Vertical light transmission profiles in structured mixed deciduous forest canopies assessed by UAV-based hemispherical photography and photogrammetric vegetation height models

Light availability below and within forest canopies governs many biological processes. Its estimation, however, is often time consuming, especially in complex canopy structures where vertical profiles of light transmission need to be known to estimate plant performance, habitat suitability or biophysical properties across the canopy. To overcome the technical limitations of assessing vertical canopy space by ground-based systems like ropes, towers or cranes, we developed a prototype to take hemispherical photographs (HP) from an unmanned aerial vehicle (UAV). With the new airborne system, we aimed to assess the differences of vertical light availability profiles from bottom to the top of the canopy in even-aged, uneven-aged and coppice-with-standards forests. Measured vertical light profiles typically followed a sigmoid shape that differed markedly depending on canopy gap size and forest structure. The results showed important variations in the transmission of sunlight at different heights within forest canopies, highlighting the importance of vertical canopy stratification when considering radiation transfer, plant area index (PAI), leaf area index (LAI) and light availability. To further cover cases where branches hinder UAV access to dense canopy spaces, we present a workflow to estimate light availability from photogrammetrically reconstructed high-resolution vegetation height models (VHM) and validate the outputs against HP. We conclude that VHMs based on high-resolution aerial photography, photogrammetry and a structure-from-motion algorithm (SfM) deliver a solid foundation to derive a wide range of canopy metrics. With an appropriate VHM point density, direct or beam light index (BLI), diffuse light index (DLI), gap or global light index (GLI) and PAI in mixed broadleaved forests can be reliably calculated. The novel development of UAV-based HP and the workflow to calculate light regimes from VHMs thus open a wide range of new applications in forest and agricultural research and management.