Barriers to seedling establishment in grasslands: Implications for Nothofagus forest restoration and migration

Tree seedling establishment outside forest boundaries is controlled by many interacting factors. Understanding the relative importance of different pressures is essential for improving techniques for forest restoration and for understanding the potential of forest boundaries to shift and adapt to changing climate conditions. We investigated constraints on the spread of a key southern hemisphere forest type into grasslands by undertaking a multifactorial experiment sowing Nothofagus cliffortioides (Nothofagaceae) in a historically deforested retired pasture. We manipulated factors to determine the relative effects of sheltered microsites, competition with pasture species, availability of ectomycorrhizal fungi, soil nutrients and rabbit herbivory, on seedling emergence and survival. Overall survival was low (11.7% after 6 months and 1.5% after 10 months), but there were strong treatment effects on both seedling establishment and survival. The availability of shelter and competition with pasture species had strongest effects, with the presence of pasture species initially aiding seedling emergence due to a sheltering effect, but negatively affecting survival at later stages. Most remaining seedlings at the conclusion of the experiment had formed ectomycorrhizae regardless of whether inoculum was supplied, and seedling survival and health was positively related to ectomycorrhizal colonization. Fertilizing had less of an effect than other factors, and results regarding rabbit herbivory were inconclusive. Synthesis and applications. This study provides new insights into how factors interact to limit tree seedling establishment in grasslands and forest expansion into neighbouring ecosystems. This improves understanding of the ability of an ectomycorrhizal keystone forest species to migrate and adapt to climate change. This study also assists restoration practitioners in selecting techniques that will enhance seedling establishment, and highlights that direct seeding approaches in open grasslands are unlikely to result in high rates of Nothofagus establishment.