Runion, G. Brett (USDA-ARSUSDA-ARS, NSDL, 411 S. Donahue Drive, Auburn, AL, 36832; Phone: 334-844-4517; Fax: 334-887-8597; Email: gbrunion@ars.usda.gov)

Effects of Elevated Atmospheric CO2 on Biomass and Carbon Accumulation in a Model Regenerating Longleaf Pine Ecosystem

G.B. Runion*, M.A. Davis, S.G. Pritchard, S.A. Prior, R.J. Mitchell, H.A. Torbert, H.H. Rogers, R.R. Dute

Community response to elevated CO₂ was examined in a model regenerating longleaf pine ecosystem exposed to two CO₂ regimes (ambient, 365 ìmol mol^-1 and elevated, 720 ìmol mol^-2) for three years using open-topped chambers. Total aboveground and belowground biomass was 70% and 49% greater, respectively, in CO₂-enriched chambers; carbon (C) content followed a similar CO₂ response pattern which resulted in a significant increase of 12.2 Mg C ha^-1 sequestered in standing biomass, with an additional increase of 1.6 Mg C ha^-1 in litter. Responses of individual species, however, varied. Longleaf pine (Pinus palustris) was primarily responsible for the positive response to CO₂ enrichment (88%). Wiregrass (Aristida stricta), rattlebox (Crotalaria rotundifolia), and butterfly weed (Asclepias tuberosa) all exhibited negative aboveground (-26%, -53%, and -65%, respectively) and belowground (-36%, -43%, and -52%, respectively) biomass responses to elevated CO₂; C content responses again followed patterns similar to biomass. While sand post oak (Quercus margaretta) had positive responses to CO₂ enrichment (39% and 12% for above- and belowground, respectively), these did not differ significantly between treatments due to high variability. Elevated CO₂ resulted in alterations in community structure; 88% of total biomass in CO₂-enriched plots was allocated to longleaf pine with only 8% allocated to wiregrass, rattlebox, and butterfly weed. In comparison, ambient CO₂ plots allocated only 76% of total biomass to longleaf pine but had 19% allocated to wiregrass, rattlebox, and butterfly weed. Therefore, while longleaf pine may perform well in a high CO₂ world, other members of this community may not be able to compete as well as atmospheric CO₂ concentration continues to rise. Regardless of individual species response, the entire system gained 11.4 Mg C ha^-1 under elevated CO₂ suggesting that this ecosystem should be a sink for atmospheric CO₂.