Future shifts from snow to rain may decrease survival and population growth of two dominant grasses in sagebrush steppe

Background/Question/Methods Forecasting the ecological effects of climate change is an urgent challenge for conservation and management. Throughout the sagebrush steppe, the dominant form of precipitation is winter snow. However, climate change will bring warmer temperatures causing a greater proportion of winter precipitation to fall as rain. This shift in precipitation type will alter the timing and magnitude of soil water availability and change soil temperatures, increasing the risk of frost damage in years with no insulating snowpack. To examine the potential impact of a shift in precipitation type on a sagebrush steppe community, we used a unique historical dataset from the US Sheep Experiment Station in eastern Idaho comprising 22 years of demographic data to parameterize stochastic integral projection models (IPMs) for three dominant species, Pseudoroegneria spicata (bluebunch wheatgrass), Hesperostipa comata (needle-and-thread grass) and Artemisia tripartita (three-tipped sagebrush). We then used regression analysis to determine the effects of winter snow and other climate variables on inter-annual variability in survival, growth, recruitment, and population growth.

Results/Conclusions Elasticity analysis revealed that perturbations of survival and growth would have larger effects on the stochastic growth rate than perturbations of recruitment for all three species: percentage of the elasticity attributable to survival and growth for P. spicata was 87%, H. comata, 75%, and A. tripartita, 77%. Higher combined snowfall in February and March increased survival (P = 0.015, r² = 0.33) and growth (P = 0.036, r² = 0.26) for P. spicata, and increased survival for H. comata (P = 0.009, r² = 0.37). Higher total precipitation in the previous year decreased survival of A. tripartita (P = 0.025, r² = 0.26). Multiple linear regression analysis of the annual population growth rates for each species revealed that February and March snowfall was more important than precipitation for explaining inter-annual variation in population growth for both bunch grass species. Our research shows that likely future decreases in snowfall in sagebrush steppe communities have the potential to negatively impact the populations of the two dominant grasses that we studied. Decreases in population growth rate will likely lead to decreased cover of these species resulting in decreased forage availability for livestock and wildlife.