Host identity has been recognized as a key determinant of the structure of ectomycorrhizal (ECM) fungal communities, but the importance of neighboring ECM hosts is less well understood. To investigate the relative importance of host and neighborhood effects, we examined the ECM fungal communities associated with Alnus rhombifolia, a host of specific ECM fungi, and Betula occidentalis, a host of generalist ECM fungi. We hypothesized that the host-specific Alnus-associated ECM fungal community would not be susceptible to the influence of plant neighborhood, while the generalist Betula-associated community would. ECM fungal communities on both hosts were characterized using ITS sequences derived from conspecific and heterospecific host settings at a field site in western Idaho, USA, and from a growth chamber bioassay. In the field study, the Betula neighborhood added minor constituents to the Alnus ECM fungal community, while in the bioassay, late planting of Betula generated strong priority effects that allowed the established Alnus neighborhood to control the structure of the Betula community. Our results indicate that while host identity acts as a primary filter on the composition and diversity of ECM fungal communities, proximity to a closely related host can mediate significant changes in community structure.

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AimIt has been proposed that co‐invasion with ectomycorrhizal (EM) fungi is a common mechanism by which non‐indigenous trees overcome symbiont limitation, yet virtually all prior evidence has come from a single plant family, the Pinaceae. We tested the co‐invasion hypothesis by examining the EM fungal communities associated with a specialized host, Alnus glutinosa (black alder), and a generalist host, Salix fragilis (crack willow), in New Zealand, where both trees are invasive. We aimed to find out if these two hosts, which often co‐occur on invaded riverbanks, (1) were forming novel EM fungal associations in New Zealand and (2) had potential to facilitate each other through shared EM fungi.LocationNew Zealand.MethodsWe collected root tip samples from both host plants at riparian sites on the North Island and South Island and used DNA sequence‐based identification to characterize EM fungal communities.ResultsBoth trees relied upon exotic EM fungi from their indigenous ranges and did not associate with any known endemic New Zealand EM fungi. Alnus had highly similar communities on both islands, while the Salix communities were distinct. All EM fungi on South Island Alnus were also present on South Island Salix, while North Island Salix did not substantially share EM fungal associates with Alnus.Main conclusionsOverall, our study indicates that plant hosts with specialized and more generalist EM fungal communities can both successfully invade new habitats with non‐indigenous EM fungi. While there may be some potential for facilitation between these two EM plants hosts via shared non‐indigenous fungi, this outcome was context specific. Our findings suggest that the specificity of fungal mutualists is not a major barrier to the spread of invasive plants and cannot be taken as evidence an introduced plant will not become invasive.

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