Abstract: We proposed a method that used the widths of trunk shadow at different positions as the proxy of corres ponding diameters to compute stem volume with the sectional method, which complements current methods for the measurement of stand volume. Given that sun light is not of absolute point source but of near parallel, combined with diffraction, the width of penumbra zone in the trunk shadow increases with tree height. This leads to large deviations for diameters represented by shadow widths. To accurately estimate the diameter at any height of the trunk based on the shadow width, an experiment was conducted using cylindric objects (PVC). We constructed five regression models for estimating the shrinkage of core shadow width of the cylinder referred to penumbra zones. Diameters at higher positions were predicted using the models after measuring shadow widths with certain intervals, and the tree volume was computed by the sectional method. Our results showed that all regression models performed well in estimating shadow shrinkage, with accuracy higher than 90%. The optimum solar zenith angle was determined as 48° when applying the method. The accuracy for estimating tree volume was as high as 93%. The accuracy of estimating tree volume decreased with tree height, and relative errors were larger towards upper sections.

Key words: shadow width, accuracy, nondestructive sampling, core shadow, penumbra.