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This study employs low-cost UAV and Crane-based photogrammetry to create detailed 3D reconstructions of deciduous trees, enabling the monitoring of shoot elongation across entire canopies. By collecting data over a full growing season, the researchers achieved impressive 3D point accuracies of 5-6 mm and reconstruction completeness of 92-98% for different UAV types. The introduction of a 3D-printed ground-truth branch allows for the evaluation of fine-detail structures, addressing a significant gap in understanding the impact of climate change on primary tree growth.
Achieving 3D reconstructions of trees with millimeter accuracy using consumer-grade UAVs could revolutionize how we monitor the effects of climate change on tree growth.
Tree growth determines how much CO2 is sequestered from the atmosphere and temporarily stored in woody biomass. At the same time tree growth is affected by increasing temperatures, more frequent drought periods, late frosts and other extreme events associated with climate change. While continuous measurements of radial (secondary) tree growth using dendrometers are well established, monitoring of shoot elongation (primary growth) has largely been neglected because suitable measurement techniques are lacking. As a result, the effects of climate change on primary tree growth remain insufficiently understood. This work aims at reconstructing native deciduous trees in 3D as a basis for measuring and monitoring shoot elongation over entire tree canopies. Here we explored the use of low-cost UAV photogrammetry and of a multi-camera CraneCam system under real-world conditions. Data were collected in two study areas over an entire growing season. We present sensor evaluations, photogrammetric data acquisition and processing strategies. A special focus is placed on the analysis of the resulting photogrammetric 3D point clouds in terms of accuracy, resolution and completeness. Results demonstrate 3D point accuracies of 5-6 mm for entire trees using consumer-grade UAVs weighing less than 250 g and a 3D reconstruction completeness between 92% and 98% depending on the UAV type. The paper introduces a novel 3Dprinted ground-truth branch to evaluate the capability to reconstructing fine-detail structures such as thin tree shoots. Finally, we discuss operational challenges and initial experiments towards a skeletonization of entire trees based on photogrammetric point clouds.
