Washington, DC--Scientists have created a map of the height of the world's forests by combining light detection and ranging (LIDAR) data from three NASA satellites. The map will 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.
Maps of local and regional forest canopy have been produced before, but the new map is the first that spans the entire globe using one uniform method. The map was based on data collected by NASA's Terra and Aqua satellites, along with the Ice, Cloud and land Elevation Satellite, or ICESat. Michael Lefsky, a remote-sensing specialist from Colorado State University (Ft. Collins, CO), produced the final product. Lefsky describes his results in a journal paper to be published next month in Geophysical Research Letters.
The new map shows the world's tallest forests are clustered in North America's Pacific Northwest and portions of Southeast Asia. Shorter forests are found in broad swaths across northern Canada and Eurasia.
The primary data Lefsky used was from LIDAR on the ICESat—capturing vertical slices of forest canopy height by shooting pulses of light at the ground and observing how much longer it takes for light to bounce back from the surface than from the top of the forest canopy. Since LIDAR can penetrate the top layer of forest canopy, as well as be used in applications as diverse as robotics movement to providing speeding-ticket data admissible in court, it provides a detailed snapshot of the vertical structure of a forest. Lefsky based the map on data from more than 250 million laser pulses collected during a seven-year period. Because each pulse returns information about a tiny portion of the surface, LIDAR offered direct measurements of only 2.4% of the Earth's forested surfaces. To complete the map, Lefsky combined the LIDAR data with information from the Moderate Resolution Imaging Spectroradiometer (MODIS), an instrument aboard NASA's Terra and Aqua satellites. MODIS observes a broad swath of Earth's surface, even though it does not supply the vertical profile.
Measuring canopy height has implications for efforts to estimate the amount of carbon tied up in Earth's forests and for explaining what absorbs 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 remaining 2 billion tons of carbon go, although scientists suspect forests capture and store much of it as biomass through photosynthesis.
SOURCE: NASA; http://www.nasa.gov/topics/earth/features/forest-height-map.html
