Bacteria and fungi living inside plant roots help plants capture mineral nutrients from the soil while benefiting from the food that the plant produces using energy from the sun. Trees can establish several types of symbiotic relationships with fungi and bacteria. Researchers constructed a global map of the types of tree symbioses across the world. With the map, they determined that the type of fungal symbiosis found in trees depends on how quickly the organic matter in the soil decomposes. The team also found that bacteria that convert nitrogen gas from the atmosphere into plant-usable products form tree symbioses in arid environments.
The health of the world’s forest is of critical importance for the well-being of the planet. Symbiotic microbes that associate with trees affect their capacity to acquire essential nutrients from the soil and air around them. Our knowledge of the effects of geographic and climatic factors on these symbioses has so far come from analyses of a limited number of environments. The study provides a global map of tree symbioses across large numbers of species and ecosystems. This comprehensive analysis not only sheds light on the important role of microbes in shaping the landscape of the world’s forests, but it will also help improve global biogeochemical models.
To understand how tree-microbial symbioses affect the state of forests at the global scale, an international consortium of researchers surveyed tree-microorganism symbioses in 1.1 million locations around the world. These ecosystems included over 28,000 tree species and a vast climatic and geographic diversity. This comprehensive study demonstrated that the majority of tree symbioses are ectomycorrhizal, although they represent a small percentage of all tree species. This type of tree symbiosis is predominant in seasonally cold and dry climates as well as at high latitude and elevation. In these conditions, decomposition of soil organic matter is slow. In warm tropical forests, where decomposition is faster, trees prefer to establish arbuscular mycorrhizal associations, and the researchers observed a fairly sudden geographical transition between the two types of symbioses. On the other hand, the research showed that symbioses with nitrogen-fixing Rhizobia and Actinobacteria occur in arid and hot ecosystems. The global microbial biogeographical map of forest symbioses constructed in this study shows that forests transition from low-latitude arbuscular mycorrhizal through nitrogen-fixer to high-latitude ectomycorrhizal ecosystems, confirming predicted rules of mycorrhizal distribution.
Kabir Gabriel Peay
This work was supported in part by the Department of Energy, Office of Science, Office of Biological and Environmental Research, Early Career Research Program. The authors also acknowledge support from other sources listed in https://static-content.springer.com/esm/art%3A10.1038%2Fs41586-019-1128-0/MediaObjects/41586_2019_1128_MOESM3_ESM.pdf.
B.S. Steidinger, T.W. Crowther, J. Liang, et al. “Climatic controls of decomposition drive the global biogeography of forest-tree symbioses.” Nature 569, 404 (2019). [DOI: 10.1038/s41586-019-1128-0]