Trends in soil microclimate and modeled impacts on germination timing in the sagebrush steppe

Authors: TERRY, TYSON - Brigham Young University; Hardegree, Stuart; MADSEN, MATTHEW - Brigham Young University; ROUNDY, BRUCE - Brigham Young University; ST. CLAIR, S - Brigham Young University

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2022
Publication Date: 9/6/2022
Citation: Terry, T., Hardegree, S.P., Madsen, M., Roundy, B., St. Clair, S. 2022. Trends in soil microclimate and modeled impacts on germination timing in the sagebrush steppe. Ecosphere. 13(9). Article e4226. https://doi.org/10.1002/ecs2.4226.
DOI: https://doi.org/10.1002/ecs2.4226

Interpretive Summary: Annual grass invasion is the major cause of ecological degradation in the Great Basin sagebrush steppe. Restoration and rehabilitation of these rangelands by seeding after wildfire is difficult due to the low probability of favorable weather for seedling establishment in any given year. A shifting climate could exacerbate this problem, and make restoration of these rangelands even more difficult. We simulated historical seedbed conditions of soil temperature and moisture at multiple sites across the Intermountain western US to determine whether there were detectable trends in soil favorability for seed germination over the last 37 years. Since 1979, seedbed favorability has increased in the fall, when seeds are usually planted, and soil climate in the summer has become more harsh. Both of these may result in decreased seedling establishment as more seeds may germinate in the fall and become killed by freezing conditions in the winter. Seedlings will also have more difficulty surviving the first summer dry season in this region. Premature germination may be mitigated by later-fall planting, but these trends may ultimately result in a shift in habitat and alteration of plant materials suitability, and may reinforce the dominance of invasive annual weeds, in these systems.

Technical Abstract: Past and projected changes in climate alter soil microclimate, and as a result impact seeds that use soil moisture and soil temperature as cues for germination. Seed germination and establishment are among the most important processes regulating plant community assembly in response to climate change. Changes in germination timing lead to seedling exposure to new abiotic and biotic factors that affect recruitment success. Species that have developed seed dormancy strategies to avoid harsh winter conditions could experience more mortality if changes in germination timing lead to early germination and subsequent exposure to freezing conditions. We analyzed the changes and trends of soil microclimate in the sagebrush steppe during the period 1979-2016 using a soil physics model (SHAW) and calculated germination timing of native species using wet thermal response curves. Our results indicate that warmer and wetter falls are becoming more prevalent and are leading to accelerated germination of native plants in the sagebrush steppe. Changes to spring soil conditions favor slightly more germination in early spring, but strong trends of earlier onset of spring soil conditions were not evident. Trends in climate are creating drier and warmer seedbed conditions in the summer, which will likely increase seedling mortality. Historic cycles of alternating slow or fast germination appear less frequently during the last 12 years of the study (2004-2016). Seedbed microenvironment trends have impacted seeds by altering germination timing, exposing seeds to harsher abiotic conditions, and increasing competition from invasive annual grasses. Changing temperature and precipitation timing has likely altered recruitment success of native plants and may drive changes in species composition in arid systems.