Background/Question/Methods Plants rely on microbial associations to mediate their interactions with their abiotic environment, for instance for nutrient and water acquisition from soil. Soil microbial communities assemble in response to local conditions, and over many generations plant and microbial populations can adapt to each other. However, climate change leads to the redistribution of species, both plant and microbial, across landscapes, resulting in novel interactions occurring in novel conditions. The consequences of these novel interactions for the establishment and growth of tree species, and thus the structure and function of future forests, are unclear and inherently complex. Nevertheless, understanding the potential consequences may be necessary to accurately predict and manage future forests. To explore the potential consequences of shifting microbial associations on tree seedling establishment in current and novel conditions, we performed paired multi-year, multi-species field and greenhouse experiments. In the field, we pre-inoculated seedlings of 12 tree species with microbial communities sourced from six locations, and tested the seedling survival in ambient and rainfall reduced conditions. This was paired with a greenhouse experiment to isolate microbial effects. The experiment was conducted in two locations – northern Wisconsin, near or beyond the northern range of our seedling species, and in central Illinois, in the center of the range for some species and on the southern range edge for others. Results/Conclusions Whether novel microbial communities increased or decreased seedling survival and growth depended on the tree species and conditions. In our northern site, which was climatically less stressful during the growing season, novel microbial communities were never better than the local community, and often worse for seedlings. In our southern site, especially in rainfall reduced conditions, seedling survival was enhanced when pre-inoculated with microbial communities sourced from more arid sites. These results were more pronounced for seedling species that were near their southern range edge. We found similar results in our greenhouse experiments, further supporting a role for microbial community composition in shaping tree drought tolerance. These results suggest that disruption of historical microbial relationships may inhibit tree range expansion into newly amenable areas, perhaps due to a mismatch between novel microbes and abiotic soil conditions. However, migration of microbial populations from hotter and drier sites may help tree populations on trailing range edges gain enhanced tolerance to novel climate conditions.
