Mycorrhizal Association Not Soil Specialisation Determines Spatial Aggregation of Tropical Trees

Understanding how tree species aggregate spatially is key to identifying the mechanisms that drive forest community assembly. Previous research on mycorrhizal fungal networks have shown seedlings recruiting near conspecific adults benefit from nutrient access and pathogen protection, suggesting that positive plant-soil feedback could affect forest structure. We measured aggregation of trees across an edaphic gradient of sandy-loam to clay soils in Lambir Hills, Malayisa, where every stem > 1 cm DBH is censused in a 52-hectare ForestGEO plot. The stable co-dominance of a diverse arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) tree community in the dipterocarp-dominated forests of South-East Asia is unique amongst tropical biomes. We show that mycorrhizal association type and seed dispersal mode explained spatial aggregation patterns. Surprisingly, soil specialisation by trees did not influence conspecific clustering. EM trees clustered more intensely than AM trees, supporting earlier experiments that show EM networks enhance seedling establishment. The pattern of greater EM clustering persists among wind-dispersed species, showing that even with dispersal limitation, mutualistic microbial interactions can determine spatial distribution. Our analysis provides landscape-scale evidence that positive density-dependence mediated by mycorrhizal fungi contribute to structuring tropical forests dynamics.