Thinning- and tree-growth-caused changes in canopy cover and stand height and their estimation using low-density bitemporal airborne lidar measurements–a case study in hemi-boreal forests

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 (CCALS) caused by thinning was similar for the leaf-off and leaf-on phenophases, and for different height thresholds. The point cloud height percentile (HPx) 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 HPx increment or CCALS change using the leaf-on data (p-value >0.05). The minimum height filter for pulse return selection had a substantial influence on HPx increment in stands thinned between the two ALS measurements. Ground points are usually excluded from HPx calculation, but for stand-level analyses, their inclusion can provide additional information.