Background and Aims Terrestrial laser scanners (TLSs) have successfully captured various properties of individual trees and have potential to further increase the quality and efficiency of forest surveys. However, TLSs are limited to line of sight observations, and forests are complex structural environments that can occlude TLS beams and thereby cause incomplete TLS samples. We evaluate the prevalence and sources of occlusion that limit line of sight to forest stems for TLS scans, assess the impacts of TLS sample incompleteness, and evaluate sampling strategies and data analysis techniques aimed at improving sample quality and representativeness. Methods We use a large number of TLS scans (761), taken across a 255 650-m2 area of forest with detailed field survey data: the Harvard Forest Global Earth Observatory (ForestGEO) (MA, USA). Sets of TLS returns are matched to stem positions in the field surveys to derive TLS-observed stem sets, which are compared with two additional stem sets derived solely from the field survey data: a set of stems within a fixed range from the TLS and a set of stems based on 2-D modelling of line of sight. Stem counts and densities are compared between the stem sets, and four alternative derivations of area to correct stem densities for the effects of occlusion are evaluated. Representation of diameter at breast height and species, drawn from the field survey data, are also compared between the stem sets. Key Results Occlusion from non-stem sources was the major influence on TLS line of sight. Transect and point TLS samples demonstrated better representativeness of some stem properties than did plots. Deriving sampled area from TLS scans improved estimates of stem density. Conclusions TLS sampling efforts should consider alternative sampling strategies and move towards in-progress assessment of sample quality and dynamic adaptation of sampling.

中文翻译：

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使用地面激光扫描对森林进行采样

背景和目标 陆地激光扫描仪 (TLS) 已成功捕捉到单棵树木的各种特性，并有可能进一步提高森林调查的质量和效率。然而，TLS 仅限于视线观察，而森林是复杂的结构环境，可能会遮挡 TLS 光束，从而导致 TLS 样本不完整。我们评估了 TLS 扫描限制视线到森林茎干的遮挡的普遍性和来源，评估 TLS 样本不完整性的影响，并评估旨在提高样本质量和代表性的采样策略和数据分析技术。方法 我们使用大量 TLS 扫描 (761)，在 255 650 平方米的森林区域内进行了详细的实地调查数据：哈佛森林全球地球观测站 (ForestGEO)（美国马萨诸塞州）。将 TLS 返回集与田间调查中的茎位置相匹配，以得出 TLS 观察到的茎组，并将其与仅从田间调查数据中获得的两个额外茎组进行比较：一组在 TLS 固定范围内的茎组和一个基于视线二维建模的茎集。比较茎组之间的茎数和密度，并评估四种可供选择的面积推导，以校正茎密度对闭塞的影响。还比较了茎组之间从田间调查数据中提取的胸高和物种的直径表示。主要结果 来自非茎源的遮挡是对 TLS 视线的主要影响。横断面和点 TLS 样本表现出比地块更好的某些茎特性的代表性。从 TLS 扫描中导出采样面积改进了对茎密度的估计。结论 TLS 抽样工作应考虑替代抽样策略，并转向正在进行的样本质量评估和抽样的动态适应。