Arbuscular mycorrhizal inoculum potential: a mechanism promoting positive diversity-invasibility relationships in mountain beech forests in New Zealand?

Mycorrhizal fungi are important symbionts for the majority of plant species, but their role in determining the susceptibility of habitat to plant invasion is poorly understood. Hieracium lepidulum is an arbuscular mycorrhizal herb, currently invading the understorey of ectomycorrhizal Nothofagus solandri var. cliffortioides (mountain beech) forest in New Zealand. Mountain beech is solely ectomycorrhizal, and other plant species within the understorey occur sporadically. Hieracium has been shown to establish preferentially in microsites with higher plant species richness at a scale of less than 1 m(2) within mountain beech forest, and we tested the hypothesis that more diverse microsites (<1 m(2)) are associated with higher levels of arbuscular mycorrhizal fungal (AMF) inoculum. We found low levels of AMF inoculum across all microsites, and over a third of samples contained no inoculum at all. Higher vascular-plant species richness (but not biomass) was associated with higher AMF spore densities in field soil, and greater AMF colonization of H. lepidulum seedlings in a bioassay. Absence of AMF inoculum from much of the soil and the positive association of inoculum potential with species richness provide a potential mechanism for the establishment of a positive diversity-invasibility relationship in the mountain beech forest.