Measuring and Monitoring Biomass and Biomass Change Using Three-dimensional Forest Reconstructions Assembled from Ground-Based Full-Waveform Lidar Scans

Credible and repeatable methods of verifying sustained carbon sinks (increases in above-ground biomass) and quantifying the carbon released during disturbance and deforestation across different landscapes are increasingly required. This study presents a three-dimensional (3-D) forest reconstruction methodology done with the new and emerging science of ground-based full-waveform lidar scanning, which can provide the rapid and efficient measurements of above-ground biomass that are essential to accurately measure and monitor biomass and biomass change over large regions.

Full-waveform, ground-based scanning lidars have several unique advantages that can enhance the accuracy of retrieval of above-ground biomass and related structural parameters. Unlike first-return (architectural) lidars, full-waveform lidars, such as the Echidna® Validation Instrument (EVI), are able to capture more of the variability of dense and structurally complex vegetation. By merging point clouds constructed from overlapping EVI scans, vegetation stands are reconstructed in 3-D space, allowing virtual direct representation of biomass distribution. These reconstructions not only provide more accurate measures of biomass than usually accomplished with allometric relationships, they can also provide estimates of fallen trees and branches on the ground, thus providing better estimates of the volume of biomass undergoing direct decay.

A voxelization process based on laser footprint and gap probability of laser shots is applied to transform the irregular, unorganized cloud of data points in the 3-D forest reconstruction into volumetric datasets. The voxelized 3-D forest reconstructions have been shown to provide an improved evaluation of both vertical and horizontal distribution of biomass. Comparisons of voxelized 3-D reconstructions through time have been used to document forest degradation and internal change. These EVI reconstructions also allow for differentiation between foliage and woody signals, which is of interest to both land biogeoscientists who require bulk vegetation biomass measures and to atmospheric biogeoscientists, who require information on surface roughness, photosynthesis, and respiration processes. Moreover, the EVI can be deployed to sample regions where disturbance has already been detected by optical sensors, such as MODIS or Landsat, to provide better calibration of the type and nature of change. Ground-based full-waveform lidar can also be combined with existing airborne and space-borne lidar systems to increase the potential to accurately measure and monitor biomass over large regions.