MEASUREMENT OF GAP FRACTION OF FRACTAL GENERATED CANOPIES USING DIGITALIZED IMAGE-ANALYSIS

A fractal based computer graphical model was used to generate poplar tree stands. From the model, the downward cumulative leaf area index (L) and leaf normal inclination distribution were determined. By graphically projecting downward geometrical layers of the simulated poplar stands on a horizontal plane, a set of pictures were obtained. The gap fractions of the projection pictures were analyzed using a digitalized image analysis system (DIAS). These measured gap fractions and the corresponding downward cumulative leaf area indices were used to test the two most often used canopy light penetration models, namely the exponential model and the negative binomial model. Results showed that at high values of L neither the exponential nor the negative binomial model could adequately describe the relationship between the gap fraction and L. These two theoretical models generally underestimated gap fraction, especially at high L. Viewed from zenith, at L = 6 approximately 8, the theoretical models predicted a gap fraction about 1-12%, whereas the gap fraction measured by DIAS was about 15-22%.