Comparison of optical methods for estimating canopy openness and leaf area index in broad-leaved forests.

Based on inversion of gap fraction data (Poisson model of foliage distribution), three optical methods using the Demon. the Plant Canopy; Analyzer LAI-2000 (PCA) and hemispherical photographs, have been compared to estimate canopy openness (CO) and leaf area index (LAI) in a mature. neutrophil. oak-beech-hornbeam forest on mull in eastern France. Mean CO over the whole hemisphere a as similar for PCA (7.9 %) and hemispherical photographs (8.0 %). The needle method, 3 vertical point quadrat method, applied to the litter after leaf fall has served as a reference to LAI (4.7). The Demon provided the estimate (4.9) closest to the reference value. The PCA and hemispherical photographs underestimate mean LAI by 30 % (3.3) and 19 % (3.8). respectively. if used without correction. Based on fish-eve sensors. LAI estimates can be improved if 3 annuli (4.2) or 2 annuli (4.5) Lire used in place of 5 with the PCA. or by means of logarithmic averaging of gap fractions over azimuth at an appropriate angular resolution (180 degrees. 4.6, or 120 degrees: 5.;2) with hemispherical photographs. Not taking into account azimuthal variation in gap fraction distribution generates 3 more important error than the error induced buy light scattering near horizon In order to improve;e LAI estimates. an original iterative procedure is presented. which allows the simultaneous calculation of LAI over 3 broad range of angular azimuthal resolutions. (C) 2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.