Influence of Geographical Aspect and Topography on Canopy Openness in Tropical Rainforests of Sri Lanka along an Altitudinal Gradient

Solar radiation energy is a limiting factor for forest growth in humid tropical environments. Radiation incident on a forest canopy varies with azimuth angle of incident radiation and aspect of the forest terrain. The objective of this work was to determine the influence of the geographical aspect and azimuth of incident radiation on the canopy structure of tropical rainforests of Sri Lanka (TRFSL). Hemispherical photography was used to capture canopy images of TRFSLs in ten permanent sampling plots across an altitudinal range from 117 m to 2132 m above mean sea level. Fraction of visible sky (Vsky) in 144 sectors of the canopy hemisphere, defined by 18 zenith angle (ZnA) x 8 azimuth angle (AzA) segments, was determined using image analysis. Canopy openness, quantified as Vsky of the overall hemisphere, increased with increasing altitude. Canopy leaf area index, decreased linearly with altitude and had a negative relationship with Vsky. Canopy openness of the top one-third (ZnA=0{degrees}-30{degrees}) of the hemisphere (Vsky(0-30)) was significantly (p<0.05) lower on the east (AzA=90{degrees}) than on the west (AzA=270{degrees}) in a majority of plots. Similarly, Vsky(0-30) was lower on the northern (AzA=0{degrees}) than the southern (AzA =180{degrees}) canopy segment. These patterns were altered in plots where nearby mountains and slope aspect of the plot influenced incident radiation. These observations suggest a hypothesis that canopies of TRFSL are structured to maximize radiation capture by allocating more leaf area, and therefore having lower canopy openness, on sides of the canopy which face directions of greater radiation receipt.