The interplay of biotic and abiotic factors shapes tree root endophyte communities and seedling growth

Root endophytic microbes can alleviate plant abiotic stresses, thus potentially supporting adaptation to a changing climate during plant range shifts. While climate change is extending plant fundamental niches northward, the distribution and colonization of mutualists (e.g., arbuscular mycorrhizal fungi) and pathogens may constrain plant growth and regeneration. Yet, the degree to which biotic and abiotic factors impact plant performance and associated microbial communities at the edge of their distribution remains unclear. Here, we used root microscopy analysis, coupled with short marker sequencing, to study bacterial, fungal, and mycorrhizal root endophytic communities from sugar maple seedlings distributed across two temperate-to-boreal elevational gradients in southern Québec, Canada. Our findings demonstrate that soil pH, soil Ca, and distance to sugar maple trees are key drivers of root endophyte microbial communities, overshadowing the influence of elevation. Interestingly, changes in root fungal community composition mediated an indirect effect of soil pH on seedling growth, a pattern consistent at both sites. Overall, our findings highlight a complex role of biotic and abiotic factors in shaping tree-microbe interactions, which are in turn impacting seedling growth. These findings have important implications for tree range expansion in response to shifting climatic conditions.