Assessing the Potential of UAV Thermal Imagery for Upscaling Tree-Level Physiological Measurements

Forests worldwide are increasingly threatened by biotic and abiotic stressors, yet many operational monitoring approaches rely on visual surveys or spectral and structural proxies that often reflect anomalies only after physiological dysfunction has become pronounced. Wireless sensor networks such as TreeTalker^[1] enable continuous observation of tree physiological and hydrological states that may reveal stress responses before visible symptoms emerge. However, the individual tree-level nature of these observations necessitates upscaling approaches capable of extending physiological insights to unsensed trees across forest stands. In this study, we present thermal infrared remote sensing as a promising method for this purpose, as canopy temperature is theoretically linked to transpiration and stomatal regulation^[2].

A physiological and hydrological baseline was first established using TreeTalker observations collected between July 2021 and August 2022 across two temperate forest plantations (Long and Belt plantations, UK). Soil moisture dynamics were characterised using changepoint-based segmentation to describe event-scale drydown behaviour, rainfall response timing, and drought occurrence. At the tree level, soil-stem coupling was quantified using night-time stem water content, while sap flow regulation and recovery were examined using cross-correlation and Granger causality frameworks that incorporated atmospheric demand via vapour pressure deficit. Together, these analyses revealed strong inter-tree and seasonal variability in hydraulic coupling and regulation strategies, providing a baseline characterisation of site and tree condition.

Building on the TreeTalker baseline, thermal surveys were conducted in May 2025 using a laboratory-calibrated DJI Matrice 210-mounted Zenmuse XT sensor over the same plantations. Tree-level mean canopy temperatures were then extracted from processed thermal orthomosaics and compared with contemporaneous TreeTalker measurements of sap flow and stem water content acquired at or near the UAV overpass. Results show that an inverse relationship between canopy temperature and sap flow is more consistently expressed among Belt Plantation trees, although notable exceptions indicate heterogeneous regulation strategies. In Long Plantation, this pattern is not dominant across the population and is largely driven by a single high-sap flow tree exhibiting a markedly cooler canopy, while most other trees show substantial scatter. No clear relationships between canopy temperature and stem water content are observed at both sites. Additional information from visual surveys of tree condition indicates that some trees exhibiting pronounced structural symptoms deviate from the general thermal-sap flow tendency, suggesting that canopy structure may contribute to tree-specific decoupling without explaining site-wide patterns.

These results indicate that UAV-derived canopy temperature primarily reflects instantaneous hydraulic flux and regulatory behaviour rather than buffered internal water storage. By anchoring thermal observations within a multi-season physiological baseline, this work demonstrates how thermal imagery can be used to upscale continuous tree-level sap flow measurements.

References

[1] Valentini, R., Belelli, M.L., Gianelle, D. et al. New tree monitoring systems: from Industry 4.0 to Nature 4.0. Ann. Silvic. Research 43(2), 84–88 (2019). https://doi.org/10.12899/asr-1847

[2] Smigaj, M., Agarwal, A., Bartholomeus, H. et al. Thermal Infrared Remote Sensing of Stress Responses in Forest Environments: a Review of Developments, Challenges, and Opportunities. Curr. For. Rep. 10, 56–76 (2024). https://doi.org/10.1007/s40725-023-00207-z