BIOMASS PRODUCTION IN A TALLGRASS PRAIRIE ECOSYSTEM EXPOSED TO AMBIENT AND ELEVATED CO2

Responses to elevated CO2 have not been measured for natural grassland ecosystems. Global carbon budgets will likely be affected by changes in biomass production and allocation in the major terrestrial ecosyste ms. Whether ecosystems sequester or release excess carbon to the atmos phere will partly determine the extent and rate that atmospheric CO2 c oncentration rises. Elevated CO2 also may change plant community speci es composition and water status. We determined above- and belowground biomass production, plant community species composition, and measured and modeled water status of a tallgrass prairie ecosystem in Kansas ex posed to ambient and twice-ambient CO2 concentrations in open-top cham bers during the entire growing season from 1989 through 1991. Dominant species were Andropogon gerardii, A. scoparius, and Sorghastrum nutan s (C-4 metabolism) and Poa pratensis (C-3). Aboveground biomass and le af area were estimated by periodic sampling throughout the growing sea son in 1989 and 1990. In 1991, peak biomass and leaf area were estimat ed by an early August harvest. Relative root production among treatmen ts was estimated using root ingrowth bags which remained in place thro ughout the growing season. Latent heat flux was simulated with and wit hout water stress. Botanical composition was estimated annually. Compa red to ambient CO2 levels, elevated CO2 increased production of C-4 gr ass species, but not of C-3 grass species. Species composition of C-4 grasses did not change, but Poa pratensis (C-3) declined, and C-3 forb s increased in the stand with elevated CO2 compared to ambient. Open-t op chambers appeared to reduce latent heat flux and increase water-use efficiency similar to the elevated CO2 treatment when water stress wa s not severe, but under severe water stress, the chamber effect on wat er-use efficiency was limited. In natural ecosystems with periodic moi sture stress, increased water-use efficiency under elevated CO2 appare ntly would have a greater impact on productivity irrespective of photo synthetic pathway.