cm. In the second, lasting 2 years, pure Eragrostis and pure Aristida con-torta (annual) communities were sampled for dry weight of tops and roots and soil water content. Plots of naturally occurring mixtures of Eragrostis and Aristida were also studied. During periods of prolonged high soil water content, perennial communities greatly out-yielded annuals (1200 and 260gm^-2 respectively). During prolonged droughts all communities yielded less than 100 gm^-2. Perennial root systems were larger than for annuals. The weight of annual roots varied little with season as distinct from perennials which varied greatly. Maximum weight of annual roots occurred in the top 10 cm of soil; the zone of maximum rooting by perennials was 10–30 cm. Water loss from annual, perennial and bare plots was similar and extended throughout the measured profile. Two gradients of soil water tension of opposite signs developed during prolonged drought. The greater was due to evapo-transpiration and the lesser developed below the zone of recharge. On average. Eragrostis grew 4.6 times faster than the annuals which enjoyed a marginally more favourable water regime. In a mixture which had received one heavy simulated grazing the presence of annuals reduced the growth of Eragrostis by 68%, suggesting that the invasion of Eragrostis by annuals would seriously reduce productivity.     

Genetic variability at neutral markers,quantitative trait land trait in a subdivided population under selection

Genetic variability in a subdivided population under stabilizing and diversifying selection was investigated at three levels: neutral markers, QTL coding for a trait, and the trait itself. A quantitative model with additive effects was used to link genotypes to phenotypes. No physical linkage was introduced. Using an analytical approach, we compared the diversity within deme (H(S)) and the differentiation (F(ST)) at the QTL with the genetic variance within deme (V(W)) and the differentiation (Q(ST)) for the trait. The difference between F(ST) and Q(ST) was shown to depend on the relative amounts of covariance between QTL within and between demes. Simulations were used to study the effect of selection intensity, variance of optima among demes, and migration rate for an allogamous and predominantly selfing species. Contrasting dynamics of the genetic variability at markers, QTL, and trait were observed as a function of the level of gene flow and diversifying selection. The highest discrepancy among the three levels occurred under highly diversifying selection and high gene flow. Furthermore, diversifying selection might cause substantial heterogeneity among QTL, only a few of them showing allelic differentiation, while the others behave as neutral markers.     

浙北地区不同土地利用下乡村草本层植物多样性研究

该文以浙江省安吉县两个典型乡村—繅舍村和赋石村为例,选取公园、农耕区、河道、人工林地四种不同土地利用类型,用Shannon-Wiener指数、Simpson指数、Sorensen指数为标识多样性的指标,分析不同生境下草本植物多样性差异。结果表明：乡村生境中共记录物种162种,分属47科123属,其中禾本科与菊科物种数占总数的31.48％;农耕区生境的Shannon-Wiener指数、Simpson指数最高,分别为2.76和0.91,农耕区与河道的Sorensen指数最高为0.57。从农耕区、河道、公园到林地生境,草本物种多样性整体呈减小趋势。另外,发现人工绿化措施会导致草本物种减少,如从种植水稻单一作物到复合作物,草本层植物随之改变。单一稻田与复合种植区比较,草本植物主要的科属组成无明显变化,莎草科、菊科、禾本科植物仍占主体;但草本植物群落主要物种组成发生变化,稻田转变成农耕区后,牛筋草、黑麦草,碎米荠、小飞蓬、喜旱莲子草成为草本层群落主要构成植物,水竹叶覆盖度减少,小飞蓬和喜旱莲子草的覆盖度增加。此外,还发现4种生境中共有喜旱莲子草、加拿大一枝黄花、土荆芥三种入侵物种,但未能对本土物种构成显著影响。这表明不同土地利用方式会严重影响草本群落物种组成及物种多样性;土地利用方式的变化,会引起草本植物主要群落构成发生改变;土地利用类型的多样化有利于本土草本物种多样性生存。该研究结果有助于为城镇化进程中保护乡村植物多样性、加速推进美丽中国建设提供技术支撑。     

Opposing community assembly patterns for dominant and nondominant plant species in herbaceous ecosystems globally

Biotic and abiotic factors interact with dominant plants—the locally most frequent or with the largest coverage—and nondominant plants differently, partially because dominant plants modify the environment where nondominant plants grow. For instance, if dominant plants compete strongly, they will deplete most resources, forcing nondominant plants into a narrower niche space. Conversely, if dominant plants are constrained by the environment, they might not exhaust available resources but instead may ameliorate environmental stressors that usually limit nondominants. Hence, the nature of interactions among nondominant species could be modified by dominant species. Furthermore, these differences could translate into a disparity in the phylogenetic relatedness among dominants compared to the relatedness among nondominants. By estimating phylogenetic dispersion in 78 grasslands across five continents, we found that dominant species were clustered (e.g., co‐dominant grasses), suggesting dominant species are likely organized by environmental filtering, and that nondominant species were either randomly assembled or overdispersed. Traits showed similar trends for those sites (<50%) with sufficient trait data. Furthermore, several lineages scattered in the phylogeny had more nondominant species than expected at random, suggesting that traits common in nondominants are phylogenetically conserved and have evolved multiple times. We also explored environmental drivers of the dominant/nondominant disparity. We found different assembly patterns for dominants and nondominants, consistent with asymmetries in assembly mechanisms. Among the different postulated mechanisms, our results suggest two complementary hypotheses seldom explored: (1) Nondominant species include lineages adapted to thrive in the environment generated by dominant species. (2) Even when dominant species reduce resources to nondominant ones, dominant species could have a stronger positive effect on some nondominants by ameliorating environmental stressors affecting them, than by depleting resources and increasing the environmental stress to those nondominants. These results show that the dominant/nondominant asymmetry has ecological and evolutionary consequences fundamental to understand plant communities.     

Differential water resource use by herbaceous and woody plant life-forms in a shortgrass steppe community

We conducted a study to test the predictions of Walter's two-layer model in the shortgrass steppe of northeastern Colorado. The model suggests that grasses and woody plants use water resources from different layers of the soil profile. Four plant removal treatments were applied in the spring of 1996 within a plant community codominated by Atriplex canescens (a C₄ shrub) and Bouteloua gracilis (a C₄ grass). During the subsequent growing season, soil water content was monitored to a depth of 180 cm. In addition, stem and leaf tissue of Atriplex, Bouteloua and the streamside tree Populus sargentii were collected monthly during the growing seasons of 1995 and 1996 for analysis of the δ¹⁸O value of plant stem water (for comparison with potential water sources) and the δ¹³C value of leaves (as an indicator of plant water status). Selective removal of shrubs did not significantly increase water storage at any depth in the measured soil profile. Selective removal of the herbaceous understory (mainly grasses) increased water storage in the top 60 cm of the soil. Some of this water gradually percolated to lower layers, where it was utilized by the shrubs. Based on stem water δ¹⁸O values, grasses were exclusively using spring and summer rain extracted from the uppermost soil layers. In contrast, trees were exclusively using groundwater, and the consistent δ¹³C values of tree leaves over the course of the summer indicated no seasonal changes in gas exchange and therefore minimal water stress in this life-form. Based on anecdotal rooting-depth information and initial measurements of stem water δ¹⁸O, shrubs may have also had access to groundwater. However, their overall δ¹⁸O values indicated that they mainly used water from spring and summer precipitation events, extracted from subsurface soil layers. These findings indicate that the diversity of life-forms found in this shortgrass steppe community may be a function of the spatial partitioning of soil water resources, and their differential use by grasses, shrubs, and trees. Consequently, our findings support the two-layer model in a broad sense, but indicate a relatively flexible strategy of water acquisition by shrubs. Received: 23 December 1997 / Accepted: 16 September 1998     

Immigration and local competition in herbaceous plant communities: a three-year seed-sowing experiment

Understanding the relative roles of local and regional processes in determining local species diversity is now of strong relevance in basic ecology. To address this question, we have tested the influence of immigration on species diversity dynamics in a three-year experiment using herbaceous plant communities. We manipulated the intensity of seed rain (i.e., immigration) and the relative contribution of each species to the seed rain. For each of three levels of intensity of immigration (seed rain), we considered three cases, in which immigration was either negatively correlated, positively correlated, or uncorrelated with local competitive ability. Our experiment illustrates how both immigration and local competition contribute to explaining species diversity in herbaceous plant communities. Communities were more diverse when they received more seeds and when immigration was inversely correlated or uncorrelated with local competitive ability. Only species of intermediate or low competitive ability responded positively to the seed-addition treatment. Community-level functional properties were not strongly modified by immigration: immigration treatments did not differ in total above-ground biomass and plant cover, soil surface occupation was higher at high immigration intensities. A comparison of our results with theoretical models of plant community structure suggests that the mechanisms underlying the species dynamics in our communities were probably a mixture of colonization-extinction and competitive weighted lottery.     

Evidence for community assembly constraints during succession in dune slack plant communities

Community assembly during succession can be constrained by both local and regional factors. Despite an increasing regional species pool size during succession, we found a limit on the number of species in 1 × 1 m plots in dune slacks. Three alternative hypotheses (habitat heterogeneity, dispersal limitation and niche limitation) explaining this community saturation were tested. A null model analysis showed that species richness in the plots had an unusually low variance suggesting that beta habitat diversity was not likely to explain the limitation on species richness. Because we did not find a correlation between the distribution of species over the slack and their dispersal capacity, we also excluded the dispersal limitation hypothesis. Finally, a guild proportionality analysis revealed that the abundances of forb, graminoid and ruderal species showed low an unusually low variance over all age classes involved. This provides evidence for nonrandom community assembly during succession, likely to be determined by competitive exclusion among species of the same guild.     

Mechanisms driving plant functional trait variation in a tropical forest

Plant functional trait variation in tropical forests results from taxonomic differences in phylogeny and associated genetic differences, as well as, phenotypic plastic responses to the environment. Accounting for the underlying mechanisms driving plant functional trait variation is important for understanding the potential rate of change of ecosystems since trait acclimation via phenotypic plasticity is very fast compared to shifts in community composition and genetic adaptation. We here applied a statistical technique to decompose the relative roles of phenotypic plasticity, genetic adaptation, and phylogenetic constraints. We examined typically obtained plant functional traits, such as wood density, plant height, specific leaf area, leaf area, leaf thickness, leaf dry mass content, leaf nitrogen content, and leaf phosphorus content. We assumed that genetic differences in plant functional traits between species and genotypes increase with environmental heterogeneity and geographic distance, whereas trait variation due to plastic acclimation to the local environment is independent of spatial distance between sampling sites. Results suggest that most of the observed trait variation could not be explained by the measured environmental variables, thus indicating a limited potential to predict individual plant traits from commonly assessed parameters. However, we found a difference in the response of plant functional traits, such that leaf traits varied in response to canopy‐light regime and nutrient availability, whereas wood traits were related to topoedaphic factors and water availability. Our analysis furthermore revealed differences in the functional response of coexisting neotropical tree species, which suggests that endemic species with conservative ecological strategies might be especially prone to competitive exclusion under projected climate change.     

Contrasting trait assembly patterns in plant and bird communities along environmental and human‐induced land‐use gradients

Human‐driven environmental changes can induce marked shifts in the functional structure of biological communities with possible repercussion on important ecosystem functions and services. At the same time it remains unclear to which extent these changes may differently affect various types of organisms. We investigated species richness and community functional structure of species assemblages at the landscape scale (1 km² plots) for two contrasting model taxa, i.e. plants (producers and sessile organisms) and birds (consumers and mobile organisms), along topography, climate, landscape heterogeneity, and land‐use (agriculture and urbanization) gradients in a densely populated region of Switzerland. Our study revealed that agricultural and urban land uses drove marked shifts in the functional structure of biological communities compared to changes along climate and topography gradients, especially for plants, while for birds these changes were comparable. Agricultural and urban land uses enhanced divergence in traits related to resource use for birds (diet and nesting), growth forms, dispersal, and reproductive traits for plants, while it induced convergence in vegetative plant traits (plant height and leaf dry matter content). These results suggest that contrasting assembly patterns may arise within and across taxonomic groups along the same environmental gradients as result of distinct underlying processes and ‘organism‐specific’ environmental perceptions. Our results further suggest a potential homogenization of biological communities, as well as low functional diversity and redundancy levels of bird assemblages in our human‐dominated study region. This might potentially compromise the maintenance of key ecological processes under future environmental changes.     

The factors controlling species density in herbaceous plant communities: an assessment

This paper evaluates both the ideas and empirical evidence pertaining to the control of species density in herbaceous plant communities. While most theoretical discussions of species density have emphasized the importance of habitat productivity and disturbance regimes, many other factors (e.g. species pools, plant litter accumulation, plant morphology) have been proposed to be important. A review of literature presenting observations on the density of species in small plots (in the vicinity of a few square meters or less), as well as experimental studies, suggests several generalizations: (1) Available data are consistent with an underlying unimodal relationship between species density and total community biomass. While variance in species density is often poorly explained by predictor variables, there is strong evidence that high levels of community biomass are antagonistic to high species density. (2) Community biomass is just one of several factors affecting variations in species density. Multivariate analyses typically explain more than twice as much variance in species density as can be explained by community biomass alone. (3) Disturbance has important and sometimes complex effects on species density. In general, the evidence is consistent with the intermediate disturbance hypothesis but exceptions exist and effects can be complex. (4) Gradients in the species pool can have important influences on patterns of species density. Evidence is mounting that a considerable amount of the observed variability in species density within a landscape or region may result from environmental effects on the species pool. (5) Several additional factors deserve greater consideration, including time lags, species composition, plant morphology, plant density and soil microbial effects.Based on the available evidence, a conceptual model of the primary factors controlling species density is presented here. This model suggests that species density is controlled by the effects of disturbance, total community biomass, colonization, the species pool and spatial heterogeneity. The structure of the model leads to two main expectations: (1) while community biomass is important, multivariate approaches will be required to understand patterns of variation in species density, and (2) species density will be more highly correlated with light penetration to the soil surface, than with above-ground biomass, and even less well correlated with plant growth rates (productivity) or habitat fertility. At present, data are insufficient to evaluate the relative importance of the processes controlling species density. Much more work is needed if we are to adequately predict the effects of environmental changes on plant communities and species diversity.     

Diversity and occurrence of herbaceous communities in West African savannas in relation to climate,land use and habitat

In the face of the current changes in land use and climate as well as habitat destruction, it is important to study herbaceous vegetation as an indicator of changes occurring in savanna ecosystems. We investigated the effects of climate, land use and habitat, both alone and in combination, on the diversity and occurrence of West African savanna herbaceous plant communities. Floristic data and environmental variables were sampled in Burkina Faso and subjected to ordination and indicator species analysis to explore the variation in nine vegetation types. Regression analyses showed that climate, land use, humidity gradient, soil fraction and vegetation structure discriminate herbaceous plant communities. Climate, habitat and their interaction had the greatest effect on the occurrence of these communities. Changes in species richness of the studied communities were mainly due to climate, land use and their interaction, which were more important for increasing rather than decreasing diversity. In all cases, climate conditions remained the most important environmental factor driving vegetation variation in West African savannas. Beside this, the effects of habitat degradation in interaction with land use and climatic conditions indicate land use to be a threat for the diversity of the herbaceous vegetation.     

A dominant dwarf shrub increases diversity of herbaceous plant communities in a Trans-Himalayan rangeland

Plant communities are structured by both competition and facilitation. The interplay between the two interactions can vary depending on environmental factors, nature of stress, and plant traits. However, whether positive or negative interactions dominate in regions of high biotic and abiotic stress remains unclear. We studied herbaceous plant communities associated with a dwarf shrub Caragana versicolor in semi-arid, high altitude Trans-Himalayan rangelands of Spiti, India. We surveyed 120 pairs of plots (within and outside shrub canopies) across four watersheds differing in altitude, aspect, and dominant herbivores. Herbaceous communities within shrub canopies had 25% higher species richness, but similar abundance when compared to communities outside the canopy, with the shrub edge having higher diversity than the centre of the canopy. Grasses and erect forbs showed positive associations with the shrub, while prostrate plants occurred at much lower abundance within the canopy. Rare species showed stronger positive associations with Caragana than abundant species. Experimental removal of herbaceous vegetation from within shrub canopies led to 42% increase in flowering in Caragana, indicating a cost to the host shrubs. Our study indicates a robust pattern of a dwarf shrub facilitating local community diversity across this alpine landscape, increasing diversity at the plot level, facilitating rare species, and yet incurring a cost to hosts from the presence of herbaceous plants. Given these large influences of this shrub on the vegetation of these high altitude rangelands, we suggest that the shrub microhabitat be explicitly considered in any analyses of ecosystem health in such rangelands.     

Disturbance as a driver of trait assembly in liana communities in a semi-deciduous Atlantic Forest

Plant Ecology - Subtropical forests have been under constant pressure due to drastic changes in land use, fragmentation, and logging. These impacts can alter the canopy cover, structure, species...     

Differential effects of goat browsing on herbaceous plant community in a two-phase mosaic

The impact of herbivores on herbaceous plant communities is usually attributed to direct consumption of plants. We hypothesized that goats affect herbaceous plants both directly (consumption by foraging) and indirectly, by changing environmental conditions through modification of woody plant structure. We assessed the effects of goats browsing on environmental conditions, landscape structure, and herbaceous plants to link the direct and indirect effects of goats on herbaceous communities. Our model system was the Mediterranean woodland in Mt. Carmel, Israel. This is a two-phase mosaic landscape, composed of herbaceous (open) and woody patches. We delineated 10 plots of 1000 m², goats were introduced to five plots and five plots remained without goats. We monitored plant species richness and composition in two adjacent patch types (woody and open) in each plot. For each patch type, in all plots, we collected data on environmental conditions. We analyzed landscape structure using landscape metrics derived from a high-resolution vegetation map. We found that goats modified the structure of woody plants and hence the landscape mosaic. This alteration was associated with changes in environmental conditions, with more light penetration and higher temperatures. The impact of goats on the herbaceous plant community depended on patch type. In open patches, goats affected the herbaceous community mostly by direct consumption, whereas in woody patches they affected the herbaceous community mainly by modification of abiotic conditions. Our results stress the importance of considering landscape and patch structure in analyzing the effect of herbivory on plant communities.     

Vole mound effects and disturbance rate in a mediterranean plant community under different grazing and irrigation regimes

A factorial field experiment was used to assess the influence of soil-disturber mammals in the structure of a 9-year-old Mediterranean annual plant community subjected to different sheep grazing and irrigation regimes. We estimated the disturbance rate (mound building activity) by Mediterranean voles, their effects on vegetation and the mechanisms of these effects during a period of vole outbreak. The effects on vegetation were analysed at the levels of species, functional groups and plant community. Disturbance rate was high and voles can disturb the entire soil surface once every four or five years. The availability of certain trophic resources (perennial plants) appeared to drive vole expansion in the experimental plots and it was independent of the irrigation and grazing treatments. Mound building activities largely affected vegetation but conserved plot differences. Total vegetation cover, absolute cover of all functional groups, mean vegetation height and species richness were less on mounds than on undisturbed ground. These effects did not change the relative abundance of annuals, perennials, grasses and forbs. Only the relative abundance of small-seeded species decreased on mounds. As the proportion of these seeds was similar in both types of patches, we suggest that small-seeded species had more difficulties for germinating or emerging when they are buried during mound formation. Irrigation and sheep grazing promoted large changes in the vegetation parameters but these effects were, in general, similar on mounds and undisturbed ground. Our results show that the availability of germinable seeds may be the major limitation for mound revegetation, probably due to the scarcity of seeds existing at the depths from which soils are excavated. Our results also suggested a resource limitation on mounds. The results provide additional evidence that soil disturbances by small herbivore mammals exert relevant ecological effects on abandoned Mediterranean croplands. We discuss the ecological implications of vole mound-building activities for plant succession, plant species conservation and forage resource availability for livestock. This revised version was published online in June 2006 with corrections to the Cover Date.     

Invertebrate community responses to land use at a broad spatial scale: trait and taxonomic measures compared in New Zealand rivers

1. Large‐scale comparative studies of ecological responses to anthropological stressors in rivers require measures that are consistent across a range of spatial scales. The biological trait profile of communities offers an alternative approach to traditional measures of macroinvertebrate taxonomic identity and is less constrained by biogeographic influences. 2. We compared the capacities of taxonomic composition and biological trait composition to discriminate the effects of land use (measured as percentage of the catchment in pasture) across a large geographic zone (the whole of New Zealand) in 30 sub‐catchments grouped into five ecoregions throughout the North and South Islands of New Zealand. In addition, we investigated trait consistency (i.e. whether similar traits had similar trait responses to land use at local (catchment) and broad scales). 3. The analysis of taxonomic composition showed that community structure was indeed influenced by land‐use intensity, but that relationships differed among ecoregions. In contrast, traditional assessment metrics (Macroinvertebrate Community Index, richness in Ephemeroptera, Plecoptera and Trichoptera taxa) and trait composition were uninfluenced by region. Trait responses were consistent at the broad and catchment scales, with similar traits responding to pastoral land use at both scales. 4. We used general linear modelling to investigate individual trait responses to land‐use intensity, catchment area and region, focussing on 15 trait categories known to be influenced by land‐use intensity at the catchment scale. Several trait categories varied with land‐use intensity and demonstrated consistency at both catchment and broad scales. Of these, the representation of shorter generation time, asexual reproduction and hermaphroditism, ability to lay eggs beneath the water surface, egg protection and respiration types tolerant of oxygen depletion generally increased in assemblages exposed to more intense pastoral land use. At the same time, the representation of short life duration of adults, prevalence of laying eggs at the water surface, sexual reproduction and low body flexibility decreased in assemblages exposed to land‐use intensification. 5. Our results highlight the value of developing predictive response measures that are relevant at multiple scales and provide the basis for new measures of river condition that are as effective as taxonomic identity in terms of response reliability.