Feedback from Plant Species Change Amplifies CO 2 Enhancement of Grassland Productivity

Dynamic global vegetation models simulate feedbacks of vegetation change on ecosystem processes, but direct, experimental evidence for feedbacks that result from atmospheric CO 2 enrichment is rare. We hypothesized that feedbacks from species change would amplify the initial CO 2 stimulation of aboveground net primary productivity (ANPP) of tallgrass prairie communities. Communities of perennial forb and C 4 grass species were grown for 5 years along a field CO 2 gradient (250-500 microL/L) in central Texas USA on each of three soil types, including upland and lowland clay soils and a sandy soil. CO 2 enrichment increased community ANPP by 0-117% among years and soils and increased the contribution of the tallgrass species Sorghastrum nutans (Indian grass) to community ANPP on each of the three soil types. CO 2-induced changes in ANPP and Sorghastrum abundance were linked. The slope of ANPP-CO 2 regressions increased between initial and final years on the two clay soils because of a positive feedback from the increase in Sorghastrum fraction. This feedback accounted for 30-60% of the CO 2-mediated increase in ANPP on the upland and lowland clay soils during the final 3 years and 1 year of the experiment, respectively. By contrast, species change had little influence on the ANPP-CO 2 response on the sandy soil, possibly because Sorghastrum increased largely at the expense of a functionally similar C 4 grass species. By favoring a mesic C 4 tall grass, CO 2 enrichment approximately doubled the initial enhancement of community ANPP on two clay soils. The CO 2-stimulation of grassland productivity may be significantly underestimated if feedbacks from plant community change are not considered.