Mycorrhizal dominance and species richness inconsistently modify tree competition–facilitation balance along climatic gradients

Plant interactions are pivotal to community assembly, but they are largely influenced by above‐ and belowground biotic responses to climate changes. Despite this, the roles of mycorrhizal dominance and species richness in modulating tree competition–facilitation balance along regional climatic gradients remain unclear. Here, we disentangled how precipitation and temperature affect the responses of tree competition and facilitation to mycorrhizal dominance and species richness, using 319 tree species associated with ectomycorrhizal (EcM) and arbuscular mycorrhizal (AM) fungi from forest plots across the contiguous USA. We observed that competition intensity increased among EcM species with rising EcM proportion, whereas it decreased both among AM species and between EcM and AM species (EcM × AM). Conversely, tree species richness consistently enhanced facilitation across mycorrhizal types. Across climatic gradients, precipitation altered the effects of EcM proportion and species richness on EcM or AM species interactions, while temperature only modified the effects of EcM proportion on EcM × AM interactions. This clarifies contexts mediate the direction and magnitude of plant dependence on species composition, with precipitation having a greater effect on conspecific mycorrhizal associations and temperature more strongly affecting heterospecific mycorrhizal interactions. Our study provides novel insights into how climate changes alter species interactions through above‐ and belowground biotic strategies.