US: Using NASA satellite data, scientists have produced a first-of-its kind map that details the height of the world’s forests. Although there are other local and regional scale forest canopy maps, the new map is the first that spans the entire globe based on one uniform method.
The work — based on data collected by NASA’s ICESat, Terra and Aqua satellites should help scientists build an inventory of how much carbon the world’s forests store and how fast that carbon cycles through ecosystems and back into the atmosphere.
The new map shows the world’s tallest forests clustered in the Pacific Northwest of North America and portions of Southeast Asia, while shorter forests are found in broad swaths across northern Canada and Eurasia. The map depicts average height over 5 square kilometres (1.9 square miles) regions), not the maximum heights that any one tree or small patch of trees might attain.
Temperate conifer forests — which are extremely moist and contain massive trees such as Douglas fir, western hemlock, redwoods, and sequoias–have the tallest canopies, soaring easily above 40 meters (131 feet). In contrast, boreal forests dominated by spruce, fir, pine, and larch had canopies typically less than 20 meters (66 feet). Relatively undisturbed areas in tropical rain forests were about 25 meters (82 feet), roughly the same height as the oak, beeches, and birches of temperate broadleaf forests common in Europe and much of the United States.
Estimation of carbon
Scientific interest in the new map goes far beyond curiosities about tree height. The map has implications for an ongoing effort to estimate the amount of carbon tied up in Earth’s forests and for explaining what sops up 2 billion tons of missing carbon each year.
Humans release about 7 billion tons of carbon annually, mostly in the form of carbon dioxide. Of that, 3 billion tons end up in the atmosphere and 2 billion tons in the ocean. It’s unclear where the last two billion tons of carbon go, though scientists suspect forests capture and store much of it as biomass through photosynthesis.
Seeing lasers through the trees
Michael Lefsky of the Colorado State associated with this map, used data from a laser technology called LIDAR that’s capable of capturing vertical slices of surface features. It measures forest canopy height by shooting pulses of light at the surface and observing how much longer it takes for light to bounce back from the ground surface than from the top of the canopy. Since LIDAR can penetrate the top layer of forest canopy, it provides a fully-textured snapshot of the vertical structure of a forest — something that no other scientific instrument can offer.