Fringe factor and view angle effect on gap fraction measurement for mature trees in California

Canopy gap fraction is a key parameter for hydrology and energy budget studies. For example, free throughfall in rainfall interception processes and energy impacts of tree shade are directly proportional to this parameter, and also to crown projection and crown size. Gap fraction can be easily measured by using photographic methods. These methods determine the boundary of crown projection area by linking the branch tips with line segments of arbitrary maximum length (fringe factor). Photos are typically taken from a horizontal perspective so that information is lacking on how the gap fraction changes with different view angles.

The objectives of this study were 1) quantity how the fringe factor affects gap fraction measurement and, 2) how gap fraction changes with view angle. Twenty different tree species (30 trees of each species) from Santa Monica, California were selected for studying the fringe factor affect on gap faction. Two photos with horizontal view angle were taken for each tree. For studying the view angle affect on gap fraction, an eight-year old Cork oak tree in Davis, California was cut down and suspended from a crane to allow photos to be taken at a range of angles with clear sky background. Phones were taken from 0 to 90 degree angles at five-degree increments.

The initial results indicate that view angle had a larger affect on gap fraction than the fringe factor does. Fringe factor affects on gap fraction was examined by connecting the branch tips with fringe factors range from 0 to 100cm at 5cm intervals. Compared with gap fraction measured without connecting branch tips, gap fraction changed less than 5% for 15 species, and less than 10% for all but one species. However, gap fraction changes more than 62% when view angle (zenith angle) change from 0 to 80 degree.