The biogeography of ecological functioning is a growing field, aimed at generating mechanistic linkages between biological communities and the functioning of global ecosystems. Given that the activity of most trees is underpinned by their associated symbiotic microorganisms in soil, the identity of symbiotic partners are integral to understanding forest ecosystem functioning. Here we generate the first spatially explicit map of forest symbiotic status using a global database of 1.2 million forest inventory plots with over 31 million stems representing over 28 thousand tree species.
Results/Conclusions
We reveal striking biogeographic turnover in major symbioses, with ectomycorrhizal and arbuscular mycorrhizal symbiosis dominating at high and low latitudes, respectively. However, while major symbioses dominate different forest ecosystems, each guild has higher than expected species richness in the tropics than predicted based on their local densities. Our analyses indicate that climate, more so than soil chemistry and nutrient availability, determines which symbiotic guild dominates a given biome. Ectomycorrhizal trees, which we estimate constitute 60% of all stems globally (and >80% outside of the tropics), dominate forests with high seasonal variability in temperature and cold wet-seasons. Arbuscular mycorrhizal trees and N-fixers, by contrast, occur in a-seasonal forests with hot wet-seasons. Symbiotic N-fixers reach peak abundance in alkaline soils that have high maximum temperatures. The existence of a global symbiosis gradient, which we call Read's Rule, represents one of the most prominent biogeographic patterns on the planet, and demonstrates the tight coupling of nutrient exchange mutualisms with the global distribution of plant communities.