Since humans have walked this planet, we have loved categorizing things and ecology is no exception. One of the most basic ecological questions has always been the simple question of “how many?”

One of the most important and well-studied forms of life on Earth are trees. Trees, have also been one of the more difficult forms of life to document due to their vast species richness, abundance, and global spread. There have been many challenges in the past when trying to document how many tree species there are on Earth, ranging from logistical and financial difficulties to the very taxonomic definition of what one would consider a unique tree species.

IIASA researchers contributed to a study that takes a closer look at this question using global data to determine a closer estimate of the number of tree species left on Earth.

Diversity in any ecosystem is vital information. Forest ecosystems span all five continents (Africa, Australia, Eurasia, North America, and South America). Getting a better grasp of forest ecosystems during the age of climate change is extremely important. Tree diversity at large geographic domains would provide a bigger picture regarding global tree richness. Current tree estimates are still unreliable as they have severe limitations that rely too heavily on published lists of species descriptions that are geographically uneven in coverage. The scientists mention that finding where global species diversity and hotspots are highest are important when wanting to reverse trends occurring, among others, due to climate change, deforestation, and habitat encroachment.

“Tree species diversity is the key to maintaining healthy, productive forests, and it is important to the economy and environment,” says project lead Jingjing Liang, who is a professor of quantitative forest ecology in Purdue University’s Department of Forestry and Natural Resources, founder of the Global Forest Biodiversity Initiative (GFBI), and part of Purdue’s Next Moves’ digital forestry initiative.

The data used in the study involves multiple individual datasets combined into one massive worldwide data set. Each individual set includes the measurement and description of every tree in a forest stand. The scientists note that despite large numbers of grid cells, extensive data, and high mean global sample coverage (96.4%), sampling within the grid cells in many regions of the world remains extremely sparse.

Results came as expected with the highest amounts of diversity coming from the tropical and subtropical zones of continents. South America has the highest number of estimated tree species. Much of the South American datasets lack saturation of numbers of species found; a lot of this is due to high numbers of uncommon species, especially around the Amazon rainforest in Brazil, incomplete sampling, or both. These findings suggest that South America contains more undiscovered species, endemic species, and rare species than any other continent. Other areas of high tree biodiversity to note include Central America and the Southeast Asian archipelago.

Overall, the study’s main findings estimate higher absolute numbers of tree species than previously reported (14.3%). The work establishes a quantitative benchmark and can be used to contribute towards conservation efforts and future discoveries. The results also highlight the vulnerability of global tree species diversity to anthropogenic land use changes, especially in global hotspots. This makes forest conservation a paramount priority in places like South America, particularly in biodiverse regions like the Amazon basin and the Andes-Amazon interface. Lastly, the results indicate that there is still a large knowledge gap regarding tree species in global forest systems and the value of approaches to help fill those gaps.

These research findings can only be done by collective efforts, as is also evident from the long list of coauthors on this study, their affiliations, and their acknowledged projects. The next step would be policy implementation, including, but not limited to, the increase in funding opportunities for ecological field research with data sharing incentives, the prioritization of conservation measures, and trade and import regulation for biodiversity threat commodities,” concludes study coauthor Dmitry Schepaschenko, a researcher in the IIASA Agriculture, Forestry, and Ecosystem Services Research Group of the Biodiversity and Natural Resources Program.   

Schepaschenko was recently interviewed about the paper on Austrian TV (please note that the program is in German).


Cazzolla Gatti, R., Reich, P., Gamarra, J., Crowther, T., Hui, C., Morera, A., Bastin, J.-F., de-Miguel, S., et al. (2022). The number of tree species on Earth. Proceedings of the National Academy of Sciences 119 (6) e2115329119. 10.1073/pnas.2115329119.

Gardner, E. K. (2022, January 31). First-of-its-kind estimate of the total number of tree species. Purdue University News. Retrieved February 16, 2022, from

Note: This article gives the views of the author, and not the position of the Nexus blog, nor of the International Institute for Applied Systems Analysis.