More than 3 trillion trees exist on Earth - roughly 422 per person - but the total number of trees has plummeted by 46 per cent since the start of human civilisation, according to a new Yale-led study.

The new estimate of more than 3 trillion trees on Earth is about seven and a half times more than some previous estimates of 400 billion. An international team of researchers mapped tree populations worldwide at the square-kilometre level using a combination of satellite imagery, forest inventories, and supercomputer technologies.

“Trees are among the most prominent and critical organisms on Earth, yet we are only recently beginning to comprehend their global extent and distribution,” said Thomas Crowther, a postdoctoral fellow at the Yale School of Forestry & Environmental Studies (F&ES) and lead author of the study.

The study was inspired by a request by Plant for the Planet, a global youth initiative that leads the United Nations Environment Programme’s ‘Billion Tree Campaign.’ Two years ago the group approached Crowther asking for baseline estimates of tree numbers at regional and global scales so they could better evaluate the contribution of their efforts and set targets for future tree-planting initiatives.

At the time, the only global estimate was just over 400 billion trees worldwide or about 61 trees for every person on Earth. That prediction was generated using satellite imagery and estimates of forest area, but did not incorporate any information from the ground. The new study used a combination of approaches to find that there are 3.04 trillion trees - roughly 422 trees per person. Crowther and his colleagues collected tree density information from more than 400,000 forest plots around the world. This included information from several national forest inventories and peer-reviewed studies, each of which included tree counts that had been verified at the ground level. The highest densities of trees were found in the boreal forests in the sub-arctic regions of Russia, Scandinavia, and N America. But the largest forest areas, by far, are in the tropics, which are home to about 43 per cent of the world’s trees.

Researchers found climate can help predict tree density in most biomes. In wetter areas, for instance, more trees are able to grow. But, the positive effects of moisture were reversed in some regions as humans typically prefer the moist, productive areas for agriculture.

In fact, human activity is the largest driver of tree numbers worldwide, said Crowther. Tree densities usually plummet as the human population increases. Deforestation, land-use change, and forest management are responsible for a gross loss of over 15 billion trees each year. “We’ve nearly halved the number of trees on the planet, and we’ve seen the impacts on climate and human health as a result,” Crowther said.

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