Aeolian erosion increases drought susceptibility of grass seedlings relative to that of shrub invaders in a Chihuahuan Desert grassland

Background and aims: Shrub encroachment is a widespread ecological state change in global arid grasslands, often driven by changes in disturbance regimes (e.g., livestock grazing). While various direct and indirect effects of grazing may promote grassland-to-shrubland transitions, the relative importance of these mechanisms remains unclear. A well-documented consequence of grazing and other disturbances is aeolian erosion, which reduces soil water and nutrient availability. We conducted a greenhouse experiment to ascertain whether such changes might favor shrubs establishment over that of grasses under drought conditions, potentially promoting shrub encroachment. Methods: We quantified seedling establishment and growth of an aggressive grassland invader (Prosopis glandulosa) to that of two grass species (Bouteloua eriopoda and Sporobolus airoides) on wind-eroded (winnowed) and intact (non-winnowed) soils under contrasting watering regimes: control (FC, field capacity), medium drought (MD, 80% of FC), and severe drought (SD, 50% of FC). Results: The combination of soil winnowing and drought negatively impacted growth of grass seedlings more than that of shrub seedlings. In well-watered (FC) conditions, seedling biomass was comparable (p > 0.05) between soil types for all species. Under MD and SD, above- and belowground biomass declined, with the greatest reductions observed for seedlings on winnowed soils, particularly for the two grass species. Additionally, grass leaf production was reduced on winnowed soils, while that of the P. glandulosa shrub seedlings was unaffected. Conclusion: These disparities in drought susceptibility on wind-eroded soils should give shrub seedlings a survival advantage in drought periods, thereby setting the stage for grassland-to-shrubland transitions.