Using soil moisture and seed germination timing to understand annual grass invasion post-fire and restoration implications

Abstract In semiarid ecosystems, precipitation and soil-water storage in shallow soils contribute to differences in seed germination of native perennial species and invasive annuals grasses. Invasive annual grasses contribute to wildfire, which displaces native vegetation and changes surface soils through post-fire erosion and loss of organic matter, decreasing surface soil moisture storage, impacting seed-based restoration, and the prevalence of invasive grasses. To understand how soils, precipitation, and fire contribute to surface soil moisture and seed germination, we used the Soil Heat and Water model (SHAW) and seed germination models to estimate shallow soil moisture and seed responses of three native species relative to Bromus tectorum. Across the semiarid regions of the Great Basin, USA, we found variation in sand content produced differences in surface soil moisture duration comparable to those generated by the wettest and driest years on record. The shallow soil environment and precipitation regimes impacted seeds differently, with greater reductions of germination probabilities in sandier soils and during years with lower-than-average spring precipitation. On average, germination probability decreased by 4% for every 10% increase in sand content. The negative effects of sand content on germination were reduced when soils had higher water content following a wetter-than-average winter. Using a literature review, we also estimated an average fire-effect on soil texture and bulk density. Based on the average fire-effects on soils, the modeled number of days of plant-available soil moisture decreased by four days, and germination probability decreased by 6% across species, soils, and precipitation regimes. Because resource availability, contributes to differences in post-wildfire recovery and annual grass invasion, soil plays a critical role during restoration. Understanding how soils and annual weather shape ecosystem resistance and resilience both pre- and post-wildfire will inform management practices, including invasive plant control and restoration.