Drought stress resistance and resilience of permanent grasslands are shaped by functional group composition and N fertilization

Central European old permanent grasslands are of considerable economic importance and ecological value. So far, there are conflicting results on their resistance and resilience to future drought events. Especially the influence of functional diversity on these attributes has yet to be fully understood, as most studies originate from experimental grassland communities. During three consecutive years we induced spring and summer drought events on three permanent grassland sites typical for Northern Germany. We observed a larger drought resistance in swards that were grass-dominated than in functionally diverse swards. Grasses determined the drought resistance potential of a sward, and their performance was impaired by the presence of forbs and legumes. Fertilization increased the resistance to drought stress of swards either through direct positive effects on the productivity or indirect effects through changes of functional sward composition. Sward functional composition was not important for sward resilience. Grasses and whole swards were resilient to drought stress only if previously fertilized. We found that the grass functional group is responsible for the swards' resistance and resilience to drought, but its behaviour was partly regulated not only by the presence but also by the biomass share of the forb and legume functional group. The differences among the functional groups' share of the total sward biomass might be important determinants of responses to drought stress. We therefore hypothesize that there is no uniform reaction of permanent grassland to drought stress events.