Trees are one of the most important lifeforms on our planet. As primary producers, they provide oxygen and food for countless species. Their roots hold soil in place, preventing erosion. Their canopies create habitats and moderate climate. Trees also have deep cultural significance in many human societies. With so many benefits, it’s clear that trees are essential for life on Earth.
But how many different tree species exist? Getting an accurate count is challenging, but scientists estimate there are 60,065 to 100,000 tree species worldwide. This number may seem surprisingly large! So how can we break it down and get a better sense of tree diversity?
What is a tree?
To estimate global tree diversity, we first need to define what constitutes a tree. Botanists consider a tree any perennial woody plant that develops many secondary branches supported clear of the ground on a single main stem or trunk. Based on this definition, trees can be found in hundreds of plant families.
Trees also vary enormously in height, from tiny dwarf willows under 6 inches to towering redwoods over 350 feet. But in general, a tree is considered a woody plant over 13 feet tall with a single stems and a more or less definite crown shape.
Using this flexible definition allows us to categorize many different growth forms as trees, from upright trees with central trunks to spreading shrubs. There are also palm trees, bamboo, tree ferns and other monocots that meet the tree criteria despite anatomical differences from most dicot trees.
Counting tree species
With a working definition of a tree, scientists can attempt to tally up all the tree species on Earth. But this is far from straightforward, as new species are constantly being discovered while the status of known species is frequently revised.
One authoritative estimate comes from botanist Peter Raven, president emeritus of the Missouri Botanical Garden. In his 2020 book “Trees of the World,” Raven suggests there are 60,065 tree species globally. This number is based on current scientific descriptions as well as projections for unknown species, especially in the tropics.
A more recent estimate comes from a 2015 study published in Proceedings of the National Academy of Sciences. The researchers used a complex computational analysis to predict a total of 73,300 tree species worldwide. Factoring sampling biases and undiscovered species, they give a final best estimate of 100,000 tree species globally.
Other experts argue the true number is likely somewhere between these two estimates, in the range of 70,000 to 80,000 species. But there is ongoing debate, as more comprehensive field surveys and genetic analyses will help refine the count.
Where are all the tree species found?
Tropical regions are believed to contain the vast majority of tree species diversity. Estimates suggest the tropics are home to:
- South America – 27,000 species
- Africa – 17,000 species
- Southeast Asia – 15,000 species
In contrast, Europe and North America combined only have about 1,300 native tree species. Higher plant diversity in the tropics is attributed to warmer climates as well as the lack of mass extinction events like glaciation.
Brazil likely has the most tree species of any country, with recent estimates ranging from 8,000 to 9,000 native species. Other particularly diverse countries are Colombia, Indonesia, Malaysia and Australia.
These tropical countries continue to yield new species as botanists survey intact forests. For example, over 100 new tree species have been described in the Amazon rainforest in the past decade.
Why do estimates vary so much?
Getting an exact count of global tree diversity is nearly impossible for several reasons:
- Undiscovered species: Scientists believe there are thousands of undiscovered tree species hidden in remote jungles and forests.
- Unresolved taxonomy: Many known tree species are poorly studied and not correctly classified, skewing diversity measurements.
- Rapid deforestation: Habitat loss inevitably leads to extinction of unknown species before discovery.
These factors make determining definitive tree numbers quite challenging. Progress requires extensive field studies as well as advanced molecular genetics to better understand relationships between closely related species.
Which families have the most tree diversity?
Certain plant families contain an outsized number of the world’s tree species. Five key families with high tree diversity are:
- Legumes: About 13,000 tree species, including acacias and mesquites.
- Myrtaceae: Over 5,500 species, including eucalyptus and guava.
- Rubiaceae: Around 4,000 species, including coffee and gardenia.
- Oaks: More than 600 oak tree species in the genus Quercus.
- Laurels: Over 3,000 species, including cinnamon and avocado.
These five families account for at least one-fourth of all tree species. Understanding the exceptional diversity in these groups can reveal patterns about wider species radiations.
Which are the largest tree genera?
Looking at tree diversity by genus, groups with the most species include:
- Eugenia (Myrtaceae family): ~1,000 species
- Ficus (Mulberry family): ~800 species
- Syzygium (Myrtaceae): ~500 species
- Psychotria (Rubiaceae): ~1,600 species
- Ocotea (Laurels): ~350 species
Notably, the two largest tree genera – Eugenia and Ficus – both belong to families identified as highly diverse. This illustrates the tendency for tree diversity to be concentrated in certain lineages.
How does tree diversity vary by region?
To visualize geographical patterns, here is a breakdown of tree diversity by major world regions:
| Region | Estimated Tree Species |
|---|---|
| Neotropics (Central & South America) | 40,000 |
| Afrotropics (Sub-Saharan Africa) | 16,000 |
| Indo-Malaya (South Asia & Southeast Asia) | 15,000 |
| Australasia | 9,000 |
| North America | 1,000 |
| Europe | 500 |
This summary illustrates the disproportionate diversity in tropical regions compared to temperate areas. South America alone may contain almost 10 times as many tree species as Europe and North America combined.
Threats to global tree diversity
While the exact number is uncertain, it’s clear Earth harbors tens of thousands of unique tree species. Unfortunately, many are threatened by human activities like:
- Deforestation for agriculture and development
- Logging, both legal and illegal
- Climate change altering forest habitats
- Spread of invasive pests and pathogens
Scientists estimate over 8,000 tree species globally are threatened with extinction. The loss of abundant foundational tree species can degrade entire ecosystems.
Preserving tree diversity is crucial for maintaining productive forests, robust wildlife populations, clean air and water, and human livelihoods. Sustainable forestry practices, setting aside protected areas, fighting illegal logging and curbing climate change are key to conserving trees worldwide.
Why tree diversity matters
With so many tree species on the planet, why does diversity matter? Reasons we should value global tree diversity include:
- Ecosystem stability: Diverse forests tend to be more resilient to disturbances.
- Wildlife habitat: More tree species supports more animal specialization and biodiversity.
- Genetic resources: Broad gene pools provide raw material for adaptation and crop breeding.
- Cultural value: Many human groups have deep ties to regional tree species.
- Ecotourism appeal: Visitors flock to experience highly biodiverse tropical forests.
Preserving global tree diversity is both an ecological and ethical priority. Their minimal management needs compared to intensive crops also make diverse natural forests highly economical.
Conclusion
Estimating Earth’s total tree diversity is challenging, but most experts agree over 60,000 species exist based on available data. The vast majority are found in tropical regions, with many still undiscovered.
Precise species numbers remain elusive due to factors like incomplete sampling, disputed taxonomy and habitat loss. But ongoing field studies and genetic analysis promise to yield a clearer global picture over time.
What is certain is that thousands of unique and ecologically important tree species exist worldwide. Conserving these irreplaceable resources in the face of extensive threats is imperative for the future of both forests and human societies.
