Environmental stress strengthens plant-mycorrhizal associations: A novel extension of the stress gradient hypothesis.

Understanding how environmental stress alters the strength of local interactions is key to explaining diversity in current and future plant communities. Along gradients of increasing environmental stress, traditional theory posits that plants experience stronger facilitative interactions and weaker antagonistic interactions. However, it remains unclear whether this pattern extends to the relative host-specificity of plant-microbe interactions along stress gradients. Understanding these dynamics is particularly important for plant interactions with pathogenic and mycorrhizal fungi, which can drive opposing density-dependent processes that shape plant community compositions. We posit that increases in abiotic environmental stress are associated with stronger associations between plants and mutualists as plants increasingly rely on facilitative resource partnerships to cope with abiotic environmental stressors. We tested this prediction along an abiotic stress gradient in the central Cascade Range of Oregon, USA, using overlapping datasets of large high-resolution forest inventory plots, soil chemistry, and amplicon sequencing. Consistent with our predictions, in low-elevation forest stands with benign abiotic conditions and abundant nutrients, tree composition was more correlated with pathogenic fungal composition than ectomycorrhizal fungal composition. However, in forests at high elevations with limited nutrients and harsher climates, tree community composition was more correlated with ectomycorrhizal fungal composition than pathogenic fungal composition. Additionally, we find that spatial aggregation of ectomycorrhizal fungi increases as abiotic stress increases with elevation and opposing patterns of pathogen and ectomycorrhizal relative abundance in different substrate layers. Together, our findings suggest that facilitative interactions in stressful environments extend to mutualist-plant interactions and such interactions play a key role in shaping forest composition along environmental stress gradients.