Ziffer-Berger, J., Herbarium of the Hebrew University, Giv'at Ram, Jerusalem, Israel
Weisberg, P.J., Dept. of Natural Resources and Environmental Science, University of Nevada, Reno, NV, United States
Cablk, M.E., Division of Earth and Ecosystem Sciences, Desert Research Institute, Reno, NV, United States
Osem, Y., Institute of Plant Sciences, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel

We examined variations in the relative importance of facilitation vs. competition, in light of the Stress-Gradient Hypothesis (SGH) by assessing plant interactions along an aridity gradient over biogeographic scales. We surveyed the relationship between a shrub species (. Artemisia tridentata) and pine seedlings (. Pinus monophylla) across the Great Basin and the Mojave Desert, USA, encompassing the entire range of P.monophylla. Using 69 sites we evaluated the spatial association between P.monophylla seedlings and A.tridentata shrubs, quantified with an electivity index, and implemented multiple regression analysis on the effects of macro- and micro-environmental factors: precipitation, temperature, monsoonality index, topography, substrate and litter cover. We identified annual precipitation as a main factor, which was negatively related to shrub-seedling association. Additionally, shrub-seedling association was stronger in the hot- than in the cold-desert, and was negatively related to litter cover. Effects of monsoonality, summer temperature, and bedrock type were not significant. We also considered nonlinear functional forms of a precipitation-electivity relationship, but the negative linear model proved most predictive. Our observations match SGH predictions. Studying the role of interspecific interactions in shaping species range shifts may lead to improved predictions of distribution ranges and changes in dryland vegetation under global change scenarios. © 2013 Elsevier Ltd.