Response of Canopy Leaf Area Index and Architecture of Tropical Rainforests in Sri Lanka to Climatic Variation along an Altitudinal Gradient

Abstract

The foliage canopy of a forest is important for radiation capture, carbon sequestration and regulation of water relations. The objective of this work was to quantify the response of key canopy properties of tropical rainforests of Sri Lanka (TRFSL) to the variation in altitude and associated climatic variables. Canopy hemispherical photography was used to measure leaf area index (LAI), ellipsoidal leaf angle distribution parameter (ELADP) and mean leaf angle (MLA) in ten 1 ha permanent sampling plots (PSPs) established across an altitudinal gradient from 117 m to 2132 m above mean sea level. All three canopy properties varied significantly (p<0.1) among PSPs. Both LAI and MLA showed second-order polynomial responses to altitude, with maxima at 658 m and 773m, respectively. The ELADP showed an inverse second-order polynomial response to altitude, with a minimum at 716 m. The long-term mean annual precipitation (RF) was the main determinant of LAI, showing a positive, linear relationship, confirming the influence of water availability in determining canopy size. Long-term solar irradiance (SR) was a key climatic factor determining canopy architecture, where the proportion of relatively horizontally oriented leaves increased (ELADP>1) and MLA decreased with decreasing SR from mid- to upper altitudes (>700 m amsl). This was an adaptation to maximize radiation interception. The LAI decreased from mid to lower altitudes (<700 m amsl) as an adaptation to reduce transpiration under higher SR and TAV in the lower altitudes, forcing canopy architecture towards horizontally-oriented leaves to maximize radiation interception. These responses could be influential in the overall response of TRFSL to climate change.