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

Measurement of carbon dioxide (CO2) fluxes over sagebrush steppe ecosystems has received little attention, and seasonal dynamics of CO2 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 CO2 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 CO2 fluxes measured by BREB instrumentation, except later in the season at Burns when soil moisture was depleted. Maximum mid-day CO2 assimilation occurred in June at both locations, with rates up to 0.4 and 0.5 mg CO2 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, It = 190). These two independent measurements of CO2 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 CO2 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 CO2 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, (C) 2001 Elsevier Science B.V. All rights reserved.