The type of competition modulates the ecophysiological response of grassland species to elevated CO2 and drought

The effects of elevated CO2 and drought on ecophysiological parameters in grassland species have been examined, but few studies have investigated the effect of competition on those parameters under climate change conditions. The objective of this study was to determine the effect of elevated CO2 and drought on the response of plant water relations, gas exchange, chlorophyll a fluorescence and aboveground biomass in four grassland species, as well as to assess whether the type of competition modulates that response. Elevated CO2 in well-watered conditions increased aboveground biomass by augmenting CO2 assimilation. Drought reduced biomass by reducing CO2 assimilation rate via stomatal limitation and, when drought was more severe, also non-stomatal limitation. When plants were grown under the combined conditions of elevated CO2 and drought, drought limitation observed under ambient CO2 was reduced, permitting higher CO2 assimilation and consequently reducing the observed decrease in aboveground biomass. The response to climate change was species-specific and dependent on the type of competition. Thus, the response to elevated CO2 in well-watered grasses was higher in monoculture than in mixture, while it was higher in mixture compared to monoculture for forbs. On the other hand, forbs were more affected than grasses by drought in monoculture, while in mixture the negative effect of drought was higher in grasses than in forbs, due to a lower capacity to acquire water and mineral nutrients. These differences in species-level growth responses to CO2 and drought may lead to changes in the composition and biodiversity of the grassland plant community in future climate conditions.