Experimental assessment of drought‑induced changes in Sun‑Induced Fluorescence (SIF), 3‑D canopy structure and UAV‑based thermal imaging of Pseudotsuga menziesii and Fagus sylvatica seedlings

Sun‑induced chlorophyll‑a fluorescence (SIF) is used as a proxy for photosynthetic activity measurements from space‑ and airborne platforms. However, its signal is affected not only by the underlying leaf biochemistry but also by geometric factors such as canopy architecture and leaf orientation, which can lead to misinterpretations of SIF signals. To evaluate how these interacting influences affect SIF signals and subsequent drought‑stress detection, we combined SIF measurements with high‑resolution structural monitoring and UAV‑based thermal imaging during a four‑week drought experiment on seedlings of two ecophysiologically contrasting tree species, Pseudotsuga menziesii (Douglas‑fir) and Fagus sylvatica (European beech). Soil moisture was recorded continuously with SMT100 sensors, while top‑of‑canopy SIF spectra were captured under clear sky conditions on four days using a spectroradiometric setup (FLOX). Leaf‑level chlorophyll fluorescence (effective quantum yield)  was assessed with a Junior‑PAM fluorometer. Photogrammetric reconstruction of RGB images allowed for the analysis of 3‑D point clouds that permitted a quantitative comparison of structural parameters from the onset to the end of the treatment. After the drought period, a multi‑sensor UAV flight acquired LiDAR point clouds, multispectral reflectance, and thermal imagery to provide spatial context for the physiological observations. Structural changes were modest, whereas apparent SIF yields declined markedly in the drought‑stressed seedlings relative to well‑watered controls. Thermal maps showed slightly increased canopy temperature in stressed plants, corresponding closely with observed SIF reductions, particularly for F. sylvatica. By a combined analysis of temporal SIF dynamics and thermal signatures, we were able to jointly interpret observed signs of stress. From this plant level analysis, an outlook is given towards continuous observations, which were conducted over the temperate mixed ECOSENSE forest in SW Germany in 2025. In summary, the synergistic, multi-sensor approach presented here enhances the reliability of fluorescence-based remote sensing of plant stress and provides a scalable framework for monitoring drought impacts across heterogeneous forest ecosystems.