The computation of forest leaf area index on slope using fish-eye sensors

Leaf area index (LAI) measured with fish-eye sensors is inevitably affected by slope. In order to quantify this effect, LAIs measured in contrasted fo rests were corrected for slope. Digitised hemispherical photographs were an alysed to compute LAI according to azimuth and as a function of slope. The topographic mask (part of hemisphere obstructed by slope) and the path leng th of a ray of light at a given zenith angle (longer upslope and shorter do wnslope) were taken into account. Results indicate a strong slope effect, b oth in temperate conifer forest plantation and in tropical rain forest. On regular slope, this effect is easily corrected, although the topographic ma sk affects data. On irregular topography, corrected and uncorrected LAIs st rongly differ, suggesting a need for reference measurements; corrected figu res appear nevertheless realistic and normality is reached only for correct ed values. LAIs should systematically be corrected for slope above 15-20 de grees, more so if topography is irregular or the forest cover heterogeneous . (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevi er SAS.