How many tree species are there on Earth?
Breakthrough research has revealed the first estimate of how many species of tree exist — but finding the answer has been a global challenge.
A new study, with major contributions by research networks RAINFOR, AfriTRON, and ForestPlots.net, has produced the first “scientifically credible” estimate of the number of tree species on the planet.
The researchers, including Professor Oliver Phillips from the University of Leeds, have found that there are 14% more tree species than previously thought.
Here, Professor Phillips describes the challenges and importance of global collaboration when trying to answer the question: “How many tree species are there on Earth?”
A simple question that’s hard to answer
At the core of the problem is that we can’t simply just count trees. There are far too many for any one group of experts to ever get close to the total. And, even if we could assemble the information from different groups in one place, there are still other practical limitations to a full count. Most grow in tropical forests, and many of these are still very hard to reach.
Most species are rare, and rare trees need their leaves, flowers and fruits to be collected in order to be identified. To be sure often requires comparing collections with existing knowledge and records scattered across the world. All this is very difficult to achieve.
It’s vital to try though. Tree diversity is fundamental for the stability of forest ecosystems and the many services they provide humanity.
Knowing how many species there are — and especially where diversity and rare species are concentrated — is essential if we are to protect them, the carbon they store, and the many unique plants and creatures sheltered beneath their boughs.
With old-growth forests still being destroyed across the world, our lack of awareness about what is being lost is of great concern. Meanwhile, politicians often talk about “planting a trillion trees” — as if simply covering land with new trees will ever compensate for losing natures’ masterpieces.
Because of the limited available data, estimates of global tree diversity still rely heavily on published lists of species descriptions that are geographically uneven in coverage.
We know that some areas are very rich. In the western Amazon, it is possible to encounter more than 300 tree species in a single hectare of forest (a 100 by 100 metre patch), many times more than in the entire British Isles. But just how many are found across the whole Amazon Basin, and where, has long been the focus of enquiry and dispute.
A new attempt to answer a big question
By combining tree collection records from across the planet — with an unprecedented compilation of hundreds of thousands of samples by the world’s ecologists — it has been possible to piece together a new picture of how many rare species there are, including those yet to be identified, and where they are concentrated.
Major contributions have come from the growing networks of tropical researchers and ecological plots, including RAINFOR, AfriTRON, and ForestPlots.net — all coordinated from the University of Leeds.
This brings generations of scientific expertise and their extraordinary effort together, to help answer questions that were impossible to achieve alone.
Our new estimate of global tree richness — roughly 73,300 species — means that around 9,000 are still to be discovered by science.
Almost all these undiscovered species are rare, restricted to quite small areas, and tropical. This highlights the great vulnerability of tropical tree diversity to deforestation and global heating.
The neglected grand challenge
At least 31,000 tree species grow in South America alone — representing an astonishing 43% of the Earth’s total tree diversity.
With 3,900 tree species yet to be discovered in this one continent, our analysis shows that many are concentrated in endangered hotspots of diversity where the Amazon forest meets the Andes in Peru, Colombia, Ecuador, Venezuela and Bolivia.
This makes forest conservation of paramount priority in South America.
In light of the Amazon — Andes epicentres of tree diversity, and huge scientific deficit in understanding what is there, a key challenge now is to support Latin American researchers in their vital work. Many colleagues working with RAINFOR have been critical for this discovery, yet all too often the contributions from botanical experts in the tropics are undervalued.
In fact, collecting, identifying, monitoring and protecting the world’s richest forests represents the neglected grand challenge of our age. It has never been more urgent to invest properly in this vital work, and those leading it on the ground.
Oliver Phillips is Professor of Tropical Ecology in the School of Geography at the University of Leeds.
The paper: The number of tree species on Earth (Gatti, R.G. et al. (2022)) is published in the journal Proceedings of the National Academy of Sciences, USA.
