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CelI &borne near-IR spectrometers can now scan the Earth at high resolution and could soon be used to create regional and global maps of environmental responses
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hen the first British colonists settled in North America, legend has it that they learned from local tribes how t o fertilize their vegetable patches by burying spoiled fish-rich in amines-near the plants. Nitrogen enrichment has been used ever since to replenish crop soils and boost harvest yields. Perhaps because nitrates and ammonium salts have been such successful fertilizers, it seems hard t o believe that nitrogen addition could actually be harmful to some plantsparticularly forest trees. Efforts by the US. government to curb acid rain have focused on sulfur emissions, which are produced primarily by large industrial point sources, much more than they have on nitrates, which are produced primarily by automobiles a n d other small sources. But recent research in the United States confirms what was found at least 10 years ago in Austria, Germany, and Switzerland Nitrogen deposition from air pollution is a major contributor to the decline of forests. Environmental chemist John Aber and his group at the Complex Sys-
tems Research Center, University of New Hampshire, Durham, hope to trace regional patterns of nitrogen deposition and its impact on the environment by following the changes it produces in the chemical composition of forest foliage. To track foliar response to increased nitrogen, these researchers have begun to use visiblelnear-IR spectrometry in conjunction with standard wet chemical methods for determining nitrogen,
lignin, and cellulose in leaf samples from targeted forest sites. Aber’s ongoing studies of experimental nitrogen additions to forested watersheds at Lead Mountain in Maine and at Harvard Forest in Massachusetts have revealed some surprises because both sites were assumed to be nitrogen-limited. “At Lead Mountain, when you add a small amount of nitrogen, half of it comes right back out in the stream,” Aber says, even though the site is much less exposed to automobile pol-
lution and other nitrate sources than is Harvard Forest. This finding indicates t h a t t h e Maine site may be near nitrogen saturation, and that even small amounts of nitrogen deposition may tip the balance in seemingly pristine forests. The added nitrogen can have a serious cumulative effect on forests. Although it promotes plant growth, too much nitrogen can cause trees to put out abundant new leaves without enough functional chlorophyll to support them-possibly because the availability of magnesium, a necessary cofactor, does not increase along with the nitrogen. In effect, the trees become overextended and they drop their yellowed leaves. In a deciduous forest, that may not be such a serious problem, but conifers such as spruce renew their foliage completely only once in seven years. In highly nitrogen-enriched soil, they may lose too many needles after only a year or so to be able to grow them back before they starve. Excess nitrogen also affects fine root production. Spruce trees may also be particularly vulnerable to nitrate deposition because of low genetic variability, ac-
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cording to Aber. “Increases in nitrogen usually stimulate the trees to produce metabolic enzymes in higher quantities to accommodate it,” he says. But spruce varieties, used to growing in a fairly nitrogen-limited environment, do not seem t o be responding this way. It may be that excess nitrogen in these areas is such a new environmental problem that the trees have not had time t o adapt through natural selection. Space age spectrometry In addition t o monitoring nitrogen saturation by measuring nitrate leaching in forest streams, Aber’s group measures the nitrogen, lignin, and cellulose content in leaves as earlier signs of excess nitrogen. Concentrations can be plotted to create maps indicating the nitrogen flux in each region. The gmup uses standard wet chemistry methods to determine these components, but is also studying near-IR techniques as a faster way to analyze the leaf samples. The speed, automation, low reagent use, and comparatively low cost of near-IR spectrometry are reasons enough to try the method, particularly when samples start to accumulate from expanded field studies. But Aber‘s group has another reason for trying it out. Advances in the use of near-IR spectrometers mounted in aircraft for remote sensing have made it possible to catch a bird’s-eye view of chemical changes taking place on the ground or, in this case, in the treetops or “canopy.” Working with NASA and the J e t Propulsion Laboratory (JPL), the designers of t h e Airborne Visible/ Infrared Imaging Spectrometer (AVIRIS), Aber’s group is using its correlation results from in-laboratory spectrometers to calibrate the airborne near-IR technology. Eventually, AVIRIS may be used to map out changes in foliar chemistry over large regions the way meteorological instruments aboard satellites are used to produce global and regional weather maps. If it works, the new method could help redefine the role of nitrate emissions and influence global environmental policy. AVIRIS contains four visible and near-IR spectrometers mounted in a U-2 (“the old spyplane,” Aber says) that view the ground from an altitude of 50,000 ft. A system with higher spectral resolution than the s p e c t r o m e t e r s aboard L a n d s a t , AVIRIS scans pixels of only 20 m x 20 m in a swath 614 pixels wide (about 10 km) across the airplane’s line of flight. The system scans each
pixel in 224 contiguous spectral channels, each 10 nm wide, over the range of 400-2450 nm (see Figure 1). Mary E. Martin, a graduate student of Aber’s who is working with the data from AVIRIS overflights, explains, “The spectral data from the 224 channels form a ‘data cube’ or ‘image cube’ for each 10 km x 10 km scene covered. We recreate and analyze the spectra from the individual bands in each cube.” The spectrometers are connected to a scanner and a calibration source with optical fibers, as well as to a microprocessor-driven data acquisition system that records the spectral data on tape, noting the time, altitude, and location for start and stop of the scan. Each flight may cover up to 30 km x 10 km of forest terrain. The system compensates for the airplane’s motion with roll/gym correction in addition t o the calibration data. Temperature a n d pressure f l u c t u a t i o n s a r e p r e v e n t e d by mounting the line array detector for each spectrometer in a Dewar with pressure relief valves. Although the microprocessor designed by JPL takes care of these interferences in midair, Martin must
do considerable groundwork to correct for atmospheric fluctuations, altitude, and other fundamental differences between the airborne system and the in-lab visiblehear-IR spectrometer. Cloud cover is an obvious problem for the team. “Even if the airport reports only 10% cloud cover, you might be out of luck if the one cloud in the sky is over your target forest stands,” Martin says. She and others in the group scout the ground area for dark parking lots, sand pits, and other nonreflecting features to use for correction-any spectral features recorded from them can usually be subtracted out as humidity or clouds. Atmospheric models for water vapor and carbon dioxide are also factored in t o correct for predicted interference with solar reflectance spectra. Because Aber’s group may be allotted only a few flights per year, Martin says they may get only one day per year of good data from t h e AVIRIS overflights. Timing is critical; t h e flight data are compared with in-lab results that reflect the actual chemical s t a t e of samples taken from each pixel of forest canopy at the time of the flight or, at
Figure 1. AVlRlS data collection. Data from lhe 224 spectral bands can be reconfiguredlo provide a lull-range spenrum for each 20 rn x 20 m pixel. (Adapted with pnission fmm Alrtwme Vis;ble/lnlrared /ma& spectmrnemr; Vane, G., Ed.: Jet Propulsion Laboratory: Pasadena. CA, 1987: JPL Publication 87-38. p. 4.)
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most, within a day or two afterwa Martin saw samples collected on ground before the time marked as the start of the scan do not represent what AVIRIS sees, and flights are sometimes aborted if clouds are sitting over the target zone. "You only know after the flight is confirmed whether the ground samples are usable," she explains. Planning the flights and ground collections for midsummer ensures that the forest canopy is chemically stable. In the northern states, some deciduous trees may not have all their leaves out until June, and by September, leaves are beginning t o change, returning about half their nitrogen content back into the tree before falling. "By early September, the chemical changes in the leaves go day to day in the northeast, so the timing is tricky for collecting samples then," Martin points out. Even trickier is the sample collection itself. To match her in-lab results with the AVIRIS scan data, she must collect fresh foliage samples from the canopy level, which is often 100 ft. or more above the ground. Martin wields a 12-gauge shotgun,
aiming in the upper canopy to bring down small branches with 10-12 leaves on them. "After three years of practice, I can now hit thd ones I want most of the time," she says.
D : i t e the waterpeaks, near-IR spectroscopy of fiesh foliage reveals sign$icant spectral features.
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Method callbration from the ground up Aber's investigation of near-IR for foliar chemical analysis-in the lab and in the air-began in the early 1980s, when the technique began to show possibilities for tissue chemism studies. David Peterson, of NASA/ Ames Research Center in Moffett Field, CA, suggested the use of near-IR remote sensing for monitoring changes in forest chemistry at a 1982 conference on global habitability that Aber attended. Aber decided t o design projects that would predict the feasibility and analytical accuracy of using airborne near-IR for that kind of monitoring. He is now a science adviser for the High-Resolution Infrared Imaging Spectrometer (HIRIS) project, part of NASA's Earth Observing System (EOS). Whether it ever "gets off t h e ground," near-IR seems to have potential in the laboratory as a substi-
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__ tute for the more cumbersome and expensive wet chemistry methodswet digestion and a CHN pyrolysis metbod-commonly used in agricultural analyses for nitrogen, lignin, and cellulose. The wet digestion method for carbon fraction analysis involves extraction of leaves, first in dichloromethane and then in hot water, before digestion in boiling concentrated sulfuric acid. For nitrogen determinations, samples are dried in a lab oven, ground, and pyrolyzed for CHN analysis to determine the carbonlnitrogen ratio and to indicate lignin and cellulose content. Before the results of near-IR spectroscopy can be considered valid, Martin and other graduate students from h e r ’ s group must develop calibration equations for t h e i n - l a b near-IR method based on strong correlations between prominent near-IR spectral features a n d the actual chemical contents of the leaf samples. To do this, they first convert raw spectra from calibration sets to firstor second-difference spectra by calculating the differences among ab-
Figure 2. Lignin concentration map created from Airborne Imaging System scan data of forest stands on Blackhawk Island, WI. (Reprinted with permission fmm Wessman, C. et al.. Nafura1988, 335.154-56.1
sorbance values for adjacent spectral bands. Then they test the correlations between selected wavelengths and chemical composition by performing multiple linear regressions on values from the converted spectra with respect to nitrogen, lignin, and cellulose concentrations obtained by the wet chemical methods. In general, near-IR and wet cbemistry results for dry samples correlate well. Carol Wessman, a former graduate student of Aber’s, devel-
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oped a calibration equation for near-IR analysis t h a t she used in 1988 to pick significant wavelengths for data scanned by the Airborne Imaging System (AIS), a precursor to AVIRIS t h a t recorded data in 128 spectral bands. Using the data from significantly correlated bands, Wessm a n was able to create a n aerial pixel-by-pixel map of lignin concentrations-which are strongly correlated w i t h nitrogen mineralization-in forest stands on Blackhawk Island, WI (see Figure 2). But Wessman, who directly compared wet chemistry results from the leaf samples with the AIS spectra of fresh foliage a t the canopy level, acknowledged that more work would have to be done to validate the AIS data. The pitfall in her work, she noted, was that lignin has never been isolated in any pure form; thus, the lignin spectrum itself is not well characterized. “Lignin is defined only by proximate analysis,” Martin explains. “Basically, it’s whatever’s left behind when you digest leaf samples in concentrated sulfuric acid. It‘s the undissolved residue.” To make matters
worse, she says, the harsh chemicals used to remove everything else probably react with the residue to form artifactual compounds, so the spectrum of isolated lignin may not bear much similarity to that obtained by overflight s c a n s of i n t a c t fresh leaves. Aber says he has started to get a predictive “true lignin” spectrum by back-calculating from the weighted means of combined near-IR spectra of fresh green leaves taken before and a t each intermediate extraction step of the carbon fraction analysis. “The third step in the carbon fraction analysis yields cellulose. When we compare the estimated spectrum with that of dissolved filter paper, which is essentially pure cellulose, we can validate t h a t this method yields a n accurate spectrum, and we can then proceed to calculate a true spectrum for lignin,” he explains. Martin’s approach for extrapolating in-lab methods to AVIRIS is to develop stepwise calibration equations between the methods, first for analyzing fresh leaves by near-IR in the laboratory and then for making AVIRIS scans of the green canopy.
Until three years ago, Aber’s group concentrated its near-IR measurements on dry fallen leaves or “litterfall” and on fallen conifer needles, in part because near-IR is subject to such large, broad peaks from water interference t h a t they thought it might not work for fresh foliage. Martin found that despite the water peaks, several spectral features correlated strongly with the chemical concentrations of nitrogen, lignin, and cellulose. However, she cautions, wavelengths chosen purely for their statistical correlation may have no real relation to the chemical bonds present in the molecules of interest. In her Calibration equations, she has selected wavelengths for both statistical significance and known biochemical absorbance features. Some calibration wavelengths, such a s the nitrogen peak a t 2170 nm, are fairly constant from instrument to instrument; others, particularly for lignin, vary. However, Martin has reported that near-IR, regardless of whether its results correlate well for lignin, gives better precision than do wet chemistry methods. Martin says her research has al-
most reached the point where she can s t a r t to create her own maps from significant chemical data in the AVIRIS spectra. The calibration equations she derived are based on data from interlaboratory comparisons of near-IR and wet chemistry methods. The laboratory recently acquired the capability to do wet chemistry methods in house, so she can eliminate some of the interlaboratory variation and refine the equations. She also has larger data sets at her disposal. Last summer, NASA funded Aber’s lab for three weeks of AVIRIS overflights instead of the usual one or two days, and the agency’s interest is increasing. The accelerated canopy chemistry program, which Aber helps to direct in support of NASA’s EOS, is being increased to 75 forest sites in Florida, Maine, Wisconsin, California, a n d other areas of the country in order to determine whether the HIRIS instrument, or something similar, will be able to monitor forest chemistry from its satellite orbit. “We hope to be able to say yes or no within about a year and a half,” Aber says. Deborah Noble
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