Climatic controls of decomposition drive the global biogeography of forest-tree symbioses

The identity of the dominant root-associated microbial symbionts in a forest determines the ability of trees to access limiting nutrients from atmospheric or soil pools 1,2 , sequester carbon 3,4 and withstand the effects of climate change 5,6 . Characterizing the global distribution of these symbioses and identifying the factors that control this distribution are thus integral to understanding the present and future functioning of forest ecosystems. Here we generate a spatially explicit global map of the symbiotic status of forests, using a database of over 1.1 million forest inventory plots that collectively contain over 28,000 tree species. Our analyses indicate that climate variables—in particular, climatically controlled variation in the rate of decomposition—are the primary drivers of the global distribution of major symbioses. We estimate that ectomycorrhizal trees, which represent only 2% of all plant species 7 , constitute approximately 60% of tree stems on Earth. Ectomycorrhizal symbiosis dominates forests in which seasonally cold and dry climates inhibit decomposition, and is the predominant form of symbiosis at high latitudes and elevation. By contrast, arbuscular mycorrhizal trees dominate in aseasonal, warm tropical forests, and occur with ectomycorrhizal trees in temperate biomes in which seasonally warm-and-wet climates enhance decomposition. Continental transitions between forests dominated by ectomycorrhizal or arbuscular mycorrhizal trees occur relatively abruptly along climate-driven decomposition gradients; these transitions are probably caused by positive feedback effects between plants and microorganisms. Symbiotic nitrogen fixers—which are insensitive to climatic controls on decomposition (compared with mycorrhizal fungi)—are most abundant in arid biomes with alkaline soils and high maximum temperatures. The climatically driven global symbiosis gradient that we document provides a 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. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.33Smithsonian’s National Zoo and Conservation Biology Institute, Washington, DC, USA. 34Institute of Tropical Forest Conservation, Mbarara University of Sciences and Technology, Mbarara, Uganda. 35Isotope Bioscience Laboratory - ISOFYS, Ghent University, Ghent, Belgium. 36Integrated Center for Research, Development and Innovation in Advanced Materials, Nanotechnologies, and Distributed Systems for Fabrication and Control (MANSiD), Stefan cel Mare University of Suceava, Suceava, Romania. 37Department of Forest Sciences, Luiz de Queiroz College of Agriculture, University of São Paulo, Piracicaba, Brazil. 38Bavarian State Institute of Forestry, Freising, Germany. 39Manchester Metropolitan University, Manchester, UK. 40Institute of Biology, Geobotany and Botanical Garden, Martin Luther University Halle-Wittenberg, Halle-Wittenberg, Germany. 41German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Leipzig, Germany. 42Department of Agriculture, Food, Environment and Forest (DAGRI), University of Firenze, Florence, Italy. 43Biological Institute, Tomsk State University, Tomsk, Russia. 44Department of Spatial Regulation, GIS and Forest Policy, Institute of Forestry, Belgrade, Serbia. 45Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT, USA. 46Tropical Forests and People Research Centre, University of the Sunshine Coast, Maroochydore, Queensland, Australia. 47Faculty of Natural Resources Management, Lakehead University, Thunder Bay, Ontario, Canada. 48Key Laboratory for Humid Subtropical Eco-Geographical Processes of the Ministry of Education, Fujian Normal University, Fuzhou, China. 49Institute of Integrative Biology, ETH Zürich, Zurich, Switzerland. 50IFER - Institute of Forest Ecosystem Research, Jilove u Prahy, Czech Republic. 51Global Change Research Institute CAS, Brno, Czech Republic. 52Nicholas School of the Environment, Duke University, Durham, NC, USA. 53Department of Biology, University of Missouri-St Louis, St Louis, MO, USA. 54Department of Plant Biology, Institute of Biology, University of Campinas, UNICAMP, Campinas, Brazil. 55Smithsonian Tropical Research Institute, Balboa, Panama. 56Department of Plant Sciences, University of Cambridge, Cambridge, UK. 57Andes to Amazon Biodiversity Program, Madre de Dios, Peru. 58Facultad de Ciencias Forestales, Universidad Juárez del Estado de Durango, Durango, Mexico. 59Department of Physical and Biological Sciences, The College of Saint Rose, Albany, NY, USA. 60Department of Biology, West Virginia University, Morgantown, WV, USA. 61Biology Department, Concordia University, Montreal, Quebec, Canada. 62Natural Science Department, Universidade Regional de Blumenau, Blumenau, Brazil. 63Cirad, UMR EcoFoG, Kourou, French Guiana. 64Department of Geographical Sciences, University of Maryland, College Park, MD, USA. 65Institute of Forestry, Belgrade, Serbia. 66National Institute of Amazonian Research, Manaus, Brazil. 67IRET, Herbier National du Gabon (CENAREST), Libreville, Gabon. 68Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, USA. 69The Santa Fe Institute, Santa Fe, NM, USA. 70Department of Environment and Science, Queensland Herbarium, Toowong, Queensland, Australia. 71Ecole de Foresterie et Ingénierie du Bois, Université Nationale d’Agriculture, Ketou, Benin. 72Biology Centre of the Czech Academy of Sciences, Institute of Entomology, Ceske Budejovice, Czech Republic. 73Geography, College of Life and Environmental Sciences, University of Exeter, Exeter, UK. 74Natural Resources Institute Finland (Luke), Joensuu, Finland. 75Institute of Plant Sciences, University of Bern, Bern, Switzerland. 76Forest Research Institute Malaysia, Kuala Lumpur, Malaysia. 77Department of Forest Resource Management, Swedish University of Agricultural Sciences SLU, Umea, Sweden. 78Department of Sustainable Agro-Ecosystems and Bioresources, Research and Innovation Center, Fondazione Edmund Mach, San Michele all’Adige, Italy. 79School of Forestry and Environmental Studies, Yale University, New Haven, CT, USA. 80Royal Botanic Garden Edinburgh, Edinburgh, UK. 81Department of Plant Sciences, University of Oxford, Oxford, UK. 82Department of Plant Systematics, University of Bayreuth, Bayreuth, Germany. 83Centre for Conservation Science, The Royal Society for the Protection of Birds, Sandy, UK. 84Instituto de Investigaciones de la Amazonía Peruana, Iquitos, Peru. 85Centre for Invasion Biology, Department of Mathematical Sciences, Stellenbosch University, Stellenbosch, South Africa. 86Theoretical Ecology Unit, African Institute for Mathematical Sciences, Cape Town, South Africa. 87Division of Forest Resources Information, Korea Forest Promotion Institute, Seoul, South Korea. 88Institut Agronomique néo-Calédonien (IAC), Equipe Sol & Végétation (SolVeg), Nouméa, New Caledonia. 89Department of Forest Science, Tokyo University of Agriculture, Tokyo, Japan. 90Institute of Dendrology, Polish Academy of Sciences, Kórnik, Poland. 91Poznań University of Life Sciences, Department of Game Management and Forest Protection, Poznań, Poland. 92Faculty of Biology, Białowieża Geobotanical Station, University of Warsaw, Białowieża, Poland. 93Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark. 94Centre for Environment and Life Sciences, CSIRO Land and Water, Floreat, Western Australia, Australia. 95Forestry Faculty, Bauman Moscow State Technical University, Mytischi, Russia. 96CAVElab – Computational and Applied Vegetation Ecology, Department of Environment, Ghent University, Ghent, Belgium. 97CTFS-ForestGEO, Smithsonian Tropical Research Institute, Balboa, Panama. 98Department of Physical and Environmental Sciences, Colorado Mesa University, Grand Junction, CO, USA. 99School of Natural and Built Environments and Future Industries Institute, University of South Australia, Adelaide, South Australia, Australia. 100Department of Botany, Dr Harisingh Gour Central University, Sagar, India. 101Department of Forest Sciences, Seoul National University, Seoul, South Korea. 102Interdisciplinary Program in Agricultural and Forest Meteorology, Seoul National University, Seoul, South Korea. 103National Center for Agro Meteorology, Seoul, South Korea. 104Research Institute for Agriculture and Life Sciences, Seoul National University, Seoul, South Korea. 105Graduate School of Agriculture, Kyoto University, Kyoto, Japan. 106Institute for World Forestry, University of Hamburg, Hamburg, Germany. 107Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, Tartu, Estonia. 108Ecosystems Services and Management, International Institute for Applied Systems Analysis, Laxenburg, Austria. 109Department of Geography, University College London, London, UK. 110Faculty of Forestry, Qingdao Agricultural University, Qingdao, China. 111Center for Forest Ecology and Productivity, Russian Academy of Sciences, Moscow, Russia. 112School of Geography, University of Oxford, Oxford, UK. 113UMR EcoFoG, AgroParisTech, Kourou, France. 114Departamento de Ciências Biológicas, Universidade do Estado de Mato Grosso, Nova Xavantina, Brazil. 115Department of Environment & Geography, University of York, York, UK. 116Department of Wildlife Management, College of African Wildlife Management, Mweka, Tanzania. 117Departamento de Ecología y Recursos Naturales, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, Mexico. 118Universidad del Tolima, Ibagué, Colombia. 119Colegio de Profesionales Forestales de Cochabamba, Cochabamba, Bolivia. 120Jardín Botánico de Missouri, Oxapampa, Peru. 121Universidad Nacional de San Antonio Abad del Cusco, Cusco, Peru. 122Department of Environmental Management, School of Environmental Science and Management, Independent University Bangladesh, Dhaka, Bangladesh. 123Instituto de Silvicultura e Industria de la Madera, Universidad Juárez del Estado de Durango, Durango, Mexico. 124Universidad Estatal Amazónica, Puyo, Pastaza, Ecuador. 125Department of Evolutionary Biology and Environmental Studies, University of Zürich, Zürich, Switzerland. 126Forestry School, Tecnológico de Costa Rica TEC, Cartago, Costa Rica. 127Climate, Fire, and Carbon Cycle Sciences, USDA Forest Service, Durham, NC, USA. 128Centre for Forest Research, Université du Québec à Montréal, Montréal, Quebec, Canada. 129V. N. Sukachev Institute of Forest, FRC KSC, Siberian Branch of the Russian Academy of Sciences, Krasnoyarsk, Russia. 130Department of Forestry, World Research Institute, Washington, DC, USA. 131Department of Ecology and Environmental Sciences, Pondicherry University, Puducherry, India. 132Instituto Nacional de Tecnología Agropecuaria (INTA), Universidad Nacional de la Patagonia Austral (UNPA), Consejo Nacional de Investigaciones Científicas y Tecnicas (CONICET), Rio Gallegos, Argentina. 133School of Social Sciences and Psychology (Urban Studies), Western Sydney University, Penrith, New South Wales, Australia