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United States Department of Agriculture

Agricultural Research Service

Title: Carbon and Nitrogen Dynamics of a Co2-Enriched Forest Ecosystem: Implications for Air, Water and Soil Quality

Authors
item Gjerstad, Dean - AUBURN UNIVERSITY
item Runion, George
item Green, Thomas - ALABAMA A&M UNIV.
item Prior, Stephen
item Rogers Jr, Hugo
item Torbert, Henry

Submitted to: Experiment Station Bulletins
Publication Type: Experiment Station
Publication Acceptance Date: April 16, 2003
Publication Date: April 16, 2003
Citation: Gjerstad, D.H., Runion ,G.B., Green, T.H., Prior, S.A., Rogers, H.H., and Torbert, H.A. 2003. Carbon and nitrogen dynamics of a carbon dioxide-enriched forest ecosystem: Implications for air, water and soil quality. Final Technical Report, Agricultural Experiment Station, Auburn University, AL. 2 pp.

Interpretive Summary: The increased growth of longleaf pine under elevated CO2 suggests that this species will become more attractive for timber growers as atmospheric CO2 continues to rise, especially if global temperatures also continue to increase. Elevated CO2 enhances the drought and heat tolerance of longleaf pine via reduced water loss (reductions in stomatal conductance and increased water use efficiency). Additionally, the insect and disease tolerance of longleaf pine may also be enhanced under elevated CO2 by increased production of foliar defensive compounds such as resins and tannins. Despite increases in loss of carbon from soil respiration, longleaf pine ecosystems can act as sinks for atmospheric carbon and may help mitigate rising levels of atmospheric carbon dioxide and the potential climatic impacts associated with such increased greenhouse gas emissions.

Technical Abstract: As atmospheric [CO2] continues to rise (it is predicted to double in the next century), it is critical to further our understanding of plant community responses to CO2-enrichment. Differences in morphology, physiology, life form, and symbiotic relationships generate differences in species responses to CO2-enrichment, which can alter competitive interactions, thus affecting community structure and function and ultimately the fate of C and N cycling within ecosystems. We examined the responses of a model regenerating longleaf pine community to three years exposure to ambient or twice ambient levels of atmospheric CO2. The model community was constructed from an assemblage of early successional forest species representing major functional guilds within a typical longleaf pine- wiregrass community: (1) a C3 evergreen conifer (Pinus palustris); (2) a C4 bunch grass (Aristida stricta); (3) a C3 broadleaf tree (Quercus margaretta); (4) a C3 perennial herbaceous legume (Crotalaria rotundifolia); (5) a C3 herbaceous perennial (Asclepias tuberosa). After three years, CO2-enrichment increased both above- and belowground biomass (70 and 50%, respectively) primarily due to increases in pine biomass. Community structure was largely unaffected by CO2 enrichment; however, Aristida, Crotalaria, and Asclepias had higher mortality and less biomass in high CO2 plots, suggesting that not all species will perform well as global [CO2] rises. Temporal and spatial rooting dynamics were examined using a combination of minirhizotrons and soil coring; CO2-enriched plots exhibited 35% higher standing crop root length, 37% greater root length production per day, and 47% greater root length mortality per day. Relative root turnover (flux/standing crop) was unchanged by elevated CO2. Root biomass of pine was higher (while that of Aristida, Crotalaria, and Asclepias were lower) in elevated compared to ambient CO2 plots; no difference in root biomass of oaks were observed. Both total carbon and nitrogen within these plant communities followed patterns similar to biomass (i.e., C and N in oaks were not affected, pines were increased (~90%), and the other species were decreased (~30%) under elevated CO2); overall, the CO2-enriched ecosystems contained 65% more C and 57% more N than their ambient counterparts. Soil respiration was significantly increased (26.5 %) for plant communities exposed to elevated CO2. These data indicate that, while increasing levels of CO2 will increase the feedback of CO2 to the atmosphere via soil respiration, terrestrial ecosystems remain potential sinks for atmospheric CO2. Longleaf pine realized the greatest and most consistent benefit from exposure to elevated CO2, suggesting that the ability of longleaf pine to compete for resources in regenerating ecosystems may be significantly enhanced by rising atmospheric CO2concentrations. Our data suggest that longleaf pine communities as a whole will perform well in a future higher CO2 world, but some species may fall prey to competitive interactions for light and soil moisture. These changes in community composition may affect overall productivity and nutrient cycling.

Last Modified: 10/25/2014