Mapping wood area in forests from ground lidar and estimating their light interception using radiative transfer modeling

Abstract

Leaves in forests are commonly believed to shade many stems and branches, and thus most of the absorption of photosynthetically active radiation (PAR) within a canopy is done by leaves. Near-infrared radiation (NIR) on the other hand is not used in photosynthesis, and leaf level absorptance of NIR is much lower than it is for PAR. Still, temperate broadleaf canopies absorb about 50–70 % of incoming NIR, and how this absorption is partitioned between leaves and woody structures is unclear. Here, I show that of the NIR absorbed within the canopy space, woody structures contribute about 30–35 %. The results also confirm that leaves account for about 90 % of the absorbed PAR within the canopy space. To establish these figures, I used ground lidar measurements to map leaf area and stem and branch area in 3D in two structurally contrasting broadleaf forests, and from radiative transfer modeling I estimated the fractions of PAR and NIR absorbed by leaves and wood based on illumination conditions measured at each site over multiple years. The findings have implications for the development of land surface models that consider the storage of heat by woody biomass in forests as part of the canopy energy balance.