Does escaping enemies mean forsaking friends? Mutualistic specificity and co-invasion of ectomycorrhizal fungi with Alnus glutinosa and Salix fragilis in New Zealand

Background/Question/Methods

“Enemy escape” is often cited as a key biotic contributor to the invasion success of non-indigenous species, but the presence or absence of specialized mutualists may also play a significant role in determining invasional outcomes. This study examines how mutualistic specificity is influencing the invasion of two European ectomycorrhizal (EM) angiosperms in New Zealand: Alnus glutinosa (black alder) and Salix fragilis (crack willow). In their native ranges, Alnus species associate with a small, host-specific set of EM fungi, while Salix species usually associate with a broader group of generalist EM fungi. As such, we hypothesized that A. glutinosa would retain a limited, specific EM fungal community as it invaded, while S. fragilis would associate with a broader subset of its usual EM fungi, perhaps acquiring some native New Zealand associates. To test our hypothesis, we performed a field study at two invaded riverbeds in New Zealand, using molecular techniques to identify both EM fungi (species-level taxa) and plant roots.

Results/Conclusions

Alnus glutinosa and Salix fragilis were both well colonized by EM fungi in New Zealand, but associated with distinctive EM fungal communities at both field sites. The A. glutinosa-associated community was taxon poor (6-7 EM fungal taxa per site) and was nearly identical at both locations, while the S. fragilis community was more taxon rich (19-25 EM fungal taxa) and differed substantially from site to site. Both of these communities had lower taxon richness than they typically would have in the native ranges of the host plants. A phylogenetic analysis of the best-represented EM fungal family (Thelephoraceae) in the data set suggested that neither A. glutinosa nor S. fragilis has acquired native New Zealand EM fungi in that family, even though the family is abundantly associated with native New Zealand trees (genus Nothofagus). Taken together, our data indicate that, despite having left many of their mutualists behind in their native European ranges, A. glutinosa and S. fragilis are relying primarily on EM fungal partners from those areas, and are not acquiring new associates as they invade New Zealand.