Methodology comparison for canopy structure parameters extraction from digital hemispherical photography in boreal forests

The retrieval of canopy architectural parameters using off-the-shelf digital cameras with fish-eye lens is investigated. The technique used takes advantage of the sensorʹs linear response to light of these cameras to improve the estimation of gap fraction using: (1) the digital numbers of mixed sky-canopy pixels to estimate the within-pixel gap fraction; and (2) this process is done considering the variation in view zenith angle to take into account the sky radiance distribution and the canopy multiple scattering effects. The foliage element clumping index is retrieved over a wide range of view zenith angles using: (1) the accumulated gap size distribution theory developed for the TRAC by Chen and Cihlar (1995a); (2) the Lang and Xiang (1986) finite-length averaging method; and (3) a method combining the gap size distribution and the Lang and Xiang finite-length methods. Using data from Canadian and Russian boreal forests, comparisons of gap fraction, clumping index and plant area index measured with the tracing radiation and architecture of canopies (TRAC) and digital hemispherical photography are presented. Evaluation of the LAI estimated from digital hemispherical photography with allometric LAI of two boreal forest stands suggest that that the clumping index combined method may be more accurate.