Burning and Grazing Effects on Water Balance of a Sagebrush-Grass Ecosystem

A Bowen ratio–energy balance system was used to obtain continuous measurements of carbon and water vapor fluxes at a sagebrush-grass plant community during the growing seasons of 1997, 1998, and 1999. The instrumentation was located at three sites originally dominated by sagebrush. One location was burned, and grasses took over the area. In another the grasses were grazed by sheep, and sagebrush remained as the only vegetation. The third one was left undisturbed, with a ratio of about 60:40 of sagebrush vs. grasses. During the three years of research there was a variation in water vapor fluxes across the sites and during the seasons. The highest precipitation occurred in 1998 where water vapor values were the highest for all the three sites. During 1998 carbon efflux values in the fall (respiration) were higher than during 1997 and 1999. This can be explained by the sufficient amount of water and its constant availability to plants. During all three seasons the results of the three sites showed reduced water use during the summer. After fall precipitation, water use was slightly increased, but the plant carbon assimilation remained similar to values during the late summer when there was little or no precipitation. A comparison of the three sites showed little or no difference between the undisturbed and the grazed sites regarding water use, while there was quite a significant difference between those two sites and the burned site. The difference was greatest during the first year of research (1997) when burning occurred and there was little or no vegetation. Thus, the water vapor values for burned site were lower than at the other two sites, and the main part of water vapor in a total ET amount was coming from the soil evaporation. Also, carbon assimilation at that site during the 1997 was much lower than in the other two following years, because there was almost no vegetation. During 1998 and 1999 the LE and carbon fluxes values at the burned site were higher than in the first year of the research because grasses became the predominant vegetation and there was less bare soil. However, those values were still not as high as at the grazed and undisturbed sites where sagebrush was predominant, due to the ability of sagebrush to access water from deeper soil layers. Generally, water vapor fluxes were similar in timing and magnitude among the three years, but significantly different across the three sites. The grazed site showed lower water use when compared to the undisturbed and burned sites