Variation of canopy openness of tropical rainforests of Sri Lanka with altitude and azimuth angle

Solar radiation energy is a limiting factor for forest growth in humid tropical environments. Canopy openness, defined as the unobstructed visible sky fraction through a forest canopy when viewed from the ground level, influences the understory light regime and regeneration dynamics of a forest. In this work, our objectives were to determine the variation of canopy openness of tropical rainforests of Sri Lanka (TRFSLs) with altitude and azimuth (i.e. the compass direction). Furthermore, we explored the possible influence of differential radiation regimes, caused by variation of aspect and topography, on the observed variations in canopy openness with azimuth. We sought evidence from our data to support the hypothesis that forest canopies allocate greater leaf area to canopy segments facing azimuth angles which receive greater solar radiation. Hemispherical photography was used to capture canopy images of TRFSLs in ten 1 ha permanent sampling plots across an altitudinal range from 117 m to 2132 m above mean sea level. Fraction of visible sky (V_sky) in 144 sectors of the canopy hemisphere, defined by 18 zenith angle (ZnA) × 8 azimuth angle (AzA) segments, was determined using image analysis. Canopy openness, quantified as V_sky of the overall hemisphere, increased with increasing altitude. Canopy leaf area index, decreased linearly with altitude and had a negative relationship with V_sky. In eight out of the ten plots, canopy openness of the top one-third (ZnA = 0°–30°) of the hemisphere (V_sky(0-30)) was lower on the east (AzA = 90°) than on the west (AzA = 270°) with the difference ranging from 3 % to 52 %. Estimated solar irradiance levels and maps of projected shade on a majority of these plots and on-site solar irradiance measurements on selected plots showed that radiation receipt was greater during the pre-noon period than during the post-noon period. In the two plots which showed lower V_sky(0-30) on the west, pre-noon solar irradiance was higher probably due to the presence of nearby mountains to the east of the plots which shaded the eastern side of the forest canopies during the pre-noon period. These results provide partial support for our hypothesis that forest canopies allocate greater leaf area to the side which receives greater irradiance. However, there was no consistent difference between V_sky(0-30) the northern (AzA = 0°) and the southern (AzA = 180°) canopy segments. Therefore, we conclude that our observations do not provide conclusive evidence to confirm the hypothesis that forest canopies allocate greater leaf area to canopy azimuth segments which receive greater solar radiation.