Ce. Owensby et al., BIOMASS PRODUCTION IN A TALLGRASS PRAIRIE ECOSYSTEM EXPOSED TO AMBIENT AND ELEVATED CO2, Ecological applications, 3(4), 1993, pp. 644-653
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.