How many tree species are there? (PNAS)

Do you know how many tree species are there? The global Forest Biodiversity Initiative (GFBI) database has annotated about 28,000 tree species, while some studies indicate there could be around 60,000. But why this difference? Many of the species are found in unapproachable areas. Additionally, human errors such as giving different names to the same species or considering two species to be the same, affect the ability to determine the true number of species. A recent large collaborative study by Gatti et al, with more than 100 authors, documented about 64,100 tree species. The authors estimated that there are about 9200 more tree species yet to be discovered, and that about one-third of these are likely to be rare species that need conservation. Interestingly, a large portion (about 40%) of discovered as well as undiscovered species are in the South American subcontinent. Less than 0.1% of tree species are likely present across all five continents. Identification of a true number of tree species can help in understanding the evolution of trees, their response to global environmental changes, and their biodiversity. Summary by Kamal Kumar Malukani, @KamalMalukani. PNAS, 10.1073/pnas.2115329119.