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

Authors: SULLIVAN, RANAE - University Of Nevada; Newingham, Beth

Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/26/2025
Publication Date: 8/1/2025
Citation: Sullivan, R., Newingham, B.A. 2025. Shrub facilitation and drought drive divergent soil seed bank responses of native forbs and invasive grass in the Mojave desert. Journal of Arid Environments. Volume 231, 2025. https://doi.org/10.1016/j.jaridenv.2025.105449.
DOI: https://doi.org/10.1016/j.jaridenv.2025.105449

Interpretive Summary: Shifting percipitation patterns threaten ecosytems with increased drought periods. Seeds in Desert soils require rainfall at specific times for seeds to germinate, grow, and produce more seed. Native annual plant species can often wait years to germinate in order to survive until conditions are right for germination. Non-native annual species, like the invasive grass red brome, do not have the ability to delay germination and instead rely on rapid growth and high seed production to survive in years with little rain. Shrubs can aid seed germination of native and non-native species by sheltering seeds from harsh climate conditions, which increases plants underneath shrubs. However, it is unclear if drought can change the soil seed bank patterns under shrubs. We asked how shrubs and drought affect seed composition under shrubs and in open spaces in the Mojave Desert. We collected soil samples under shrubs and in open spaces at four sites in Gold Butte National Monument for three years, which included a drought year. We counted and identified seeds from all samples, and used the Palmer Drought Seversity Index (PDSI) to investigate how drought and shrubs affect seed composition in the soil. We found that red brome seed density decreased during drought, but was still greater under shrubs even in drought, meaning that shrubs aided seed density of red brome. Seed counts from native species generally remained stable or increased even in during drought, and the density of native seeds was not as affected by shrubs. This study shows that the role of microsites (under shrubs or in open spaces) as well as how specific species respond to drought are both important for shaping composition of seeds in soil and offers important insights into invasive species management.

Technical Abstract: In arid ecosystems, soil seed banks enable plant regeneration, serving as propagule reservoirs and relying on episodic rainfall for seed germination, plant growth, and succession. Changes in future precipitation threaten these ecosystems with shifting rainfall patterns and increased drought. Native annuals use bet-hedging strategies (e.g., delayed germination) to persist through unfavorable conditions. However, invasive species like Bromus rubens lack persistent seed banks, relying on rapid growth and high reproductive output. Shrubs facilitate native and invasive germination and growth by buffering against harsh climate conditions, increasing plant abundance under shrub canopies. However, it is unclear if drought alters these seed bank patterns. We investigated how shrub facilitation and drought affect seed banks in the Mojave Desert. We collected soil samples from beneath shrubs and interspaces at four sites in Gold Butte National Monument over three years, spanning a drought. 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 regardless of drought. Shrub facilitation effects on B. rubens seed density intensified with drought. Native forb density generally remained stable or increased despite drought, though their response to shrub facilitation was less pronounced and unaffected by drought. Species richness and diversity fluctuated, largely driven by B. rubens seed declines. This study emphasizes the role of microsite variability and species-specific responses in shaping seed bank dynamics, offering important insights for restoration efforts aimed at managing invasive species and conserving native biodiversity in arid regions during drought.