Soil‐mediated effects of invasive ungulates on native tree seedlings

Invasive browsing ungulates can have strong impacts on the structure and composition of forest ecosystems, particularly where ungulates are not native ecosystem components as in New Zealand. Ungulate impacts on plant communities have been considered mostly from an above‐ground perspective. However, understanding below‐ground effects of these invasive herbivores is critical as they may drive feedbacks to above‐ground ecosystem components. We measured growth responses of seedlings of five common tree species in a greenhouse experiment in soils collected from 26 plots fenced to exclude invasive ungulates for at least 17 years and from paired, unfenced control plots. We then further investigated soil‐mediated effects of ungulates on one tree species, Melicytus ramiflorus, by partitioning these effects into soil abiotic and biotic components, as well as measuring arbuscular mycorrhizal fungal (AMF) root infection. Biomass of seedlings of all five species was greater in soils from within exclosures, although this was only significant for two species. These soil‐mediated effects were partially driven by changes in physical and chemical soil properties; soil bulk densities were lower inside exclosures than in controls. Effects of invasive ungulates on seedling biomass of M. ramiflorus were positively related to effects on per cent AMF root infection. The biomass of M. ramiflorus seedlings was positively related to the AMF infection of its roots, which in turn was related to greater organic matter content and lower bulk density of soils from within exclosures. Results for M. ramiflorus indicated that soil‐mediated effects of ungulates on seedling biomass were of abiotic origin, but were mediated by the biotic soil component, that is, through effects on AMF. Synthesis. Invasive herbivores may potentially impact on plant performance and community structure not only directly but also indirectly through influencing soil abiotic and biotic properties. Our results show that shifts in plant–soil interactions and feedbacks represent important but understudied pathways by which invasive ungulates can have wide‐ranging impacts on forest ecosystems. Future studies should consider the importance of soil‐mediated effects of invasive ungulates relative to direct effects of herbivory.