2015 Volume 35 Issue 1 Pages 10-23
In this study, we demonstrate a method for quantifying the solar shading effect of urban trees. Leaf area density (LAD) distribution is estimated using multi-return airborne light detection and ranging (LiDAR); then, the estimated distribution is applied to a radiative transfer model of vegetation to calculate the direct photosynthetically active radiation (PAR) transmittance. When first and single returns are used to estimate the LAD distribution, which is the same as the previously developed methods by other researchers, LAD is estimated with a large error in the lower part of the crown. The estimation error is an obstacle for an accurate calculation of direct PAR transmittance. Therefore, a method using the last and intermediate returns in addition to the first and single returns was examined. We verified the estimation accuracy of the LAD distribution using the terrestrial LiDAR data of a single Japanese zelkova (Zelkova serrata) tree. We confirmed that using the last and intermediate returns improves the estimation accuracy of the entire crown area, especially in the lower part of the crown. Improvement of the spatial resolution of the external crown geometry and correction of the LAD for the voxels where there are no airborne LiDAR returns from the leaves were also conducted. Subsequently, the estimated LAD distribution was applied to a radiative transfer model, and then, the direct PAR transmittance was calculated. The PAR under the Z.serrata tree was measured using a device with a 1-m probe in which PAR sensors are embedded. We obtained PAR distribution by moving the device in the direction perpendicular to the probe, and the calculated transmittance was then compared with the measured one. The comparison showed that when the estimated LAD distribution with the LAD correction is used, direct PAR transmittance is accurately calculated, regardless of solar altitude.
