The earth is not only a blue planet, but also a green one: Large parts of the land surface are covered by forests. But how many tree species grow in forests around the world? An international research team has now investigated this. According to their balance sheet, there are a good 73,000 tree species worldwide, but at least 9,200 of them could still be undiscovered and undescribed. The greatest variety of tree species can be found in the tropical forests of South America: the species growing there account for around 43 percent of the total tree species diversity, as the team reports. At the same time, the proportion of rare, undiscovered species could be highest there.
How many species are there on earth? This is one of the fundamental questions of ecology – and one that remains unanswered to this day. Because humanity does not even know a fraction of all living things on the planet. “Even for trees, which are among the largest and most widespread organisms on the planet, enabling much of the terrestrial biodiversity and providing us humans with a wealth of ecosystem services, we still don’t know exactly how many species there are,” they state Roberto Cazzolla Gatti of Purdue University and his colleagues. Although the forests of the middle and higher latitudes have been studied relatively well, many species-rich forest areas in the tropics are still largely unexplored. “But knowledge of the species richness and diversity of trees is important in order to maintain the stability and functionality of ecosystems,” says Gatti.
Data collected from all over the world
To close this knowledge gap, a team of more than 100 scientists from all over the world compiled and evaluated the most extensive and comprehensive data collection on arboreal biodiversity. To do this, they combined information from two global databases, one from the Global Forest Biodiversity Initiative (GFBI) and one from the TREECHANGE project. For these datasets, the tree populations of tens of thousands of test areas around the world were recorded. “Each of these datasets comes from someone who has recorded every single tree in a piece of forest and collected information about species, size and other characteristics,” explains
Gatti’s colleague Jingjing Liang, coordinator of the Global Forest Biodiversity Initiative.
However, much of the data from such surveys has not yet been published and has only been shared by scientists on request or as part of special projects. “There was no central repository for the valuable data these people collected,” says Liang. He therefore made it his task to set up such a central database. With approximately 38 million trees mapped and recorded from 90 countries, the GFBI database is the largest global repository of information about trees in the world to date. By combining this with a second large data collection, the team was able to extend this further. In the next step, Gatti and his colleagues divided the global land surface into 9,353 grid cells measuring 100 by 100 kilometers and used a statistical method to estimate the number of tree species at the biome, continent and world level.
South America has the most tree species
The evaluations showed that there are at least 73,274 tree species worldwide, of which around 9,200 have not yet been discovered or described. “The absolute number of tree species is thus considerably higher than previously assumed, there are 14.3 percent more species than were known to researchers,” write Gatti and his colleagues. The greatest diversity of species can be found in the tropical forests of South America in particular: “43 percent of all tree species grow in South America, followed by Eurasia with 22 percent, Africa with 16 percent and North America with 15 percent,” reports the team. South America is also the continent with the highest proportion of native species – almost half of all tree species growing there are endemic to the continent. There could also be more than 4000 tree species undiscovered. “Many of them probably grow in the Amazon basin and in the Andes and thus in biodiversity hotspots,” explains co-author Peter Reich from the University of Michigan.
The evaluations also showed that although many forests are dominated by a few common species, rare species represented by only a few specimens account for around a third of the global diversity of tree species. Some appeared only once or twice in the entire database, the researchers report. “This underscores the susceptibility of global forest diversity to anthropogenic changes, especially through land use and climate,” says Reich. “Because the survival of rare species is disproportionately threatened by such factors.” The research team hopes that their results can contribute to advancing knowledge about arboreal diversity, but also about the protection of forests.
Source: Roberto Cazzolla Gatti (Purdue University, West Lafayette) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2115329119