How Many Tree Species are There in the World?

A joint effort of a global tree scientist community finally answers a long-standing question about the total number of the world’s tree species.

The advancements in tracking and recording everything happening on Earth have been stunning over the last few decades. As for species detection, we have come a long way since Carl Linnaeus and his contemporaries’ painstaking work of naming all the plant species they could get their hands on, waiting for months for the samples to arrive via sailing boats.

Today, we have much more people doing the work and much more technical resources – the planes, the satellites, all the advanced techniques and technology. 

In that light, the fact that the total number of the world’s tree species has been unknown for so long is perplexing. 

When we say “undiscovered species,” we often imagine elusive, fast-moving, and likely small animals able to escape the scientists’ observations. We intuitively tend to think that most of the world’s plant species – especially the large ones such as trees – are already known. After all, they’re unable to run away and are basically standing there waiting to be discovered, right?

It turns out that the story is much more complicated. Fortunately, in the last few years we have gained a much better insight into the richness of the world’s tree species – but also into the problems faced by scientists doing this fundamental research.

The first estimation of tree species

In 2017, the scientist crew from the UK’s Botanic Gardens Conservation International have compiled the first comprehensive list of all the world’s tree species. 

The organization gathered the data from more than 500 publications, with the involvement of 80 experts from their associate network over a period of two years.

They have estimated that there are 60,065 species of trees globally. Many species on their list are endangered and more than half are present only within a single country.

BCGI has made its database public under the name The Global Tree Search. The database is continuously updated and the base of the Global Tree Assessment, an ambitious project by BGCI and IUCN that aims to have the first IUCN Red List assessment for every tree species published by 2023.

Although nearly on-spot, the problem with the BCGI study is that it hasn’t accounted for all the tree species that have remained unknown to science.

The unknown tree species problem

While the number of undiscovered animals is undoubtedly higher than the number of plants, it doesn’t mean that we have even come close to discovering all the tree species in the world.

Besides discovering the number of the described tree species, the latest and biggest-yet study on the global tree biodiversity came with the idea to try to model those “missing” tree species and their numbers – to finally get the proper estimate of total tree species on Earth, whether we’ve found them or not.

How many species of trees are out there?

The study titled “The number of tree species on Earth” was published in the Proceedings of the National Academy of Sciences this February. The international team used a ground-sourced global database to assess the number, with significant contributions from research networks RAINFOR, AfriTRON, and ForestPlots.net.

That led them to note ~64,000 tree species are already known to science – quite close to the results of the UK BCGI study.

However, since there are undiscovered tree species too, they had to compare species accumulation curves (SACs) of tree species across different spatial scales to estimate the number of species that have not (yet) been recorded in the data compilation they used for the research. 

Using this methodology, they have assessed that more than 9,200 tree species remaining to be discovered. 

Adding the two figures leads to the conclusion that there are approximately 73,300 extant tree species in the world.

So, why did we have to wait so long to get these figures?

Like other similar studies on tree species richness, this one also faced the challenge of uneven data. While some regions are well-explored, other areas – especially in the highly biodiverse tropics – remain painfully undersampled. 

Oliver Phillips, a professor of Tropical Ecology in the School of Geography at the University of Leeds who participated in the study, explains the challenge of tracking down and list all of the world’s tree species. 

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, 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 inquiry and dispute.

Oliver Phillips, School of Geography, the University of Leeds

It is precisely these poorly surveyed and highly biodiverse areas that hide the undiscovered tree species. Although they come from different meridians, these species share some common traits.

• They come from tropical or subtropical regions.
• Most are local or endemic, with low populations and limited spatial distribution.
• Their distribution is most likely to be in remote tropical lowlands and mountains. 
• Roughly 40 percent of undiscovered tree species are in South America.
• Over one-third of them are rare, with many at risk of extinction – even before they get discovered.

The importance of the tree diversity studies

Understanding global tree diversity with more precision has several practical implications for biodiversity and conservation.

First, on a fundamental level, it can help better comprehend the evolutionary mechanisms behind the tree diversity and predict how they could impact tree populations in the future. 

That is especially important in times of global climate change – and the second potential bonus of this study is helping predict which tree communities may be the most resilient to it. 

Thirdly, having a better idea about the undetected species, their rarity and distribution is essential for sound environmental management and taking the suitable measures to prevent species loss. It can boost local and regional conservation efforts and improve the ability to predict extinctions, manage biodiversity hotspots, or set priorities for seed collection.

Conclusion

For all the centuries of studying trees and millennia of worshiping them, it is strange and nearly shocking that we haven’t been sure of the tree species number for so long. But don’t think that scientists were lazy. Underfunding this kind of fundamental research has been a lingering issue for an embarrassingly long time – as the words of professor Phillips testify. 

“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.”

These words can be a foreboding conclusion to this story. It would be a shame for a civilization that can venture into space to lose its tree species before it has even gotten to know them.

Resources

The Studies:

Beech, E., Rivers, M., Oldfield, S., Smith, P.P. (2017) GlobalTreeSearch: The first complete global database of tree species and country distributions, Journal of Sustainable Forestry, 36:5, 454-489, DOI: 10.1080/10549811.2017.1310049 https://www.tandfonline.com/doi/full/10.1080/10549811.2017.1310049

Gatti, R.C., et al (2022) The number of tree species on Earth. PNAS vol. 119 (6) e2115329119 | https://doi.org/10.1073/pnas.2115329119 https://www.pnas.org/doi/10.1073/pnas.2115329119

Articles:

Earth may have 9,200 more tree species than previously thought. Jude Coleman. Science News. 7 February 2022  https://www.sciencenews.org/article/tree-species-earth-biodiversity

First-of-its-kind estimate of the total number of tree species. Purdue University. 31 January 2022. https://www.purdue.edu/newsroom/releases/2022/Q1/first-of-its-kind-estimate-of-the-total-number-of-tree-species.html

How many tree species are there on Earth?. University of Leeds. Medium. 2 February 2022 https://medium.com/university-of-leeds/how-many-tree-species-are-there-on-earth-37c581b8c9af

New Survey Estimates Earth Has 60,065 Tree Species. Jason Daley. Smithsonian Mag. 11 April 2017. https://www.smithsonianmag.com/smart-news/earth-has-60065-tree-species-180962830/

There Are More Than 60,000 Tree Species Worldwide, Scientists Say. Merrit Kenenedy. NPR. 12 April 2017 https://www.npr.org/sections/thetwo-way/2017/04/12/523616033/there-are-60-000-species-of-tree-worldwide-scientists-say

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