One mechanism proposed to explain high species diversity in tropical systems is strong negative conspecific density dependence (CDD), which reduces recruitment of juveniles in proximity to conspecific adult plants. Although evidence shows that plant-specific soil pathogens can drive negative CDD, trees also form key mutualisms with mycorrhizal fungi, which may counteract these effects. Across 43 large-scale forest plots worldwide, we tested whether ectomycorrhizal tree species exhibit weaker negative CDD than arbuscular mycorrhizal tree species. We further tested for conmycorrhizal density dependence (CMDD) to test for benefit from shared mutualists. We found that the strength of CDD varies systematically with mycorrhizal type, with ectomycorrhizal tree species exhibiting higher sapling densities with increasing adult densities than arbuscular mycorrhizal tree species. Moreover, we found evidence of positive CMDD for tree species of both mycorrhizal types. Collectively, these findings indicate that mycorrhizal interactions likely play a foundational role in global forest diversity patterns and structure.

Biology Centre Institute of Entomology Czech Academy of Sciences Budějovice Czech Republic

Biology Department Wilfrid Laurier University Waterloo Canada

Botanical Garden Division Taiwan Forestry Research Institute Taipei City Taiwan ROC

Center for Conservation and Sustainability Smithsonian Conservation Biology Institute National Zoological Park Washington DC USA

Centre National de la Recherche Scientifique et Technologique Ouagadougou Burkina Faso

Conservation Ecology Center Smithsonian Conservation Biology Institute National Zoological Park Washington DC USA

Department of Biological Sciences Marquette University Milwaukee WI USA

Department of Biology Indiana University Bloomington IN USA

Department of Biology Middlebury College Middlebury VT USA

Department of Biology Temple Ambler Field Station Temple University Ambler PA USA

Department of Biology University of Hawaii Hilo HI USA

Department of Biology University of Oregon Eugene OR USA

Department of Biology Washington University in St Louis St Louis MO USA

Department of Ecology and Evolutionary Biology Tulane University New Orleans LA USA

Department of Ecology and Evolutionary Biology University of California Los Angeles Los Angeles CA USA

Department of Ecology and Evolutionary Biology University of Michigan Ann Arbor MI USA

Department of Ecology Evolution and Environmental Biology Columbia University New York NY USA

Department of Environmental Sciences University of Puerto Rico Rio Piedras Puerto Rico

Department of Environmental Sciences Wageningen University Wageningen The Netherlands

Department of Forest Ecology Silva Tarouca Research Institute Průhonice Czech Republic

Department of Forest Management W A Franke College of Forestry and Conservation University of Montana Missoula MT USA

Department of Life Science Tunghai University Taichung City Taiwan ROC

Department of Natural and Applied Sciences University of Wisconsin Green Bay Green Bay WI USA

Department of Natural Resources and Environmental Management University of Hawaii at Manoa Honolulu USA

Department of Natural Resources and Environmental Studies National Dong Hwa University Hsinchu Taiwan ROC

Department of Plant and Microbial Biology University of Minnesota St Paul MN USA

Department of Renewable Resources University of Alberta Edmonton Canada

Department of Science and Technology Uva Wellassa University Badulla Sri Lanka

Environmental Change Institute School of Geography and the Environment University of Oxford Oxford UK

Environmental Studies Department University of California Santa Cruz Santa Cruz CA USA

Estación Experimental de Zonas Áridas Consejo Superior de Investigaciones Científicas Almería Spain

ETH Zurich Department of Environmental Systems Science Zurich Switzerland

Forest Ecology Group Smithsonian Environmental Research Center Edgewater NJ USA

Forest Global Earth Observatory Smithsonian Conservation Biology Institute National Zoological Park Washington DC USA

Forest Global Earth Observatory Smithsonian Environmental Research Center Edgewater NJ USA

Forest Global Earth Observatory Smithsonian Tropical Research Institute Panama City Panama

Gabon Biodiversity Program Center for Conservation and Sustainability Smithsonian National Zoo and Conservation Biology Institute Gamba Gabon

Geography and Environmental Systems University of Maryland Baltimore County Baltimore MD USA

Harvard Forest Harvard University Petersham MA USA

Herbier National du Gabon Institut de Pharmacopée et de Médecine Traditionelle Libreville Gabon

Institute of Pacific Islands Forestry USDA Forest Service Hilo HI USA

National Biobank of Thailand National Science and Technology Development Agency Khlong Nueng Pathum Thani Thailand

Programa Ciencias de la Biodiversidad Instituto de Investigacion de Recursos Biologicos Alexander von Humboldt Bogota Colombia

School of Biological Sciences and Center for Plant Science Innovation University of Nebraska Lincoln Lincoln NE USA

School of Biological Sciences University of Aberdeen Aberdeen UK

School of Forest Fisheries and Geomatics Sciences University of Florida Gainesville USA

School of Forest Resources and Conservation University of Florida Gainesville FL USA

School of Life Sciences Sun Yat sen University Guangzhou China

School of the Environment Washington State University Pullman WA USA

State Key Laboratory of Biocontrol School of Ecology School of Life Sciences Sun Yat sen University Guangzhou China

Taiwan Forestry Research Institute Taipei City Taipei Taiwan ROC

The Ecology Center Utah State University Logan UT USA

The Royal Society SEARRP Kota Kinabalu Sabah Malaysia

The Wilderness Institute W A Franke College of Forestry and Conservation University of Montana Missoula MT USA

Tiantong National Forest Ecosystem Observation and Research Station School of Ecological and Environmental Sciences East China Normal University Shanghai China

U S Forest Service Institute of Pacific Islands Forestry Pacific Southwest Research Station Hilo HI USA

UK Centre for Ecology and Hydrology Bailrigg UK

University of Hawaii at Manoa 1910 East West Rd Honolulu HI USA

Water Resources Research Center University of Hawaii at Manoa Honolulu USA

Wildland Resources Department Utah State University Logan UT USA

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