In 2006, the United Nations Environment Program (UNEP) started the billion trees campaign with the goal of planting — you guessed it — one billion trees around the world. Plant for the Planet, which inherited the campaign, wanted to know how big of an effect a billion trees would have on the total population of trees. But without an accurate baseline population, it was hard to assess the magnitude of impact that planting all these trees will have.
This obstacle motivated postdoctoral researcher Thomas Crowther at the Yale School of Forestry and Environmental Studies to generate an accurate tree count. In a study published in Nature, Crowther and colleagues estimate that there are more than three trillion trees in the world. Although this figure is more than seven times greater than previous estimates, the total number of trees has decreased by about 46 percent since the beginning of human civilization.
Crowther’s team calculated this estimate by modeling tree density based on about 50 variables, including environment, soil content, climate, and topography, combining satellite and ground data. The estimate is much more accurate than ones that came before it, which were solely based on satellite images. Crowther’s research correlated each variable with tree density, and then extrapolated these findings to areas for which they lacked ground data.
“I thought [the study was going to] deter people leading massive reforestation efforts because there are so many more trees than expected,” Crowther said. “But when I told the people from the billion tree campaign, they just seemed excited. Now they have new meaningful targets, which they are rapidly working towards.”
Going forward, Crowther will expand his current density map, and will obtain more samples. He also hopes to add tree types and sizes to the density map to better estimate the amount of carbon stored in these forests. In the end, these models could help identify where reforestation would be most beneficial to aid global efforts like the UNEP’s billion tree campaign — now called the trillion tree campaign.