3-D mapping of a multi-layered Mediterranean forest using ALS data

Towards Automated Characterization of Canopy Layering in Mixed Temperate Forests Using Airborne Laser Scanning

“…For example, echoes from the vegetation in the lower canopy parts can be either from the forest floor or young growth or caused by low branches of larger trees. For the ALS-based canopy stratification we applied, it is difficult to distinguish between these different sources, even if a combination with methods at the individual tree level could help to address this issue, as, for example, understory and understory trees can be partly detected [32,78,79].…”

“…Furthermore, the length of the determined layers was estimated, whereby only changes in absolute point density (i.e., inflexion points in the height distribution function) were used as the criterion to define the borders of a layer, which are not necessarily coincident. Maltamo et al [36] also distribution functions of echo heights, in a manner comparable to Whitehurst et al [30], Ferraz et al [32], and Wang et al [35], to relate the modes of the function (i.e., unimodal or multimodal) to the amount of canopy layers. In contrast, however, a fundamental part of their algorithm was the prior definition of a threshold to separate the dominant tree layer and understory trees, which limits the determination of canopy layers to a maximum of two layers.…”

“…ALS-based assessment of canopy layering can be either derived from LiDAR waveform data (mostly the case for large or medium-footprint LiDAR systems) [29][30][31], or based on ALS point clouds directly [32,33] or indirectly, e.g., by generating a kind of synthetic waveform using classified echo height distributions or height distribution probability functions [34][35][36]. The assessment of canopy layering with ALS is mainly performed using area-based approaches (ABA).…”

“…The peak definition parameters, however, were determined by visual analysis [30]. Ferraz et al [32] applied a mean shift segmentation algorithm to determine the modes of the distribution functions of echo heights and afterwards defined thresholds to divide the canopy into overstory, understory and ground vegetation. In this context, the definition of these thresholds remains critical as one component of the algorithm is linked to a function of canopy layer depth and thus depends on the tree species and/or forest biome [32].…”

“…Ferraz et al [32] applied a mean shift segmentation algorithm to determine the modes of the distribution functions of echo heights and afterwards defined thresholds to divide the canopy into overstory, understory and ground vegetation. In this context, the definition of these thresholds remains critical as one component of the algorithm is linked to a function of canopy layer depth and thus depends on the tree species and/or forest biome [32]. Wang et al [35] also used the modes of echo height distribution functions to divide the canopy into single-layered and double-layered canopies.…”