The GEDI laser will penetrate tropical forest treetops to map the understory’s 3D structure.
BRUSINI Aurélien/hemis.fr/Hemis/Alamy Stock Photo
Tallying up the biomass in a forest—and monitoring changes to it—is no easy task. You can cordon off a patch of forest and use tape measures to assess tree growth, hoping your patch is representative of the wider forest. Or you can turn to aerial or satellite photography—if the pictures are available and sharp enough. But even the best cameras can’t see past the forest canopy to the understory below.
On 5 December, scientists gained a new tool for this tricky business when NASA’s Global Ecosystem Dynamics Investigation (GEDI) was launched on a SpaceX rocket. The instrument, the size of a large refrigerator, will be attached to the International Space Station, where it will begin to gather data on the height and 3D structure of tropical and temperate forests. The campaign will help scientists understand whether forests are slowing or amplifying climate change, and identify prime habitat for valued species. “We’ve wanted this data set desperately,” says Ralph Dubayah, a geographer at the University of Maryland in College Park and the project’s principal investigator.
GEDI will harness a technology called light detection and ranging (lidar). Like its cousin radar, lidar sends out pulses of electromagnetic energy and measures the reflections. But whereas radar uses radio waves, GEDI’s lidar uses laser light, firing 242 times per second in the near-infrared. The focused, high-frequency radiation offers sharp resolution and can penetrate dense forests, bouncing not only off the treetops, but also off midstory leaves, branches, and the ground. Dubayah and his colleagues will combine GEDI data with ground measurements and statistical models to produce maps of tropical forest carbon that, at 1 kilometer resolution, should vastly shrink the errors of previous maps.
Countries that want to use the carbon stored in their forests to help meet Paris agreement climate targets may use those maps to gauge progress, says Naikoa Aguilar-Amuchastegui, director of forest carbon science at the World Wildlife Fund in Washington, D.C. Researchers tracking forest degradation, due to the selective logging of individual trees and fuelwood harvesting from the understory, are eager for the data, too. Those activities are invisible to imaging satellites such as Landsat, says Laura Duncanson, a research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “GEDI gets you that third dimension,” she says.
The 3D maps could also identify the rich structure and variety of forests that harbor at-risk species such as the orangutan, says Scott Goetz, an ecologist at Northern Arizona University in Flagstaff and a mission deputy principal investigator. The maps could find priority areas for conservation, and even help plan habitat corridors for wildlife migrating because of climate change.
The finely tuned laser will also resolve the heights of treetops and the ground more precisely than previous instruments—crucial for monitoring the health of the carbon-dense mangrove forests that shroud tropical coastlines, says Goddard research scientist Lola Fatoyinbo Agueh. Knowing how high the mangroves sit above the water could determine whether they will keep pace with sea level rise or die back, releasing stored carbon—a key input for climate models, she says.
GEDI’s perch on the space station—chosen to keep its cost below a $94 million cap—comes with a drawback, however. Its view will be confined to latitudes between 51.6° north and south. That means it will miss the boreal forests of North America and Asia. And it will likely get booted after 2 years to make room for a Japanese instrument. The short mission will make it harder to answer an urgent question: Are tropical forests overall a carbon sink, capturing some of the emissions from vehicles and industry, or a source? That depends on whether forest growth is sequestering more carbon than deforestation and degradation are releasing. But seeing such a trend requires years of continuous data, says Wayne Walker of the Woods Hole Research Center in Falmouth, Massachusetts. “Nothing’s better than a long-term record.”
GEDI also can’t distinguish tree species, which vary in carbon density. Dubayah is using species-specific measurements from about 5000 field plots to calibrate the GEDI data. But with more than 40,000 tree species in the tropics, that’s just a start, says Oliver Phillips, an ecologist at the University of Leeds in the United Kingdom, who runs a large tropical forest plot network. “A large ground effort is needed to get maximum value from this,” Phillips says.
Researchers may be able to work around some of these limitations. Alessandro Baccini, a remote sensing scientist also at Woods Hole, hopes to train machine-learning algorithms to extend carbon estimates into the past and future by using GEDI’s carbon maps to calibrate long-term forest-cover data from imaging satellites. He adds that by combining data from GEDI and ICESat-2, a NASA lidar satellite launched in September that primarily measures ice sheets but is flying over the whole planet, investigators could construct a global carbon map—one that includes the boreal forest. Still, Baccini wants more. “Why can’t we have a proper mission designed for vegetation that is global?” he asks.
Source: Science Mag