Tree-associated fungi, including mycorrhizal mutualists and pathogenic antagonists, are increasingly recognized as important drivers of tree demography and forest composition. Yet, the extent to which fungal communities mediate elevational and latitudinal gradients in tree diversity remains understudied. One way that tree-associated fungi may mediate tree diversity is by altering the magnitude and directionality of biotic processes including conspecific density dependence (CDD). CDD is a population-level process defined as the influence of conspecific (same species) population densities on population growth or demographic rates. When strongly negative, CDD decreases per-capita population growth rates of locally common species compared to locally rare species, thus stabilizing population sizes, and promoting local plant diversity. One primary driver of negative CDD are natural enemies, including host-specific pathogenic fungi, which exert a relatively negative effect on hosts compared to non-hosts (or suboptimal hosts). Host-specific mutualisms, including mycorrhizal fungi, have the opposite effect, advantaging common, densely aggregated tree populations relative to uncommon, sparse populations. Despite a robust understanding of tree-fungi species interactions the influence of these interactions on CDD, and the resulting implications for forest diversity, are rarely synthesized in a single system.
Emerging evidence suggests that negative-CDD neutralizes, and even flips to positive-CDD, with increasing elevation. Here, we test the hypothesis that neutralized negative-CDD with increasing elevation is explained by a decrease in diversity of generalist arbuscular mycorrhizal and pathogenic fungi relative to species-specific ectomycorrhizal fungi. To test this hypothesis, we leverage long-term forest inventory datasets and high-resolution soil sequencing across a 1000-meter elevation gradient at the H.J. Andrews Experimental Forest LTER in Blue River, Oregon. We examine the changing composition and relative abundance of the fungal guilds capable of generating CDD as well as underlying soil nutrient and chemical properties. This study fundamentally advances our understanding of how tree-associated fungi influence the maintenance of tree diversity in forests, a critical conservation concern of the 21st century.
