Evaluation of hemispherical photography for determining plant area index and geometry of a forest stand

Hemispherical photographs were taken beneath an 80-years-old Douglas-fir (Pseudotsuga menzie sii) forest stand under clear and overeast sky conditions, and under clear sky with the canopy partially covered with snow. The photographs were digitized using an optical scanner. The ‘effective plant area index’ (Le) and the distribution of the inclination angle of plant elements were obtained with Norman and Campbell's linear least-square inversion technique. Photographic exposure was determined by matching the Le values obtained from the photographs with those measured using the Plant Canopy Analyzer (PCA) (Li-Cor LAI-2000). Agreement between the inverted and measured values was found when the photographs were underexposed by 4–5 stops compared with readings of a light meter facing vertically upward under the canopy. These values of Le were about half the plant area index calculated from the measurements of tree diameter at breast height (Dbh) using published relationships for Douglas-fir trees. The distribution of sky radiance under clear and overcast conditions had a considerable effect on the determination of the angular distribution of plant elements. Pronounced differences were found between inclination angle distributions calculated from the PCA gap fraction measurements and the digitized photographs taken with and without a blue filter (Wratten 80B). These differences may have resulted from the effect of the scattering and possibly diffraction of the visible radiation by the foliage. The scattering and diffraction caused more leaves to blend into the sky at small than at large zenith angles.