Establishment of a desert foundation species is limited by exotic plants and light but not herbivory or water

Questions The biodiversity of deserts is becoming increasingly threatened due to global change including the introduction of invasive species. Desert shrubs are foundational species that can facilitate native plant communities but can also benefit exotic species. The influence of exotic plants on the establishment of benefactors from seeds or seedlings is a critical knowledge gap. We tested if the establishment, survival, or growth of seedlings for a benefactor shrub species in California was reduced by the invasive grasses that they facilitate in the field. Location San Joaquin Desert, California, USA. Methods We conducted a field survey to determine if a native shrub,Ephedra californica, facilitated the invasive grassBromus madritensis. Using seed collected from the field, we conducted a competition experiment onEphedra californica, using a densities series ofBromus madritensisand under manipulated conditions of light, water, and simulated herbivory. We measured seedling establishment, survival, and biomass ofEphedra californicaandBromus madritensis. Results In the field,Ephedra californicafacilitatedBromus madritensiswithin the shrub canopy. In the competition experiment,Bromus madritensishad consistent negative effects onEphedra californicaemergence and seedling survival at all resource and herbivory levels. The emergence and survival ofEphedra californicawas reduced in low light, but none of the manipulated conditions increased the competitive effect ofBromus madritensis. Conclusions Reciprocal costs of facilitation by shrubs were evident in emergence and seedling survival but not in growth once established. Water, herbivory, and shade did not mitigate these costs, but also did not exacerbate competition from exotics. Direct competition with exotic plant species is the most significant impact tested here on dryland shrub species and manipulations of resources or herbivory may not effectively promote shrub recruitment. Native shrubs are well adapted to variable desert conditions and could be effective foundational species if invasive grasses are reduced.