The skies over Hawaii buzz with the propellers of small aircraft. Most of them ferry people among the islands, or give tourists a glimpse of inaccessible locales. But there is one among the swarm that is unlike anything else in the sky, distinguishable from the tourist planes only by a small opening underneath that bristles with lenses and sensors. This unassuming vessel will soon help researchers probe deeper into the structure and chemistry of ecosystems than ever before.
|Asner lab researcher Matt Jones guides the Carnegie Airborne Observatory (CAO) along its flight path. While the pilots, seated in front, actually steer the plane, Jones provides them with precise, real-time directions via a heads-up display to ensure that valuable flight time is not wasted.|
The Carnegie Airborne Observatory (CAO) wields some of the most powerful remote sensing instruments available today. Conceived, developed, and brought to life by Greg Asner and his team at the Department of Global Ecology, the CAO can efficiently image large swaths of forest in pinpoint three-dimensional detail. On a typical flight, the instruments gather gigabytes of data that can reveal patterns in native species diversity, foreign species invasion, forest growth rates, and overall ecosystem health. For the past several months, the Big Island of Hawaii— which contains examples of more than two-thirds of all ecosystem types on Earth—has served as the CAO’s proving ground.
“It’s as if the information is written in invisible ink, and all we have to do is figure out how to read it,” Asner explains. “When we fly overhead, we see the tops of trees. But the CAO can see physical structures down below the canopy, and can pick up on biochemical information that is invisible to the naked eye.”
The heart of the CAO consists of two main instruments. One, a waveform light detection and ranging (LiDAR) system, maps the three-dimensional physical structure of the trees. LiDAR is a close cousin of radar, except that it bounces laser light off of its targets instead of radio waves. The second device is a hyperspectral imager, which gathers information about the biochemistry of the forest by measuring the wavelengths of light reflected by trees and other vegetation. The CAO integrates the LiDAR and hyperspectral data on the fly to construct a complete three-dimensional picture. “Greg’s team is using the CAO to gather and blend incredibly detailed physical and chemical data in a way that hasn’t been done before,” says Global Ecology department director Chris Field. “They’ve essentially devised a way to do a virtual ‘CAT scan’ of an ecosystem.” Strictly speaking, the plane itself is not part of the CAO; rather, the instruments are portable and designed to plug into just about any small aircraft. This will enable Asner’s team to deploy the CAO anywhere in the world. It also allows razorsharp scanning that other remote sensing equipment, much of which rides on satellites or in high-altitude aircraft, cannot equal. Depending on how low the plane flies, resolution can range from 1 meter per pixel to as fine as 20 centimeters per pixel—precise enough to make out the individual branches and leaves on trees.
Much as a doctor can use a CAT scan to detect and diagnose myriad ailments in the human body, the CAO is designed to ferret out a variety of ecological threats. A crucial issue is the rampant thinning— and in some cases, disappearance—of native plant species. Asner plans to use the CAO to map the species diversity of rain forest ecosystems and discover which species are disappearing, and how quickly. Similarly, his team wants to know where they are surviving, and what factors are vital for their health. This information can aid conservation efforts in Hawaii, the Brazilian Amazon, northern Australia, and elsewhere by identifying the most threatened areas, as well as diversity “hot spots” where many species beat the odds and persevere. In many areas, the biggest threat is from foreign, “invasive” species that outcompete the natives for space and resources. In the Hawaiian test sites, for example, two particular introduced tree species can get their nitrogen from the air rather than from the limited supplies in the ground, as the native species do. This has allowed the faster-growing invaders to push out the natives at an alarming rate. Using the CAO, Asner’s group can map and track these species, gathering data that can be used to inform the effort to stem the invasion. “The CAO has an unparalleled ability to measure and map the structure, composition, and physiology of ecosystems,” says Flint Hughes, an invasive species ecologist and Asner’s collaborator from the U.S. Forest Service’s Institute of Pacific Islands Forestry. “The conservation and management potential is unique in Hawaii—and in the world, for that matter.”
Asner’s group will need a way to definitively identify plant species from the air in order to study species diversity and patterns of invasion. In theory, each species should have its own unique fingerprint based on the chemicals in its leaves and stems, which will reflect slightly different wavelengths of light. To develop a key to these fingerprints, Asner’s team is collecting leaf samples the old-fashioned way—on foot—and matching each species and their biochemistry with the spectra of light they reflect. With this information in hand, it is testing the CAO’s ability to accurately detect plant fingerprints from the air. As it refines the system, the team hopes to eventually be able to identify all the tree species in a study area with just one pass.
While the CAO specializes in vegetation, it can also identify hidden geomorphologic features in the landscape. Asner’s colleague and frequent collaborator, Stanford ecologist Peter Vitousek, recently looked at a CAO scan and could scarcely believe his eyes. There, in the middle of a study site he had traversed on foot literally hundreds of times, he saw that a series of depressions—water catchments and channels on the forest floor—was organized in a way that he had never seen as a whole. “The CAO has the advantage of aerial access and largearea coverage, but also provides levels of detail beyond what you can achieve on foot,” Vitousek says. “It’s going to prove incredibly useful in the study of fragile ecosystems, and learning more about how they respond to conservation efforts.” — MATTHEW EARLY WRIGHT