Enhanced facilitation at the extreme end of the aridity gradient in the Atacama Desert: a community-level approach

Plant facilitation is now recognized as an important process in severe environments. However, there is still no agreement on how facilitation changes as conditions become increasingly severe. The classic stress gradient hypothesis (SGH) predicts a monotonic increase in facilitation, which rises in frequency as conditions approach the extreme end of the environmental gradient. However, few studies have evaluated the validity of the SGH at the community level, the level at which it was formulated. Moreover, few studies have tested the SGH at either extreme of the gradient, and very few have excluded the effect of livestock on community response to stress. In line with the SGH, we hypothesized that several spatial pattern summary statistics would change monotonically from the least to the most arid sites, indicating increasingly aggregated patterns. In this study, we performed an evaluation of the SGH both within communities of shrub species and across a large portion of the Atacama Desert, and we isolated the abiotic component of the SGH. Our environmental gradient covered an extreme aridity gradient (