Microbe-mediated plant-soil feedback (PSF) is an important driver of plant community dynamics. Variation in strength and direction of feedback impacts plant communities, and may drive or reinforce the dominance or rarity of individual species. Previous studies have suggested that negative PSF may be a common mechanism supporting coexistence in tropical forests dominated by trees forming arbuscular mycorrhizas (AM), whereas positive feedbacks are rare and associated with monodominant stands of ectomycorrhizal (EM) host species. However, a majority of studies on tropical PSF have taken place in the Neotropics, and few have characterized PSF among both AM and EM hosts in non-monodominant tropical forest where EM hosts are diverse and abundant.
We assessed the generality of negative PSF in tropical forests using a large-scale shade-house experiment in Borneo, where diversity of both AM and EM hosts is high. Our study included both EM and AM tree species from phylogenetically varied lineages to better understand the role of fungal mutualists in determining feedback strength. We tested the influence of evolutionary history on strength and direction of feedback using a phylogenetically structured design, and utilized soils collected across a soil-resource gradient to examine how resource availability affects PSF.
We documented significant effects of seedling-soil phylogenetic distance on seedling growth, though this effect had significant interactions with host mycorrhizal type. Further, the strength and direction of feedback was influenced by soil chemistry, suggesting that feedbacks may be context dependent. Within ectomycorrhizal taxa, phylogenetic relatedness to soil source and soil type had significant effects on percent root colonization by EM fungi, which correlated strongly with seedling growth rates. Our results suggest that in this system the strength and direction of plant-soil feedback is significantly dependent on host mycorrhizal type. We document general trend toward positive feedbacks among EM host taxa, questioning the generality of localized negative plant-soil feedback as a mechanism maintaining plant species diversity in species-rich tropical forests.