Spectral characteristics of forest vegetation in moderate drought condition observed by laboratory measurements and spaceborne hyperspectral data

Although there have been several studies on the spectral characteristics related to leaf water content, it remains unclear whether the spectral property of leaves can be extended to the canopy-level. In this study, we attempt to compare the spectral characteristics of forest vegetation in moderate drought condition observed by laboratory measurement and satellite hyperspectral image data. Spectral reflectance data were measured from detached pine needles and oak leaves in the laboratory with a spectroradiometer. Canopy reflectance spectra of the same species were collected from temperate forest stands with dense canopy conditions using EO-1 Hyperion imaging spectrometer data obtained during the moderate drought season in 2001, and then compared with those obtained in the normal precipitation season of 2002. The relationship between leaf-level spectral reflectance and leaf water content was the clearest at the shortwave infrared (SWIR) regions. However, the canopy-level spectral characteristics of forest stands did not quite correspond with the leaf-level reflectance spectra. Further, four water-related spectral indices (WI, NDWI, MSI, and NDII) developed mainly with leaf-level reflectance were not very effective to be used with the canopy-level reflectance in dense forest condition. Forest canopy spectra under moderate drought status may be more influenced by canopy foliage mass, rather than by canopy moisture level.

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