Examining Mechanisms of Positive Plant Interactions in Deserts

Deserts are ecologically important ecosystems that contain high levels of endemism and that are sensitive to global change. Positive interactions among plants have been cited as factors that support desert biodiversity by buffering against climate variability. However, there is limited understanding on the underlying mechanisms that determine positive plant interactions. Herein, we proposed a conceptual framework that describes multiple mechanisms of facilitation among plants. We then empirically tested in multiple deserts of California the different facilitation mechanisms in the context of extreme climate events, multiple stressors, and spatial gradients. We also conducted species distribution modelling to assess the role of positive interactions in expanding the niche and geographic range of beneficiary species. We expanded upon the previous literature by describing six mechanisms of facilitation and two meta-mechanisms. We found in experimentation that shrubs can buffer against extreme drought using the described mechanisms, but that facilitation effects are strongest at intermediate or low levels of abiotic stress. The shrub species used was found to deter herbivory and ameliorate abiotic stress, but not increase soil moisture. We also found shrub facilitation to be species specific and typically increased the biomass of plant species with more competitive traits. Consequently, non-native species were found to be frequently facilitated and shrubs were observed to have lower species richness. Positive interactions were determined to increase the geographic range of annual plant species that have been previously reported as facilitated in the literature. We challenged previous research that suggests positive interactions increase linearly with abiotic stress and that facilitation can buffer against climate variability. Shrubs were determined to be significant foundation species in these desert ecosystems supporting annual productivity and the unique occurrence of annual species. However, these interactions are more sensitive to global change than previously thought and could collapse at environmental extremes. The proposed framework and experiments provides better understanding into the predictability of positive plant interactions and an opportunity for future applied research into the restoration and conservation of desert ecosystems.