ABSTRACT

Repeated airborne laser scanning (ALS) measurements during leaf-on and leaf-off phenophases were studied. A 15 km × 15 km test site located in northern Estonia was used that included a reference set of stands, and 870 stands with thinning carried out before, between, and after two ALS flights. The decrease in ALS-based canopy cover estimate (CC_ALS) caused by thinning was similar for the leaf-off and leaf-on phenophases, and for different height thresholds. The point cloud height percentile (H_Px) values increased in almost all thinned stands, and the increase was present for the leaf-off and leaf-on phenophases. ALS point cloud metrics (skewness, kurtosis, mode, and canopy relief ratio) showed no response to thinning (p-value >0.05). Stand-dominating species had no significant influence on H_Px increment or CC_ALS change using the leaf-on data (p-value >0.05). The minimum height filter for pulse return selection had a substantial influence on H_Px increment in stands thinned between the two ALS measurements. Ground points are usually excluded from H_Px calculation, but for stand-level analyses, their inclusion can provide additional information.

摘要

研究了在叶上和叶下表相期间重复进行的机载激光扫描（ALS）测量。使用了一个位于爱沙尼亚北部的15 km×15 km的测试站点，其中包括一组参考机架，并且在两次ALS飞行之前，之间和之后进行了870机架稀疏。由稀疏引起的基于ALS的树冠覆盖估计值（CC _ALS）的减少对于叶期和叶期表位期以及不同的高度阈值而言都是相似的。在几乎所有稀疏林分中，点云高度百分位数（H _Px）值均增加，并且叶期和叶期表位期均出现增加。ALS点云指标（偏度，峰度，众数和冠层缓解率）对稀疏无响应（p-值> 0.05）。使用叶片数据，以林分为主的物种对H _Px增量或CC _ALS变化无显着影响（p值> 0.05）。在两次ALS测量之间变细的机架中，用于脉冲返回选择的最小高度滤波器对H _Px增量有很大影响。通常不将地面点从H _Px计算中排除，但是对于站级分析，将其包括在内可以提供更多信息。