Water and Carbon Fluxes Along a Precipitation Gradient in a Sagebrush Steppe Environment.

Environmental gradients exert controls on water, carbon and energy fluxes across montane landscapes, impacting the magnitude and timing of evapotranspiration, carbon uptake, water stress, and water use efficiency.  Four eddy covariance systems were situated along an elevation gradient in Idaho’s Owyhee Mountains.  The sites are part of the Reynolds Creek Critical Zone Observatory and contribute to an ongoing long-term environmental monitoring network in the USDA’s Reynold’s Creek Experimental Watershed. The sites include a Wyoming big sagebrush site, a low sagebrush site, a post-fire mountain big sagebrush site, and a mountain big sagebrush site located at elevations of 1425, 1680, 1808 and 2111 m.  Variations in climate follow the montane elevation gradient; mean annual precipitation at the sites is 290, 337, 425, and 795 mm, respectively, and mean annual temperature is 8.9, 8.4, 6.1, 5.4°C.  Evapotranspiration (ET) peaked about a month earlier at the lower elevation sites, but with limited precipitation the vegetation also encountered water stress much earlier. The ratio of ET to potential evapotranspiration (PET) indicated that plants experienced less water stress for a large part of the growing season at higher sites. Water use efficiency tended to be lower at the Wyoming big sagebrush site, presumably due to the more bare ground and water loss to soil evaporation.  WUE was similar at the three higher elevation sites, except that a July rain caused a decrease in WUE at the two lowest elevation sites due to wet surface soil and increased soil evaporation.