Evaluation of digital hemispherical photography and plant canopy analyser for measuring Vegetation area index of orange orchards

This study presents the results of an evaluation of two indirect methods (Plant Canopy Analyser (PCA) and hemispherical photographs) for measuring vegetation area index VAI of two orange orchards, differing by their ground fraction cover (fc). The evaluation is based on reference data obtained by destructive measurements on limited samples, which allow to calibrate and validate an exponential relationship between the diameter of branches and associated areas of leaves (R2 = 0.99). The obtained results show that the two indirect methods underestimate the reference values of VAI. For the PCA device, the best estimates of VAI are obtained using the five rings for high fc (fc = 0.7) and using only four rings for low fc (fc = 0.3). For both case, the hemispherical photographs give accurate estimates of VAI: the relative errors are about 11 and 14% for high and low fc, respectively. Alternatively, a simple method consisting of calculating VAI as the weighted average of the maximum (VAImax, below the tree) and minimum (VAImin, at the center of four trees) values using fc as a weighting factor, was successfully tested (R2 = 0.90). For both indirect methods, the tree volumes are well estimated with comparison to the values calculated assuming an ellipsoidal form.