Trait‐based approach confirms the importance of propagule limitation and assembly rules in old‐field restoration

Community assembly theory is suggested as a guiding principle for ecological restoration to help understand the mechanisms that structure biological communities and identify where restoration interventions are needed. We studied three hypotheses related to propagule limitation, stress‐dominance, and limiting similarity concepts in community assembly in a restoration field experiment with a trait‐based null model approach. The experiment aimed to assist the recovery of sand grassland on former arable land in the Kiskunság, Pannonian biogeographic region, Europe. Treatments included initial seeding of five grassland species, carbon amendment, low‐intensity mowing, and combinations in 1 m by 1 m plots in three old fields from 2003 to 2008. The distribution of 10 individual plant traits was compared to the null model and the effect of time and treatments were tested with linear mixed effect models. Initial seeding had the most visible impact on species and trait composition confirming propagule limitation in grassland recovery. Reducing nutrient availability through carbon amendment strengthened trait convergence for length of flowering as expected based on the stress‐dominance hypothesis. Mowing changed trait divergence to convergence for plant height with a strengthening impact with time, supporting our hypothesis of increasing dominance of limiting similarity with time. Our results support the idea that community assembly is simultaneously influenced by propagule limitation and multiple trait‐based processes that act through different traits. The limited impact of manipulating environmental filtering and limiting similarity compared to seeding, however, supports the view that only targeting the dispersal and environmental filters in parallel would improve restoration outcome.     

Using functional trait diversity to evaluate the contribution of multiple ecological processes to community assembly during succession

Successional chronosequences provide a unique opportunity to study the effects of multiple ecological processes on plant community assembly. Using a series of 0.5 × 0.5 m² plots (n = 30) from five successional sub‐alpine meadow plant communities (ages 3, 5, 9, 12, and undisturbed) in the Qinghai‐Tibetan Plateau, we investigated whether community assembly is stochastic or deterministic for species and functional traits. We tested directional change in species composition, functional trait composition, and then functional trait diversity measured by Rao's quadratic entropy for four traits – plant height, leaf dry matter content, specific leaf area, and seed mass – along two comparable successional chronosequences. We then evaluated the importance of species interactions, habitat filtering and stochasticity by comparing with random communities and partitioning the environmental and spatial components of Rao's quadratic entropy. We found no directional change in species composition, but clear directionality in functional trait composition. None of the abiotic environmental variables (except P) showed linear change with successional age, but soil moisture and nitrogen were positively related to functional diversity within meadows. Functional trait diversity increased significantly with the increase in successional age. Comparison with random communities showed a significant shift from trait divergence in early stages of succession (3‐ and 5‐yr) to convergence in the later stages of succession 9‐, 12‐yr and undisturbed). The relative importance of abiotic variables and spatial structure for functional trait diversity changed in a predictable manner with successional age. Stochasticity at the species level may indicate dispersal limitation, but deterministic effects on functional trait distributions show the role of both habitat effects and biotic interactions.     

Recruitment in species-rich grasslands: the effects of functional traits and propagule pressure

Aims The assembly of plant communities is a complex process which combines impacts from the species pool, dispersal and propagule pressure, niche requirements of colonizing species and the niche structure of the community. Recent theory development has incorporated all these aspects, e.g. in 'stochastic niche theory'. We investigated recruitment into a species-rich grassland community, using an experimental approach where we manipulated the trait composition of the community and examined the success of colonizing species entering with various propagule pressure. Specifically, we examined two predictions: (i) colonization success increases with increasing difference between traits of the colonizing species and the trait profile of the community and (ii) colonization success increases with increasing propagule pressure.Methods The examined communities were species-rich semi-natural grasslands located in southern Sweden. After a careful documentation of the composition of the plant communities at the experimental sites, we manipulated the trait profile of species-rich grassland plots based on the plant functional trait specific leaf area (SLA), which is correlated with several key life history functions. In addition to SLA, seed mass was also used to describe the trait profile of grassland plots. Seeds of 12 plant species from the regional species pool, varying in SLA and seed mass, were sown into plots using four different levels of propagule pressure. Recruitment was examined after 1 year. We also planted juvenile 'plug plants' of the same species which allowed us to examine survivorship and growth beyond the seedling stage.Important findings Overall we found very limited evidence for relationships between the traits of the colonizing species and the trait profile of the community and for recruitment after sowing these relationships were contrary to the prediction. Survival of plug plants after two seasons of growth was high irrespective of the trait profile of the community, but growth of plug plants was affected by the trait profile of the surrounding community. For four of the species there was a positive effect of increased propagule pressure on colonization. The results suggest that species assembly in species-rich grasslands is not strongly dependent on the niche structure of the community. However, the finding that colonization of only a third of the species responded positively to increased propagule pressure indicates that there might be niche-related effects that were not captured by our treatments. Overall, our results indicate that the factors determining colonization in this community are species specific. Some species are able to colonize irrespective of niche relationships, provided that the propagule pressure is sufficiently high to overcome stochastic mortality after seed arrival. For other species, however, we cannot exclude that niche assembly occurred, but we failed to identify the relevant niche factor.     

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.     

A functional trait-based approach to understand community assembly and diversity–productivity relationships over 7 years in experimental grasslands

Several multi-year biodiversity experiments have shown positive species richness–productivity relationships which strengthen over time, but the mechanisms which control productivity are not well understood. We used experimental grasslands (Jena Experiment) with mixtures containing different numbers of species (4, 8, 16 and 60) and plant functional groups (1–4; grasses, legumes, small herbs, tall herbs) to explore patterns of variation in functional trait composition as well as climatic variables as predictors for community biomass production across several years (from 2003 to 2009). Over this time span, high community mean trait values shifted from the dominance of trait values associated with fast growth to trait values suggesting a conservation of growth-related resources and successful reproduction. Increasing between-community convergence in means of several productivity-related traits indicated that environmental filtering and exclusion of competitively weaker species played a role during community assembly. A general trend for increasing functional trait diversity within and convergence among communities suggested niche differentiation through limiting similarity in the longer term and that similar mechanisms operated in communities sown with different diversity. Community biomass production was primarily explained by a few key mean traits (tall growth, large seed mass and leaf nitrogen concentration) and to a smaller extent by functional diversity in nitrogen acquisition strategies, functional richness in multiple traits and functional evenness in light-acquisition traits. Increasing species richness, presence of an exceptionally productive legume species (Onobrychis viciifolia) and climatic variables explained an additional proportion of variation in community biomass. In general, community biomass production decreased through time, but communities with higher functional richness in multiple traits had high productivities over several years. Our results suggest that assembly processes within communities with an artificially maintained species composition maximize functional diversity through niche differentiation and exclusion of weaker competitors, thereby maintaining their potential for high productivity.     

Impact of grazing on germination trait selection in an alpine grassland on the Tibet Plateau

放牧对青藏高原高寒草地种子萌发性状选择的影响 以前的研究表明放牧能够引起草地生物与非生物环境的显著变化,但这种变化影响草地群落对种子萌发特征的选择机制尚不清楚。因此,我们旨在回答：放牧是否对草地群落中萌发特征的组成和多样性产生显著影响。我们在实验室检测了研究草地群落内主要物种种子的萌发特性,并比较了这些植物在放牧和非放牧草地上的表现。在此基础上,比较了放牧草地和非放牧草地的各萌发性状的群落加权平均值和萌发性状多样性,从而了解放牧草地和非放牧草地是否存在不同的萌发性状结构。研究结果表明,在物种水平上,放牧和非放牧草地各物种的多度变化与物种的萌发性状无显著关系。但在群落水平上,与非放牧草地相比,放牧草地的物种普遍具有较高的种子萌发率;放牧草地种子萌发对变温的正响应显著大于非放牧草地,而且放牧草地种子萌发温度生态位宽度小于非放牧草地。与非放牧草地相比,放牧草地种子萌发性状多样性增加,萌发性状均匀度降低。放牧可以改变微生境,从而通过环境过滤改变草地群落对萌发性状的选择,导致草地群落的萌发性状于放牧前后在群落水平上存 在显著差异。     

Grassland restoration after afforestation: No direction home?

The coastal grasslands in north‐eastern South Africa are a severely threatened vegetation type rich in plant species, particularly forbs. Many of the forbs have underground storage organs which allow them to resprout rapidly after fires. A significant portion of this land was placed under commercial pine afforestation in the 1950s. The pine plantations have since been removed starting 17 years ago and restored to grasslands within a conservation area. We assessed the effects of plantations on grassland plant diversity and functional trait composition by sampling 64 circular quadrats of 5 m radius distributed equally in restored versus natural grasslands. The difference in plant diversity was dramatic with the natural grassland supporting 221 species of which 163 were forbs compared with 144 and only 73 forb species in restored grasslands. Major differences in species composition were recorded, especially for forb species. Natural grasslands were dominated by resprouters (130 species) but these were rare in the restored grasslands (36 species). Differences in plant species response to fire were also evident for the two grassland states. In contrast to coastal forest restoration studies in the same area which have shown near linear increases in woody species with time, restored grasslands showed no increase in forb species richness with increasing time since clear‐felling of pines. Our results indicate that current methods for restoring these grasslands are inadequate and that restoring grasslands may be a lot harder than previously thought. Considerable effort should be made in conserving what is left of natural grasslands.     

Response of plant functional traits during the restoration of calcareous grasslands from forest stands

In this survey, we studied the response of plant functional traits to calcareous grassland restoration in the Calestienne region, Southern Belgium (restoration protocol: forest clear-cutting followed by grazing at all sites). We considered traits related to dispersal, establishment, and persistence that integrate the main challenges of plants to re-establish and survive in restored areas. Functional traits were compiled from databases and compared among (i) pre-restoration and young restoration forests; (ii) restoration areas of different ages; and (iii) old restorations and reference grasslands. The following questions were addressed: (i) What is the early response (2–4 years) in terms of plant functional trait following one restorative clear-cut event? (ii) What plants functional trait responses occur from restorative management (i.e., sheep and goat grazing)? (iii) Which differences still persist between the oldest restored parcels (10–15 years), and the historical reference grasslands? Forest clear-cuts induced several changes among functional traits, including decreased mean seed mass and certain vegetative traits (i.e., decreased phanerophytes, branching species; and increased short lifespan species i.e., annuals and biennials). During restorative management, clonal, epizoochorous and autumn germinating species were favored. Despite numerous other changes during this phase, many differences remained compared to reference grasslands. In particular, geophytes, mycorrhizal and evergreen species abundance were not approaching reference grassland values. The observed pattern helped to draw inferences on the possible mechanisms operating under vegetation recovery following restorative forest clear-cut and subsequent management were identified and described in this study. Results indicated grazing was an important factor, which increased epizoochorous species, and autumn germinating taxa that filled niches in vegetation opened by summer grazing animals. Finally, differences between old restoration and reference grasslands emphasized that management should focus on reduction in soil fertility, and geophyte rhizomatous grasses. Long-term monitoring is vital to assess if management plans are effective in the complete restoration of species functional trait assemblages.     

Sustained functional composition of pollinators in restored pastures despite slow functional restoration of plants

Habitat restoration is a key measure to counteract negative impacts on biodiversity from habitat loss and fragmentation. To assess success in restoring not only biodiversity, but also functionality of communities, we should take into account the re‐assembly of species trait composition across taxa. Attaining such functional restoration would depend on the landscape context, vegetation structure, and time since restoration. We assessed how trait composition of plant and pollinator (bee and hoverfly) communities differ between abandoned, restored (formerly abandoned) or continuously grazed (intact) semi‐natural pastures. In restored pastures, we also explored trait composition in relation to landscape context, vegetation structure, and pasture management history. Abandoned pastures differed from intact and restored pastures in trait composition of plant communities, and as expected, had lower abundances of species with traits associated with grazing adaptations. Further, plant trait composition in restored pastures became increasingly similar to that in intact pastures with increasing time since restoration. On the contrary, the trait composition of pollinator communities in both abandoned and restored pastures remained similar to intact pastures. The trait composition for both bees and hoverflies was influenced by flower abundance and, for bees, by connectivity to other intact grasslands in the landscape. The divergent responses across organism groups appeared to be mainly related to the limited dispersal ability and long individual life span in plants, the high mobility of pollinators, and the dependency of semi‐natural habitat for bees. Our results, encompassing restoration effects on trait composition for multiple taxa along a gradient in both time (time since restoration) and space (connectivity), reveal how interacting communities of plants and pollinators are shaped by different trait–environmental relationships. Complete functional restoration of pastures needs for more detailed assessments of both plants dispersal in time and of resources available within pollinator dispersal range.     

Testing for the influence of niche and neutral factors on sapling community assembly beneath isolated woody plants in grasslands

Questions: Perching and nursing effects drive initial steps of forest expansion over grasslands. Nursing effect is obviously related to niche mechanisms, while perching effect is likely to result both from neutral and niche factors. This study assessed the effect of neutral and niche factors on species composition in sapling communities developing beneath isolated trees/shrubs (ITS) in grassland. Location: A mosaic of Campos grassland and Araucaria forest in São Francisco de Paula, southern Brazil (29°28′S, 50°13′W). Methods: We described sapling communities beneath 32 ITS using mean number of forest woody saplings of different species. We performed a stepwise canonical correspondence analysis (CCA) to select ITS traits that maximized the association with species composition. Then we evaluated the contribution of distance from seed source, ITS traits and distance‐structured ITS traits on sapling community assembly using a variation partitioning method based on CCA and partial CCA. Results: Sapling species composition was significantly explained by ITS traits (ITS dispersal mode, ITS growth form, crown area:ITS height ratio, crown area, ITS height and crown area:volume ratio). Distance from seed sources explained only a minor, non‐significant fraction of sapling species composition. Distance‐structured trait variation was negligible. Conclusions: Sapling community assembly beneath ITS was mostly explained by niche factors related to both nursing and perching effects. Dispersal limitation explained only a small fraction of variation in species composition beneath ITS, suggesting that neutral‐based perching effect had a minor role in community assembly.     

Intraspecific trait variation and allocation strategies of calcareous grassland species: Results from a restoration experiment

Intra- and interspecific trait variation express the response of plants dealing with different environmental conditions. We measured root and leaf traits on 14 species of calcareous grasslands in a restoration experiment. We aimed at identifying intraspecific differences in biomass allocation and functional plant traits under contrasting soil conditions by comparing plants growing in ancient grassland and two restored grasslands on ex-arable land, one of them with topsoil removal. Relative importance of trait variation within and among species, and among site was assessed by variance partitioning. Interspecific variation was more important than intraspecific variation, but the contribution of the latter to total variation was considerable, especially for specific leaf area. Changes in soil properties due to topsoil removal resulted in lower values of plant height, specific leaf area and specific root length compared to the control (ancient grassland). Soil fertility found in the treatment without top soil removal did not affect plant plasticity compared to the control. The study species showed two allocation strategies in relation to resource stress, while the responses of individual traits to the soil treatments were consistent across species. We conclude that caution must be taken when using mean trait values for plastic species or when working with environmental gradients.     

Species and functional trait re-assembly of ground beetle communities in restored grasslands

Ecosystem restoration provides unique opportunities to study community dynamics under succession and can reveal how consumer communities re-assemble and respond to successional changes. Studying community dynamics from both taxonomic and functional trait perspectives also may provide more robust assessments of restoration progress or success and allow cross-system comparisons. We studied ground beetle (Coleoptera: Carabidae) communities for three years in a restored grassland chronosequence with sites from 0 to 28 years old. We measured traditional community metrics (abundance, richness, Shannon diversity) and functional trait metrics based on species’ body length, wing morphology, activity time, phenology, and diet. Communities had high species richness and abundance in early successional stages, but these declined in later stages to low levels comparable to an adjacent grassland remnant. Species composition also shifted with time, converging with the remnant. Although functional richness, like species richness, declined as succession progressed, functional divergence quickly increased and was maintained over time, suggesting niche differentiation in established communities. Young sites were typified by small, macropterous, phytophagous species, while older sites contained larger species more likely to be flightless and carnivorous. Prescribed burns also affected traits, decreasing prevalence of larger species. This study demonstrates that functionally diverse consumer communities can self-assemble under restoration practices. In a relatively short amount of time both morphological and trophic level diversity are established. However, prescribed fire intended to control non-desirable plants may also shape beetle community functional composition, and restoration managers should consider if plant community benefits of fire outweigh potential declines in consumer function.     

Resource availability determines the importance of niche‐based versus stochastic community assembly in grasslands

Niche‐based selection and stochastic processes can operate simultaneously to generate spatial and temporal variation in species composition. Yet, the conditions under which ecological dynamics are dominated by niche‐based versus stochastic processes are poorly understood. Using a field experiment in early‐successional temperate grassland and null models of beta diversity, this study investigates the effects of soil nutrient supply on the relative importance of niche‐based selection versus stochastic dynamics for variation in species composition among sites. Nutrient availability was manipulated experimentally, individual seed mixtures with 25 species were sown in each experimental plot, and then stochastic and deterministic niche‐based assembly processes were allowed to happen. We found that compositional variation among grassland plots with low nutrient supply was driven by stochastic immigration and extinctions. In contrast, nutrient enrichment reduced the importance of stochasticity and imposed a deterministic environmental filter that homogenized communities through the selection of few species with greater competitive ability for light. This demonstrates that soil nutrient availability is a critical environmental feature that dictates the degree to which terrestrial plant communities are controlled by niche‐based selection versus stochastic assembly processes. Our study shows further that alternative states of eutrophic grasslands emerge from initial stochastic variation in the composition of a particular functional group of species that can become dominant at high nutrient supply. We discuss potential mechanisms underlying the shift from stochastic to niche‐driven dynamics along soil nutrient gradients.     

A functional analysis of vegetation dynamics in abandoned and restored limestone grasslands

Question: How do various plant functional groups and types are related to regeneration of limestone grasslands and to temporal changes in cover of trees and shrubs? Which mechanisms are responsible for changes in differently treated grasslands? Location: Southern Poland, 50° 01'N, 19° 50'E. Methods: A 12‐year restoration experiment after the clearing of a 35‐yr‐old secondary pine wood developed on limestone grassland, and a parallel observation of succession in neighbouring abandoned grassland were carried out. Changes in cover of species with different functional traits and established strategies in relation to time and cover of shrub layer were analysed using multiple linear regression. Results: Over 12 years the composition and cover of species changed in the studied grasslands, but significant differences still occurred between the old grassland and the grassland restored in former wood gaps and that developed in former closed wood. Despite the very close proximity of the old grassland, the composition of restored grasslands was still much determined by initial conditions. In these grasslands, the cover of Festuco‐Brometea species was significantly lower than in the old grassland. In all sites, the cover of species with large leaves and with leaves distributed regularly along the stem, and the cover of CS strategists increased. With increasing shrub cover in restored grasslands, the cover of annuals and biennials, tall species, and R and CS strategists decreased. Conclusions: Limited availability of seeds seems the principal reason for the weak regeneration of xerothermic limestone grasslands. Trees and shrubs inhibit the developed of both early successional species and perennials occurring in established communities. They significantly hinder regeneration of grasslands and should be cut every five or six years.     

Discriminating trait‐convergence and trait‐divergence assembly patterns in ecological community gradients

Question: Whereas similar ecological requirements lead to trait‐convergence assembly patterns (TCAP) of species in communities, the interactions controlling how species associate produce trait‐divergence assembly patterns (TDAP). Yet, the linking of the latter to community processes has so far only been suggested. We offer a method to elucidate TCAP and TDAP in ecological community gradients that will help fill this gap. Method: We evaluated the correlation between trait‐based described communities and ecological gradients, and using partial correlation, we separated the fractions reflecting TCAP and TDAP. The required input data matrices describe operational taxonomic units (OTUs) by traits, communities by the quantities or presence‐absence of these OTUs, and community sites by ecological variables. We defined plant functional types (PFTs) or species as community components after fuzzy weighting by the traits. The measured correlations for TCAP and TDAP were tested by permutation. The null model for TDAP preserves the trait convergence, the structure intrinsic in the fuzzy types, and community total abundances and autocorrelation. Results: We applied the method to trait‐based data from plant communities in south Brazil, one set in natural grassland experimental plots under different nitrogen and grazing levels, and another in sapling communities colonizing Araucaria forest patches of increasing size in a forest‐grassland mosaic. In these cases, depending on the traits considered, we found strong evidence of either TCAP or TDAP, or both, that was related to the environmental gradients. Conclusions: The method developed is able to reveal TCAP and TDAP that are more likely to be functional for specified ecological gradients, allowing establishment of objective hypotheses on their links to community processes.     

Multivariate analysis of the response of overgrown semi-natural calcareous grasslands to restorative shrub cutting

Monitoring and evaluation of restoration management often suffers from poor sampling design and a lack of statistical rigour, seldom considering the spatial and temporal variability of habitat. We tested the effectiveness of shrub cutting on the restoration of calcareous grasslands, using a 4-year Before–After/Reference-Control-Impact (BARCI) project design with replicated patches of habitat. Departure from the control sites and convergence with the reference sites were analysed using the Principal Response Curves (PRC) method.The structure of communities was compared 1 year before and 3 years after management on control scrubland, restored ex-arable land, restored and reference grassland. Results show that shrub cutting did not suffice to restore the community composition of the semi-natural calcareous grasslands. The restored ex-arable areas maintained a community structure extremely similar to the control scrubland.The restored grassland, on the other hand, deviated from the control and slightly converged to the reference grassland, though not significantly. The dominant scrub species that was cut, Prunus spinosa, showed higher cover values compared to the reference grassland in any of the treatments, even the first year after cutting. Species typical of xeric and/or calcareous grassland were more abundant in the reference than in the restored grassland, while arable and ruderal species were more frequent in the restored sites.This study has demonstrated that the BARCI approach is a powerful tool for the evaluation of restoration management, as it was possible to evaluate not only departure from the unmanaged control, but also convergence with the reference community. The PRC method provided a comprehensive overview of the response of the 153 species involved in the study.Our results also indicate that the second PRC should be considered, when significant, to not exclude important information from the assessment.     

Quantifying the role of deterministic assembly and stochastic drift in a natural community of Arctic mosses

The interpretation of natural plant communities frequently invokes species‐sorting controlled by niche differences along spatial environmental gradients. This process of niche structuring can be explained by reference to functional traits, which provide a mechanistic explanation for community structure. In contrast, models explaining species coexistence obviate the limiting effect of niche difference, by invoking processes which cause species‐level drift, e.g. demographic stochasticity. This paper investigates a simple habitat with strong gradients (moss communities in a patterned arctic wetland) to identify signature‐patterns under‐pinning the relative importance of deterministic assembly and stochastic drift in a natural community. First, ordination analysis was used to confirm community composition structured by a range of nine carefully selected functional traits. Second, to determine whether traits explaining community composition might also explain species richness, local species richness (sR) was compared to (1) observed trait diversity and (2) expected trait diversity based on permutation tests, which are used to simulate null community assembly for different values of sR. Traits explaining species composition, consistent with deterministic niche structuring, do not appear to maintain sR. This surprising result was explained by decomposing the community into individual pair‐wise comparisons, i.e. species niche‐differences and association (χ²). Results support deterministic processes via the sorting of species with similar and contrasting niches, at opposite ends of a composite environmental gradient. Nevertheless, stochastic drift is apparent in the random structure of a majority of pair‐wise associations; in addition, a species’ abundance was in general not related to environmental distance from response‐optima. We suggest therefore that spatial pattern in the moss community is a balance between deterministic forces with respect to species traits and controlling environmental gradients, and stochastic drift, which weakens this deterministic structure.     

Effect of habitat area and isolation on plant trait distribution in European forests and grasslands

A number of studies show contrasting results in how plant species with specific life‐history strategies respond to fragmentation, but a general analysis on whether traits affect plant species occurrences in relation to habitat area and isolation has not been performed. We used published data from forests and grasslands in north‐central Europe to analyse if there are general patterns of sensitivity to isolation and dependency of area for species using three traits: life‐span, clonality, and seed weight. We show that a larger share of all forest species was affected by habitat isolation and area as compared to grassland species. Persistence‐related traits, life‐span and clonality, were associated to habitat area and the dispersal and recruitment related trait, seed weight, to isolation in both forest and grassland patches. Occurrence of clonal plant species decreased with habitat area, opposite to non‐clonal plant species, and long‐lived plant species decreased with grassland area. The directions of these responses partly challenge some earlier views, suggesting that further decrease in habitat area will lead to a change in plant species community composition, towards relatively fewer clonal and long‐lived plants with large seeds in small forest patches and fewer clonal plants with small seeds in small grassland patches. It is likely that this altered community has been reached in many fragmented European landscapes consisting of small and isolated natural and semi‐natural patches, where many non‐clonal and short‐lived species have already disappeared. Our study based on a large‐scale dataset reveals general and useful insights concerning area and isolation effects on plant species composition that can improve the outcome of conservation and restoration efforts of plant communities in rural landscapes.     

Contrasting Effects of Intraspecific Trait Variation on Trait-Based Niches and Performance of Legumes in Plant Mixtures

Niche differentiation, assumed to be a key mechanism of species coexistence, requires that species differ in their functional traits. So far it remains unclear to which extent trait plasticity leads to niche shifts of species at higher plant diversity, thereby increasing or decreasing niche overlap between species. To analyse this question it is convenient to measure niches indirectly via the variation in resource-uptake traits rather than directly via the resources used. We provisionally call these indirectly measured niches trait-based niches. We studied shoot- and leaf-morphological characteristics in seven legume species in monoculture and multi-species mixture in experimental grassland. Legume species varied in the extent of trait variation in response to plant diversity. Trait plasticity led to significant shifts in species niches in multiple dimensions. Single-species niches in several traits associated with height growth and filling of canopy space were expanded, while other niche dimensions were compressed or did not change with plant diversity. Niche separation among legumes decreased in dimensions related to height growth and space filling, but increased in dimensions related to leaf size and morphology. The total extent of occupied niche space was larger in mixture than in the combined monocultures for dimensions related to leaf morphology and smaller for dimensions related to whole-plant architecture. Taller growth, greater space filling and greater plasticity in shoot height were positively, while larger values and greater plasticity in specific leaf area were negatively related with increased performance of species in mixture. Our study shows that trait variation in response to plant diversity shifts species niches along trait axes. Plastically increased niche differentiation is restricted to niche dimensions that are apparently not related to size-dependent differences between species, but functional equivalence (convergence in height growth) rather than complementarity (divergence in traits associated with light acquisition) explains increased performance of legumes in mixture.     

Non‐interacting impacts of fertilization and habitat area on plant diversity via contrasting assembly mechanisms

Aim

The local‐ and regional‐based forms of anthropogenic change reducing grassland diversity are generally identified, but these scale‐dependent processes tend to co‐occur with unclear interactive effects. Here, we explicitly test how common local and regional perturbations simultaneously affect plant alpha and beta diversity in a multiyear community assembly experiment using fragments of grassland habitat of various sizes. We hypothesized that local disturbances and decreasing patch size would interact, suppressing local diversity while homogenizing composition among patches.

Location

North America.

Methods

We conducted a three‐year grassland assembly experiment, factorially manipulating local perturbation (nitrogen addition and mowing) and patch area for 36 patches over 13 ha. We quantified the individual and interactive effects of these local and regional factors on plant alpha and beta diversity within (quadrat scale) and among patches (patch scale). We also used a null model approach to disentangle between stochastic‐ and niche‐based assembly mechanisms.

Results

We detected a gradient of assembly outcomes driven by two non‐interacting factors—the effects of N fertilization on alpha (negative) and beta (positive) diversity regardless of spatial scale and the scale‐dependant effect of increasing patch size on alpha (positive) and beta (positive) diversity. These effects unfolded over time, with the constraints on richness and composition shifting from dispersal‐based during the first sampling year to perturbation‐and size‐based factors at year two and three. Fertilization effects were driven by a mixture of deterministic (i.e., selection at the species level) and stochastic (i.e., random extinctions) processes resulting in a decline in local richness but an increase in spatial heterogeneity in species composition. Area appeared to influence alpha diversity mainly via stochastic “sampling effect”—larger patches represented a larger sample of the regional pool. Niche‐based processes, however, led to convergence in beta diversity among smaller patches driving a positive overall effect of area on beta diversity.

Main conclusion

Our results illustrate how diversity regulation in contemporary grasslands can be simultaneously shaped by local and regional factors acting additively but via contrasting assembly mechanisms that operate at different spatial and temporal scales.