Microbial symbionts strongly influence the functioning of forest ecosystems. They exploit inorganic, organic and/or atmospheric forms of nutrients that enable plant growth, determine how trees respond to elevated CO2, regulate the respiratory activity of soil microbes, and affect plant species diversity by altering the strength of conspecific negative density dependence. Despite growing recognition of the importance of root symbioses for forest functioning and the potential to integrate symbiotic status into Earth system models that predict functional changes to the terrestrial biosphere, we lack spatially-explicit, quantitative maps of the different root symbioses at the global scale. Generating these quantitative maps of tree symbiotic states would link the biogeography of functional traits of belowground microbial symbionts with their 3 trillion host trees, spread across Earth’s forests, woodlands, and savannas.
Results/Conclusions
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 28,000 tree species. Our analyses indicate that climatic variables, and in particular climatically-controlled variation in decomposition rate, are the primary drivers of the global distribution of major symbioses. We estimate that ectomycorrhizal (EM) trees, which represent only 2% of all plant species, constitute approximately 60% of tree stems on Earth. EM symbiosis dominates forests where seasonally cold and dry climates inhibit decomposition, and are the predominant symbiosis at high latitudes and elevation. In contrast, arbuscular mycorrhizal (AM) trees dominate aseasonally warm tropical forests and occur with EM trees in temperate biomes where seasonally warm-and-wet climates enhance decomposition. Continental transitions between AM and EM dominated forests occur relatively abruptly along climate driven decomposition gradients, which is likely caused by positive plant-microbe feedbacks. The climatically driven global symbiosis gradient we document represents the first spatially-explicit, quantitative understanding of microbial symbioses at the global scale and demonstrates the critical role of microbial mutualisms in shaping the distribution of plant species.