Does process-based soil water modeling of sagebrush steppe inform climate envelopes

Background/Question/Methods Sagebrush steppe is one of the most widespread ecosystem types in the western United States. It provides essential habitat for wildlife and plays a crucial role in the water cycle of these water-limited systems. However, land management, habitat destruction and biological invasions have altered sagebrush steppe on a large scale. To assess how sagebrush steppe ecosystems may respond to changing climate, we need to identify the processes that define sagebrush steppe distribution under current conditions. We applied soil water modeling to characterize the range of climatic and hydrologic conditions that support sagebrush steppe. Because it is relatively well studied, sagebrush steppe provides an excellent case to investigate how process-based soil water modeling can inform climate envelope approaches to assess future ecosystem distributions. We randomly selected 1,000 sagebrush steppe grid cells (30 m x 30 m) from the regional GAP analysis programs and extracted local climate, soil type, and 50 years of daily weather data for each cell. We used a daily time step soil water simulation model to characterize water cycling in each grid cell. Results/Conclusions Annual temperatures of our grid cells averaged 6.5˚C ± 2.5 with mean annual precipitation of 35.5 cm ± 13.8. The seasonal overlap between temperature and precipitation was only 47 % ± 17 indicating the importance of cold season precipitation including snow (snow = 29 % ± 12 of annual precipitation). The upper soil layers are only intermittently wet during the summer months (46 ± 15 % of the days have wet soil versus 78 ± 9 % during the winter), whereas the deep layers provide moisture to the deep roots of sagebrush year-round. Our model results describe observed seasonal and spatial (soil depth) dynamics of important water balance variables across the geographic range of sagebrush steppe. Furthermore, these results provide biotic and biotic-abiotic interaction limits to climate envelope approaches. Integrating soil water dynamics into climate envelope approaches will be a powerful tool for understanding how sagebrush steppe, and other water-limited ecosystems, respond to changing climatic conditions.