Non-native grasses alter evapotranspiration and energy balance in Great Basin sagebrush communities

Over the last century non-native plants have invaded sagebrush communities of the American West. The widespread transformation from native sagebrush to invasive grassland communities as a result of wildfire will likely alter key ecosystem processes in the Great Basin, including hydrology and energy balance. To determine how this invasion may be disrupting ecosystem function, we coupled measurements of evapotranspiration (ET) and energy fluxes using the Bowen ratio-energy balance method with measurements of normalized difference vegetation index (NDVI) and plant cover in native sagebrush and adjacent post-fire bunchgrass communities in the northern Great Basin Desert over the growing season of 2003. Soil heat flux and soil temperatures were generally greater and sensible heat flux and net radiation were lower in the post-fire community compared to sagebrush. These differences increased through the summer. The post-fire community had greater ET than sagebrush in early May, likely driven by relatively high volumetric soil moisture (>20%) and greater transpiration by the developing vegetation. In June and July, ET was correlated with surface soil moisture (top 20 cm) in both communities. During late May and June, surface soil moisture was greater in the sagebrush; however, there was no difference after mid-June when soil moisture declined below 10%. By disrupting surface-atmosphere exchange in the early growing season, conversion of native sagebrush to post-fire invasive communities may disrupt hydrologic patterns in this semi-arid ecosystem.