One of the most fundamental questions in ecology is how many species inhabit the Earth. However, due to massive logistical and financial challenges and taxonomic difficulties connected to the species concept definition, the global numbers of species, including those of important and well-studied life forms such as trees, still remain largely unknown. Here, based on global ground-sourced data, we estimate the total tree species richness at global, continental, and biome levels. Our results indicate that there are ∼73,000 tree species globally, among which ∼9,000 tree species are yet to be discovered. Roughly 40% of undiscovered tree species are in South America. Moreover, almost one-third of all tree species to be discovered may be rare, with very low populations and limited spatial distribution (likely in remote tropical lowlands and mountains). These findings highlight the vulnerability of global forest biodiversity to anthropogenic changes in land use and climate, which disproportionately threaten rare species and thus, global tree richness.

Agriculture Food Environment Centre University of Trento San Michele all'Adige 38122 Italy

AgroParisTech INRAE Silva Université de Lorraine 5400 Nancy France

All Russian Institute of Continuous Education in Forestry 141200 Pushkino Moscow region Russia

AMAP IRD CIRAD CNRS INRAE Université de Montpellier Montpellier Cedex 5 F 34398 France

AMAP Université de Montpellier CIRAD CNRS INRAE IRD Montpellier 34090 France

Amazonia Green Landscape Protection and Governance Programme SOS Amazônia Rio Branco 69905 082 Brazil

Białowieża Geobotanical Station Faculty of Biology University of Warsaw Warsaw 17 230 Poland

Biological Institute Tomsk State University Tomsk 634050 Russia

Biometris and Forest and Nature Policy Group Wageningen University and Research Wageningen 6700 AA The Netherlands

Bioversity International IITA Regional Office in Cameroon Yaounde Cameroon

CB Departamento de Ecologia Universidade Federal do Rio Grande do Norte Natal 59072 970 Brazil

Center for Biodiversity Dynamics in a Changing World Department of Biology Aarhus University DK 8000 Aarhus C Denmark

Center for Forest Ecology and Productivity of the Russian Academy of Sciences Moscow 119991 Russia

Centre for Invasion Biology Department of Mathematical Sciences Stellenbosch University Stellenbosch 7602 South Africa

Centre for the Research and Technology of Agro Environmental and Biological Sciences CITAB University of Trás os Montes and Alto Douro UTAD Quinta de Prados 5000 801 Vila Real Viseu Portugal

Centro de Modelacion y Monitoreo de Ecosistemas Universidad Mayor Universidad de La Frontera Santiago Chile

Cirad UMREcoFoG 97 310 Kourou French Guiana

CIRAD UPR Forêts et Sociétés Yamoussoukro Côte d'Ivoire

College of Life and Environmental Sciences University of Exeter Exeter EX4 4PY United Kingdom

Compensation International Progress S A Ciprogress Greenlife Bogotá Colombia

Departamento de Silvicultura y Cons de la Naturaleza Universidad de Chile Santiago 8820808 Chile

Département des sciences biologiques Centre for Forest Research Université du Québec à Montreal Montreal QC Canada H3C 3P8

Département FOREN Institut National Polytechnique Félix Houphouët Boigny Yamoussoukro BP 1093 Côte d'Ivoire

Department of Agricultural Food Environmental and Animal Sciences University of Udine 33100 Italy

Department of Agriculture Alimentation Environment and Forestry Università degli Studi di Firenze Firenze 50144 Italy

Department of Agriculture Forestry and Bioresources Seoul National University Seoul 08826 South Korea

Department of Biological Geological and Environmental Sciences Alma Mater Studiorum University of Bologna Bologna 40126 Italy

Department of Biology and Florida Museum of Natural History University of Florida Gainesville FL 32611

Department of Biology University of Florence 50019 Sesto Fiorentino Italy

Department of Biology University of Hawai'i at Hilo Hilo HI 96720

Department of Botany Dr Harisingh Gour Central University Madhya Pradesh Sagar 470003 India

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Department of Geosciences and Natural Resource Management University of Copenhagen Copenhagen 1017 Denmark

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Department of Physical and Environmental Sciences Colorado Mesa University Grand Junction CO 81501 3122

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Department of Plant Science Faculty of Science University of Buea Buea Cameroon

Department of Plant Sciences in the Conservation Research Institute University of Cambridge Cambridge CB2 3EA United Kingdom

Department of Plant Systematics University of Bayreuth 95440 Bayreuth Germany

Department of Population Health Sciences University of Wisconsin Madison Madison WI 53704

Department of Spatial Regulation GIS and Forest Policy Institute of Forestry 11030 Belgrade Serbia

Department of Statistics University of Wisconsin Madison Madison WI 53706

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Environmental Change Institute School of Geography and the Environment University of Oxford Oxford OX1 3QY United Kingdom

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Escuela de Ciencias Ambientales Universidad Nacional Abierta y a Distancia Bogotá 2102 Colombia

Escuela de Ingeniería Forestal Insttuto Tecnologico de Costa Rica Cartago 30101 Costa Rica

European Commission Joint Research Centre 21027 Ispra Italy

Faculty of Biology Geobotany University of Freiburg 79104 Freiburg Germany

Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague 16521 Prague Czech Republic

Faculty of Natural Resources Management Lakehead University Thunder Bay ON Canada P7B 5E1

Faculty of Natural Sciences University of Stirling FK9 4LA United Kingdom

Faculty of Natural Sciences University of Stirling Stirling FK9 4LA United Kingdom

Faculty of Science and Technology Free University of Bolzano Bozen Bolzano 39100 Italy

Faculty of Science University of South Bohemia 37005 České Budějovice Czech Republic

Forest Ecology and Forest Management Group Wageningen University and Research Wageningen 6700 AA The Netherlands

Forest Research Institute University of the Sunshine Coast Sippy Downs QLD 4556 Australia

Forest Science Laboratory Multidisciplinary Center Universidade Federal do Acre Cruzeiro do Sul 69920 900 Brazil

Forestry Department FAO Rome 00153 Italy

Forêts et Sociétés Univ Montpellier CIRAD Montpellier 34000 France

FSBI ROSLESINFORG 141208 Ivanteevka Moscow region Russia

Global Change Research Institute of the Czech Academy of Sciences 603 00 Brno Czech Republic

Groupement d'Intérêt Public Ecofor Paris 75116 France

Hawkesbury Institute for the Environment Western Sydney University Penrith 2753 Australia

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Institute of Forestry and Rural Engineering Estonian University of Life Sciences Tartu 51006 Estonia

Institute of Future Environmental and Forest Resources Research Institute for Agriculture and Life Sciences Seoul National University Seoul 08826 South Korea

Institute of Integrative Biology ETH Zurich 8092 Zurich Switzerland

Institute of Tropical Forest Conservation Mbarara University of Science and Technology Kabale Uganda

Instituto de Investigaciones de la Amazonia Peruana Iquitos Peru

Instituto de Investigaciones paa el Desarrollo Forestal Universidad de Los Andes Mérida 5101 Venezuela

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Interdisciplinary Program in Agricultural and Forest Meteorology Seoul National University Seoul 08826 South Korea

International Center for Tropical Botany Department of Biological Sciences Florida International University Miami FL 33133

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Key Laboratory of Tropical Biological Resources of Ministry of Education School of Life and Pharmaceutical Sciences Hainan University Haikou 570228 China

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Laboratory for Geoinformation Science and Remote Sensing Department of Environmental Sciences Wageningen University and Research Wageningen 6700 AA The Netherlands

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Laboratory of Tropical Dendrology and Forestry Training Center in Agroforestry Sciences Federal University of Southern Bahia Ilheus 45613 204 Brazil

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Museo de Historia natural Noel Kempff Mercado Santa Cruz Bolivia

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National Center for AgroMeteorology Seoul 08826 South Korea

National Forest Centre 96001 Zvolen Slovakia

Nicholas School of the Environment Duke University Durham NC 27708

Plant Systematic and Ecology Laboratory Department of Biology Higher Teachers' Training College University of Yaounde 1 Yaounde Cameroon

Proceedings of the National Academy of Sciences U S A Washington DC 20001

Quebec Center for Biodiversity Sciences Centre for Sustainability Research CSFG and Biology Department Concordia University Montreal ON QC H3G 1M8 Canada

Research and Innovation Centre Fondazione Edmund Mach San Michele all'Adige 38010 Italy

Royal Botanic Garden Edinburgh Edinburgh EH3 5LR United Kingdom

School of Biological and Behavioural Sciences Queen Mary University of London London E1 4NS United Kingdom

School of Biological Sciences University of Bristol Bristol BS8 1TQ United Kingdom

School of Geography University of Leeds Leeds LS2 9JT United Kingdom

School of Life Sciences Chair of Forest Growth and Yield Science Technical University of Munich 85354 Freising Germany

School of Science and Engineering James Cook University Cairns QLD 4878 Australia

Section 1 4 Remote Sensing and Geoinformatics Helmholtz GFZ German Research Centre for Geosciences Telegrafenberg Potsdam 14473 Germany

Section for Ecoinformatics and Biodiversity Department of Biology Aarhus University DK 8000 Aarhus C Denmark

Silviculture Rearch Institute Vietnamese Academy of Forest Sciences Hanoi Vietnam

Spatial Ecology and Conservation Lab Department of Tourism Hospitality and Events Management University of Florida Gainesville FL 32611

Spatial Ecology and Conservation Lab School of Forest Resources and Conservation University of Florida Gainesville FL 32611

State Forests Coordination Centre for Environmental Projects Warsaw 02 362 Poland

Statistics Program King Abdullah University of Science and Technology Thuwal 23955 Saudi Arabia

Sukachev Institute of Forest of the SB Russian Academy of Sciences 660036 Krasnoyarsk Russia

Systematic Botany and Functional Biodiversity Institute of Biology Leipzig University 04103 Leipzig Germany

Tartu Observatory University of Tartu Tartu 61602 Estonia

TERRA Teaching and Research Centre Gembloux Agro Bio Tech University of Liege Gembloux 5030 Belgium

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UniSA STEM and Future Industries Institute University of South Australia 5001 Adelaide Australia

Universidad Autónoma del Beni Riberalta Beni 2W3Q VHJ Bolivia

Universidad Nacional de San Antonio Abad del Cusco Cusco 08000 Peru

Universidad Nacional de San Antonio Abad del Cusco Cusco Peru

Urban Ecosystem Research School of Social Science Western Sydney University Penrith NSW 2751 Australia

Vicerrectoría de Investigación y Postgrado Universidad de La Frontera Temuco 4811230 Chile

World Agroforestry Nairobi 00100 Kenya

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Proc Natl Acad Sci U S A. 2022 Mar 29;119(13):e2202784119. doi: 10.1073/pnas.2202784119. PubMed

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