An assessment study of three indirect methods for estimating leaf area density and leaf area index of individual trees

Abstract

Field measurements of leaf area density (LAD) and leaf area index (LAI) for individual trees have increased in importance and relevance with the advent of high spatial resolution remote sensing for the urban landscape. However, indirect field measurements of LAD/LAI for widely dispersed individual trees have not been comprehensively evaluated. The present study compares the accuracy of three indirect LAD/LAI estimation methods, including single-return terrestrial laser scanning (TLS), LAI-2200, and digital hemispherical photography (DHP) on urban trees. To this end, field measurements were inter-compared and physically modeled by discrete anisotropic radiation transfer (DART). The inter-comparisons of field data revealed substantial inconsistencies between DHP and the other two approaches. For the physical modeling, reference LAI was obtained from realistic 3-D tree objects in DART, and the LAD/LAI was derived from simulated TLS, LAI-2200, and DHP acquisitions and was evaluated against the references. The physical modeling results showed that TLS could reasonably estimate LAD/LAI for LAD < 3 and LAI < 6 using a small (0.3 m) voxel size. However, the measurements became saturated for dense foliage (LAD > 3 and LAI > 6). The LAD/LAI accuracy was sharply reduced as voxel size increased. In addition, the trunks caused overestimation for both of LAD and LAI, while branches caused LAD underestimation and LAI overestimation. Further research is needed to compensate for the effects of occlusions and clumping in the estimates of LAD/LAI for dense-foliated trees using TLS. LAI-2200 grossly underestimated LAD for all cases, while it accurately estimated LAI for LAI < 5 and became gradually saturated for LAI > 5. The estimation accuracy of LAI-2200 declined markedly with increasing uncertainty in crown shape. The 90° view cap had higher accuracy than the 180° or 270° view caps using four or all five LAI-2200 rings. Additional sensors or algorithms for crown shape measurement should be developed for LAI-2200 to reduce its reliance on other data sources. DHP is not recommended for individual trees as the LAI estimations were biased from the reference values, and improvements in reliability will depend on new algorithms to account for differences in path length. These results serve as a benchmark for evaluating the accuracy of in situ LAD/LAI measurement techniques and for optimizing the configurations required for each indirect measurement method applied to individual trees.