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United States Department of Agriculture

Agricultural Research Service

Title: Bowen Ratio and Closed Chamber Carbon Dioxide Flux Measurements over Sagebrush Steppe Vegetation

item Angell, Raymond
item Svejcar, Anthony
item Saliendra, Nicanor - UTAH STATE UNIVERSITY
item Johnson, Douglas
item Bates, Jonathan

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 2, 2001
Publication Date: June 1, 2001

Interpretive Summary: There is increased interest in evaluating terrestrial C02 fluxes and ecosystem productivity as a basis for understanding the global carbon cycle and ecosystem responses to increasing C02. ARS global change research includes the goal of determining the importance of crops, rangeland and forest in carbon cycling and greenhouse gas increases. Measurement of C02 on rangeland is difficult, and data are lacking for carbon dynamics on rangeland at this time. Techniques are being implemented which utilize direct measurement of carbon dioxide concentration, or alternatively, gradients at two elevations. We measure carbon dioxide concentration over sagebrush rangeland at two locations: Dubois, ID and Burns, OR. Our goal was to measure the seasonal dynamics of carbon dioxide using two techniques to determine the potential for measuring ecosystem dynamics at meter and hectare scales. One of the large needs of ecosystem research is to scale up from the plant to the landscape level. This research describes the first report of such an effort in the Great Basin. Results of this research indicate that chamber and energy balance techniques are complementary and that, even with the diverse canopy cover which is found on the sagebrush steppe, researchers can measure responses at two scales and can compare between them. This will greatly enhance our understanding of the dynamics of carbon flux on rangelands and will provide new info which will be of significant importance to policy makers as well as land managers.

Technical Abstract: Measurement of carbon dioxide (C02) fluxes over sagebrush steppe ecosystems has received little attention, and seasonal dynamics of C02 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 C02 fluxes during eight 24-hour sampling periods throughout the 1997 growing season on ungrazed sagebrush steppe communitie at the U.S. Sheep Experiment Station near Dubois, ID and the Northern Great Basin Experimental Range near Burns, OR. Instantaneous CC measurements closely followed 20-min average C02 fluxes measured by BREB instrumentation. Maximum midday C02 assimilation occurred in June at both locations, with rates up to 0.4 and 0.5 mg m-2 s-1 at Burns and Dubois, respectively. In August, midday assimilation was low, at about 0.1 mg m-2 s-1 at both locations. Regression equations of CC flux as a function of BREB were not different between locations, so data were pooled. The similarity of these two independent measurements of C02 flux suggests that both are valid techniques. These two techniques can be used together to obtain reliable estimates of C02 flux on these shrub-dominated communities. The BREB technique is appropriate for large-scale, continuous measurements of C02 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.

Last Modified: 4/22/2015
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