Crown level clumping in Norway spruce from terrestrial laser scanning measurements

The clumping of coniferous needles into shoots is widely acknowledged as a structural feature that cannot be ignored in radiation regime models of coniferous forests. However, higher level clumping, i.e. the aggregation of leaves and shoots in tree crowns and forest stands, is still rarely accounted for in the models. Clumping reduces the light interception of and increases the light penetration depth in a plant stand. To improve forest radiation regime models with respect to this forest structural parameter, we propose a method that can quantify clumping at different hierarchical levels by estimating the silhouette to total area ratio from point clouds acquired by laser scanners. Our method is based on estimating attenuation coefficients in a voxel grid, and subsequently computing the total leaf area and spherically averaged silhouette area of a tree crown or forest stand. We tested our method with empirical data in young Norway spruce trees, where we compared leaf area and silhouette area to destructive and photogrammetric reference measurements. The accuracy of leaf area estimates depended strongly on the voxel size, with voxel sizes below 10 cm side length exhibiting up to 100% higher estimates than the reference leaf area, and large voxels with 90 cm side length being closest to the reference measurements due to crown clumping. The silhouette area estimates varied less with voxel size and were slightly higher than the reference estimates. We analyzed possible error sources and point out ways to improve the measurements of leaf and silhouette area for conifer trees using laser scanning data.