Scientists Use NASA Satellite Data To Map Height of Forest Canopy


SAN FRANCISCO — A new map that relies on satellite data to show the varying height of forests around the world is one step in a campaign to improve climate models by offering scientists information on the amount of carbon stored in trees.

The map, which uses data drawn from NASA’s Ice, Cloud and land Elevation Satellite (ICESat) as well as the Terra and Aqua Earth-observation satellites to show forest canopy height around the globe, was created by Michael Lefsky, an assistant professor at Colorado State University in Fort Collins.

Years before ICESat was launched in 2003, Lefsky began to investigate whether the spacecraft’s instrument, known as the Geoscience Laser Altimeter System (GLAS), could provide information on forest height. The instrument was designed to give scientists detailed information on the size and thickness of polar ice sheets covering Antarctica and Greenland. “Using it to extract forest height was not something we intuitively knew we could do,” Lefsky said.

Nevertheless, Lefsky discovered that he could use GLAS data to determine forest height along the track followed by the spacecraft. Until the ICESat mission ended in February, GLAS used a laser to transmit pulses of green and infrared light toward the ground 40 times per second. It then measured the time it took that light to be reflected back to the spacecraft’s onboard telescope. GLAS provided detailed data on surface height for areas approximately 70 meters in diameter. Those areas were spaced nearly 170 meters apart.

ICESat focused on polar regions and provided direct measurements of the height of only 2.4 percent of the world’s forests. To create the first detailed study of global forest height that relies on one uniform method of measurement, Lefsky combined GLAS information with data drawn from the Moderate Resolution Imaging Spectroradiometer instrument carried aboard NASA’s Terra and Aqua satellites. The study has been accepted for publication by the journal Geophysical Research Letters, a publication of the American Geophysical Union.

Information on forest canopy height is important to scientists who are trying to gain a more thorough understanding of how carbon is stored and released on Earth. In recent years, human activity, including the burning of fossil fuels, has produced between 7 billion and 8 billion tons of carbon annually. The Earth’s atmosphere absorbs roughly 3 billion tons, and oceans absorb about 2 billion tons. While scientists know that plants, trees and soils also absorb carbon, it is difficult to measure that quantity, according to Richard Houghton, deputy director of the Woods Hole Research Center in Massachusetts.

While one can go to a plot of land and measure the amount of carbon stored, Houghton said, that would be a time-consuming and labor-intensive job. Remote sensing methods, including satellite and aerial imagery, are helping to provide scientists with additional data on the use of land, which allows them to calculate the amount of carbon stored in forests or released, for example, when a forest is cleared and replaced with crops. “Satellites are an important tool,” Houghton said. He cautioned, however, that existing satellites are more useful in showing where deforestation occurred than in measuring more subtle changes, such as the growth of trees in a forest.

While data drawn from ICESat, Terra and Aqua are helping to show the current height of forests, separating the tall, old trees from new stands, the information is not precise enough to be used for carbon monitoring. “You could not use it for carbon credits and treaty compliance,” Lefsky said. “The spatial resolution isn’t appropriate for that kind of analysis.”

That type of precise data is expected to come from NASA’s Deformation, Ecosystem Structure and Dynamics of Ice (DESDynI) mission scheduled for launch in 2017.

“DESDynI will give you orders of magnitude more information on forest canopy structure than anything available so far,” said Ralph Dubayah, geography professor at the University of Maryland in College Park and leader of the ecosystems working group for the DESDynI mission. “It will offer more observations of canopy than ICESat, and it will be optimized for canopy observation.”

DESDynI will provide data on the height of trees every 30 meters along a track. Moreover, its crisscrossing orbital path will allow it to take measurements that are never more than 500 meters apart, Dubayah said. “We will completely blanket the Earth with those lidar observations,” he added.

DESDynI, a mission recommended by the National Research Council in its 2007 Earth Science Decadal Survey, is expected to carry two sensors, an L-band interferometric synthetic aperture radar and a multibeam lidar, to gather data on deformation of the Earth’s surface, terrestrial ecosystems and ice structure. The lidar included in the DESDynI mission is expected to offer 25-meter spatial resolution and vertical accuracy of approximately 1 meter.