Retrieval of Leaf Area Index and Canopy Closure from CASI Data over the BOREAS Flux Tower Sites

Abstract

This study is to investigate the retrievals of the canopy leaf area index (LAI) and canopy closure using a canopy model. The data used were obtained by the Compact Airborne Spectrographic Imager (CASI) with a spatial resolution of 2 m over the flux tower sites in the BOREAS study area in the winter of 1994 and in the summer of 1996. The FLIM (Forest–Light Interaction Model) was adapted to describe the reflectance properties of the boreal forest canopies investigated by considering the effect of the ellipticity of the conifer crowns on shadows and crown transmittance. The LAI and canopy closure images were produced for the flux tower sites investigated from the CASI winter images using the modified FLIM. Visual evaluation shows that the spatial patterns shown in the canopy LAI and canopy closure images are consistent with those exhibited by the color composite CASI images for the same sites. The canopy LAI images were further validated by field-measured LAI along specific transects. The R 2 between the retrieved canopy LAI and the field-measured LAI ranges from 0.51 to 0.86 for the sites investigated. Furthermore, the CASI summer image over the Old Black Spruce site in the BOREAS souther study area (SOBS) was used to investigate the effect of the understory reflectance on the retrieval of the canopy LAI. The R 2 between the field-measured LAI and the retrieved LAI is 0.27, under the assumption that the reflectance is uniform for each understory vegetation in the SOBS site. The R 2 increases to 0.78 when the understory reflectance is allowed to vary within the range of variability as determined by field measurements. This result conforms to the sensitivity analysis result in which the at-canopy reflectance is expressed as a function of the understory reflectance at the CASI viewing geometry. The results imply that it is questionable to assume a uniform understory reflectance in canopy model inversions in the boreal forest summer scenes.