The arboreal census

According to a new census of all the world’s hardwoods and softwoods, there are at least 3 trillion trees on the planet—over seven times more trees than scientists had previously estimated.

Why the huge discrepancy?

Earlier counts, explains Thomas W. Crowther, relied largely on interpretations of satellite imagery alone. For the new survey, he says, “we figured out how to combine satellite information with data from more than 400,000 plots on the ground in which people physically counted all the trees.” Crowther was a postdoctoral researcher at the School of Forestry and Environmental Studies when he led the international team that undertook the survey project; he is now a postdoc at the Netherlands Institute of Ecology.

The new map shows that most of the world’s trees—about 1.4 trillion—are in tropical and sub-tropical forests. The next biggest categories are the 0.74 trillion trees in boreal zones and the 0.61 trillion in temperate-zone woodlands, with the remainder scattered in various habitats around the globe. (The study was published in the online edition of Nature on September 2.)

The map also shows something disquieting: the pervasive impact of humans on the abundance and distribution of trees. “We estimate that since the onset of civilization—when there were about 5.6 trillion trees—the total has dropped by some 46 percent,” says Crowther. “We’re currently losing more than 15 billion trees annually. We’ve cut down more trees than there are stars in the galaxy.”