Determining change in vegetation structure using small-footprint, waveform-resolving lidar

The Experimental Advanced Airborne Research Lidar (EAARL) is a raster-scanning, waveform-resolving, green-wavelength (532 nm) lidar designed to map near-shore bathymetry, topography, and vegetation structure simultaneously. For each laser pulse, the EAARL sensor records the time history of the return waveform within a small footprint (20-cm diameter at nominal flying altitude of 300 m), enabling characterization of vegetation canopy structure and “bare Earth” topography beneath vegetation. A collection of individual waveforms combined within a synthesized large footprint was used to define three metrics to quantify vegetation structure: canopy height (CH), canopy reflection ratio (CRR), and height of median energy (HOME). The appropriate size of the synthesized footprint was based on an approximate stand size of 5-m diameter. The methodology was applied to the Naval Live Oaks Area Preserve (NLO) in Gulf Islands National Seashore, where an EAARL survey was conducted in October 2005 and May 2007. Lidar-based vegetation metrics at NLO from the 2005 and 2007 surveys were used to define patches of vegetation communities that had undergone significant change. The presence/absence of vegetation change was evaluated using high-resolution digital aerial imagery acquired during the survey. The Gulf Coast Network National Park Service Inventory & Monitoring program is using this change analysis as part of its’ vegetation monitoring protocol.