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 forests were corrected for slope. Digitised hemispherical photographs were analysed to compute LAI according to azimuth and as a function of slope. The topographic mask (part of hemisphere obstructed by slope) and the path length of a ray of light at a given zenith angle (longer upslope and shorter downslope) were taken into account. Results indicate a strong slope effect, both in temperate conifer forest plantation and in tropical rain forest. On regular slope, this effect is easily corrected, although the topographic mask affects data. On irregular topography, corrected and uncorrected LAIs strongly differ, suggesting a need for reference measurements; corrected figures appear nevertheless realistic and normality is reached only for corrected values. LAIs should systematically be corrected for slope above 15-20 degrees, more so if topography is irregular or the forest cover heterogeneous.