Comparison of in situ and airborne spectral measurements of the blue shift associated with forest decline

Abstract

During August 1985, extensive field activities were conducted at spruce/fir ( Picea and Abies) sites in Vermont and Baden-Württemberg (FRG) currently undergoing rapid forest decline suspected of being due to various forms of air pollution. High-spectral resolution in situ reflectance curves and photosynthetic pigment determinations were acquired for common branch samples from specimens of spruce typical of both high- and low-damage sites. Similar spectral responses (reflectance data) were exhibited for specimens collected from high-damage sites in both the United States and FRG. Both current year and older foliage from high-damage sites in both countries showed an approximately 5 nm shift away from the normal inflection point of the red edge reflectance feature toward shorter wavelengths (a blue shift). This blue shift was associated with an observed reduction in chlorophyll b and a relative decrease in chlorophylls for needles collected from the high-damage sites, as compared with those from low-damage sites. An airborne high-resolution imaging spectrometer (the Fluorescence Line Imager or FLI) was flown over the Vermont study sites in August 1985 and has detected a blue shift of the chlorophyll absorption maximum at the high-damage site. Red edge parameters (wavelength position of the chlorophyll absorption maximum, red radiance, and NIR radiance) detected by the FLI have been used to image and map areas of damage in a highly accurate manner. Since data presented here suggest that the blue shift is a previsual symptom of damage, the ability to remotely detect such subtle spectral symptoms may serve as an early indicator of certain types of forest damage, and thus could be of considerable value in monitoring forest condition and state of health.