Testing the co‐invasion hypothesis: ectomycorrhizal fungal communities onAlnus glutinosaandSalix fragilisin New Zealand

Abstract

AbstractAimIt 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 theEMfungal 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 novelEMfungal associations in New Zealand and (2) had potential to facilitate each other through sharedEMfungi.LocationNew Zealand.MethodsWe collected root tip samples from both host plants at riparian sites on the North Island and South Island and usedDNAsequence‐based identification to characterizeEMfungal communities.ResultsBoth trees relied upon exoticEMfungi from their indigenous ranges and did not associate with any known endemic New ZealandEMfungi.Alnushad highly similar communities on both islands, while theSalixcommunities were distinct. AllEMfungi on South IslandAlnuswere also present on South IslandSalix, while North IslandSalixdid not substantially shareEMfungal associates withAlnus.Main conclusionsOverall, our study indicates that plant hosts with specialized and more generalistEMfungal communities can both successfully invade new habitats with non‐indigenousEMfungi. While there may be some potential for facilitation between these twoEMplants 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.