Estimation of coniferous shoot structures by high precision blue light 3D photogrammetry scanning

Clumping describes the heterogeneity of forest structure - the spatial arrangement of foliage elements, such as leaves or needles, within a vegetation canopy. Clumping information is essential for assessing radiation transfer through canopies, photosynthesis, and hydrological processes. The challenge in conifer stands arises from the difficulty of measuring gaps between needles within a shoot using traditional optical instruments, or LiDAR. Previous methods for estimating needle-to-shoot-area ratio were often destructive and labor-intensive. In this study, we introduce a highly efficient technique—blue light 3D photogrammetry scanning—to comprehensively characterize the structure of conifer shoots and determine shoot-level clumping. This approach significantly reduces the labor intensity associated with previous methods. To validate our technique, we compared it to the established photographic/volume displacement method for quantifying shoot-level clumping. Here, we present 3D shoot models, shoot-level clumping values, and their seasonal variations for a wide range of European native conifer species.

The demonstrated effectiveness and performance of the blue light 3D photogrammetry scanning method offer the potential for more frequent and accurate measurements of 3D shoot structures. This advancement opens doors to further improvements in measuring and upscaling optical properties for coniferous canopies. In future research, the enhanced understanding of needle shoots, a fundamental yet often overlooked aspect of foliage clumping in canopies, will significantly improve 3D radiative transfer modeling for coniferous forests.