Testing the co-invasion hypothesis: ectomycorrhizal fungal communities on Alnus glutinosa and Salix fragilis in New Zealand

It 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. Location: New Zealand. Methods: We 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. Results: Both 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 conclusions: Overall, 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