Measuring tree biodiversity

At the continental scale, the study estimates that around 43% of all Earth’s tree species are in South America, followed by Eurasia (22%), Africa (16%), North America (15%) and Oceania (11%). Most undiscovered species are probably in South America. 

The study suggests that almost one-third of all tree species yet to be discovered are likely to be rare, with limited spatial distribution, and particularly vulnerable to human-driven activities, such as deforestation and urbanisation, and climate change. 

To gain a comprehensive understanding of the number of known tree species worldwide, the study combined data from two global datasets —the Global Forest Biodiversity Initiative (GFBI) and TREECHANGE — enabling compilation of ground-sourced data on tens of millions of trees. The combined datasets indicated there are 64,100 documented tree species. 

The study subsequently used statistical methods to estimate the total number of unique tree species at biome, continental and global scales. 

“We used different estimators developed by statisticians and mathematicians to estimate the number of unknown species based on the abundance and presence of known ones, especially those of known rare species,” says Jingjing Liang of Purdue University, co-founder of the GFBI. “Some of the estimators are based on the Good-Turing theory, which was developed by Alan Turing and his colleagues during the Second World War.”

The study produced a figure of 73,274 as a conservative estimate of the total number of tree species on Earth.

Alexandre Antonelli, director of science at Royal Botanic Gardens, Kew, who was not involved in the study, comments, “Despite inherent uncertainties with the data and methods, the take-home of this study is clear: global tree diversity is still far from known.” He points out that “some 2,000 new plant species are identified each year, but 40% of all plants are likely to be threatened, so scientists are working around the clock to document biodiversity and research its benefits to humanity.”

Going forward, the researchers say their findings will contribute to tree and forest conservation efforts. “Our estimate provides an important benchmark value for forest conservation,” says Liang. “This value, together with future estimates of the number of tree species, makes it possible for us to know if the total number of tree species globally, or in a certain continent, is increasing or declining. This study reminds us how little we know about our own planet and its biosphere. There is so much more we need to learn about the Earth, so that we can better protect it, and conserve natural resources for future generations.”

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