Li-Strahler Model Research Articles

GOST2: The Improvement of the Canopy Reflectance Model GOST in Separating the Sunlit and Shaded Leaves

Accurately simulating the area ratios of the sunlit and shaded foliage in multiple-view angles presents a challenge in developing a geometric-optical (GO) model. GOST model by Fan et al. [1] proposed a high computationally demanding ray tracing method on this issue. In order to relax the computational restriction, a new hybrid canopy reflectance model GOST2 based on GOST is developed with a “ray tracing + GO” method, which is used for simulating the area ratios of the sunlit and shaded foliage. GOST2 shows the explicitly physical mechanism and has the capability in modeling the area ratios of the sunlit and shaded foliage on slopes. The area ratios of the four scene components of the five GO models, such as GOST2, GOST, the Li-Strahler model, the four-scale model, and Unified, are quantitatively evaluated. The canopy reflectances by the five GO models and the three-dimensional virtual canopy model are validated by the observed reflectance. It indicates that GOST2 is both reliable and computationally undemanding canopy reflectance model.

Quantitative forest canopy structure assessment using an inverted geometric‐optical model and up‐scaling

The physical‐based geometric‐optical Li–Strahler model can be inverted to retrieve forest canopy structural variables. One of the main input variables of the inverted model is the fractional component of sunlit background (K g). K g is calculated by using pure reflectance spectra (endmembers) of the viewed surface components. In this paper, the feasibility of up‐scaling from high (Quickbird) to medium (Hyperion) spatial resolution data for extracting the required endmembers is demonstrated. Furthermore, the sensitivity of the endmembers used as input for inverting Li–Strahler model is evaluated. After validating the inverted model results, namely spatially explicit forest mean crown closure and crown diameter using field measurements, it can be concluded that the regional scaling‐based endmembers derived from the linear unmixing model are the best ones to be used in combination with the inverted Li–Strahler model for quantitatively monitoring disturbance in forest canopy structure.

Scaling-based forest structural change detection using an inverted geometric-optical model in the Three Gorges region of China

We use the Li-Strahler geometric-optical model combined with a scaling-based approach to detect forest structural changes in the Three Gorges region of China. The physical-based Li-Strahler model can be inverted to retrieve forest structural properties. One of the main input variables for the inverted model is the fractional component of sunlit background, which is calculated by using pure reflectance spectra (endmembers) of surface components. In this study, we extract these endmembers from moderate spatial resolution MODIS data using two scaling-based methods (namely, a regional based linear unmixing and a purest-pixel approach) relying on corresponding high spatial resolution Landsat TM images. Then, the forest structural property crown closure (CC) is estimated by inverting the Li-Strahler model based on the extracted endmembers. Changes in CC are mapped using MODIS mosaics dated 2002 and 2004 for the whole Three Gorges region. Validation of the estimated CC using 25 sample sites indicates that the regional scaling-based endmembers extracted using linear unmixing are more suitable to be used in combination with the inverted Li-Strahler model for monitoring the forest CC than the purest-pixel approach, and results in significantly better estimates in both years ( R 2 2002 = 0.614, RMSE 2002 = 6%, R 2 2004 = 0.631 and RMSE 2004 = 5.2%). A change detection map of the model derived CC in 2002 and 2004 shows a decrease in CC in the eastern counties of the Three Gorges region located close to the Three Gorges Dam. An increase in CC has been observed in other counties of the Three Gorges region, implying a preliminary positive feedback on certain policy measures taken safeguarding forest structure.

Inversion of the Li-Strahler canopy reflectance model for mapping forest structure

As part of a larger forest vegetation mapping process based on Landsat TM and digital terrain data, inversion of the Li-Strahler model provides estimates of tree size and cover for conifer stands. The vegetation maps are intended for use in natural resource management by the US Forest Service. Analysis of extensive field data in the form of "test" stands from four National Forests indicate the following about the Li-Strahler model: (1) the underlying assumptions of independence between tree size and crown shape are valid, (2) the means for tree geometry parameters vary between forest types, (3) estimates of forest cover are reliable, and (4) estimates of tree size are unreliable due to the breakdown in the relationship between image intra-stand variance and tree size. Improvements in estimates of tree size will require additional data beyond a single Landsat TM image, with multidirectional data a promising possibility.

Modeling bidirectional radiance measurements collected by the advanced Solid-State Array Spectroradiometer (ASAS) over oregon transect conifer forests

A geometric-optical model of the bidirectional reflectance of a forest canopy, developed by Li and Strahler, fits observed directional radiance measurements with good accuracy. This model treats the forest cover as a scene of discrete, three-dimensional objects (trees) that are illuminated and viewed from different positions in the hemisphere. The shapes of the objects, their count densities and patterns of placement, are the driving variables, and they condition the mixture of sunlit and shaded objects and background that are observed from a particular viewing direction, given a direction of illumination. This mixture, in turn, controls the brightness apparent to an observer or a radiometric instrument. The Advanced Solid-State Array Spectroradiometer (ASAS) was used to validate this model. This aircraft sensor presently acquires images in 29 spectral bands in the range (465–871 nm) and is pointable fore-and-aft, allowing directional measurements of radiance as a target is approached and imaged at view angles ranging ± 45° from nadir. Through atmospheric correction, ASAS radiances were reduced to bidirectional reflectance factors (BRFs). These were compared to corresponding BRF values computed from the Li-Strahler model using, wherever possible, ground measured component BRFs for calibration. The comparisons showed a good match between the modeled and measured reflectance factors for four of the five Oregon Transect Sites. Thus, the geometric-optical approach provides a realistic model for the bidirectional reflectance distribution function of such natural vegetation canopies. Further modifications are suggested to improve the predicted BRFs and yield still better results.