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 ∼ 40,000 and ∼ 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 ∼ 19,000-25,000 tree species. Continental Africa is relatively depauperate with a minimum of ∼ 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.

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1. Enhancing knowledge on the role of evolutionary history during forest succession and its relationship with ecosystem function is particularly relevant in the context of forest landscape restoration for climate change mitigation and adaptation.2. We used fine resolution vegetation and environmental data (soil, elevation and slope) from two large-scale surveys (320 × 1000 m 2 plots in two 10 km × 10 km blocks) in the Upper Mekong to quantify (1) the role of abiotic and biotic (species interactions) factors in community assembly processes and (2) the effect of biodiversity, environmental factors and forest succession on above-ground biomass (AGB).3. We found strong correlation between soil fertility and community structure in the early successional seres, while species interactions played an increasingly important role in older seres, presumably due to species complementary. 4. We detected a significant relationship between AGB and phylogenetic diversity, elevation and soil fertility across successional gradients. Within successional stages, soil fertility was not significantly associated with AGB, while elevation was significantly associated with AGB only in forest <100 years old. Phylogenetic diversity was positively correlated with AGB in the young secondary forest (< 15 years old) but not significantly associated with AGB in older seres. Synthesis.Our results support the hypothesis that abiotic filtering influences species assembly in the initial stages of forest succession, while biotic interactions dominate community assembly processes in older seres. We found that phylogenetic diversity, soil fertility and elevation gradients were strongly predictive of AGB in a secondary tropical montane forest in Southeast Asia. However, elevation may reflect other underlying abiotic gradients, such as water availability.Phylogenetic diversity was significantly associated with AGB only in youngest 1420 | Journal of Ecology SATDICHANH eT Al.itively superior species. Nevertheless, plant AGB tends to increase rapidly early in forest succession (Li et al., 2017) but the rate of AGB accumulation may decrease as stand age increases, and AGB may seres (<15 years old). Considering phylogenetic diversity in restoration plantings and the management of forests younger than 15 years old could enhance forest biomass and the climate mitigation function forest landscape restoration. K E Y W O R D Sabiotic and biotic factors, above-ground biomass, biodiversity, community assembly, ecosystem function, phylogenetic diversity, succession, tropical forest | 1421Journal of Ecology SATDICHANH eT Al.