The role of genetic distance in modulating intraspecific plant–soil interactions among a diverse assemblage of tropical tree species seedlings

Abstract Microbial specialization on host species can promote plant diversity through plant–soil interactions that affect seedling survival and performance. While these processes are typically studied at the interspecific level, emerging evidence suggests that plant–soil interactions can be genotype-specific, contributing to intraspecific density dependence (iDD). However, iDD remains poorly understood in tropical tree species, particularly with respect to its strength and frequency. We hypothesized that genetic distance among conspecific trees should modulate the strength of iDD, and the direction of the effect—positive or negative seedling response—should reflect the net balance of plant–microbe antagonistic and mutualistic interactions. Using a reciprocal planting experiment across eight tropical tree species, we show that genetic distance between conspecific individuals significantly influences seedling responses, consistent with iDD. Yet, the direction of this effect was variable: Castilla tunu showed higher survival and Wettinia aequalis greater biomass in soils from more genetically distant conspecifics, consistent with negative iDD; Inga marginata exhibited positive iDD; five species exhibited no pattern. These findings demonstrate that genotype-specific plant–soil interactions can generate biologically relevant but variable impacts on seedling responses. This highlights the need to further investigate the drivers and ecological consequences of intraspecific genetic variation in plant–soil interactions and iDD in tropical forests.