Remote sensing and forest damage Detecting and monitoring stress to vegetation By Nicholas J. Reid Each year, damage to North American forests caused by insects, disease, pollution, and fire results in multibilliondollar losses of revenue and resources. To respond to these losses effectively, forest managers need timely information on the location, extent, and spread of the damage. Traditional techniques for monitoring forest damage include high-resolution aerial photography, color infrared photography, and visual reconnaissance mapping. These techniques require visual interpretation of the data and often are somewhat subjective. In addition, because such analyses are time-consuming and costly, many areas of forest are never mapped, and in cases where maps exist, they often are obsolete or incomplete. An airborne imager has been developed to solve the problems of time-consuming visual analysis and interpretation. The Programmable Multispectral Imager (PMI) (MONITEQ Ltd., Concord, Ont.) measures small changesinvisible by conventional detection methods-in light reflected by the forest canopy. The PMI measures the color and intensity of reflected light and records this information digitally on computer tape aboard an aircraft. This information is then available for later entry into a computer for processing and enhancement. Although airborne imagers have been available for nearly three decades, they have not been used extensively for forest damage assessment or other forestry applications because of their poor sensitivity and their limited number of fixed spectral channels. The PMI is the first of a new generation of imagers that combine high sensitivity with the flexibility of continuous spectral coverage. This allows scientists to evaluate the potential causes and effects of stress on vegetation.
How it works At the heart of the PMI are computer-controlled electrooptical mcd428 Environ. Sci. Technol.. MI. 21. No. 5. 1987
Programmable Multispectral imager The PMI offers a number of advantages over conventional forest d a m age monitoring techniques and instruments. High spectral rerolutlon. The PMI detects 288 colors in the visible and near IR wavelengths; d o r film records only three. Digital format. Color information is recorded digitally on magnetic tape, which can be read by a computer for processing. Automatic interpretation. Software developed to generate images of the forest canopy can provide damage assessment and speciation, with minimal need for human interpretation or interaction. Computergenerated maps. Using data from an aircraft's navigation system, forest damage information can be generated automatically on maps that are geometrically correct and registered to ground control points. Early detectton of damage. Because the imager can detect damage before it is evident by traditional measures, forest managers can take corrective action before damage be. comes extensive.
des. Each has an objective lens, a transmission grating, and a detector matrix. As does a conventional camera, the objective lens gathers light reflected from the target; the images are recorded on a detecting surface. The transmission grating breaks the light into its component colors, in a manner similar to a prism, and disperses the light across a photosensitive surface, in this case a matrix of 100,OOO silicon photodiodes that are sensitive to light in the visible and near-infrared wavelengths. Each photodiode produces a small voltage, in proportion to the light striking it. The voltage is recorded on tape as the image is acquired. The instrument has two modes of operation: a research mode, in which continuous spectral coverage is available, and a survey mode, which provides eight programmable spectral bands in high spatial resolution. When the imager is flown over a target of interest, it creates an image composed of pixels, each representing a very small area within the image. The pixel size and swath width are controlled by the aircraft's altitude. In a typical image acquired at an altitude of 10,OOO ft, each pixel is 7 ft wide and, depending on the aircraft's ground speed, up to 18ft long. The image swath is more than 2 mi wide. Each pixel contains information on the intensity and color of light reflected from that area on the target. Figure 1 shows how this information is displayed in the form of a spectral plot. One axis shows the intensity of light reflected by green leaves; the other shows the color or wavelength of the light. The green wavelengths of light are reflected most brightly in the visible region of the spectrum. Figure 1 also shows that there is significant reflection at the infrared wavelengths. This phenomenon, which is not visible to the human eye, is typical of vegetation. The rapid increase in reflectance that occurs between the red and infrared regions is often called the red edge. The imager detects damage by moni-
W13936x1871wz1-04281,50100 1987 American Chemical Society
FIGURE 1
FIGURE2
spearat plottypical of healthy green kaves
stressindueedspecha1changes
-
Blue = -500 nm Green = 500-6w nm Red = 600-700 nm Infrared = 7OC-1100nm
j:
40
Blue = 4C0-500 nm Green = 500-6w nm
Red=600-7Wnm
E
10
10
300400 500 m 7 w 800 9001m11w Wavelength (nm)
Wavelenath fnm) toring subtle changes in color that are indicative of vegetation stress. Stress can occur when a plant is exposed to a number of environmental factors, including air pollution, acid rain, heavymetal contamination of the soil, insect infestation, and disease. Stress is manifested by changes in the light reflected by the plant. Figure 2 is a spectral plot typical of vegetation under stress. This figure shows that the effect of stress is apparent in several areas. Most notably there is a shift in the position of the red edge toward the blue end of the
spectrum and a decrease in the reflection of infrared wavelengths. These effects have been identified as the result of cell damage within leaves (I).They can be quantified using computer algorithms that can generate images of the forest canopy, which are assigned colors based on the level of stress involved. Such images are called false color stress images. In its current state of development the PMI can perform simple data-processing algorithms as the images are acquired. In the future it is expected that
the imager will be able to generate forest damage maps in real time, providing a versatile and powerful tool for forest managers.
Reference (I)Rock,
B. N. el al. Bioscience 1986, 36,
439-45.
Nicholns J. Reid is a biologisr wirh MONI7EQ in in Concord, Onr. H e is involved wifk rke application of airborne mulrispecrrai imagery to environmenral monitoring.
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