Global balance of earthly tree species

More than 73 plants grow on our planet.200 tree species – many of them in South America

Trees

How many tree species are there worldwide? A new survey now comes a big step closer to finding the answer. © Leonhard Steinacker/ TU Munich

How many tree species are there on our planet? An answer is now provided by the most comprehensive assessment of global tree diversity to date. According to this, at least 73.200 different tree species, which is a good 14 percent more than previously estimated. The greatest diversity is found in the tropical forests of South America, with 43 percent of all tree species. However, many of these species are very rare and therefore particularly threatened, the research team reports.

Whether in the tropics, in temperate latitudes or in the taiga of the far north: trees are found almost everywhere on earth. More than three trillion of these plants are estimated to grow on our planet. They form the "green lungs" of our planet and are important climate buffers; at the same time, forests provide habitat for countless other animal and plant species.

How many species exist?

But one question remains largely unanswered: How many tree species exist on our planet? "Even among trees, which are among the largest and most widespread organisms on the planet, we still don’t know exactly how many species there are," Roberto Cazzolla Gatti of Purdue University and his colleagues state. For example, while the forests of the middle and higher latitudes have been relatively well studied, many species-rich forest areas of the tropics have not.

To fill these gaps, Gatti and his team have compiled the most comprehensive inventory of arboreal biodiversity to date. To do this, they relied on two global databases whose datasets contain information on tens of thousands of test plots around the world. With around 38 million mapped and recorded trees from 90 countries, the Global Forest Biodiversity Initiative (GFBI) database is the largest global repository of information on trees in the world to date.

The team combined these data with a second large data collection from the TREECHANGE project and used a statistical method to estimate the number of tree species at the biome, continental, and global levels.

Tree species

Data sources and determined number of tree species (outside) per continent. © Gatti et al./ PNAS

A good 14 percent more tree species than thought

The result: Worldwide, there are at least 73.274 tree species, of which, however, about 9 are.200 not yet described or named. "The absolute number of tree species is thus considerably higher than previously thought, representing 14.3 percent more species than were known to researchers," Gatti and his colleagues write. However, even this figure is likely to be incomplete and too low, he said, because data is very thin in some regions.

The greatest diversity of trees is found in the tropics: "The highest levels come from tropical and subtropical moist forest biomes," the researchers report. "About two-thirds of all tree species already known occur in such forests on all five continents."The rainforests are not only hotspots of tree diversity, but are probably also home to the largest number of undiscovered species.

South America has the greatest diversity

In a comparison of the continents, by far the most tree species grow in South America: "Our data on continental tree diversity show that around 43 percent of all tree species grow in South America," report Gatti and his colleagues. Most of these trees are found in the Amazon basin and in the forest areas on the edge of the Andes Mountains. Researchers estimate that more tree species may be undiscovered there than anywhere else.

Closely related to this is the high number of rare tree species in this region: worldwide, rare species represented by few specimens account for about one-third of global tree species diversity – and they are also concentrated in tropical forests. "This underscores the vulnerability of global forest diversity to anthropogenic changes, primarily from land use and climate," says co-author Peter Reich of the University of Michigan. "Because the survival of rare species is disproportionately threatened by such factors."

Important even beyond the trees

Scientists believe that knowledge of these rare and potentially endangered tree species, as well as of the overall arboreal diversity, can now help to better protect them. "A sound knowledge of the species richness of trees and their versatility is key to maintaining the stability and functionality of ecosystems," Gatti says. In addition, he said, this is also an important piece of the puzzle in the overall biodiversity of the planet.

"Our findings demonstrate both the lack of knowledge we still have about trees in our global forest ecosystems and the value of approaches to fill these gaps," the research team states. "Because this provides us with fundamental insights about the diversity of life on our planet and the need to protect." (Proceedings of the National Academy of Sciences, 2022; doi: 10.1073/pnas.2115329119)

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