Different mycorrhizal nutrient acquisition strategies shape tree species competition and coexistence dynamics

Mycorrhizal fungi with different nutrient acquisition strategies influence the functional separation among plant species. This might drive resource competition dynamics that cumulatively impact tree species coexistence, but few manipulative experiments have directly tested this. Combining surveys and experiments in a modern coexistence theory framework, we tested how variation in mycorrhizal strategies and nutrient conditions affect competitive outcomes between co-occurring ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) tree species. The dependency on EM symbioses increased with latitude and nitrogen (N) limitation, matching global trends. Host-specific soil microbiome conditioning and N fertilization combined to qualitatively affect coexistence outcomes. Lower N conditions favored EM over AM trees, but N fertilization reversed this outcome for southern species, consistent with regional-scale forest mycorrhizal transitions. As the magnitude and outcome of microbially-mediated competition depends on mycorrhizal differentiation and soil nutrient availability, this strongly supports the importance of mycorrhizal symbioses in driving large-scale biogeographic patterns of tree species.