News in Science

Earth has three trillion trees, and falling

Tree count There are about three trillion trees on Earth, roughly 422 for every person and eight times more than previously estimated, researchers say.

A 15-nation team led by Yale University experts used a combination of old-fashioned headcounts and state-of-the-art satellite and supercomputer technology to produce what they claim is the most comprehensive tree census ever.

"I don't know what I would have guessed, but I was certainly surprised to find that we were talking about trillions," says the study's lead author Thomas Crowther of the Yale School of Forestry and Environmental Studies in Connecticut in the United States.

But there was bad news, the researchers reported in the journal Nature.

The calculation revealed that tree cover had nearly halved since the start of human civilisation.

And the pace of deforestation has not abated: our species is currently felling some 15 billion trees every year, the study found.

The team based their research on verified tree counts from some 400,000 forest plots.

They then used satellite imagery to determine how factors like climate, topography, vegetation, soil conditions and human impact affected tree density.

Developing models to estimate tree numbers at regional levels, they then drew a global map of Earth's estimated 3.04 trillion trees.

"The highest densities of trees were found in the boreal forests in the sub-arctic regions of Russia, Scandinavia and North America," the researchers say.

"But the largest forest areas, by far, are in the tropics, which are home to about 43 percent of the world's trees."

The bad news

The team's calculations revealed that of all the factors impacting tree numbers, human activity had by far the biggest effect, largely through deforestation and land-use change.

There has been in total a 46-percent drop in tree numbers since humans began to clear land to plant seeds, the study found.

"In short, tree densities usually plummet as the human population increases," the scientists say.

"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," says Crowther.

"The study highlights how much more effort is needed if we are to restore healthy forests worldwide."

Apart from offering oxygen, fuel and shelter, trees store important quantities of carbon, which, if released, contribute to global warming.