Retrieval of leaf area index and canopy closure from CASI data over the BOREAS flux tower sites

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 b y the Compact Airborne Spectrographic Imager (CASI) with a spatial resoluti on 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 c anopies investigated by considering the effect of the ellipticity of the co nifer crowns on shadows and crown transmittance. The LAI and canopy closure images were produced for the flux tower sites investigated from the CASI w inter images using the modified FLIM. Visual evaluation shows that the spat ial patterns shown in the canopy LAI and canopy closure images are consiste nt with those exhibited by the color composite CASI images for the same sit es. The canopy LAI images were further validated by field-measured LAI alon g specific transects. The R-2 between the retrieved canopy LAI and the fiel d-measured LAI ranges from 0.51 to 0.86 for the sites investigated. Further more, the CASI summer image over the Old Black Spruce site in the BOREAS so uther study area (SOBS) was used to investigate the effect of the understor y 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 w ithin the range of variability as determined by field measurements. This re sult conforms to the sensitivity analysis result in which the at-canopy ref lectance is expressed as a function of the understory reflectance at the CA SI viewing geometry. The results imply that it is questionable to assume a uniform understory reflectance in canopy model inversions in the boreal for est summer scenes. (C) Elsevier Science Inc., 2000.