Estimation of the leaf area density distribution of individual trees using high-resolution and multi-return airborne LiDAR data

Abstract

In this paper we demonstrate a method for estimating the leaf area density (LAD) distribution of individual trees using high-resolution airborne LiDAR. This method calculates the LAD distribution from the contact frequency between the laser beams and leaves by tracing the laser beam paths. Multiple returns were used to capture the foliage distribution in the inner part of the crown. Each laser beam is traced from a location of the last return to the location of the first or intermediate return that is recorded immediately before the last return. We verified the estimation accuracy of the LAD distribution using terrestrial LiDAR data from single trees (Zelkova serrata and Cinnamomum camphora). The appropriate voxel size for representing the LAD distribution from the airborne LiDAR data was determined to be 1m×1m×0.5m. The accuracy of the estimated LAD distribution for this voxel size was then examined while considering the number of airborne incident laser beams on the voxel (N) and the return type used. When only the first and single returns were used, the LAD was overestimated even for the voxels with large N. LAD was estimated as zero for most voxels with small N, although LAD was significantly overestimated for several voxels. We found that using the last and intermediate returns improved the LAD estimation accuracy even if N was the same. The mean LAD estimation error was 0.25–0.3m2/m3 for both species. Assigning different weights to the first and intermediate returns improved the accuracy slightly. Estimation error clearly corresponded to N, and N of 8–11 could be a criterion for an accurate LAD estimation.