Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)The high species richness of tropical forests has long been recognized, yet there remains substantial uncertainty regarding the actual number of tropical tree species. Using a pantropical tree inventory database from closed canopy forests, consisting of 657,630 trees belonging to 11,371 species, we use a fitted value of Fisher's alpha and an approximate pantropical stem total to estimate the minimum number of tropical forest tree species to fall between similar to 40,000 and similar to 53,000, i.e., at the high end of previous estimates. Contrary to common assumption, the Indo-Pacific region was found to be as species-rich as the Neotropics, with both regions having a minimum of similar to 19,000-25,000 tree species. Continental Africa is relatively depauperate with a minimum of similar to 4,500-6,000 tree species. Very few species are shared among the African, American, and the Indo-Pacific regions. We provide a methodological framework for estimating species richness in trees that may help refine species richness estimates of tree-dependent taxa.11224 74727477Arnold Arboretum of Harvard UniversityBelgian Science Policy [SD/AR/01A]Botanic Garden MeiseBrazilian Science CouncilConselho Nacional de Desenvovimento Cientifico e Tecnologico of Brazil [MAUA] [PRONEX-FAPEAM 1600/2006]Conselho Nacional de Desenvovimento Cientifico e Tecnologico of Brazil [Universal] [479599/2008-4]Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)British Ecological Society [4709/5747]Coordination of Improvement of Higher Education Personnel, BrazilDEFRA Darwin InitiativeDeland Award for student research, Arnold ArboretumDepartment of Biotechnology-National Remote Sensing Agency, IndiaDepartment of National Parks, Wildlife and Plant Conservation, ThailandEl Consejo de Ciencia y Technologia Grant [33851-B]Federal Ministry of Education and Research, GermanyForest Department Sarawak, MalaysiaFulbright ProgramFundacao de Amparo a Pesquisa de Minas Gerais, BrazilGerman Agency for International Cooperation Projects [08.7860.3-001.00, 13.1432.7-001.00]German Science Foundation [CRC 552, CU127/3-1, HO 3296/2-2, HO3296/4-1, RU 816]Grant Agency of the Czech Republic Project [14-36098G]Intitut National pour L'etude et la Recherche Agronomiques, Democratic Republic CongoInstituto Nacional de Pesquisas de Amazonia/Max Planck Institute for ChemistryLarge-Scale Biosphere Atmosphere Experiment in AmazoniaMissouri Botanical GardenMuseo delle ScienzeNational Institute for Environmental Studies, JapanNational Science Foundation (NSF) [DEB-0075334, DEB-0515678, LTEB1357177, LTREB/DEB1357112, NSF DEB-0424767, NSF DEB-0639393, NSF DEB-1147429, NSF DEB-1350125, NSF-DEB-1053237, NSF DEB-0841885]NERC Human-Modified Tropical Forests ProgrammeOperation WallaceaPAPIIT-DGAPA-UNAM, Mexico [IB-200812, IN-204215]Pontificia Universidad Catolica del EcuadorRockefeller FoundationRoyal Forest Department of ThailandRoyal Society South-East Asia Rainforest Research Programme [RS243]Rufford Small Grant FoundationSt. Louis ZooFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Smithsonian Tropical Research InstituteSociety of Systematic BiologistsSystematics AssociationSwiss National Science FoundationTropenbos InternationalUniversity of MinnesotaUS Department of Agriculture Forest Service, International Institute of Tropical ForestryUSAIDWorld Wildlife FundGordon and Betty Moore FoundationSmithsonian InstitutionNational Science FoundationJohn Merck FundJohn D. and Catherine T. MacArthur FoundationAndrew W. Mellon FoundationFrank Levinson Family FoundationHSBC Climate PartnershipBromley Charitable TrustJohn Swire Sons Pty Ltd.Small World Institute FundConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)CNPq [CNPq]FAPESP [FAPESP 03/12595-7-COTEC/IF 41.065/2005-IBAMA/CGEN 093/2005]