CHARLOTTESVILLE, Va., Nov. 16 -- A new technique for gaining a three-dimensional picture of forest canopies may help answer both practical needs of forest managers and basic questions about the relationship between forests and global climate change.
The system, developed by University of Virginia environmental sciences graduate student Michael Lefsky and scientists from NASA and the Smithsonian Environmental Research Center, employs laser radars (lidar) suspended from low-flying airplanes to rapidly and accurately assess the trees' shapes. It is called Scanning Lidar Imager of Canopies by Echo Recovery (SLICER).
"Large scale surveys using SLICER could, for the first time, accurately predict the current storage of carbon in forest stands and their capacity to act a source and sink for carbon dioxide," says Lefsky, whose research concerns the physiological ecology of plants with emphasis on landscape-scale processes.
SLICER works like this: A plane flies over a forest at a height of five kilometers, sending out laser pulses; the beams bounce off layers of foliage and the ground and then return to the plane's bottom-mounted telescopes. The reflected light returns to the telescopes at slightly differing times, depending on the distance it transversed. Lefsky developed the mathematical algorithm that turns this raw laser data into an estimate of the canopy's shape.
According to its developers, SLICER is as accurate as the standard field method of gathering similar data, which takes many hours to perform. In contrast, SLICER can take up to 100 measurements per second to yield a three-dimensional picture of the canopy, and could be used even in dense and remote forests. Moveover, SLICER can predict other forest characteristics, such as the total number and size of the trees.
Information about forest structure is valuable both to forest managers, who may wish to know when the trees reach a marketable size, and to global-change scientists, who study the flow of carbon through the global ecosystem. Carbon, predominantly in the form of gaseous carbon monoxide and carbon dioxide, is one of the elements that contributes to world-wide atmospheric warming. The two principal causes of increased atmospheric carbon dioxide are fossil fuel burning and changes in land use (typically, replacing trees with cropland). Trees bind carbon dioxide in their tissues as they grow. Thus, the numerous rapidly growing trees in a young forest store more carbon that the few, mature trees in an older forest. At present, there is little quantitative understanding of precisely how much young and old forests differ in their capacities to store carbon. SLICER could help answer this question.
Lefsky's partners in this research are Geoffrey Parker, of the Smithsonian Environmental Research Center, and David Harding and J. Brian Blair, of NASA Goddard Space Flight Center. Results of initial experiments carried out at sites in North Carolina and Maryland were presented at a meeting of the Ecological Society of America in August.
November 15, 1996
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