Combination approach uses ground-based surveys and satellites to find many more trees than anticipated.
There are roughly 3 trillion trees on Earth — more than seven times the number previously estimated — according to a new tally by an international team of scientists. The study also finds that human activity negatively affects tree abundance from the boreal forests to the equator. Roughly 15 billion trees are cut down each year, the researchers estimate; since the onset of human civilization, the global number of trees has dropped by roughly 46%.
“The scale of human impact is astonishing,” says Thomas Crowther, an ecologist at the Netherlands Institute of Ecology in Wageningen who led the study while at Yale University in New Haven, Connecticut. “Obviously we expected humans would have a prominent role, but I didn’t expect that it would come out as the as the strongest control on tree density.”
The widely accepted previous estimate of the world’s tree population, about 400 billion, was based largely on satellite imagery. Though remote imaging reveals a lot about where forests are, it does not provide the resolution of a person counting trunks in a particular plot. Crowther and his colleagues merged these approaches by first gathering data from more than 400,000 ground-based counts reported in forestry inventories and the scientific literature for every continent except Antarctica. These counts allowed them to improve tree-density estimates based on satellite imagery. Then the researchers applied those density estimates areas that lack good ground inventories. For example, ground-truthed data from forests in Canada and northern Europe were used to revise estimates from satellite imagery for similar forests in remote parts of Russia.
“It’s not like we discovered new trees,” says Crowther. “Rather, we added another layer of information that allowed us to revise much of the previous estimates.”
Improved population estimates could help managers weigh the economic benefits that forests provide in terms of water purification, soil conservation and other functions against those of harvesting timber or clearing trees for farmland, says ecosystems services ecologist Becky Chaplin-Kramer of Stanford University in California. “It’s great when we can fill in gaps like this,” she says.
The highest tree densities, calculated in stems per hectare, were found in the boreal forests of North America, Scandinavia and Russia. These forests are typically tightly packed with skinny conifers and house roughly 749.34 billion trees, 24.28% of the global total. Tropical and subtropical forests, with the greatest area of forested land, are home to 1.29 trillion trees, or 42.82% of the total.
The new numbers raise questions about which species are represented where and how particular forest types evolve, says biogeochemist Susan Trumbore of the Max Planck Institute for Biogeochemistry in Jena, Germany.
The number of trees is just one piece of the puzzle,” Trumbore says. “A tree in the tundra is not the same as a tree in the rainforest.”
Crowther cautions that even though the new figures do not change the current science on carbon storage or diminish the impact of deforestation. “We’re not saying, ‘Oh, everything’s fine’.”
In fact, the work suggests that in some places where one might expect trees to thrive, such as warm regions with lots of moisture, human activities like farming have pushed forest to the margins.
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Ehrenberg, R. Global forest survey finds trillions of trees. Nature (2015). https://doi.org/10.1038/nature.2015.18287