A calibration method for the crown illumination index for assessing forest light environments

The crown illumination index (CII) is an ordinal scale frequently used to qualitatively assess the light environment within a forest. Although this method provides a simple and rapid approach, it can be subjective and needs to be calibrated against quantitative measurements of canopy structure. A study was undertaken in north-western Amazonia to provide a means to calibrate the CII in tropical forests. Hemispherical photographs were taken to represent each CII class, which were then analysed using HemiView 2.1 software in order to calculate light availability factors. The proportion of visible sky and indirect, direct and global site factors (the proportion of indirect, direct and total radiation reaching a point, compared to an open location) were all found to correlate strongly with CII class and therefore offer a viable method for calibration. CII classes could only be weakly defined with respect to leaf area index (LAI), however, and so conversion of CII to LAI or vice versa is problematic. The results of this study provide a quantified description of each index class, significantly improving interpretation of the crown illumination index.