Using ground surveys along with satellite data yielded a better estimate of the number of trees in the world.

Using ground surveys along with satellite data yielded a better estimate of the number of trees in the world.

Thomas Crowther

Earth home to 3 trillion trees, half as many as when human civilization arose

It’s a good news, bad news report. Earth today supports more than 3 trillion trees—eight times as many as we thought a decade ago. But that number is rapidly shrinking, according to a global tree survey released today. We are losing 15 billion trees a year to toilet paper, timber, farmland expansion, and other human needs. So even though the total count is large, the decline is “a cause for concern,” says Tom Spies, a forest ecologist with the U.S. Forest Service in Corvallis, Oregon, who was not involved with the work.

Forests are important to humans, not just for their products, but also for their ability to foster biodiversity, store carbon, preserve water quality, and perform other ecosystem services. Up to 45% of the carbon stored on land may be tied up in forests. But figuring out how many trees grace our planet has been a difficult task. To date, most forest assessments have estimated tree cover based on satellite images. In 2005, one group converted that coverage into a measurement known as tree density and concluded the planet was home to 400 billion trees, or 61 trees per person alive at the time.

The newest count, published online today in Nature, had a more down-to-earth component. Almost 3 years ago, Plant-for-the-Planet, which took over the United Nations’s Billion Tree Campaign, wanted to know whether it was making an impact. In its first 5 years, the campaign had planted 12 billion trees in 193 countries to promote the removal of carbon dioxide from the atmosphere. The group asked Yale University foresters for help.

There, postdoc Thomas Crowther and remote sensing expert Henry Glick and colleagues compiled not just satellite data, but also tree density information from researchers measuring and counting trees in forest plots around the world. Using 400,000 ground-based data points, they converted other satellite images into tree density estimates to get more accurate tree counts in areas not surveyed by hand. In addition, they incorporated data from a 2013 report on changes in forest cover to get a sense of whether forests were expanding or shrinking.

The densest areas of tree coverage are the boreal forests of the Arctic, whereas deserts support the fewest trees per hectare. Even so, cold places have just 24% of the trees, whereas the tropics and subtropics support almost 43%. The rest are distributed throughout other places, including temperate zones.

In all areas, human impact was the dominant influence on tree density, the researchers found. By combining tree density measures with forest cover estimates for the past 12 years, Crowther, Glick, and colleagues concluded that humans are responsible for the loss of 15 billion trees a year. They think that about 5 billion new trees are planted or sprout annually, yielding a net loss of 10 billion. Since the beginning of human civilization, the number of trees has dropped by 46%, they estimate.

Although these numbers are impressive, not even Crowther thinks they tell the whole story. “Tree size, species identity, and tree qualities matter hugely in accounting for the importance of forests and trees outside of forests,” says Robin Chazdon, a forest ecologist on leave from the University of Connecticut, Storrs. “This study does not take those attributes into account.”

Crowther and his colleagues plan to start looking into how to quantify those aspects, particularly the sizes of trees, because that determines how much biomass—and consequently carbon storage—in each one. But one thing is clear: Even the 14.2 billion trees planted in the Billion Tree program is just a drop in the bucket.