Mycorrhizal symbiont type is an important trait that differentiates tree species’ responses to their neighbours. In temperate forests, tree species associated with arbuscular mycorrhizae (AM) fungi tend to show reduced performance in conspecific relative to heterospecific neighbourhoods, while species associated with ectomycorrhizal (EcM) fungi do not suffer such 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 within mixed dipterocarp (EcM host trees of family Dipterocarpaceae) forests of Sri Lanka. 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 growth and survival over ~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 over two years beneath conspecific and heterospecific tree crowns. 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, suggesting that above-ground enemies or input from leaf litter drive neighbourhood effects. AM host seedlings had lower growth and survival in conspecific vs. heterospecific neighbourhoods in the field, but the opposite pattern was observed for growth 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. These results support mycorrhizae-mediated neighbourhood effects in tropical mixed dipterocarp forests. 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.

All species must balance their allocation to growth, survival and recruitment. Among trees, evolution has resulted in different strategies of partitioning resources to these key demographic processes, i.e. demographic trade-offs. It is unclear whether the same demographic trade-offs structure tropical forests worldwide. Here, we used data from 13 large-scale and long-term tropical forest plots to estimate the principal trade-offs in growth, survival, recruitment, and tree stature at each site. For ten sites, two trade-offs appeared repeatedly. One trade-off showed a negative relationship between growth and survival, i.e. the well-known fast−slow continuum. The second trade-off distinguished between tall-statured species and species with high recruitment rates, i.e. a stature−recruitment trade-off. Thus, the fast-slow continuum and tree stature are two independent dimensions structuring most tropical tree communities. Our discovery of the consistency of demographic trade-offs and strategies across forest types in three continents substantially improves our ability to predict tropical forest dynamics worldwide.