Takashi Asawa

Detailed reproduction of three-dimensional crown shape and foliage distribution of trees in an urban area using high-resolution airborne LiDAR

This study examines the accuracy of the tree crown shape and crown structure (i.e., the distribution of foliage and branches) derived from airborne LiDAR for an urban district. First, airborne LiDAR points distributed on the crown surface were extracted. The spatial accuracy of these points was then verified using terrestrial LiDAR data. Next, the point cloud of the crown was divided into voxels. The voxels that included one or more points were then extracted as areas in which foliage or branches were distributed. The accuracy of the distribution of the extracted voxels was verified by a comparison with the distribution derived from the terrestrial LiDAR. The foliage density of the voxels that were not identified by the airborne LiDAR data was also evaluated.

Estimating the Solar Transmittance of Urban Trees Using Airborne LiDAR and Radiative Transfer Simulation

This paper presents a method for estimating the solar transmittance of urban trees using airborne light detection and ranging (LiDAR), and the radiative transfer simulation of vegetation. The leaf area density (LAD) distribution of trees with voxel size 1 m × 1 m × 0.5 m is estimated using high-resolution and multireturn airborne LiDAR data. The LAD of voxels having few incident laser beams is corrected from the surrounding voxels. The LAD of the periphery of the crown is discretized into 0.5 m × 0.5 m × 0.5 m voxels to accurately calculate the shaded area. The resulting LAD distribution is used in a radiative transfer simulation to calculate the solar transmittance of the trees. We verified the accuracy of the calculated transmittance by comparing it with empirical data for a Zelkova serrata. The comparisons were conducted under different angles of incidence of laser beams and solar radiation. When the angle between the incident laser beams and solar radiation was small, the transmittance could be accurately estimated. The LAD correction enabled the method to be applied to a broader range of the angle between beams and solar radiation. When the zenith angle of the incident laser beams was small (<; 10°) and the LAD correction was carried out, the errors in transmittance were within 0.06 for solar altitudes greater than 40°. Next, we examined the difference in solar transmittance among streets caused by the layout of trees and buildings and the growth condition of the trees. It was shown that the present method is able to quantify the solar shading provided by urban trees and take into account LAD, tree layout, and the spatial geometry of the surrounding buildings.

Continuous Measurement of Whole-tree Water Balance for Studying Urban Tree Transpiration

Street and garden trees in urban areas are often exposed to advection of strong vapor pressure deficit (VPD) air that can raise the whole-tree transpiration rate (ET), known as the oasis effect. However, urban trees tend to have small soil volume compared with natural conditions and so they are believed to strongly regulate stomata. ET characteristics of such urban trees have not been well understood because of a lack of reliable measurement methods. Therefore, we propose a novel weighing lysimeter method and investigate the whole-tree water balance of an isolated container-grown Zelkova serrata to examine (a) which biotic and abiotic factors determine ET, and (b) which spatial and temporal information is needed to predict ET under urban conditions. Whole-tree water balance and environmental conditions were measured from 2010 to 2012. Although leaf area substantially increased in the study period, daily ET did not vary much. ET increased with VPD almost linearly in 2010, but showed saturation in 2011 and 2012. Root water uptake lagged ET by 40 min in 2012. These results suggest that the small planter box interfered with root growth and that hydraulic supply capacities did not increase sufficiently to support leaf area increase. From analysis of water balance, we believe that neglecting soil drought effects on street trees without irrigation in Japan will overestimate ET over 4–5 sunny days at the longest. This is unlike previous studies of forest.

Estimating PAR transmittance of individual trees using multi-return airborne LiDAR and radiative transfer simulation

In this study we demonstrate a method for estimating the photosynthetically active radiation (PAR) transmittance of individual trees in an urban district. The leaf area density (LAD) distribution, derived from airborne light detection and ranging (LiDAR) data using a method proposed by our previous study, is applied to radiative transfer simulation of vegetation. Estimation accuracy of the PAR transmittance is verified using a single broad-leaved tree. The PAR distribution under a Zelkova tree is measured by a device that consists of 1 m probe, in which 64 PAR sensors are distributed. The estimated transmittance is then compared with the measured one. We conclude that when the derived LAD distribution is used, PAR transmittance is estimated within an error of 0.06, regardless of solar altitude. The influence of airborne LiDAR observation specification and method for deriving LAD distribution on the estimation accuracy of PAR transmittance is also examined.

Accuracy of external form of individual trees acquired by high-resolution airborne LiDAR

This study examined the accuracy of external form assessments of individual trees acquired by high-resolution airborne LiDAR in an urban area. The airborne LiDAR points obtained for the crown surface were extracted using multiple extraction intervals. The spatial accuracy of these points was then verified using the terrestrial LiDAR data. The external form of the tree crown was then reconstructed using voxels. External voxels that include one or more points were extracted as areas in which foliage was distributed. The distribution accuracy of the extracted voxels was verified by comparing with the data against terrestrial LiDAR data. The reasons for voxels not being extracted by the airborne LiDAR were also investigated using the Leaf Area Density in voxels derived from terrestrial LiDAR.

Estimating cooling effects of urban trees using airborne LiDAR and radiative transfer simulation

We examined a method for evaluating the cooling effects of trees for urban spaces using airborne LiDAR data. Voxel-based LAD distribution was derived from the data, and the distribution was applied to radiative transfer simulation to quantify solar transmittance and APAR. Subsequently, direct solar transmittance and transpiration rate of trees were calculated for an actual urban district, and several evaluations of these effects were performed. It was shown that the present method was able to quantify the cooling effects of trees while considering their structure, layout, and spatial geometry of the surrounding buildings.

A multi-echo airborne LiDAR-based method for estimating leaf area density distribution of individual trees

We demonstrate a method for estimating the leaf area density (LAD) distribution of individual trees using multi-echo airborne light detection and ranging (LiDAR). This method improves upon the previously developed method which calculates LAD based on the contact frequency between the laser beams and leaves by tracing the paths of the laser beams. The proposed method exploits the last and intermediate echoes in addition to the first and single echoes to capture the foliage distribution in the inner part of the crown. We verify the estimation accuracy using terrestrial LiDAR data of a single tree. The estimated LAD distribution is compared to the terrestrial LiDAR-derived LAD distribution. We confirmed that using the last and intermediate echoes improves the estimation accuracy of the entire crown area.

Estimation of tree crown structure in urban areas using high resolution airborne LiDAR

A method for estimating three-dimensional tree crown structure using airborne LiDAR (Light Detection and Ranging) in urban areas is presented. This latest high-resolution airborne LiDAR observation system was used to acquire information on small trees distributing in urban areas. The voxel-based method was applied to estimate the density distribution of tree crowns. Three types of numerical values that indicated the density of a voxel were calculated using airborne LiDAR data. Then, the values were compared with the number of points in for each voxel calculated from terrestrial LiDAR data. The results show that the airborne-derived estimation matched the distribution and density of leaves and branches obtained by the terrestrial LiDAR.