Vascular plant diversity structures bryophyte colonization in experimental grassland

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 (one to four; grasses, legumes, tall herbs, small herbs) as well as in additional bare ground and spontaneously colonized plots and semi-natural control meadows. 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 were more 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 6 yr, 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 grassland may reduce net species diversity

Convergent high diversity in naturally colonized experimental grasslands is not related to increased productivity

Initial plant diversity might control subsequent community assembly processes and plant productivity. To study these effects, we used a biodiversity experiment (Jena Experiment) with subplots of different sown diversity that were never weeded and spontaneously colonized control plots of different size (3.5 × 3.5 m, 20 × 20 m) with and without mowing in an 8-year study. On non-sown bare plots without mowing, colonizer accumulation depended on plot size resulting in a loss of diversity in large, undisturbed (unmown) control plots after initial colonization. On sown plots that were mown, species richness converged to high levels due to the accumulation of internal colonists (species belonging to the experimental pool of sown species), while initially high species richness of external colonists (species not belonging to the experimental pool) and residents (species sown on the plot) declined over time. The convergence of total species richness at higher levels was paralleled by increased taxonomic (Simpson index, QSimp), phylogenetic (QPhylo) and trait (FDQ) diversity, whereby FDQ was greatest on plots with low resident species richness after several years. Rates of change in terms of species colonization and extinction decelerated over time irrespective of resident species richness, mowing or plot size. While the contribution of residents declined and that of colonists increased, community biomass production did not change over time. The biomasses of residents and colonists were greatest at higher levels of species richness, partial QPhylo and FDQ of the respective species group, but community biomass was consistently weakly related to total species richness and diversity indices. Our study shows that in contrast to the period of succession, “mature” plant communities resulting from natural assembly processes, favouring the coexistence of multiple species and thus high biodiversity, do no longer show significant relationships between species richness and variables related to ecosystem functioning such as primary productivity