Tree‐fungal interactions across climatic gradients: What is the potential for tree niche expansion via varying fungal associations?

Tree species persist across wide climatic gradients, potentially facilitated by varying species interactions. We investigated how tree species' associations with fungal communities varied with climatic gradients across their range. We partnered with volunteers to sample tree roots across the eastern United States, with >1000 participants providing samples from 20 states. We characterized fungal communities via amplicon sequencing and tested how fungal guilds and individual taxa varied across climate gradients, as well as how fungal guilds and taxa correlated with seedling performance in a field experiment representing four climate conditions. Relative abundance of non‐mycorrhizal endophytes (NME) increased in hotter and drier locations, guild‐wide and for individual taxa. Relative abundance of ectomycorrhizal (EM) fungi declined in drier sites, while that of many individual EM fungi increased with colder temperatures. Seedling performance increased with greater relative abundance of arbuscular mycorrhizal (AM) fungi and intermediate abundance of NME in hot, dry conditions and individual NME taxa had on average positive correlations with seedling growth in these conditions. One of the NME, Cladosporium , emerged as a candidate taxon for niche expansion in Acer trees due to its increased abundance and positive association with seedling growth in hot, dry conditions in field conditions. Synthesis . Climatic niche breadth may reflect both intrinsic tree traits and varying relationships with microbes. NME represents an intriguing group whose impact on current and future tree ranges deserves further investigation.