Phylogenetic Relatedness Influences Plant Interspecific Interactions Across Stress Levels in Coastal Ecosystems: a Meta-Analysis

Positive and negative interactions can occur simultaneously between plant species. According to the stress gradient hypothesis (SGH), species interactions shift towards more facilitative interactions or reductions in competition with increasing stress, whereas debate continues over whether evolutionary history influences the strength of species interactions. However, few studies have investigated the effects of phylogenetic relatedness (i.e., the sum of branch lengths separating species on a phylogeny) on the outcomes of interspecific interactions across stress levels. Therefore, we conducted a Bayesian meta-analysis on data collected from publications on plant interactions within coastal ecosystems in order to investigate the effects of phylogenetic relatedness on interspecific interactions across different stress levels. These analyses showed the effect sizes of species interactions on survival and growth to increase with stress increment, supporting the SGH in coastal ecosystems. However, phylogenetic relatedness did not lead to these differences of interspecific interactions between low and high stress. We found that species interactions affecting plant survival were not significantly influenced by phylogenetic relatedness; however, when evolutionary relationships of target and neighbor species were more phylogenetically distant, their interactions were more likely to facilitate growth of target species. Furthermore, the effect of the interaction between phylogenetic distance and stress on species interactions was negative. This suggests the observed net effects of phylogenetically distant neighbor species on target species were not due to true facilitation but reductions in competition when moving from low stress to high stress environments. According to these results, phylogenetic relatedness should be considered in choosing species for restoration of coastal ecosystem plant communities. Specifically, increasing the phylogenetic breadth of the assemblage is more likely to include species that have evolved to reduce stress on surrounding species through modification of the environment.