- The most comprehensive survey of Earth’s tree life has just been published, showing that there are some 9,000 species that scientists still haven’t described.
- Nearly half of these unknown trees are found in South America, which in turn accounts for 43% of the estimated 73,000 trees found on Earth, according to the study.
- Almost 150 researchers from across the globe collaborated on the study, which increases the previous estimate for total tree species by 14%.
- The study authors say the unidentified species are mostly rare and more vulnerable to the risk of extinction, hence there’s an urgent need to implement stricter protection and enforcement of environmental laws.
“Since I was a kid, I have always been fascinated by the diversity of living beings on our planet and I dreamed to work to explore Earth’s unexplored places and discover the unknown species living on our planet,” says Roberto Cazzolla Gatti. “Then, as a biologist, during the last years, I dedicated much work to the open questions of biodiversity and one of them is how to estimate it with the smallest error and how to protect them.”
With this goal in mind, Cazzolla Gatti, from the University of Bologna in Italy, led a study that culminated in the most comprehensive global georeferenced survey of trees ever done.
The result of the work surprised even the nearly 150 researchers from around the world who were involved in it. It took years to create this gigantic database, involving the participation of all kinds of experts, as well as the help of supercomputers and artificial intelligence.
Previous estimates put the number of tree species worldwide at about 64,000. This new study, however, bumps up that figure by 14%, at 73,300 species. Of these, 9,000 are still unknown to science, and thus very rare.
“The result left us stunned,” Cazzolla Gatti says. “We would never have imagined that there would be so many species of trees yet to be discovered. In fact, this study offers the scientific community, and humanity in general, more knowledge about their incredible diversity. It is even more incredible to think that we don’t even know all of them yet [in 2022]!”
Renowned for its biodiversity, South America is home to the highest number of tree species, according to the study, at 43% of the total. Next comes the Eurasia region (22%), Africa (16%), North America (15%), and Oceania (11%).
South America is also the continent with most trees still undescribed by science — somewhere around 4,000.
“The richest areas are where the Andes meet the Amazon,” says Oliver Phillips, professor of tropical ecology at the University of Leeds, U.K. “This is where the diversity is high and also the number of undescribed species is likely to be higher.
“Because of the complex topography, climates and geology, there is more diversity and many species will have a smaller distribution,” he adds. “This makes them more difficult to discover and also more vulnerable.”
A specialist in tropical forests and coordinator of the Amazon Forest Inventory Network (Rainfor), Phillips is one of the 148 authors of the study published in early February in the Proceedings of the National Academy of Sciences (PNAS), which also includes several Brazilian researchers.
Amazon Basin: A biodiversity hotspot
At least 31,000 species of trees grow in South America alone. Many of them are endemic to the continent, meaning they don’t occur natively anywhere else on the planet. Their habitats include the tropical and subtropical forests of the Amazon, in countries such as Peru, Colombia, Ecuador, Venezuela, Bolivia and Brazil, as well as the high altitudes of the Andes region.
In this green immensity — there are an estimated 390 billion trees in the Amazon Basin alone — two-thirds of the species are considered very rare.
“These are species that have less than 1 million individuals,” says co-author Wendeson Castro, a botanist at the Federal University of Acre in Brazil. This is the case for the species Pouteria sessilis, commonly called abiu and endemic to Peru, Castro says. “We are still very ignorant about the number of species we have and how many we are losing,” he adds.
This is why the new study, effectively an inventory of Earth’s tree life, is so relevant. With the certainty that we still lack knowledge about the biodiversity of global forests, and especially those of South America, it’s essential to implement stricter protection and inspection laws, the authors say.
“Our results reinforce the absolute necessity of preserving tropical forests,” says co-author Jorcely Barroso, a professor at the Federal University of Acre. “Unidentified species are mostly rare and more vulnerable to the risk of extinction. Having a better understanding of these numbers and possible locations is essential in developing protection strategies, even without knowing them.”
Barroso says the large number of species not yet documented, probably occurring in the Amazon Basin and in the transition zone between the Andes and the Amazon, makes these areas a priority for conservation.
Unknown and endangered
Despite the good news that there are more tree species on Earth than previously estimated, one of the main concerns that the study highlights is the impact of human activities on their preservation.
For instance, the climate in South American forests, where most of the unknown species are found, has been stable for several centuries. But in the face of more recent phenomena, such as climate extremes, there’s concern that the plant life of this region is facing difficulty adapting to the impacts of these changes, such as prolonged droughts and more frequent fires.
“They are more vulnerable to habitat loss, especially since their range is small to begin with,” Phillips says. “But if we can keep some elevation corridors of forest habitat protected, the species are more likely to migrate upward to avoid the heat and drought of climate change.”
The researchers say species going extinct isn’t the only problem. In the face of all these vulnerabilities and threats, trees may cease to provide the essential environmental services that humankind needs for survival and the planet needs to remain in balance.
“We often think that trees and forests are just our oxygen producers, but they are actually much more than that,” Cazzolla Gatti says. “Without them we would have no clean water, no safe mountain slopes, no habitat for many animals, fungi and other plants. We would lose the most biodiverse terrestrial ecosystems, the sinks for our excess carbon dioxide, and the purifiers of our polluted air.”
He adds that if we want to protect the diversity of trees and the incredible wealth of other plant and animal life linked to them, we must immediately stop deforestation and forest degradation and start considering forests as untouchable ecosystems that deserve protection, like coral reefs.
As an example, Cazzolla Gatti says, “Wood, pulp and paper should only be harvested from human-grown plantations and not from natural or semi-natural forests.
“If we stop burning forests by considering them ‘biofuels,’ buying products containing palm oil, using tropical timber for our homes, eating large amounts of meat every day, replacing nature with urbanized and industrial areas, and killing and polluting the wildlife that tree ecosystems preserve, we will have time to discover new species and allow our planet to have a more balanced existence, as well as the human species itself.”
Cazzolla Gatti, R., Reich, P. B., Gamarra, J. G. P., Crowther, T., Hui, C., Morera, A., … Liang, J. (2022). The number of tree species on Earth. Proceedings of the National Academy of Sciences, 119(6), e2115329119. doi:10.1073/pnas.2115329119
Ter Steege, H., Pitman, N. C. A., Sabatier, D., Baraloto, C., Salomão, R. P., Guevara, J. E., … Silman, M. R. (2013). Hyperdominance in the Amazonian tree flora. Science, 342(6156), 1243092. doi:10.1126/science.1243092
Banner image of the rainforest in the Middle Amazon region, by Neil Palmer/CIFOR.