Probing How Much Carbon The World's Forests Store

Using new remote sensing techniques, scientists for the first time have successfully uncovered the intricate architecture of a large area of tropical rainforest in Costa Rica. The research promises to help answer a
key question in global warming: how much carbon do the world's forests store?

Results from the Costa Rica mission were presented Thursday by scientists from the University of Maryland and NASA's Goddard Space Flight Center during the Fall Meeting of the American Geophysical Union in San Francisco. "Having a baseline estimate of forest biomass will be extraordinarily useful for future carbon modeling," said University of Maryland geographer Ralph Dubayah.

Determining the amount of carbon in tropical forests is very important, Dubayah said, because most changes in Earth surface carbon occur in the tropics as a result of changing land use. Scientists need to understand whether forests and human activities in forests, such as deforestation, are acting to increase or decrease carbon in the atmosphere, and thus potentially accelerating or inhibiting global warming.

Excessive deforestation adds carbon into the atmosphere, then some deforested areas rebound, and the growing trees act as carbon sinks, pulling carbon out of the atmosphere. Satellite images have long been able to see how much land area is covered by forest. "But how do you know how much carbon there is in a certain patch of land?" asked Dubayah. Estimating the amount of carbon from conventional two-dimensional images is unreliable, he said, because they cannot measure tree heights, how densely the trees grow, or the thickness of the tree canopy.

Using an instrument called the Laser Vegetation Imaging Sensor (LVIS) aboard a NASA C-130 aircraft, scientists took measurements over 1500 hectares (3700 acres) of tropical forest within La Selva Biological Research Station in Costa Rica this spring. Using a new digital data analysis technique developed at Goddard, scientists from Goddard, the University of Maryland and other universities, and University of Maryland graduate students Birgit Peterson, Laura Rocchio and Jason Drake, were able to make estimates of tree canopy height, the amount of leaves and branches in a vertical column, and even the underlying topography of the land surface. (The aboveground biomass of a tree is closely related to its height, and about fifty percent of a tree's biomass is made up of carbon. So knowing the height and vertical structure, scientists can estimate biomass and estimate the amount of carbon contained in the forest.)

The LVIS instrument, developed by Goddard engineer Bryan Blair, is a wide-swath, airborne laser altimeter or lidar, which stands for light detection and ranging. Lidar works by sending pulses of laser energy to the Earth's surface. The laser energy interacts with leaves and branches and reflects back to the instrument. What are uncovered are not only tree heights and the ground level below, but what's in between. "The variability of the return signal going down through the canopy reveals the architecture of the canopy," said Blair.

The Costa Rica mission is a test for a laser instrument that will fly on the first of NASA's Earth System Science Pathfinder missions in May 2000. The Vegetation Canopy Lidar's (VCL) main objective is to measure how much carbon is locked up in vegetation on the Earth's land surface by making measurements of forest structure.

The mission is led by the University of Maryland, which is responsible for all elements of the program, from the spacecraft to the final data products. Dubayah, principal investigator on the VCL mission, said that researchers have used the new method to study coniferous forests in the Sierra Nevada of California and eastern deciduous forests of the mid-Atlantic region around the Chesapeake Bay.

The Costa Rica project was the greatest challenge because of the extremely dense canopy of trees. The denser the canopy, the more difficult it is for the laser light to penetrate all the way to the ground because it is intercepted by leaves and branches, just as sunlight is filtered by a thick canopy, leaving the forest floor in darkness.

John Weishampel, a biologist from the University of Central Florida and a project co-investigator, said that the LVIS data is unique because it is the first three-dimensional data for forests. The 3-D view helps scientists understand how some parts of the forest may be more fragile than other parts.

Weishampel declared that small disturbances in a rainforest could avalanche into large-scale disturbances like massive tree falls or spreading fire. "On a global scale, we're giving scientists a method to look at the canopies over a large scale instead of taking scattered measurements here and there," Weishampel said.

Robert Knox, Goddard forest ecologist, said accurate topographical maps are difficult to make in such remote areas with dense canopies and haven't been done this well in the past. In addition, the lidar data, because it can see down into the forest, can distinguish undisturbed tropical forests from secondary, regrowing forests, something that is difficult using other remote sensing techniques, he said.

La Selva forest ecologist David Clark from the University of Missouri, St. Louis, works from the ground to measure tree heights and determine how LVIS distinguishes between old-growth forests and young, re-growing forests. The two forests look completely different from the ground, said Clark, but in conventional images from space, they look the same. VCL will be able to tell the relative age of forests much better than these 2-dimensional imaging satellites because it measures the canopy structure, he said.

Clark said that an old-growth forest contains about 2-10 times more biomass than a young, re-growing forest. "There's an internal dynamic of an old forest that's missing in a new forest," he said.

11-Dec-1998