Host plant phylogeny and abundance predict root-associated fungal community composition and diversity of mutualists and pathogens

Interactions between plants and their root-associated fungi (RAF) may influence the relative abundance of tree species and determine forest community diversity. Such plant-soil feedbacks in turn depend on the degree to which spatial distance and phylogenetic relatedness of host trees structure pathogen and mutualist communities, but research detailing these aspects of RAF communities is lacking. Here, we characterize plant-RAF associations across a diverse plant community, focusing on the degree to which RAF communities are structured by spatial distance, host phylogenetic relatedness, and host abundance. We compare results for different functional groups, including both putative mutualists and pathogens, an aspect poorly examined hitherto. We collected roots at regular intervals along ten 50 m by 2 m transects, then used DNA barcoding to identify host plants, and characterize the associated fungal community. Variance partitioning was used to measure the relative contributions of host phylogenetic relatedness and spatial distance to explaining RAF community composition. A weighted linear regression was used to measure the correlation between host abundance and RAF diversity. Phylogenetic distance among hosts was a better predictor of RAF community composition than spatial distance, but this relationship was stronger for putative pathogens than for mutualists, suggesting that pathogens show stronger host preference than mutualists. Across all functional groups, RAF showed similar levels of spatial structure. Additionally, RAF communities of locally abundant plants were less diverse than RAF communities of rare plants. Synthesis. We found that RAF communities are structured by the phylogenetic relatedness of hosts and, to a lesser extent, by spatial distance, with pathogens showing stronger host preference than mutualists. Abundant hosts had less diverse RAF communities than rare hosts, which is notable because abundant plants tend to experience weaker negative plant-soil feedback. Going forward, mechanisms underlying the host abundance-RAF diversity relationship warrant further investigation. Additionally, the survey approach presented here could be paired with experiments linking RAF community composition to plant recruitment.