World Has Many More Trees Than Previously Thought, New Report Says
There are slightly more than three trillion trees in the world, a figure that dwarfs previous estimates, according to the most comprehensive census yet of global forestation. 
Using satellite imagery as well as ground-based measurements from around the world, a team led by researchers at Yale University created the first globally comprehensive map of tree density. Their findings were published in the journal Nature on Wednesday.
A previous study that drew on satellite imagery estimated that the total number of trees was about 400 billion. The new estimate of 3.04 trillion is multiple times that number, bringing the ratio of trees per person to 422 to 1.
While the density of foliage was surprisingly high overall, the researchers cautioned that global vegetation is still in decline. The number of trees on Earth has fallen by 46{154653b9ea5f83bbbf00f55de12e21cba2da5b4b158a426ee0e27ae0c1b44117} since the beginning of human civilization, according to the report. The researchers said they believed the findings would provide a valuable baseline for future research on environment and ecosystems.
“The previous information that we had about the global forest system all came from satellite images…we’re using all this information to provide more detailed understanding of what’s going on below the surface [of the canopy],” saidThomas Crowther, a postdoctoral fellow at the Yale School of Forestry and Environmental Studies and the lead author of the study. “The numbers that we’ve generated are going to be useful for a wide variety of applications.”
The map was generated using 429,775 ground-based measurements in more than 50 countries, collected from a variety of sources, including the Smithsonian Tropical Research Institute, the National Forest Inventory and several peer-reviewed studies, said Henry Glick, co-director of the Ucross High Plains Stewardship Initiative, a research program within the Yale School of Forestry and Environmental Studies.
The effort paired existing tree-count data, in which a person either counted or estimated the number of trees in a given area, with environmental characteristics such as temperature and elevation. This enabled them to get a more accurate count than the rough forest-cover estimates via satellite. To fill in the gaps where there were no field measurements, they made estimates based on tree-density trends in regions with similar environmental characteristics, Mr. Glick said.
Although the tree census yielded a much higher population than expected, the researchers said that human activity—whether through deforestation or general land use—has drastically reduced tree populations. They estimate that humans cut down more than 15 billion trees a year. Dr. Crowther said it was hard to accurately count regrowth and reforestation, in part because saplings are small, but the researchers estimated that new trees might reduce the net loss to about 10 billion.
“These numbers only seem large because we had no strong previous estimates,” said Dr. Crowther. “We’ve not discovered a load more trees. The current state of forests is exactly what it was before. We’ve just provided the information that scientists or environmental practitioners can use to start to rebuild forest ecosystems.”
Dr. Crowther said the findings should be useful for scientists in a variety of disciplines. Biologists will be able to use the tree density estimates to determine habitat suitability for different plants and animals. The estimates will also be critical in assessing forests’ impact on carbon in the atmosphere in different ecosystems, as well as projecting how tree populations will be affected by future climate conditions, he said.
“It’s exactly the kind of information we need to take the next step in terms of predicting the responses of forests to climate change,” saidScott Saleska, an associate professor in the department of Ecology and Evolutionary Biology at the University of Arizona who wasn’t involved in the study.
Write to Mark Armao at [email protected]
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