Feedback from plant species change amplifies CO2 enhancement of grassland productivity

Authors: Polley, Herbert; Jin, Virginia; Fay, Philip

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2012
Publication Date: 9/1/2012
Citation: Polley, H.W., Jin, V.L., Fay, P.A. 2012. Feedback from plant species change amplifies CO2 enhancement of grassland productivity. Global Change Biology. 18:2813-2823.

Interpretive Summary: The concentration of carbon dioxide (CO2) gas in the atmosphere has increased by 40% since industrialization and is anticipated to reach double the pre-Industrial concentration by mid-century. CO2 stimulates plant growth by increasing carbon fixation and reducing plants’ water requirement. These benefits of CO2 enrichment are better expressed in some plant species than others, leading to changes in the relative abundances of species in multi-species plant communities, such as grasslands. Change in species abundances could feedback to either amplify or reduce the response of community productivity to CO2, but experimental evidence is rare. We tested the prediction that feedbacks from species change would amplify the initial CO2 stimulation of aboveground net primary productivity (ANPP) of grassland communities. Communities of perennial forb and warm-season grass species were grown for 5 years along a field CO2 gradient in central Texas USA on each of three soil types, including upland and lowland clay soils and a sandy soil. The ANPP-CO2 response increased between first and fifth years of the experiment on the two clay soils. CO2 increased the contribution of a tall grass to communities, which in turn doubled the initial enhancement of community ANPP. By contrast, species change had little influence on the ANPP-CO2 response on the sandy soil, possibly because soil nitrogen or other constraints on plant growth limited the increase in grassland productivity. Our results highlight the importance of multi-year CO2 experiments for detecting feedbacks that alter initial responses. Potential CO2 effects of grassland productivity would have been significantly underestimated if feedbacks from plant community change had not been considered.

Technical Abstract: Dynamic global vegetation models simulate feedbacks of vegetation change on ecosystem processes, but direct, experimental evidence for feedbacks that result from atmospheric CO2 enrichment is rare. We hypothesized that feedbacks from species change would amplify the initial CO2 stimulation of aboveground net primary productivity (ANPP) of tallgrass prairie communities. Communities of perennial forb and C4 grass species were grown for 5 years along a field CO2 gradient (250-500 µL L-1) in central Texas USA on each of three soil types, including upland and lowland clay soils and a sandy soil. CO2 enrichment increased community ANPP by 0-117% among years and soils and increased the contribution of the tall-grass species Sorghastrum nutans (Indian grass) to community ANPP on each of the three soil types. CO2-induced changes in ANPP and Sorghastrum abundance were linked. The slope of ANPP-CO2 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 CO2-mediated increase in ANPP on the upland and lowland clay soils during the final three years and one year of the experiment, respectively. By contrast, species change had little influence on the ANPP-CO2 response on the sandy soil, possibly because Sorghastrum increased largely at the expense of a functionally-similar C4 grass species. By favoring a mesic C4 tall grass, CO2 enrichment approximately doubled the initial enhancement of community ANPP on two clay soils. The CO2-stimulation of grassland productivity may be significantly underestimated if feedbacks from plant community change are not considered.