Pathogens and herbivores exact density-dependent mortality and provide an away-site advantage, contributing to diversity maintenance in plant communities across the globe. These biotic factors are especially important in tropical forests where abiotic niches are insufficient by themselves in explaining coexistence at local scales.
Below-ground biotic interactions, particularly plant-mycorrhizal associations have received little attention in tropical ecosystems. Of particular interest are forests where a majority of the trees associate with ectomycorrhizae (EM), a highly beneficial form of mutualistic fungi. In most forests with EM dominants, the host-trees exclude all competitors and reduce local diversity, with one notable exception: lowland mixed-dipterocarp forests of Asia. Here, Dipterocarpaceae species (all EM-host trees) dominate as a family, but also co-occur with other non-EM dominants such that local diversity is maintained. Why this is so, is not known.
We hypothesize that in these forests, pathogens do maintain diversity at early life stages (seedlings) but larger individuals (saplings and larger) tap into below-ground hyphal networks and experience higher survival. Additionally, habitat- and not host-specific ectomycorrhizae explain the co-occurrence of several dipterocarp species within the same habitat. We wish to test these questions in the mixed-dipterocarp forests of Sri Lanka, using experiments in the shadehouse and in the field, as well as observational studies of seedling survival, growth and recruitment through time.