Species pools and environmental sorting control different aspects of plant diversity and functional trait composition in recovering grasslands

Succession theory argues that the taxonomic and functional structure of assembling communities is regulated by the interplay of local environmental conditions and regional factors such as available species pools or landscape configurations that moderate dispersal. Yet, the relative importance of these factors and whether they control different aspects of community structure, for example species diversity or the composition of plant traits, is not well understood. This limits predictions of spatial variation in successional outcomes. We explored patterns of species diversity and traits in recovering calcareous grasslands that varied in environmental conditions, age, past and present spatial landscape configuration, as well as in the propagule pressure they receive from historical grasslands that have large and functionally distinct species pools. Mixed-effects modelling and multimodel inference were used to dissect the leading drivers of species diversity and functional composition. Propagule pressure from the regional species pools of old grasslands during early stages of grassland re-assembly was an important determinant of community structure, with significant effects not only on species diversity but also on the composition of traits related to resource economics. Such species pool effects exceeded positive influences of environmental heterogeneity on species richness and confounded well-known diversity–environment and trait–environment relationships. As an orthogonal axis of community assembly, environmental sorting occurred along a gradient of soil texture that discriminated species according to their canopy height and – to a smaller extent – phenology. Additionally, high grassland perimeter–area ratios favoured specialist species richness and increased the proportion of species with predicted poor epizoochorous and wind dispersal. Synthesis. We provide empirical evidence that both species pool effects and local environmental sorting operate during community assembly of recovering calcareous grasslands, but that they influence different aspects of plant community structure. Further, we show that geographic variation in propagule pressure from large and functionally distinct source species pools can confound diversity–environment and trait–environment relationships. Therefore, besides accounting for environmental factors, considering the location of source species pools, as well as their compositional and functional characteristics, is a prerequisite for robust predictions of contingencies in taxonomic and functional community structure of successional habitats.