Plant diversity effects on grassland productivity are robust to both nutrient enrichment and drought

Global change drivers are rapidly altering resource avail-ability and biodiversity. While there is consensus thatgreater biodiversity increases the functioning of ecosystems,the extent to which biodiversity buffers ecosystem pro-ductivity in response to changes in resource availabilityremains unclear. We use data from 16 grassland experimentsacross North America and Europe that manipulated plantspecies richness and one of two essential resources—soilnutrientsorwater—toassessthedirectionandstrengthof the interaction between plant diversity and resourcealteration on above-ground productivity and net biodiver-sity, complementarity, and selection effects. Despite strongincreases in productivity with nutrient addition anddecreases in productivity with drought, we found thatresource alterations did not alter biodiversity–ecosystemfunctioning relationships. Our results suggest that theserelationships are largely determined by increases in com-plementarity effects along plant species richness gradients.Although nutrient addition reduced complementarityeffects at high diversity, this appears to be due to highbiomass in monocultures under nutrient enrichment. Ourresults indicate that diversity and the complementarityof species are important regulators of grassland ecosystemproductivity, regardless of changes in other drivers ofecosystem function.