How do you count every tree on Earth? You do not check them one by one. Scientists combine satellite images, forest records and more than 400,000 measurements taken on the ground.
In 2015, an international team led by ecologist Thomas Crowther used this method to create a global map of tree density. Their estimate was huge: about 3.04 trillion trees. That was almost eight times higher than an older estimate of 400 billion. Satellite images can show where forests are, but ground measurements reveal how closely the trees grow together.
Trees are not spread evenly. The highest densities appear in the cold northern forests of Canada, Alaska, Scandinavia and Russia. But tropical and subtropical forests contain the largest total number. Forests in South America, Central Africa and Southeast Asia hold more than 40% of the world's trees.
The figure is not a permanent count. Trees grow and die, while forests can regenerate naturally or be cleared and replanted by people. Using historical models, the same 2015 study estimated that Earth had lost about 46% of its trees since the beginning of human civilization. It also estimated that more than 15 billion trees were removed each year, although natural growth and replanting made up for part of the loss.
Could we simply plant replacements? It is not that easy. Young trees take decades to mature. Forests planted with only one or two species often support fewer animals and cope less well with drought, disease and fire than natural forests.
Planting trees can help, but it cannot quickly replace an old forest. Protecting forests that already exist, while restoring native trees where they belong, may be the more valuable goal.