The question is one that has long eluded botanists: how many tree species are there? The answer is 60,065, provided by Botanic Gardens Conservation International (BGCI) in a recent article (Beech et al., 2017, Journal of Sustainable Forestry, dx.doi.org/10.1080/10549811.2017.1310049).

The number of tree species has been calculated from GlobalTreeSearch, a new publicly available database at www.bgci.org/globaltreesearch. This is the first comprehensive list of tree species and their country-level distributions. Previous estimates were between 40,000 and 100,000 species, and were generally based on broad estimates or models. BGCI began compiling the database over 2 years ago and GlobalTreeSearch now comprises 375,500 records collated from 500 sources.

GlobalTreeSearch offers some interesting statistics. Nearly half of all tree species are found in just 10 families, with the Leguminosae, Rubiaceae and Myrtaceae having the most. Brazil, Colombia and Indonesia are the countries with the greatest diversity of trees. Surprisingly, 58% of tree species are endemic to a single country, with hotspots in Brazil, Madagascar and Australia. Some of the results are as expected; the Neotropic biome is the most diverse, with 23,000 species, and the region with the least tree diversity is the Nearctic region of North America, with fewer than 1,400 species. There are no tree species in the Antarctic.

Although it seems extraordinary that it has taken until 2017 to publish the first global, authoritative list of tree species, it is worth remembering that GlobalTreeSearch represents a huge scientific effort encompassing the discovery, collection and describing of tens of thousands of plant species. This is big science, involving the work of thousands of botanists over centuries. One of the challenges for the creation of the database was a paucity of available information on the floras of certain regions. The database contains country-level records but also incorporates province-level data for Brazil, China, South Africa and the USA. BGCI encourages submissions from regions where data may not be as readily available, to improve the database. In addition, there is scope for increasing the amount of regional data for other countries as new data become available. For example, island-level data for countries such as Indonesia and the Philippines would be useful for conservation planning and forestry.

BGCI's main reason for undertaking the challenge of documenting tree diversity was to provide a tool for people trying to conserve rare and threatened tree species. GlobalTreeSearch will form the backbone of the Global Tree Assessment (Oryx, 2015, 49, 410–415), an initiative to assess the conservation status of all tree species by 2020. This will allow the prioritization of the tree species that are most in need of conservation action so we can ensure that no tree species is lost forever. Current knowledge suggests that at least one in five tree species are threatened with extinction, although this is likely to be a substantial underestimate.

The database will also be used by forestry and restoration practitioners. GlobalTreeSearch contributes to Target 1 of the Global Strategy for Plant Conservation (An online flora of all known plants) and will have direct uses in monitoring and managing tree species diversity, forests and carbon stocks. The database is not a static entity, and will be updated as new information becomes available.