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

Clumping describes the spatial distribution of foliage elements (leaves or needles) within a vegetation canopy. Clumping information is important for determining the radiation transfer through canopies, photosynthesis, and hydrological processes. Clumping of needles in shoots in conifer stands has posed a challenge because optical instruments have generally been incapable of measuring gaps between needles within a shoot. Previous methods for estimating the needle-to-shoot-area ratio have had in common destructive and/or highly labor-intensive aspects. We introduce blue light 3D photogrammetry scanning as a highly efficient technique for estimating shoot-level clumping, which significantly reduces the labor intensity aspect of the previous approaches. We validate the approach by comparing it to the combined photographic/volume displacement method – an established methodology for quantifying shoot-level clumping. We used shoots of two species - Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst.) - collected from trees in the Järvselja RAdiation transfer Model Intercomparison (RAMI) pine stand in Estonia. The needle-to-shoot area ratio values were similar to those measured using the traditional combined photographic/volume displacement method. The demonstrated effectiveness and performance of the blue light 3D photogrammetry scanning method shall lead to more frequent actual measurements of 3D shoot structures. Growth in knowledge about this most elementary yet often overlooked level of foliage clumping in canopies shall improve coniferous forest 3D radiative transfer modeling.