Spatial variation of tropical forest canopy structure in relation to remotely sensed laser waveforms

摘要

The Vegetation Canopy Lidar (VCL) satellite, scheduled to launch in fall 2000, will be the only space-based sensor capable of measuring tree heights in closed canopy forests as well as providing profiles of vertical canopy structure. The Lidar Vegetation Imaging Sensor (LVIS), an airborne simulator of VCL, uses laser altimetry to record vegetation height and canopy structure by timing round trip transference of laser pulses. Unlike previous laser altimeters, LVIS is geared towards mapping canopy characteristics at the stand or watershed scale by producing a 1 km swath of overlapping 25 m diameter laser footprints. In March 1998, LVIS mapped canopy structural patterns for tropical wet and dry forests in the Panama Canal Zone. In March 1999, canopy-based measurements were made using Smithsonian Tropical Research Institute/United Nations Environmental Program construction cranes located at tropical wet and dry forest sites to compare with the remotely sensed data. Digital hemispherical photograph measurements were taken at intervals as short at 2 m as the crane gondola descended into the canopy. Significant differences existed in the three-dimensional structure at the wet and dry forest sites. At both forests, the degree of structural heterogeneity changed as the scale increased from stand to forest level. Although there was no significant correlation between individual waveforms and crane-based measurements at the footprint scale, there were similarities in trends in aggregated canopy-based and remotely-based measures at the landscape scale.