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Abstract

Questions: Numerous grassland biodiversity experiments have explored how plant diversity influences colonization by vascular plants, but no such studies have examined how sown vascular plant diversity structures colonization by bryophytes, which can contribute greatly to grassland diversity. Location: Grassland biodiversity experiment (Jena Experiment), Germany. Methods: We studied bryophyte composition in experimental grasslands encompassing a gradient of vascular plant species richness (1, 2, 4, 8, 16 and 60 species), functional group richness and composition (1 to 4; grasses, legumes, tall herbs, small herbs) as well as in additional bare ground and spontaneously colonized plots and semi-natural controlmeadows. Results: Increasing vascular plant species richness and functional group richness as well as the presence of legume species decreased bryophyte species richness and increased the spatial variability of this richness. Bryophyte species richness and cover responded positively to grass presence, which also decreased the spatial variability of both. The proportion of acrocarpous species was largest in bare ground plots and decreased with increasing vascular plant species richness and with grass and legume presence. Non-metric multidimensional scaling revealed that bryophyte species composition followed the gradient in sown vascular plant species richness and was dependent on grass and legume presence. The effect of plant diversity on bryophytes was only partly attributable to increased vascular plant species cover. Bryophyte habitat indicator values suggested that conditions in communities of higher vascular plant diversity that included grasses weremore shady,moist and nutrient-rich than in communities with lower vascular plant diversity and without grasses, whereas bryophyte assemblages in communities with legumes indicated well-lit nutrient-rich conditions. Even after six years, bryophyte species richness and cover was considerably lower in experimental grasslands than in semi-natural meadows, suggesting a role for dispersal limitation or habitat filtering. Conclusions: Our study shows that bryophyte colonization is not random, rather it is structured by vascular plant species diversity and composition. Not all components of a plant community respond positively to increased vascular plant diversity and advocating increased richness of one taxonomic group over another in grasslandmay reduce net species diversity.