Future regeneration of big sagebrush under climate variability

Background/Question/Methods Big sagebrush dominates much of the semiarid U.S. intermountain region; however, several studies predict important changes in sagebrush geographic distribution under future conditions. Maintenance and expansion of sagebrush distribution requires successful migration, recruitment, and competition with other plants. While each of these steps can regulate big sagebrush, recruitment may be especially important for understanding future distribution because recruitment is episodic and thought to be under climatic control, mostly limited by dry summer soils, but also by cold spells. We simulated germination and first-year seedling survival to understand how changing climate means and climate variability may influence recruitment patterns in time and across big sagebrush geographic distribution. We utilized a daily time step, multiple soil layer process-based ecosystem water balance model. We simulated soil water at each of 49 random sites in the U.S. intermountain region forced by 500 years of generated daily weather for all combinations of four treatments: climate (current, future A2 and future B1), continuous duration of dry days (ambient, 50% and 200%), duration of dry or wet periods (in years; ambient and doubled mean length), and severity of dry or wet years (ambient and 200% mean deviation from normal). We calculated percentage of years supporting potential germination and seedling survival based on published relationships with temperature and soil water.

Results/Conclusions Simulation results indicated strong dependence of percentage of suitable years for germination and seedling survival on geographic location and total precipitation, particularly for the colder season. The simulations predicted under current conditions among sites an average of 72%±26SD of years suitable for germination and 34%±32 of years suitable for seedling survival. Among treatment factors, germination and seedling survival were mostly influenced by duration of wet and dry spells, followed, only for seedling survival, by climate scenarios. Magnitude and direction of changes in percentage of suitable years depended on geographic location. Under future climates, seedling survival showed an increase in correlation with summer precipitation. We found no evidence of interactions among the four treatments. Our results suggest the importance of climate variability for regeneration processes in semiarid big sagebrush shrublands and emphasize that responses are dependent on overall site conditions and can vary strongly with geographic location. Thus, potential changes in big sagebrush regeneration could be strongly related to variability and timing of future local intra- and interannual precipitation patterns which are still poorly understood.