Species interactions amplify functional group responses to elevated CO2 and N enrichment in a 24-year grassland experiment

Plant functional groups (FGs) differ in their response to global changes, although species within those groups also vary in such responses. Both species and FG responses to global change are likely influenced by species interactions such as inter-specific competition and facilitation, which are prevalent in species mixtures but not monocultures. As most studies focus on responses of plants growing in either monocultures or mixtures, but rarely both, it remains unclear how interspecific interactions in diverse ecological communities, especially among species in different FGs, modify FG responses to global changes. To address these issues, we leveraged data from a 16-species, 24-year perennial grassland experiment to examine plant FG biomass responses to atmospheric CO₂, and N inputs at different planted diversity. FGs differed in their responses to N and CO₂ treatments in monocultures. Such differences were amplified in mixtures, where N enrichment strongly increased C3 grass success at ambient CO₂ and C4 grass success at elevated CO₂. Legumes declined with N enrichment in mixtures at both CO₂ levels and increased with elevated CO₂ in the initial years of the experiment. Our results suggest that previous studies that considered responses to global changes in monocultures may underestimate biomass changes in diverse communities where interspecific interactions can amplify responses. Such effects of interspecific interactions on responses of FGs to global change may impact community composition over time and consequently influence ecosystem functions.