How many trees are there in the world? Three trillion, according to an international study led by a team from the Yale School of Forestry and Environmental Studies.
Researchers, including members from Chile, Russia and New Zealand, used satellite imagery and more than 400,000 ground plots to find the figure, which is seven times higher than previous estimates. The study also reported that farming practices have depleted over half of the world’s forests in a span of 12,000 years, and that present deforestation and forest management practices are decimating trees at a rate of 15 billion per year. The study was published in the journal Nature on Sept. 10.
“This study is a significant contribution to bring the fundamental understanding of our planet and its forest ecosystems one step further,” said co-author Markus Huber of the Swiss Federal Institute for Forest, Snow and Landscape Research.
Past research has mainly been based on remote sensing data in the form of satellite imagery, which can lead to underestimation. This study, in contrast, used tree density data collected through on-the-ground counting. This allowed the researchers to more accurately estimate the number of trees in the world. Still, the more diligent methods came with additional challenges.
Researchers had to find enough sample plots and compare different modeling approaches to arrive at a reasonable estimate, said Christian Salas, study co-author and a professor at Universidad de la Frontera in Chile.
The study’s results may be particularly useful when analyzed in tandem with similar work, given the increasing popularity of studies on climate change and carbon storage in forests. The findings could serve as a benchmark for future environmental research. Information on the number of trees could direct work on the relationship between tree density and factors like population, climate variables, income and location, said Salas. Knowing the exact number of trees should also allow researchers to make better estimates of the amount of carbon stored in trees, said Susan Wiser, co-author of the study and programme leader at Landcare Research in New Zealand.
“By bringing the individual tree into the spotlight, this discussion might be more comprehensive for some people and they might understand the impact of human activities on forests,” said Huber.
Though the study’s scope was international, the data collected will also be valuable for researchers working at the local level, said Wiser.
The study was inspired by the U.N. Billion Tree Campaign, an effort to plant one billion trees. Organizers wanted to know how that number would match up with the number of trees already on earth. Previous estimates had put the number of trees at approximately 400 billion. This research has encouraged the campaign to revise its efforts to increase the number of trees it hopes to plant.
“The information that we provide is valuable for regreening efforts … Setting meaningful goals and reforestation targets requires an understanding of the number of trees that already exist at global scales,” said Tom Crowther, the study’s lead author and postdoctoral researcher at the Yale School of Forestry and Environmental Studies. “For example, based on these data, the U.N. Billion Tree Campaign has been upscaled to the Trillion Tree Campaign.”
The new figure puts the tree to person ratio at 430 to one.