Over 9,000 of the world's tree species are yet to be discovered, according to a major international study involving more than 100 scientists and the largest forest database ever created.
The research, which scientists compared to a jigsaw puzzle “with pieces spread all over the world”, estimates that there are around 73,000 tree species on the planet in total.
This is around 14 per cent higher than the current number of known tree species.
The study suggests most of the undiscovered tree species are rare, with very low populations and limited distribution. This makes them particularly vulnerable to human-caused disruptions such as deforestation and mounting greenhouse gas emissions causing the global climate crisis.
Around 40 per cent of the undiscovered species are likely to be in South America, the research team said, highlighting the importance of preventing illegal logging and unregulated clearances for agricultural purposes.
The findings will help inform forest conservation efforts, the authors of the study said.
“These results highlight the vulnerability of global forest biodiversity to anthropogenic changes, particularly land use and climate, because the survival of rare taxa is disproportionately threatened by these pressures,” said University of Michigan forest ecologist Peter Reich, who was one of the senior authors on the paper.
“By establishing a quantitative benchmark, this study could contribute to tree and forest conservation efforts and the future discovery of new trees and associated species in certain parts of the world,” he said.
In order to come up with the estimates the researchers combined tree abundance and occurrence data from two global datasets. Together these databases yielded a total of 64,100 documented tree species worldwide, a total similar to a previous study that found about 60,000 tree species on the planet.
“We combined individual datasets into one massive global dataset of tree-level data,” said the study's other senior author, Jingjing Liang of Purdue University, and coordinator of the Global Forest Biodiversity Initiative.
“Each 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.”
After combining the datasets, the researchers used novel statistical methods to estimate the total number of unique tree species at biome, continental and global scales – including species yet to be discovered and described by scientists. A biome is a major ecological community type, such as a tropical rainforest, a boreal forest or a savanna.
Their “conservative estimate” of the total number of tree species on Earth is 73,274, which means there are likely about 9,200 tree species yet to be discovered.
The team said their study uses a “vastly more extensive” dataset and more advanced statistical methods than previous attempts to estimate the planet's tree diversity.
They said they used modern developments of techniques first devised by mathematician Alan Turing during World War II to crack Nazi code, Dr Reich said.
As well as being the continent with the greatest proportion of undiscovered tree species, South America is also the continent with the highest estimated number of rare tree species (about 8,200) and the highest estimated percentage (4 per cent) of continentally endemic tree species, meaning species found only on that continent.
The researchers said hot spots of undiscovered South American tree species likely include the tropical and subtropical moist forests of the Amazon basin, as well as Andean forests at elevations between 1,000 metres (about 3,300 feet) and 3,500 meters (about 11,480 feet).
“Beyond the 27,000 known tree species in South America, there might be as many as another 4,000 species yet to be discovered there,” said Dr Reich.
“This makes forest conservation of paramount priority in South America, especially considering the current tropical forest crisis from anthropogenic impacts such as deforestation, fires and climate change,” he said.
Worldwide, roughly half to two-thirds of all already known tree species occur in tropical and subtropical moist forests, which are both species-rich and poorly studied by scientists.
Tropical and subtropical dry forests likely hold high numbers of undiscovered tree species as well.
“Extensive knowledge of tree richness and diversity is key to preserving the stability and functioning of ecosystems,” said study lead author Roberto Cazzolla Gatti of the University of Bologna in Italy.
Forests provide many “ecosystem services” to humanity. In addition to supplying timber, fuelwood, fibre and other products, forests also clean the air, filter the water, and help control erosion and flooding, the scientists said.
They help preserve biodiversity, store climate-warming carbon, and promote soil formation and nutrient cycling while offering recreational opportunities such as hiking, camping, fishing and hunting.
The research is published in the Proceedings of the National Academy of Sciences.