Science Snippet: Mapping the World’s Trees

Who would tackle the task of mapping tree density on planet Earth? Well, almost 40 scientists from all over the globe did just that. After defining trees as plants with woody stems larger than four inches in diameter, the scientists from every continent except Antarctica walked through forests counting trees within pre-established grid patterns. All told, counts were obtained from 429,775 global sites. Counts were correlated with overflights of forests by aircraft equipped with heat monitoring instruments, as well as geographic factors (like tundra, swamp, rainforest, etc.). The data allowed the authors to generate a statistical model that provided density estimates for the many regions of the earth. Here are their results:

• The global number of trees is about 3.04 trillion.
• About 1.39 trillion exist in tropical and subtropical forests.
• There are 0.74 trillion trees in boreal regions.
• Only 0.61 trillion are found in temperate regions.

The authors estimate that more than 15 billion trees are cut down each year, and the global number of trees has diminished by 46 percent since the start of human civilization. Considering the importance of trees as producers of oxygen, their ability to soak up carbon dioxide, and their use as fuel, without them the planet would be an unlikely place for man to live. (Crowther, T.W., and 27 other scientists, 2015, “Mapping tree density at a global scale,” in Nature, vol. 525, pp. 201-205)