Assessing methods for estimating leaf area density and index

With the advent of high spatial resolution remote sensing for urban landscapes, field measurements of leaf area density (LAD) and leaf area index (LAI) for individual trees have increased in importance and relevance. LAD is defined as half the total leaf area per unit volume (m²m^-3) while LAI is defined as half the total leaf area per unit projection area on the ground (m²m^-2).

Extensive studies have been conducted to study various indirect field methods used to measure LAD and LAI with regards to crops and continuous forests. In contrast, the accuracy and limitations of these indirect methods is not completely understood when used for individual trees and needs more comprehensive analysis.

In this study, Maria Angela Dissegna, researcher with the Ecosystem Services in Urban Landscapes team at the Future Cities Laboratory (FCL) programme and co-authors compared the accuracy of three indirect LAD/LAI estimation methods, including single-return terrestrial laser scanning (TLS), LAI-2200, and digital hemispherical photography (DHP) on individual tropical urban trees.The field measurements were inter-compared and physically modeled by discrete anisotropic radiation transfer (DART).  

Upon comparing field measurements from the three indirect methods, significant inconsistencies were observed between DHP and the other two approaches. In addition, the researchers observed that tree trunks caused overestimation for both of LAD and LAI, while branches caused LAD underestimation and LAI overestimation.

For physical modelling, reference LAI was obtained from DART and evaluated against the LAD/LAI derived from simulated TLS, LAI-2200, and DHP acquisitions. The evaluation showed that TLS could reasonably estimate LAD/LAI but accuracy decreased with voxel size, and performed best at 0.3 m voxel size.

LAI-2200 significantly 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 of trees. The 90° view cap had higher accuracy than the 180° or 270° view caps using four or all five LAI-2200 rings.

The researchers propose that additional sensors or algorithms for crown shape measurement should be developed for LAI-2200 to reduce its dependence on other data sources. Also, the researchers do not recommend using DHP for the LAI estimations of individual trees as they were biased from the reference values.

The main advantage of indirect realistic modelling is the ease of extracting reference LAI/LAD values compared to tedious actual direct measurements, which are not always available. This study provided a benchmark for the use of a realistic scene for a radiative transfer model that evaluates the algorithm for individual tree LAD/LAI estimation. It also identified the parameters controlling the accuracy of indirect LAD/LAI estimation and serves as a guide in optimising each indirect field measurement method applied to individual trees.

The paper external page "An assessment study of three indirect methods for estimating leaf area density and leaf area index of individual trees" was published in Agricultural and Forest Meteorology.