Topographic and Host Effects on Arbuscular Mycorrhizal and Ectomycorrhizal Fungal Communities in a Forested Watershed

In a forested watershed, identity of tree species and topographical position could be important driving factors shaping mycorrhizal fungal communities. Here we aimed to disentangle the contributions of these two factors to mycorrhizal fungal community structure. We collected tree roots colonized by either arbuscular mycorrhizal (AM) or ectomycorrhizal (EM) fungi in a small, temperate, forested watershed of the Susquehanna-Shale Hills Critical Zone Observatory. Relative abundances of fungal OTUs were assessed using high-throughput DNA sequencing. The structures of fungal communities, both AM and EM, were compared between different host species at the same slope position, and within the same host species at different slope positions that vary in soil moisture, nutrient content and belowground biomass. We found that structures of AM fungal communities were significantly affected by host species but not by slope position. Although the structures of EM fungal communities were not significantly affected by either host identity or slope position, there were three core EM fungal OTUs (occurrence ≥ 50%) for which their relative abundances were significantly affected by slope position and three for which their relative abundances were significantly affected by host species. In our system, the effects of host identity and slope position were only moderately strong and varied between mycorrhizal types. Our findings provide guidance to those attempting to link the fine-scale distribution of mycorrhizal fungi and mycorrhizal-mediated ecosystem functions to both host species and topographic position.