The stress gradient hypothesis explains plant-plant interaction networks in edapho climatic gradients

The stress-gradient hypothesis (SGH) predicts a shift from facilitation to competition as the environment turns from severe to favorable. Different authors have reported conflicting results when testing the SGH, mainly concerning the role of facilitation in stressful environments. We aimed to infer the mechanisms best explaining the variation of interspecific interactions across gradients through the joint analysis of network metrics and abiotic variables. We analyzed six plant-plant interaction networks (size, number of links, connectance, average degree, nestedness, and modularity) in edapho-climatic gradients in northeastern Brazil. Nestedness occurs when many species connect both with each other and with a set of other species with few connections, indicating facilitation. Modularity occurs when the network is split into groups of species linking to each other only within the groups, indicating competition. We used correlation, multiple linear regressions, and a generalized linear model to understand how the abiotic variables could drive network alterations and if the abundance of a possible benefactor species (Fabaceae trees) could increase facilitation. All the climate, soil, and network variables varied widely across the sites. All the sites had aluminium-toxic, nutrient-poor and nitrogen-limited soils, and Holdridge's life zones varied from very dry to moist tropical forests. Climate, soil, and network variables were correlated to each other, but the soil was the main driver of variation in the network metrics. Nestedness and modularity were negatively correlated, and both were correlated with the number of Fabaceae trees in each community. Deciduous species in very dry and dry forests require more nitrogen than evergreen species in dry/moist and moist forests, so the soil was more limiting to the deciduous species, which associated with Fabaceae trees and increased network nestedness. Our data corroborated the SGH, which, in this case, was mediated by the soil nutrient status and had Fabaceae as the benefactor species.