Neighbourhood Effects on Tree Seedlings Differ by Mycorrhizal Symbiont Type in a Mixed Dipterocarp Forest

Aims Mycorrhizal symbiont type is an important trait that differentiates tree species' responses to their neighbours. In temperate forests, tree species associated with arbuscular mycorrhizal ( AM ) fungi show reduced performance in conspecific relative to heterospecific neighbourhoods, while species associated with ectomycorrhizal ( EcM ) fungi suffer weaker negative effects from conspecific neighbours. However, our understanding of such mycorrhizae‐mediated neighbourhood effects in tropical forests is limited. We address this knowledge gap through a shade house plant–soil feedback experiment and an observational field study of seedling dynamics in a mixed dipterocarp ( EcM host trees of family Dipterocarpaceae) forest. Location Sinharaja, Sri Lanka. Methods In the shade house, we transplanted 478 seedlings of three EcM host and two AM host species into conspecific‐ and heterospecific‐trained soils and monitored their total biomass and survival after ~1.5 years. In the field, we tagged, identified and measured seedlings in 576 seedling plots established within known adult neighbourhoods, and monitored growth and survival of eight EcM and 14 AM host taxa beneath conspecific and heterospecific tree crowns over three annual surveys. Results We found that the response of seedling growth and survival to neighbourhood (conspecific or heterospecific) depended on mycorrhizal symbiont type ( AM or EcM ) in the field, but not in the shade house. AM host seedlings had lower growth and survival in conspecific versus heterospecific neighbourhoods in the field, but the opposite pattern was observed for biomass in the shade house. In contrast, growth and survival of EcM host seedlings did not vary with neighbourhood type in the field or shade house. Conclusions The ability of EcM host dipterocarp species to tolerate conspecific neighbours likely contributes to their dominance, whereas negative effects of conspecific neighbours may allow AM host seedlings to persist in enemy‐free space beneath heterospecific adults.