Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/22/2022
Publication Date: 9/16/2022
Citation: Hardegree, S.P., Boehm, A.R., Glenn, N., Sheley, R.L., Reeves, P.A., Pastick, N., Hojjati, A., Boyte, S., Enterkine, J., Moffet, C., Flerchinger, G.N. 2022. Elevation and aspect effects on soil microclimate and the germination timing of fall-planted seeds. Rangeland Ecology and Management. 85:15-27. https://doi.org/10.1016/j.rama.2022.08.003.
DOI: https://doi.org/10.1016/j.rama.2022.08.003

Interpretive Summary: Millions of hectares of sagebrush/bunchgrass rangeland in the Intermountain Western US have transitioned to annual weed dominance after repeated wildfire. These weeds, however, appear to be constrained by elevation and aspect effects on ecological resilience and resistance to weed invasion. Our inability to develop a mechanistic understanding of these effects has limited positive restoration outcomes on these rangelands, particularly on drier, lower elevation sites. In this study, we examined annual variability in elevation and slope effects on seedling establishment and identified topographic patterns of seedbed favorability and potential post-germination freezing mortality that may play a role in determining long-term ecological resilience and resistance. This information may be used in the development of rangeland restoration strategies that would have a higher probability of initial seeding success. Improved restoration outcomes in this region will benefit rangeland livestock and wildlife, reduce fire frequencies, and protect soils that are vulnerable to wind and water erosion when plant cover is reduced by wildfire or other disturbance.

Technical Abstract: Invasive annual grasses now dominate millions of hectares of rangeland in the Intermountain Western US that require restoration. Local annual grass distribution, however, has been shown to follow landscape patterns of slope, aspect and elevation that are correlated with ecological resilience and resistance to annual grass invasion. Although these patterns have previously been linked to soil-climate classes, there are several mechanistic factors in early seedling establishment that are also associated with both topography and seasonal weather patterns in the year following planting. In this study we modeled long-term weather effects on soil microclimate and predicted germination response as a function of planting date, slope, aspect and elevation in the Boise Foothills in southwestern Idaho, USA. Higher elevation and northerly aspect sites are more likely to defer germination of seeded species until late enough in the fall that they avoid post-germination/pre-emergence 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 conceptual patterns of ecological resilience and resistance but seedbed modeling can produce both quantitative and finer-scale metrics for mapping these patterns over space. The probabilistic nature of these metrics also may yield useful insights into successful restoration approaches for re-establishment of native plant communities in lower-elevation ecosystems with inherently lower ecological resilience and resistance. Weather limitations, however, may limit successful restoration outcomes at lower elevation even under conditions of long-term, persistent, annual grass control.