- In the biggest survey of the world's forests, scientists estimated there are 9,200 tree species unknown to them.
- Many of these mystery trees are likely rare and some could go extinct before they're catalogued.
- South America probably has about 40% of the unknown tree species, mostly in the Amazon rainforest.
Earth's forests are concealing about 9,200 species of trees that scientists have not yet discovered, according to a new estimate.
More than 100 scientists from all over the world built the largest global forest database to date, and in a study published Monday in the Proceedings of the National Academy of Sciences, they made a science-backed guess at how many types of trees they haven't discovered yet.
Many of the roughly 9,200 tree species are probably unknown to science because they are very rare, with small numbers of trees growing in limited territories, according to the new study.
That makes these mystery trees especially vulnerable to climate change.
"It is very possible we could lose undiscovered tree species to extinction before we even find them," Jingjing Liang, a forest ecologist at Purdue University and coauthor of the new study, said in a press release.
The largest database yet of Earth's trees
To create the largest known database of the world's trees, the researchers combined data from two global tree surveys — one from the Global Forest Biodiversity Initiative and one from TREECHANGE, an independent, worldwide tree-counting effort. The surveys include more than 38 million trees across 90 countries and 100 territories.
"Each [data] set comes from someone going out to a forest stand and measuring every single tree — collecting information about the tree species, sizes, and other characteristics. Counting the number of tree species worldwide is like a puzzle with pieces spread all over the world," said Liang, who coordinates the Global Forest Biodiversity Initiative.
The combined surveys documented 64,100 species of tree. But as with any type of plant, animal, fungus, insect, or bacteria, scientists know they haven't found every species on Earth.
So Liang's team used their new comprehensive data set to calculate tree-species diversity in different types of forests across different continents. They also analyzed the occurrence of rare species — trees that scientists on the ground only found once or twice as they surveyed forests.
By analyzing and modeling these trends, the scientists determined there are about 73,300 species of tree on the planet — 14% more than are currently catalogued.
"An accurate estimation of the number of tree species globally is a great piece of knowledge," Mo Zhou, associate professor of forest economics and management at Purdue University, said in a press release.
"No matter what will happen in future, our children and their children will know the Earth harbors approximately 70,000 tree species in the dawn of the 21st century."
Conservation is paramount in South America, home to many of the world's undiscovered trees
The researchers estimate that 40% of undiscovered tree species live in South America — a greater share of secret trees than any other continent. South America also has the highest number of rare tree species (about 8,200) and the largest share of tree species found only on that continent (about 49%), according to previous estimates.
Many of these unknown South American tree species are likely unique to the Amazon and the regions where the Andes mountains meet the rainforest, according to Peter Reich, a forest ecologist at the University of Minnesota and the University of Michigan.
That makes forest conservation a paramount priority in South America, Reich said in a statement. Protecting the Amazon is especially urgent, as logging, human-ignited fires, and climate change have culled about 17% of the rainforest in the last 50 years. The deforestation reached a new peak last year, when Brazil reported its highest land area lost since 2006.
In addition to forests performing key ecological functions, like filtering water and absorbing carbon from the atmosphere, there are economic benefits to preserving them.
In a 2016 data analysis published in the journal Science, Liang and his colleagues calculated that forest biodiversity creates five times more economic value than it costs to implement global conservation efforts — because forests provide lumber, fuel, paper, and other materials humans rely on.
"This study could contribute to tree and forest conservation efforts and the future discovery of new trees," Reich said.