Laser altimeter canopy height profiles - Methods and validation for closed-canopy, broadleaf forests
Laser altimeter canopy height profiles - Methods and validation for closed-canopy, broadleaf forests
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DOI：
10.1016/s0034-4257(00)00210-8
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发表时间：
2001-06-01
影响因子：
13.5
通讯作者：
Blair, JB
中科院分区：
文献类型：
--
作者：
Harding, DJ;Lefsky, MA;Blair, JB
对植被景观的波形记录激光高度计观测提供了来自树冠表面和下面地面的激光脉冲背向散射能量的时间分辨测量。机载激光高度计的波形数据是由Echo Recovery(切片器)使用激光雷达冠层扫描成像仪获取的，记录了马里兰州东部四个闭合的落叶林木林分的演替序列。选择四个林分是为了包括一系列对森林生态系统功能重要的树冠结构，包括最外层树冠表面高度和粗糙度的变化以及树冠层和林隙的垂直组织。描述了切片器后向散射信号的特点，提出了一种考虑激光能量被冠层遮挡的方法，将后向散射信号转换为定量表示冠层表面积相对垂直分布的冠层高度剖面(CHP)。该变换将增加的权重应用于作为通过树冠闭合的函数的后向散射幅度，并假定树冠组件的水平随机分布。切片CHP，平均在高度计地面轨道相交的重叠区域，被证明是高度重复性的。四个林分的CHP横断面显示了林内和林间植被的空间差异，其规模相当于单个10米直径的激光足迹。将平均切片器CHP与从地面目击和在四个林分内的地块上测量的植物截获得到的类似高度剖面结果进行比较。这些地块位于高度计地面轨道上，从中得出平均切片器CHP，地面观测值在切片器数据获取后2周内获得，以最大限度地减少时间变化。切片机和基于地面的CHP结果类似地描述了四个林分之间的冠层结构的差异。然而，相似性的卡方检验证明了具有统计学意义的差异。在定义结果CHP的平滑度的测量特性和可能垂直偏向CHP表示的冠层结构的冠层特性方面讨论了差异。统计上的差异很可能是由于地面CHP的噪声较大，特别是在冠层较高，因为地面观测很少，导致低估了冠层表面积和高度，以及偏离了冠层一致性的假设，特别是关于缺乏聚集和垂直恒定的冠层反射率，这使CHP产生偏差。结果表明，切片观测可靠地提供了一种冠层结构的测量，揭示了生态上有趣的结构变化，例如表征闭合冠层阔叶林林分演替序列的结构变化。(C)2001 Elsevier Science Inc.保留所有权利。
Waveform-recording laser altimeter observations of vegetated landscapes provide 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 forest stands in eastern Maryland. The four stands were selected so as to include a range of canopy structures of importance to forest ecosystem function, including variation in the height and roughness of the outermost canopy surface and the vertical organization of canopy stories and gaps. The character of the SLICER backscatter signal is described and a method is developed that accounts for occlusion of the laser energy by canopy surfaces, transforming the backscatter signal to a canopy height profile (CHP) that quantitatively represents the relative vertical distribution of canopy surface area. The transformation applies increased weighting to the backscatter amplitude as a function of closure through the canopy and assumes a horizontally random distribution of the canopy components. SLICER CHPs, averaged over areas of overlap where altimeter ground tracks intersect, are shown to be highly reproducible. CHP transects across the four stands reveal spatial variations in vegetation, at the scale of the individual 10-m-diameter laser footprints, within and between stands. Averaged SLICER CHPs are compared to analogous height profile results derived from ground-based sightings to plant intercepts measured on plots within the four stands. The plots were located on the segments of the altimeter ground tracks from which averaged SLICER CHPs 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 similarity documents differences that are statistically significant. The differences are discussed in terms of measurement properties that define the smoothness of the resulting CHPs and canopy properties that may vertically bias the CHP representations of canopy structure. The statistical differences are most likely due to the more noisy character of the ground-based CHPs, especially 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 and vertically constant canopy reflectance, which bias the CHPs. The results demonstrate that the SLICER observations reliably provide a measure of canopy structure that reveals ecologically interesting structural variations such as those characterizing a successional sequence of closed-canopy, broadleaf forest stands. (C) 2001 Elsevier Science Inc. All rights reserved.