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Hunt and Peek
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Carbon, the stuff of life, is a key player in one of Earth's biogeochemical cycles, along with the interwoven nitrogen and water cycles. Because carbon dioxide in the atmosphere drives the whole process of climate change, "It was recognized early on that we need to know how carbon is released and stored," says Ollinger, who specializes in forest ecosystems and remote sensing at UNH's Institute for the Study of Earth, Oceans, and Space. Some of the carbon dioxide emitted by human activity gets absorbed by the oceans, some by plants. The rest stays in the atmosphere.

"Qualitatively, we know this fairly well, but we don't have very good numbers. We don't know how much and precisely where carbon gets stored in the ecosystem, and that turns out to be very important," he says.

Shooting leaves and hyperspectral images and comparing their data is one means of getting good numbers. It is also a method that gets the biggest bang for the buck. The chemical and spectral data from a series of 20- by 20-meter plots (the same area as one AVIRIS pixel), combined with annual growth measurements of trees in those plots, provides the scientists with information about forest productivity that can be applied to a much larger region, in this case the entire White Mountain National Forest.

In its simplest terms, the research Ollinger, Martin (who pioneered the method of correlating leaf chemistry with hyperspectral images) and Smith are doing determines the level of nitrogen in leaves shot from the tops of trees on a specific plot of forest. (Nitrogen is typically the limiting nutrient with respect to plant growth; the more there is the more growth occurs.) This is then correlated with the AVIRIS images, which also identify leaf nitrogen content depending on how light is absorbed or reflected by the leaf surface. With this data for one plot, and after shooting and analyzing similar plots with a wide variety of trees having both high and low nitrogen content, the scientists can accurately gauge just how much nitrogen is in all the trees for a whole forest. The process gives them overall forest productivity or, put another way, how much carbon has been added to the trees.

Says Ollinger, "Wood production tells us a lot about carbon uptake because wood is about 50 percent carbon." He adds, "The fact that canopy nitrogen is so tightly correlated with growth rates means that if a pixel on our AVIRIS image represents a foliar nitrogen value of two percent, we know that about 2,650 pounds of carbon or 5,300 pounds of new plant biomass is added per acre annually." For a time, at least. As trees mature, their growth rate slows down and, eventually, the carbon in matches the carbon out. Thus, forests are not an infinite "sink" for atmospheric carbon.

Although this scientific marriage of low and high technology is young, Ollinger notes, "We've more than scratched the surface of what we can do with this kind of technique. But we have not investigated all possibilities."

Ollinger's pie-in-the-sky goal would be to shoot the North American continent in an effort to get a clearer picture of the continental carbon cycle. If NASA gives a thumbs-up to the research team's next renewal proposal, which will include 10 new sites strategically located around the country, Ollinger may have a piece of his pie sooner rather than later.

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