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 percent of the world’s trees. (Only 24 percent are in the dense boreal regions, while another 22 percent exist in temperate zones.)
The results illustrate how tree density changes within forest types. Researchers found that climate can help predict tree density in most biomes. In wetter areas, for instance, more trees are able to grow. However, the positive effects of moisture were reversed in some regions because humans typically prefer the moist, productive areas for agriculture.
In fact, human activity is the largest driver of tree numbers worldwide, said Crowther. While the negative impact of human activity on natural ecosystems is clearly visible in small areas, the study provides a new measure of the scale of anthropogenic effects, highlighting how historical land use decisions have shaped natural ecosystems on a global scale. In short, tree densities usually plummet as the human population increases. Deforestation, land-use change, and forest management are responsible for a gross loss of over 15 billion trees each year.
“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,” Crowther said. “This study highlights how much more effort is needed if we are to restore healthy forests worldwide.”
Researchers from 15 countries collaborated on the study. There were 14 researchers from across the Yale community who contributed to the study.
The original paper:
Crowther, T.W. et al. 2015. Mapping tree density at a global scale. Nature. 10.1038/nature14967