Modeling lidar waveforms in heterogeneous and discrete canopies

Abstract

This study explores the relationship between laser waveforms and canopy structure parameters and the effects of the spatial arrangement of canopy structure on this relationship through a geometric optical model. Studying laser waveforms for such plant canopies is needed for the advanced retrieval of three-dimensional (3-D) canopy structure parameters from the vegetation canopy lidar (VCL) mission.For discontinuous plant canopies, a hybrid geometric optical and radiative transfer (GORT) model describing the effects of 3-D canopy structure parameters of discrete canopies on the radiation environment has been modified for use with lidar. The GORT model is first used to describe the canopy lidar waveforms as a function of canopy structure parameters and then validated using scanning lidar imager of canopies by echo recovery (SLICER) data collected in conifer forests in central Canada during the boreal ecosystem-atmosphere study (BOREAS).Model simulations show that the clumping in natural vegetation, such as leaves clustering into tree crowns causes larger gap probability and smaller waveforms for discontinuous plant canopies than for horizontally homogeneous plant canopies. Ignoring the clumping effect can result in significantly lower values for the estimated foliage amount in the profile and in turn lower estimated biomass. Because of clumping, only the gap probability and apparent vertical projected foliage profile can be directly retrieved from the canopy lidar data. The retrieval is sensitive to the ratio of the volume backscattering coefficients of the vegetation and background, and this ratio depends on canopy architecture as well as foliage spectral characteristics. Extensions of the GORT model from single-layered canopies to include multilayered ones are also explored. Index Terms-Geometric optical and radiative transfer (GORT) model, heterogeneous plant canopies, lidar waveforms. NOMENCLATURE Roman AlphabetVertical crown radius (m). Foliage area volume density of a single crown (m ). Apparent foliage profile as a function height (m ). Actual foliage profile as a function height (m ).

Implications of differing input data sources and approaches upon forest carbon stock estimation

“…LIDAR data (either as a direct measure or as a data input for modeling) have been used to characterize forest attributes such as stand height (Naesset et al 2005), canopy structure (Ni-Meister et al 2001), crown closure (Lim et al 2003), biomass (Popescu 2007), and canopy volume (Lefsky et al 2005;Coops et al 2007;Heo et al 2008). Although LIDAR has largely been used for collecting highresolution geospatial data over vegetated areas, only in the last few years have natural resource scientists begun to use LIDAR for forest structure mapping and inventory purposes (Reutebuch et al 2003).…”

Estimating canopy structure of Douglas-fir forest stands from discrete-return LiDAR

“…However, owing to the inability of the vertical view to resolve foliage angle distribution, clumping and non-foliage elements, the profiles derived are not the same as the true foliage density profiles and the derived profiles are referred to here as "apparent" foliage profiles (AFP). The difference between the true and apparent profiles depends on the canopy structure and type as discussed by Ni-Meister et al (2001), since this effects the nature of its projection in the vertical direction.…”

A Monte Carlo radiative transfer model of satellite waveform LiDAR

“…Increasingly, physically-based radiative transfer models of vegetation canopies have been used to constrain retrieval of land surface biophysical parameters, either by direct inversion or use in algorithm development. For lidar, radiative transfer models have been developed originally for atmospheric simulation (Platt, 1981), and recently several models have been developed for vegetation canopies which treat the light interaction at various degrees of complexity (Govaerts and Verstraete, 1998;Ni-Meister et al, 2001;Kotchenova et al, 2003;Disney et al, 2006).…”