Elevation and Aspect Effects on Soil Microclimate and the Germination Timing of Fall-Planted Seeds

Invasive annual grasses now dominate millions of hectares of rangeland in the Intermountain Western United States. Local annual grass distribution, however, has been shown to follow landscape patterns of slope, aspect, and elevation that are correlated with ecological resilience to stress and disturbance and resistance to annual grass invasion. Although these patterns have previously been linked to soil-climate classes, several mechanistic factors in native-plant seedling establishment are also associated with both topography and seasonal weather patterns in the year following planting. In this study we used the Simultaneous Heat and Water (SHAW) model to estimate long-term weather effects on soil microclimate and hydrothermal-germination models to predict germination response of one fast- and one slow-germinating native grass as a function of planting date, slope, aspect, and elevation in the Boise Foothills in southwestern Idaho. Higher elevation and northerly aspect sites are more likely to defer germination of seeded species until late enough in the fall that they avoid postgermination/preemergence freezing mortality. These sites are also more favorable for survival of emerged seedlings through mid to late summer. Slope, aspect, and elevation effects on modeled restoration outcomes are consistent with previously modeled general patterns of ecological resilience and resistance as a function of soil hydrothermal class, but inclusion of slope and aspect effects may produce finer-scale metrics for mapping these patterns over space. The probabilistic nature of microclimatic variability as a function of elevation may yield useful insights into successful restoration approaches for reestablishment of native plant communities in lower-elevation ecosystems with inherently lower ecological resilience and resistance. The generally arid climate in this region, however, may limit successful restoration outcomes at lower elevation in most years even under conditions of long-term annual grass control.