Species specificity challenges the predictability of facilitation along a regional desert gradient

Questions Deserts ecosystems are threatened by shifts in precipitation patterns from climate change. Positive interactions among plants could buffer desert communities from environmental extremes and resource limitations. However, to improve our ability to predict the response of plant interactions and the plant community to environmental change, we must examine the role of species identity in facilitation. Here, we asked: how do species identity, soil nutrients, and aridity drive positive interactions among plants along a regional gradient of semi-arid to hyper-arid. Location California, USA. Methods We selected seven sites located across three deserts in California that cover the geographic range of the benefactor shrubEphedra californica. In two growing seasons and within 30 pairs of shrub-open microsites at each site, we planted seeds of three annual phytometer species selected for their affinity to areas that are semi-arid, arid, or both. In each microsite, we also surveyed the composition of the annual community and measured soil nutrients. Results Shrubs facilitated the semi-arid phytometer species, reduced the hyper-arid phytometer species, and had no effect on the species found throughout the arid gradient. Shrub facilitation on community-level biomass of annual species decreased linearly with aridity to neutral interactions at the most arid sites. In the semi-arid sites, shrubs negatively affected community-level species richness and native species abundance, but increased invasive grasses. Conclusions Idiosyncratic responses of annual plant species to shrub facilitation along an aridity gradient highlight the issue in generalizing plant interactions without considering species-specific differences. Additionally, the effects of shrub facilitation appear to benefit exotic species with competitive traits which can indirectly reduce native species richness in semi-arid ecosystems. Understanding positive interactions in the context of aridity gradients can provide better insights into the expected changes in species composition that will occur as a result of climate change.