Factors causing different CO2 responses in shoot regrowth after defoliation between two grassland species in Mongolia

Abstract

To address the interspecific differences in CO2 responses of plant regrowth after grazing, we grew two Mongolian grasses, Agropyron cristatum and Stipa krylovii, which are highly palatable to livestock, under ambient or elevated CO2 concentration ([CO2]) and subjected them to shoot clipping. We found two major differences under elevated [CO2]: a larger increase in shoot regrowth in A. cristatum than in S. krylovii and a decrease in the degree of biomass compensation (the ratio of shoot mass of clipped plants plus mass of clippings to shoot mass of non-clipped plants) in A. cristatum but not in S. krylovii. The former resulted from a decrease in net assimilation rate in S. krylovii. The latter was caused by a larger increase in shoot growth in non-clipped than in clipped plants under elevated [CO2] in A. cristatum, which resulted from an increase in biomass partitioning to shoots caused by elevated [CO2] in non-clipped plants. Our results imply that the productivity increase caused by elevated [CO2] would be higher in A. cristatum-dominated than in S. krylovii-dominated grasslands under grazing. Downregulation of photosynthesis at elevated [CO2] and CO2-induced changes in biomass partitioning may contribute to the interspecific differences in CO2 responses in the regrowth of grass species.