Novel fungal partnerships highlight host flexibility in the New Zealand Douglas-fir invasion

Background/Question/Methods

Mutualisms between trees and ectomycorrhizal fungi influence primary production and nutrient recycling. The maintenance of these plant-fungal partnerships depends on a suite of recognition and regulation systems developed and maintained over evolutionary time. Intriguingly, these systems still allow for partner diversity: For example, individual trees may associate with dozens of species of fungi. The extent to which this diversity represents niche partitioning or imprecise regulation is unclear. The relative importance of these regulatory limitations can be assessed by exposing host trees to non-coevolved fungi and measuring the prevalence and growth effects of novel associations. Here, we quantified the ectomycorrhizal community associated with Douglas-fir (Pseudotsuga menziesii) in New Zealand, where Douglas-fir is an invasive non-native species exposed to a suite of fungal species both endemic to New Zealand and introduced from elsewhere, including the tree's native North American range. We examined the mycorrhizal communities on wilding seedlings and exposed uncolonized seedlings to fungal propagules in soils collected from three vegetation types – native forest, fir plantation, and grassland – to measure novel and native interactions, and understand the role of canopy type in modulating these symbioses.

Results/Conclusions

Naturalised Douglas-fir seedlings in New Zealand associated with more than 150 ectomycorrhizal species from all over the world. The most dominant members of the community were species of the genera Rhizopogon and Suillus from the tree's native range. However, native New Zealand fungi, especially from the genus Cortinarius, made up a significant proportion of the fungal community, colonizing up to 20% of seedling root tips. The presence of adult ectomycorrhizal host trees enhanced total fungal diversity. In particular, the New Zealand native southern beech Nothofagus solandri is likely required for successful colonization by native fungi. The greenhouse bioassay confirmed this result: colonization of seedlings planted in intact soil cores taken from native forests was relatively low. By contrast, seedlings planted in soils from Douglas-fir plantations and grasslands were readily colonized, typically by non-native fungi. Our results suggest that, while Douglas-fir associates with a high diversity of fungi and forms novel associations with endemic fungi, a few key (and equally invasive) mycorrhizae may be most important to the Douglas-fir's future spread and invasion potential.