Net CO2 flux in a tallgrass prairie along a 230 to 550 ppm MOL MOL-1 CO2 gradient

A Texas grassland containing C₃ and C₄ species was exposed from March through December 1998 and 1999 to a continuous CO₂ concentration gradient from 230 to 550 ppm in two 50-meter long chambers. Net CO₂ flux and above-ground biomass were measured in each of ten 5-meter long sections in each chamber. In both years, year-end, above-ground biomass (excluding litter) increased linearly with CO₂, and ranged from 571 to 1760 g m^-2 in 1998 and from 130 to 1083 g m^-2 in 1999. Above-ground biomass was lower in 1999 apparently because of an increase in litter accumulation and possibly greater carbon allocation below ground. Net CO₂ fluxes, by contrast, were similar in 1998 and 1999. Total season net flux ranged from 2110 to 7803 g CO₂ m^-2 in 1998 and from 2332 to 8438 g CO₂ m^-2 in 1999. The amount of surface area covered by C3 plants along the chamber gradient was related to daily net CO₂ flux. Net flux, normalized for C₃ cover, was positively correlated with the CO₂ gradient in both years. This research is unique because it examined plant response over a wide range of CO2 concentrations in the field and includes the first field evidence of plant response of intact ecosystems over subambient CO₂.