Counting trees: 143 scientists just did

New global research estimates that the world’s tropical forests are home to between 40,000 and 53,000 species of trees, some of them extremely rare. pixabay

Forget about finding the needle in the haystack. An even more complicated task might be counting all the hay on the farm.

A new study documents the results of just such a search, undertaken across the globe, to count the number of tree species found in the planet’s tropical forests.

By the authors’ estimate—all 143 of them—Earth’s tropical forests are home to between 40,000 and 53,000 species of trees, some of them extremely rare.

The paper represents one of the first attempts to construct a global database of tropical tree species.

Previously, such surveys had counted trees only in specific areas, without any kind of broad consistency. Other studies had estimates including 37,000, 43,000, and 50,000, which were “based on expert opinion,” the researchers wrote.

“It was a little piecemeal,” says Center for International Forestry Research (CIFOR) principal scientist, Terry Sunderland, one of the study’s authors. “It’s about reassessing the state of the knowledge and understanding where the gaps are and how we can really undertake these global analyses.”

THE GLOBAL TREESCAPE

To conduct the study, researchers created a database of more than 650,000 trees from more than 11,000 species, found in the tropical regions of Africa, Asia, and Latin America.

This approach had the benefit of bringing together researchers from all over the world, and putting researchers from developing countries on the international publishing stage.

Their results upend some previous assumptions of tropical tree speciation.

Latin America, for example, had long been considered to be home to the most diverse forests, but the study showed that Southeast Asia was in many areas just as diverse, with both regions supporting between 19,000 and 25,000 species.

Africa, by contrast, had just 4,500 to 6,000 species.

That number, though, still far exceeds what’s found in forests elsewhere: Europe’s temperate forests contain just 124 species. These disparities largely come from differences in rainfall among the regions, as high rainfall is generally correlated with more plant biodiversity.

“This research tells us that if we’re interested in conserving biodiversity, this is where we need to focus,” Sunderland says. “I think it’s an incredibly important management tool.”

Previous research indicated that high biodiversity means high biomass.

Surveying tree species, then, could better inform programs like REDD+, which seeks to both mitigate climate change and protect plant and animal species by offering credits in exchange for keeping forests intact.

BUT HOW MANY, EXACTLY?

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A price tag on the environment?

The study reveals that tropical forests are home to myriad numbers of species that have very small population sizes, though it’s unclear just how many species or how small the populations are. That’s a question left to future researchers. Some of these species could be endangered and in need of protection, while others could simply be rare.

“That’s one of the major problems of forests,” Sunderland says. “There’s a lot of speciation patterns that you can’t say, ‘we’re going to protect an area here and it’s going to protect all the biodiversity.’ It doesn’t work like that.”

In addition to informing conservation decisions today, tree surveys lay the groundwork to which future studies can be compared—an especially important role as forests evolve along with the climate.

Changes in forests that had previously been documented over millennia are now occurring much faster, Sunderland says.

Warmer conditions in the tropics can lead to reduced rainfall, inciting drought that can be dangerous for forests. Combined with deforestation, drought could decimate tree populations.

Collecting knowledge about the state of the trees today will be crucial in assessing and perhaps lessening the damage over the coming decades.

“It’s not only an academic exercise, which is interesting and important, but it provides a baseline to start monitoring patterns and trends as things change with climate change, for example,” Sunderland says.

“Providing that baseline allows us to understand the speed and the nature of that change.”

For more information about the tropical species count, please contact Terry Sunderland at t.sunderland@cgiar.org