Precipitation change and biomass allocation in a temperate grassland: The role of intraspecific variation and species composition change

Grasslands have a large proportion of belowground biomass and are sensitive to precipitation changes. Community-level responses of biomass allocation can result from intraspecific variation in allocation and/or a shift in species composition. Yet, the relative importance of these two mechanisms in ecosystem responses to precipitation changes is unclear. In a temperate grassland in Central Hungary, we measured the root mass fraction (RMF) of 11 species over two years with contrasting precipitation. We also investigated how four years of precipitation manipulations (from water addition to severe drought) affected community-level RMF, using community-level root samples and aboveground biomass estimates. To assess whether change in species composition alone can explain changes in community-level RMF, we compared the realised community-level RMF in the experimental plots with the community weighted mean of species-specific RMFs (CWM RMF); based on individuals growing under ambient conditions. Annuals had lower RMF than perennials, and species had higher RMF in the dry year than in the average year. The realised community-level RMF increased with dryness, while the CWM RMF decreased; the latter was driven by the higher abundance of annuals. We conclude that intraspecific variation of persistent species overrides the importance of altered species composition in driving community-level RMF response. However, a further shift towards annual dominance may reverse this change, thereby reducing community-level RMF. Our results highlight that both intraspecific variation and altered species composition may contribute to community-level changes in RMF under climate change, and the effects of these two processes may shift in importance over time.