Direct and indirect associations between plant species richness and productivity in grasslands: regional differences preclude simple generalization of productivity-biodiversity relationships

Klaus VH, Hölzel N, Boch S, Müller J, Socher SA, Prati D, Fischer M, Kleinebecker T

Zusammenfassung

Plant species richness of permanent grasslands has often been found to be significantly associatedwith productivity. Concentrations of nutrients in biomass can give further insight into these productivity-plant species richness relationships, e.g. by reflecting land use or soil characteristics. However,the consistency of such relationships across different regions has rarely been taken intoaccount, which might significantly compromise our potential for generalization.We recorded plantspecies richness and measured above-ground biomass and concentrations of nutrients in biomass in295 grasslands in three regions in Germany that differ in soil and climatic conditions. Structuralequation modelling revealed that nutrient concentrations were mostly indirectly associated withplant species richness via biomass production. However, negative associations between the concentrationsof different nutrients and biomass and plant species richness differed considerably amongregions. While in two regions, more than 40% of the variation in plant species richness could beattributed to variation in biomass, K, P, and to some degree also N concentrations, in the third regiononly 15% of the variation could be explained in this way. Generally, highest plant species richnesswas recorded in grasslands where N and P were co-limiting plant growth, in contrast to N or K (co-)limitation. But again, this pattern was not recorded in the third region. While for two regions landuseintensity and especially the application of fertilizers are suggested to be the main drivers causingthe observed negative associations with productivity, in the third region the little variance accountedfor, low species richness and weak relationships implied that former intensive grassland management,ongoing mineralization of peat and fluctuating water levels in fen grasslands have overruledeffects of current land-use intensity and productivity. Finally, we conclude that regional replicationis of major importance for studies seeking general insights into productivity-diversity relationships.