CHARACTERIZING CANOPY NONRANDOMNESS WITH A MULTIBAND VEGETATION IMAGER (MVI)

A new method for measuring plant canopy nonrandomness and other archit ectural components has been developed using a 16 bit (65535 gray scale levels) charged-coupled device (CCD) camera that captures images of p lant canopies in two wavelength bands. This complete system is referre d to as a multiband vegetation imager (MVI). The use of two wavelength bands (visible (VIS) 400-620 nn and near infrared (NIR) 720-950 nm) p ermits identification of sunlit and shaded foliage, sunlit and shaded branch area, clouds, and blue shy based on the camera's resolution, an d the Varying spectral properties that scene components have in the tw o wavelength bands. This approach is different from other canopy imagi ng methods (such as fish-eye photography) because it emphasizes measur ing the fraction of an image occupied by various scene components (bra nches, shaded leaves, sunlit leaves) under different sky conditions ra ther than simply the canopy gap fraction under uniform sky conditions. The MVI has been used during the Boreal Ecosystem-Atmosphere Study (B OREAS) in aspen (Populus tremuloides) and balsam poplar (Populus balsa mifera) to estimate architectural characteristics of each canopy. The leaf area index (LAI), sunlit LAI, and degree of nonrandomness within a canopy are architectural properties that have been measured with the MVI. Using a crown-based Monte Carlo model for nonrandom canopies, no nrandomness factors are calculated from MVI data using two approaches (gap fraction and gap-size distribution theories) to correct total and sunlit LAT estimates from indirect methods that assume random foliage distributions. Canopy nonrandomness factors obtained from analyzing t he gap-size distribution in a Monte Carlo model are shown to be a func tion of path length (angle) through the canopy (Omega(e)(theta)); thus we suggest that LAI-2000 indirect measurements of LAI be adjusted wit h the value of Omega(e)(theta) at theta = 35 degrees because this is t he mean angle at which the canopy gap fraction is measured by the LAI- 2000. In this study, values of Omega(e)(35) = 0.69 in an aspen forest. Alternatively, corrections to indirect LAI measurements obtained with the MVI in this study are made using the value of Omega(e)(0) because the MVI is used to measure the canopy gap-size distribution and gap f raction within 15 degrees of the zenith. Values of Omega(e)(0) obtaine d with the MVI in aspen are typically between 0.55 and 0.65; while in balsam poplar, average values of Omega(e)(0) are equal to 0.82. This s tudy shows that the MVI provides an attractive indirect measurement te chnique to obtain accurate estimates of total LAI in aspen. Corrected canopy LAT and direct LAI measurements are greater than indirect estim ates based on assuming the foliage to be randomly distributed: In aspe n, total LAI is 45% larger (3.3 versus 2.0) and sunlit LAI (40 degrees Sun zenith angle) 10% larger, while in balsam poplar, total LAI is 17 % larger (2.3 versus 1.9) and sunlit LAI is only 1% larger. The import ance of these clumping characteristics is best appreciated with estima tes of canopy net CO2 assimilation derived from scaling leaf photosynt hesis versus light relations. Aspen canopy assimilation accounting for clumping is 39% larger than estimates based on indirect measurements of total LAI and the assumption that foliage is randomly distributed.