Laser altimeter canopy height profiles - Methods and validation for closed-canopy, broadleaf forests

Waveform-recording laser altimeter observations of vegetated landscapes pro vide a time-resolved measure of laser pulse backscatter energy from canopy surfaces and the underlying ground. Airborne laser altimeter waveform data was acquired using the Scanning Lidar Imager of Canopies by Echo Recovery ( SLICER) for a successional sequence of four, closed-canopy, deciduous fores t stands in eastern Maryland. The four stands were selected so as to includ e a range of canopy structures of importance to forest ecosystem function, including variation in the height and roughness of the outermost canopy sur face and the vertical organization of canopy stories and gaps. The characte r of the SLICER backscatter signal is described and a method is developed t hat accounts for occlusion of the laser energy by canopy surfaces, transfor ming the backscatter signal to a canopy height profile (CHP) that quantitat ively represents the relative vertical distribution of canopy surface area. The transformation applies increased weighting to the backscatter amplitud e as a function of closure through the canopy and assumes a horizontally ra ndom distribution of the canopy components. SLICER CHPs, averaged over area s of overlap where altimeter ground tracks intersect, are shown to be highl y reproducible. CHP transects across the four stands reveal spatial variati ons in vegetation, at the scale of the individual 10-m-diameter laser footp rints, within and between stands. Averaged SLICER CHPs are compared to anal ogous height profile results derived from ground-based sightings to plant i ntercepts measured on plots within the four stands. The plots were located on the segments of the altimeter ground tracks from which averaged SLICER C HPs were derived, and the ground observations were acquired within 2 weeks of the SLICER data acquisition to minimize temporal change. The differences in canopy structure between the four stands is similarly described by the SLICER and ground-based CHP results. However, a chi-square test of similari ty documents differences that are statistically significant. The difference s are discussed in terms of measurement properties that define the smoothne ss of the resulting CHPs and canopy properties that may vertically bias the CHP representations of canopy structure. The statistical differences are m ost likely due to the more noisy character of the ground-based CHPs, especi ally high in the canopy where ground-based sightings are rare resulting in an underestimate of canopy surface area and height, and to departures from assumptions of canopy uniformity, particularly regarding lack of clumping a nd vertically constant canopy reflectance, which bias the CHPs. The results demonstrate that the SLICER observations reliably provide a measure of can opy structure that reveals ecologically interesting structural variations s uch as those characterizing a successional sequence of closed-canopy, broad leaf forest stands. (C) 2001 Elsevier Science Inc. All rights reserved.