Bowen ratio and closed chamber carbon dioxide flux measurements over sagebrush steppe vegetation

Abstract

Measurement of carbon dioxide (CO 2) fluxes over sagebrush steppe ecosystems has received little attention, and seasonal dynamics of CO 2 uptake are not known for most portions of this expansive ecosystem. We utilized two techniques — Bowen ratio/energy balance (BREB) and closed chamber (CC) — to measure CO 2 fluxes during eight 24 h sampling periods throughout the 1997 growing season on ungrazed sagebrush steppe communities at the US Sheep Experiment Station near Dubois, ID and the Northern Great Basin Experimental Range near Burns, OR. Instantaneous CC measurements generally agreed with 20 min average CO 2 fluxes measured by BREB instrumentation, except later in the season at Burns when soil moisture was depleted. Maximum mid-day CO 2 assimilation occurred in June at both locations, with rates up to 0.4 and 0.5 mg CO 2 m −2 s −1 at Burns and Dubois, respectively. In August, mid-day assimilation was low, at about 0.1 mg m −2 s −1 at both locations. Slopes of CC fluxes as a function of BREB were not different between locations, so data were combined across locations. A significant, positive correlation was observed between CC and BREB ( R=0.82, n=190). These two independent measurements of CO 2 flux showed good agreement, except during extremely hot and dry periods in late summer. This suggests that both BREB and CC are valid techniques and can be used in concert to obtain reliable estimates of CO 2 flux on these shrub-dominated communities, although caution is advised for CC during periods when temperatures are high and soil moisture is low. The BREB technique is appropriate for large-scale, continuous measurements of CO 2 flux, and compares well with the CC technique, which can partition flux estimates between shrub canopy and interspace, thereby providing a measure of spatial variability.