Though trees are hard to miss, they are also hard to quantify. They are not even always easy to identify. “Their crowns are hundreds of feet up; they’re in between other things; they look like similar [species],” says Miles Silman, a conservation biologist at Wake Forest University, who was not involved in the new study. “It’s a rare breed of person that sits out in the wild for months on end and looks at every single tree.”
The new research drew on the efforts of hundreds among that rare breed from around the world. These contributors have cataloged trees in two huge data sets: One, the Global Forest Biodiversity Initiative, records every species found in extensively documented forest plots around the globe. The other, TREECHANGE, compiles sightings of individual species.
The study’s authors used these databases to calculate that there are approximately 64,100 known tree species on the planet—up from previous estimates of around 60,000. South America has the highest tree biodiversity, representing 43 percent of species, the team found, followed by Eurasia, with 22 percent, Africa, with 16 percent, North America, with 15 percent, and Oceania, with 11 percent.
To arrive at their estimate of 9,200 still unknown trees, the researchers extrapolated from the number of rare trees already in the databases. They used this strategy because most unknown trees on the earth are likely to be rare species, found in limited numbers in small geographical areas, says study co-author and Purdue University quantitative forest ecologist Jingjing Liang.
Searching for the new species will inform not only conservation but the basic evolutionary science of how and why species diversify and die out, Kerkhoff says. “Just the fact that there are thousands of species of something as common as trees out there that are still left to be discovered,” he adds, “I find pretty inspirational.”
A version of this article with the title “Hidden Forests” was adapted for inclusion in the May 2022 issue of Scientific American.
