Shrub facilitation and drought drive divergent soil seed bank responses of native forbs and invasive grass in the Mojave desert

Abstract

In arid ecosystems, soil seed banks support plant regeneration by storing propagules that respond to episodic rainfall. Shifting precipitation patterns and drought may alter seed banks, with cascading effects on plant communities. Native annuals often use bet-hedging (e.g., delayed germination) to cope with variable conditions, while invasives like Bromus rubens favor rapid growth and reproduction. Shrubs can mediate these responses by buffering microclimate and increasing plant abundance beneath their canopies. However, it remains unclear whether facilitative effects persist in the soil seed bank and how they are influenced by drought. Therefore, we asked: (1) Are seeds in the soil seed bank more abundant under shrub canopies compared to interspaces? (2) Are shrub facilitation effects on soil seed banks modified by drought? and (3) Do drought conditions drive abrupt changes in species specific seed density suggesting ecological thresholds? We sampled soil seed banks over three years, including during drought, under three shrub species and in open interspaces at four sites in Gold Butte National Monument (Nevada, USA). Soil seed bank density and composition were analyzed in relation to the Palmer Drought Severity Index (PDSI). Bromus rubens seed density declined during drought but remained higher under shrubs, where facilitative effects intensified. Native forbs showed stable or increasing seed density and weaker microsite differences. Species richness and diversity varied with B. rubens seed dynamics. This study emphasizes the role of microsite variability and species-specific responses in shaping soil seed banks, with important implications for restoration, invasive species management, and biodiversity conservation during drought.