Is kanuka and manuka establishment in grassland constrained by mycorrhizal abundance?

Two indigenous small tree and shrub species, kanuka (Kunzea ericoides) and manuka (Leptospermum scoparium), have potential as reforestation species in New Zealand as they are forest pioneer species that can invade grassland naturally from present seed sources. The aim of this study was to determine if establishment of kanuka and manuka from seed in grassland distant from stands of these species might be constrained by lack of appropriate mycorrhizal fungi. Both species were grown in an unsterilised grassland soil from a low-productivity montane site assumed to be devoid of appropriate mycorrhizal fungi and inoculated with sterilised or unsterilised O-horizon or mineral soil from beneath three kanuka and three manuka communities expected to contain such fungi. Inoculation with unsterilised O-horizon soil improved kanuka biomass by 36–92%, depending on the source of the inoculant. Inoculation did not improve manuka biomass. No ectomycorrhizal infection was observed on either kanuka or manuka in samples examined under binocular microscope. The biomass response by kanuka to inoculation may be due to introduction of more effective arbuscular mycorrhizal fungi from kanuka communities or possibly to the introduction of soil microorganisms. Testing of inoculation under field conditions will be essential to determine whether establishment of either species in grassland soil by seeding is seriously constrained by lack of appropriate mycorrhizal fungi or soil microorganisms.