Mycorrhizal feedbacks inﬂuence global forest structure and diversity
One mechanism proposed to explain high species diversity in tropical systems is strong negative conspeciﬁc density dependence (CDD), which reduces recruitment of juveniles in proximity to conspeciﬁc adult plants. Although evidence shows that plant-speciﬁc soil pathogens can drive negative CDD, trees also form key mutualisms with mycorrhizal fungi, which may counteract these effects. Across 43 large-scale forest plots worldwide, we tested whether ectomycorrhizal tree species exhibit weaker negative CDD than arbuscular mycorrhizal tree species. We further tested for conmycorrhizal density dependence (CMDD) to test for beneﬁt from shared mutualists. We found that the strength of CDD varies systematically with mycorrhizal type, with ectomycorrhizal tree species exhibiting higher sapling densities with increasing adult densities than arbuscular mycorrhizal tree species. Moreover, we found evidence of positive CMDD for tree species of both mycorrhizal types. Collectively, these ﬁndings indicate that mycorrhizal interactions likely play a foundational role in global forest diversity patterns and structure.