Plant community assembly and biodiversity: a spatio-temporal perspective

Abstract: Biodiversity, the variety of life at all organisational levels from genes to ecosystems, affects ecosystem processes and therefore the goods and services ecosystems provide. More research is needed to provide new insights into biodiversity changes and the processes that drive these changes, in order to formulate effective policy and conservation measures to stop the ongoing biodiversity loss. In this thesis, I focus on spatial and temporal changes in different aspects of plant biodiversity and examine the driving forces that generate and maintain observed biodiversity patterns. Multiple facets of biodiversity (taxonomic, phylogenetic, functional) were characterized in semi-natural grasslands (in plots of 0.5 × 0.5 and 2 × 2 m, and whole grassland polygons). The extent to which the present-day and historical characteristics of the sites and their surrounding landscape explain the current diversity patterns was quantified. Temporal changes in the multiple facets of diversity, and assembly processes that drive these changes, were investigated along a more than 300 year long chronosequence representing an arable to semi-natural grassland succession. Both grassland plant species richness and functional trait diversity in grassland sites were to a large extent explained by the land use history of the sites and the availability of grassland habitat in the surrounding historical landscape. It appears that not only is there a delayed loss of species diversity in response to landscape fragmentation ("extinction debt") but that there is also a delayed decline of functional diversity in response to ongoing habitat destruction (i.e. a "functioning debt") that will potentially generate a time lag in the changes in ecosystem attributes. Quantification of the linkages between the distribution and diversity of dispersal and persistence traits and current and historical properties of the grassland sites and their surrounding landscape revealed that long-distance dispersal potential as well as the diversity of different dispersal and persistence strategies within present-day grassland communities was mainly determined by the local management history and landscape history. Long-distance dispersal by wind and animals no longer appears to be contributing to the colonization of the remaining fragments of habitat within the increasingly fragmented modern landscape, and long-term persistent species are likely to dominate the grassland communities in the future. Whereas many long-distance dispersed species can still persist locally in the presence of grazing disturbance, grazing management may also promote the diversity of different dispersal and persistence strategies, but only in sites that were well connected to grassland areas in the past. The extent to which grassland management strategies can maintain a high diversity of dispersal and persistence strategies, and thereby the capacity of a plant community to buffer environmental change, will depend on the context of the site within the historical surrounding landscape. Comparative analysis of taxonomic, phylogenetic and functional diversity at different stages of arable-to-semi-natural grassland succession demonstrated that community assembly during secondary grassland succession was deterministic with respect to species traits, suggesting that it may be possible to predict changes in biodiversity, and associated alterations in ecosystems functioning in future environments, on the basis of species functional traits. Taxonomic, phylogenetic and functional diversity showed contrasting patterns of change over time. Short-term grazing management (5-50 years) promoted species richness, but did not enhance phylogenetic or functional diversity. Only long-term grazing management, over more than 270 years, promoted phylogenetic and functional diversity without further increases in species richness. I conclude that (a) multiple facets of biodiversity should be considered in order to more realistically assess the full dimensions of biodiversity loss resulting from human-driven environmental changes, (b) history is a major determinant of biodiversity, and (c) the simultaneous consideration of multiple facets of biodiversity can provide new insights into the processes that shape communities.