The effects of bark beetle stress on the foliar spectral reflectance of lodgepole pine

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Abstract
This paper reports the results of a study of the foliar reflectance properties of lodgepole pine trees under mountain pine beetle attack which aimed to determine the sequence of changes which occur. The motivation for this study was to identify spectral regions showing the earliest signs of attack in order to choose spectral bands for airborne linear array sensors. Normal illumination, diffuse hemispherical reflectance spectra were obtained of needles from trees showing varying degrees of stress from beetle attack and for needles from unattacked trees for comparison. The more pronounced changes in the spectra were interpreted visually and compared to changes reported by other authors. The more subtle changes were studied by analysing variance methods. Three spectral bands (the green peak, red edge and near-infrared (NIR) shoulder regions) have been identified as most promising for detecting early effects of bark beetle attack. Three additional bands (the blue, red and NIR plateau regions) are identified as reference bands for calibration and comparison purposes. The difference between foliar reflectance of attacked and unattacked trees is found to be most significant in the visible and red edge regions for current foliage and in the NIR for previous foliage. A red edge red shift was observed in the spectra of current foliage from attacked trees, in contrast to red edge blue shifts associated with stress in other studies. The observed sequence of subtle to more pronounced changes cannot be explained qualitatively using current knowledge of the plant pigment and anatomical changes which occur at the cellular and needle levels in stressed conifers. Further study of the detailed changes in pigments anel cellular and foliar anatomy is recommended, both to elucidate on the cause-and-effect relationships which occur, and to indicate the extent to which this paper's findings can be generalized. The findings suggest that multispectral linear array airborne scanners may be able to detect stressed conifers long before the red attack stage, but further investigation is required to determine whether the differences at foliar level between attacked and unattacked trees can also be detected at the whole-tree level, and whether confusion with other ground cover types can be avoided.

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