The world’s great forests have long been recognized as the lungs of the earth. They fix carbon and produce oxygen. So it should come as a pleasant surprise to hear that there are over three trillion trees on earth, according to a new assessment. The figure is more than seven times as big as the previous best estimate, which counted perhaps 400 billion at most.
It has been produced by Thomas Crowther and colleagues, from Yale University, who combined a mass of ground survey data with satellite pictures. (1)
“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 Crowther. “They store huge amounts of carbon, are essential for the cycling of nutrients, for water and air quality, and for countless human service,” he added. “Yet you ask people to estimate, within an order of magnitude, how many trees there are and they don’t know where to begin. I don’t know what I would have guessed, but I was certainly surprised to find that we were talking about trillions.”
This new number will form a baseline for a wide range of research applications—everything from studies that consider animal and plant habitats for biodiversity reasons, to new models of the climate, because it is trees of course that play an important role in removing the greenhouse gas carbon dioxide from the atmosphere.
The previous estimate of around 400 billion trees meant about 61 trees for every person on earth. The new study indicates that there are 3.04 trillion trees, or roughly 422 trees per person.
The highest densities of trees were found in the boreal forests in the sub-arctic regions of Russia, Scandinavia, and North America. But the largest forest areas, by far, are in the tropics, which are home to about 43% of the world’s trees.
Three decades ago ‘forest dieback’ was a hot topic, with the very survival of large forest ecosystems seemingly in doubt. But besides the new study finding a vast additional number of trees, other complimentary studies indicate forests have actually been growing at a faster rate.
Whether, how, and why forest stands have changed their growth patterns over the last century are still hotly disputed questions. Scientists are putting the growth acceleration down to rising temperatures and the extended growing season. Carbon dioxide and nitrogen are other factors contributing to the faster growth. The concentrations of these gases in the atmosphere have been rising steadily over the last century. (2)
One last item- The Crowther study also mentions that the total number of trees has plummeted by roughly 46 percent since the start of human civilization. The team estimates we are removing about 15 billion a year, with perhaps only five billion being planted back. Said Dr. Henry Glick, “This net loss is about a third of a percent of the current number of trees globally.” (3)
Some folks in the media, such as The New York Times, highlight this dark side. They report, “The study found that 15 billion trees are cut down each year by people, with another 5 billion planted. That’s a net loss of 10 billion trees a year. At that rate, all of earth’s trees will be gone in about 300 years.” (4)
Using the previous estimate of 400 billion trees prior to this study, and losing 10 billion trees a year means we would no trees in 40 years. So, I can’t get too alarmed about what the Times reports. I’ll take the glass as half-full rather rather than half-empty.
1. T.W. Crowther et al., “Mapping tree density at a global scale,” Nature, 2015;DOI:10.1038/nature 14967
2. H. Pretzsch et al., “Forest stand growth dynamics in Central Europe have accelerated since 1870,” Nat. Commun. 5, 4967, 2014
3. Jonathan Amos, “Earth’s trees number three trillion,” bbc.com, September 3, 2015
4. “Lots of trees to hug: study counts 3 trillion trees on earth,”, The New York Times, September 2, 2015