3.04 Trillion Trees Are Not Enough

Rachel Ehrenberg: “There are roughly 3 trillion trees on Earth — more than seven times the number previously estimated — according to a tally1 by an international team of scientists. The study also finds that human activity is detrimental to tree abundance worldwide. Around 15 billion trees are cut down each year, the researchers estimate; since the onset of agriculture about 12,000 years ago, the number of trees worldwide has dropped by 46%. | “The scale of human impact is astonishing,” says Thomas Crowther, an ecologist now at the Netherlands Institute of Ecology in Wageningen who led the study while at Yale University in New Haven, Connecticut. “Obviously we expected humans would have a prominent role, but I didn’t expect that it would come out as the strongest control on tree density.” | The previously accepted estimate of the world’s tree population, about 400 billion, was based mostly on satellite imagery. Although remote imaging reveals a lot about where forests are, it does not provide the same level of resolution that a person counting trunks would achieve. | Crowther and his colleagues merged these approaches by first gathering data for every continent except Antarctica from various existing ground-based counts covering about 430,000 hectares. These counts allowed them to improve tree-density estimates from satellite imagery. Then the researchers applied those density estimates to areas that lack good ground inventories. For example, survey data from forests in Canada and northern Europe were used to revise estimates from satellite imagery for similar forests in remote parts of Russia.”

[Crowther et al 2015] Anstract: The global extent and distribution of forest trees is central to our understanding of the terrestrial biosphere. We provide the first spatially continuous map of forest tree density at a global scale. This map reveals that the global number of trees is approximately 3.04 trillion, an order of magnitude higher than the previous estimate. Of these trees, approximately 1.39 trillion exist in tropical and subtropical forests, with 0.74 trillion in boreal regions and 0.61 trillion in temperate regions. Biome-level trends in tree density demonstrate the importance of climate and topography in controlling local tree densities at finer scales, as well as the overwhelming effect of humans across most of the world. Based on our projected tree densities, we estimate that over 15 billion trees are cut down each year, and the global number of trees has fallen by approximately 46% since the start of human civilization. [Nature (2015) doi:10.1038/nature14967]

Here is a pop quiz: How many trees are on the planet? | Most people have no idea. | A new study [published this week in Nature] says the answer is more than 3 trillion trees — that’s trillion with a T, and that number is about eight times more than a previous estimate. | Thomas Crowther was inspired to do this tree census a couple of years ago, when he was working at the Yale School of Forestry and Environmental Studies. He had a friend who was working with a group with an ambitious goal: trying to fight global warming by planting a billion trees. A billion trees sounded like a lot. But was it really?