Species and functional group diversity independently influence biomass accumulation and its response to CO₂ and N

The characteristics of plant assemblages influence ecosystem processes such as biomass accumulation and modulate terrestrial responses to global change factors such as elevated atmospheric CO₂ and N deposition, but covariation between species richness (S) and functional group richness (F) among assemblages obscures the specific role of each in these ecosystem responses. In a 4-year study of grassland species grown under ambient and elevated CO ₂ and N in Minnesota, we experimentally varied plant S and F to assess their independent effects. We show here that at all CO₂ and N levels, biomass increased with S, even with F constant at 1 or 4 groups. Likewise, with S at 4, biomass increased as F varied continuously from 1 to 4. The S and F effects were not dependent upon specific species or functional groups or combinations and resulted from complementarity. Biomass increases in response to CO₂ and N, moreover, varied with time but were generally larger with increasing S (with F constant) and with increasing F (with S constant). These results indicate that S and F independently influence biomass accumulation and its response to elevated CO₂ and N.