Title

Slope and Aspect Effects on Seedbed Microclimate and Germination Timing of Fall-Planted Seeds

Document Type

Article

Publication Date

3-2021

Abstract

Rangeland vegetation in the Great Basin, United States, is frequently disturbed by natural- and human-caused wildfires that facilitate the establishment and dominance of introduced annual weeds such as cheatgrass (Bromus tectorum) and medusahead wildrye (Taeniatherum caput-medusae [L.] Nevski). Ecological resilience and resistance 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 might also be correlated with these perceived spatial patterns of resistance and resilience over space. We used a 39-yr 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 for the very-fast-germinating cheatgrass, fast-germinating bottlebrush squirreltail (Elymus elymoides [Raf] Swezey), and slower-germinating Idaho fescue (Festuca idahoensis Elmer). Topographic mapping of seedbed favorability showed distinct seasonal patterns associated with both slope and aspect. Southern exposures are likely to facilitate both prewinter germination for early-fall-planted seeds and relatively more midwinter germination for seeds planted later in the fall, but these exposures are also less subject to midwinter frost effects. Northern exposures were likely to delay germination into later winter and early spring and thus avoid potential exposure to a higher probability of winter frost mortality. Microclimatic simulations of this type may provide new metrics for improving our understanding of the mechanistic causes of observed patterns of ecological resistance and resilience over space.

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