Climate change impacts on future sagebrush ecohydrology across the western US

Background/Question/Methods

In the coming century, climate change is predicted to have large impacts on precipitation and temperature regimes in the western US. Sagebrush ecosystems, which cover approximately 500,000 km², are expected to be strongly affected by altered climate through changes in soil water availability. We explored the impacts of altered climate on ecohydrology of sagebrush ecosystems using output from 10 general circulation models (GCMs) for emission scenario RCP 8.5. Specifically we asked: 1) How will climate change influence key aspects of sagebrush water balance, particularly seasonal water dynamics of upper (0-30cm) and lower (>30cm) soil layers and 2) How substantial is the uncertainty in water balance projections implied by variability among GCMs? We explored these questions across multi-decadal time frames (1980-2010, 2030-2060, 2070-2010) to understand how changes in water balance will develop through the 21^st century. We assessed ecohydrological variables at 898 sagebrush sites across the western US using a process-based soil water model, SOILWAT, which uses daily precipitation and temperature data, monthly vegetation parameters, and soil attributes to model all components of daily water balance. We quantified changes in key variables that characterize the sagebrush ecohydrological niche, particularly daily soil water potential (SWP) and daily transpiration in upper and lower soil layers.

Results/Conclusions

Throughout the 21^st century, SWP increased slightly in late fall and winter across all 898 sites, but decreased in late spring and summer. The maximum daily transpiration shifted earlier in the year for most GCMs and time periods, but particularly for the bottom soil layer in 2070-2100 (current = mid-June, projected = mid-May). Furthermore, the growing-season transpiration peak widened, such that the number of consecutive days with high transpiration increased. In addition to shifts in the timing of maximum transpiration, total annual transpiration increased across most GCMs and time periods relative to current conditions (mean increase = 17.2mm for the top soil layer, 9.75mm for the bottom soil layer). However, both daily transpiration values and SWP varied depending on the GCM chosen, particularly for the top soil layer. Our SWP and transpiration results suggest that soils, particularly the top soil layer, will dry out earlier in the year in response to altered climate, resulting in drier summer conditions. Drier summer and wetter fall conditions may negatively impact sagebrush regeneration and survival and could favor cold-season annuals (e.g. cheatgrass) at the expense of warm-season perennials, resulting in shifts in the relative abundance of sagebrush plant functional groups.