Slope and aspect effects on seedbed microclimate and germination timing of fall-planted seeds

Authors: Boehm, Alex; Hardegree, Stuart; GLENN, NANCY - Boise State University; Reeves, Patrick; Moffet, Corey; Flerchinger, Gerald

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2020
Publication Date: 3/1/2021
Publication URL: https://handle.nal.usda.gov/10113/7239085
Citation: Boehm, A.R., Hardegree, S.P., Glenn, N., Reeves, P.A., Moffet, C., Flerchinger, G.N. 2021. Slope and aspect effects on seedbed microclimate and germination timing of fall-planted seeds. Rangeland Ecology and Management. 75:58-67. https://doi.org/10.1016/j.rama.2020.12.003.
DOI: https://doi.org/10.1016/j.rama.2020.12.003

Interpretive Summary: Millions of hectares of rangeland in the Intermountain western US have been degraded by the proliferation of introduced annual weeds after wildfire. Restoration outcomes in this region are often unsuccessful, particularly in lower-elevation warm and dry Wyoming big sagebrush habitat. Restoration priorities are often assigned to higher elevation zones with higher ecological resilience and resistance to weed invasion. In this study, we examined annual variability in topographic effects on seedling establishment and determined that seedbed temperature and water availability in the year after disturbance may play a role in determining long-term ecological resilience and resistance. This information may be useful in developing weather-centric rangeland restoration plans that improve the economic efficiency of lower-elevation restoration projects and increasing the landscape footprint of successful restoration outcomes in this arid region of the western US. 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: Rangeland vegetation in the Great Basin, USA, is frequently disturbed by natural and human caused wildfires that facilitate the establishment and dominance of near-monocultures of introduced annual weeds such as cheatgrass (Bromus tectorum) and medusahead wildrye [Taeniatherum caput-medusae (L.) Nevski]. Ecological resilience and resistance to disturbance of native and seeded-non-native plant communities in this region, however, appear to follow topographic patterns associated with slope, aspect and elevation. Currently, resistance and resilience concepts are being used to prioritize rangeland restoration efforts based on soil-climate classification. We hypothesized that probabilistic patterns of shorter-term weather effects on seedbed microclimate and seedling establishment processes might also contribute to the perceived pattern of resistance and resilience as a function of topography. We used a 38-year gridded weather dataset to estimate seedbed temperature and water potential at seeding depth as a function of slope and aspect using the Simultaneous Heat and Water (SHAW) model. Seedbed temperature and water potential were then used as input to hydrothermal germination response models to generate indices of seedbed favorability for initial germination and emergence, and to estimate cumulative germination response as a function of topography and planting date. We used these simulations as a bioassay to map spatial and temporal variability in seedbed microclimate as a function of slope and aspect, and to assess potential planting date effects on germination and emergence as a function of topographic position. Topographic mapping of seedbed favorability showed distinct patterns associated with both slope and aspect and may provide additional metrics for estimating a seedling-establishment component of ecological resilience and resistance over space.