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.

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Agarwood trade has been growing rapidly due to its active ingredients composed of volatile substances, which are widely used in the cosmetics and pharmaceuticals. However, the formation time of agarwood in nature is quite long and little is known about its formation agents and mechanisms in planted Aquilaria crassna trees. In this study, biological, chemical and mechanical treatments were applied to 5-, 8-, and 11-year-old A. crassna plantations in north central, Vietnam. Agarwood samples were collected at 1 and 2 years after treatment. Oil content (O c ) in dry wood of A. crassna and sesquiterpene content in extracted oil were analyzed using hydrodistillation method and gas chromatography-mass spectrometry. The results indicated that 1 year after treatment, control (no treatment) and mechanical treatment (nails hammered to tree stems) had highest O c (0.061-0.079 %) but lowest sesquiterpene content (2.1-7.1 %). Chemical treatment (mixture of acid sulfuric and sodium methyl bisulfate) had lowest O c (0.038-0.039 %) but highest sesquiterpene content (15.8-20.8 %). While, 1 year after treatment biological treatments had O c of 0.050-0.077 % and sesquiterpene content of 2.4-11.1 %. Two years after treatment, control still had lowest sesquiterpene content (3.2-7.0 %), while highest content (13.9-44.2 %) belonged to biological treatment (a mixture of fungi Phialophora spp. and Fusarium spp.). There was a total of 56 sesquiterpenes found in extracted oil from wood samples in biological treatments, which included eight highly commerciallyvaluable sesquiterpenes on international trade. Biological treatment with a mixture of fungi P. spp. and F. spp. should be encouraged to apply to 11-year-old A. crassna plantation, which resulted in highest sesquiterpene content (44.2 %).