Direct measurement of the three-dimensional distribution of leaf area density and light conditions in a mature oak stand by the cube method

Abstract

Although the distributions of foliage and light play major roles in various forest functions, accurate, nondestructive measurement of these distributions is difficult due to the complexity of the canopy structure. To evaluate the foliage and light distributions directly and nondestructively in a mature oak stand, we used the cube method by dividing the forest canopy into small cubes (50 cm per side) and directly measured leaf area density (LAD, the total one-sided leaf area per unit volume, i.e., cube) and relative irradiance (RI) within each cube. The distribution of LAD and of RI was highly heterogeneous, even at the same canopy height. This heterogeneity reflected the presence of foliage clusters associated with multiple forking branches. The relationship between cumulative LAD at the canopy surface and average RI followed the Beer–Lambert law. The mean light extinction coefficient (K) was 0.32. However, K was overestimated by more than double (0.80) when calculated based on the classical method using RI at the forest floor. This overestimation was caused by the lower RI due to light absorption by nonleaf plant parts below the canopy. Our findings on the complex foliage and light distributions in canopy layers should help improve the accuracy of RI and K measurements and thus more accurate predictions of environmental responses and forest functions.

Keywords

Beer-Lambert law / Canopy structure / Foliage cluster / Leaf area density / Leaf area index / Relative light intensity