There are potentially 9,200 undiscovered tree species on Earth, a full 14 percent more than the total tally of known tree species, according to an exhaustive multi-year census of global arboreal diversity that involved 150 collaborators, thousands of field studies, artificial intelligence, and supercomputer power.
A third of these unidentified tree species may be vanishingly rare, a finding that spotlights the fragility of forest ecosystems in the face of escalating human pressures, such as logging, anthropogenic fires, habitat loss, and climate change.
“We were surprised not only by the number [of estimated trees], but also by the implications,” said Roberto Cazzolla Gatti, an associate professor of biological diversity and conservation at the University of Bologna in Italy who led the international collaboration, in a call. “If we don't know, in 2022, 9,000 tree species, imagine how many other species we don’t know. That is the first thing that drives us.”
Gatti and the team estimated that some 73,300 tree species currently exist on Earth, of which about 64,100 are known to humans, a number that is “considerably higher than previously reported,” according to a study published on Monday in Proceedings of the National Academy of Sciences.
The researchers were able to calculate this number based on “species accumulation curves,” which estimate the total number of species in an area by extrapolating from a finite number of species that can be observed during a survey. For instance, examining the proportion of observed abundant and rare species can help scientists predict the presence of undiscovered types of species that might not show up in any given survey.
Conservation was another motivator behind the study, and Gatti noted that an estimated 3,000 undiscovered species are likely to be rare plants that inhabit South America’s tropical or subtropical biomes. “We are destroying these areas and these forests so fast,” Gatti continued. “We could lose them before we discover them.”
Gatti and his colleagues produced the new estimate by stitching together a global dataset of 40 million trees observed in nearly 10,000 field studies of local forests. The team then statistically analyzed the data with a supercomputer at Purdue University’s Forest Advanced Computing and Artificial Intelligence (FACAI) Laboratory, which produced what Gatti called a relatively conservative estimate of 9,200 unidentified tree species.
A third of those missing species worldwide are likely to be rare, which makes them especially vulnerable to endangerment or extinction as a result of human encroachment. Losing these species also contributes to broader destabilization of forest ecosystems by reducing biodiversity.
“If we continue destroying, at this huge rate, the current forest, we risk losing important diversity, which is actually something that keeps the ecosystem alive,” Gatti explained. “Without this resilience that is based on a sort of insurance that rare species, that richness of species, gives to the ecosystem, of course, we will risk having an ecosystem that is very unhealthy and the first disturbance could really destroy it. It could be climate change or something else.”
Though the new study is the most robust estimate of global tree richness on record, the researchers plan to build on the findings to produce increasingly accurate counts in the future.
“It's very important that people don't think there are exactly 9,000 other species that we need to discover before completing our count,” Gatti said. “It could be more, or it could be less. It depends on how we'll improve our knowledge.”
For the time being, this enormous new census of trees underscores how much there is to discover in Earth’s forests, and the dire need to protect these regions from the damaging effects of human activity. In addition to serving as a foundation of ecosystems around the world, trees are also an essential part of human culture and identity. To that end, ensuring that forests remain as pristine as possible is not only important for maintaining biodiversity, but also for nurturing our own ancient connection to woodland wilderness.
“Trees are in our imagination and are also something spiritual—not only pieces of wood,” Gatti said. “We talk about the Tree of Life or the Tree of Knowledge.”
“We know the importance of trees but we accept that we can remove most of them for our short-term benefit,” he concluded. “To discover potentially 9,000 species we need years, maybe decades, but if you want to discover these species, we need also to think in the long-term, not only in the short term that the economic cycle pushes us to do.”