The temp-2-stress project

temp-2-stress is led by Dr. Alexander Röll (University of Bonn) and carried out together with Simon Swatek (University of Bonn), in cooperation with Prof. Dr. Dirk Hölscher (University of Göttingen) as senior scientific advisor and project partners at the University of Twente and the Northwest German Forest Research Institute.

The project duration is from November 15, 2024, to November 14, 2027. Drone flights, a central element of temp-2-stress, will primarily take place in the summer months of 2025 and 2026.

The investigations in temp-2-stress cover mixed deciduous forests on Level-II monitoring plots along a north-south gradient in Lower Saxony and Hesse (Germany), as well as complementary plots from a forest irrigation experiment in the Hessisches Ried region (SiZuRi project).

Central European forests are increasingly suffering from drought and heat stress as a result of ongoing climate change. This leads to a decline in vitality and increased tree mortality, which in turn results in the reduction or loss of important ecological, economic, and social forest ecosystem functions. Tree vitality and mortality, ecosystem indicators, and ecophysiological processes have been intensively monitored and studied on small-scale monitoring plots (e.g., Level-II plots) for decades. Modern remote sensing methods for measuring surface temperatures using drones and satellites offer additional opportunities to integrate these studies and further develop forest monitoring. In temp-2-stress, relatively large-scale yet high-resolution stress index maps will be created using drone and satellite imagery and will be made available through a user-friendly open-source workflow.

In temp-2-stress, small-scale ground reference measurements are combined with thermography and multispectral imaging from drone and satellite systems. This allows the study of drought and heat stress at tree, stand, and landscape scales.

General site information as well as soil moisture, matrix potential, and tree growth are collected on the Level-II monitoring plots and SiZuRi research plots. On nearby open areas, additional climatic variables (air temperature, humidity, global radiation, precipitation, wind speed) are measured, which can be used to model potential evapotranspiration.

A DJI Matrice 350 RTK drone system is equipped with a Micasense Altum PT camera system (simultaneous panchromatic, multispectral, and thermal imagery). Additionally, an Apogee 4-component net radiation system is mounted on the drone. The thermal images and radiation measurements form the basis for modeling evapotranspiration and drought stress, while the multispectral images are used to assess tree vitality.

In parallel with the drone’s thermal imagery, the satellite product ECOSTRESS is used to investigate evapotranspiration and drought stress on larger spatial scales, with a spatial resolution of approximately 70 meters. A satellite equivalent of the drone’s multispectral data (e.g., from ESA) is also freely available. In addition to expanding spatial coverage, incorporating satellite data also extends the temporal coverage, as some satellite data go back many years.

The drought and heat stress analyses will be implemented in a user-friendly, largely automated workflow by the end of the project, using open-source solutions wherever possible.

• November 15, 2024: Official project start
• January 17, 2025: Kick-off meeting with all project partners
• February 1, 2025: Project start for temp-2-stress PhD candidate Simon Swatek
• Currently ongoing: Integration of drone, camera, radiation, and stabilization systems
• Planned for March/April 2025: First test flights and system checks

View of the beech canopy from a tower near the Solling research site. 

• Ahongshangbam, J., Khokthong, W., Ellsäßer, F., Hendrayanto, Hölscher, D., Röll, A.: Drone-based photogrammetry-derived crown metrics for predicting tree and oil palm water use. Ecohydrology 12 (2019): e2115. 
• Ahongshangbam, J., Röll, A., Ellsäßer, F., Hendrayanto, Hölscher, D.: Airborne tree crown detection for predicting spatial heterogeneity of canopy transpiration in a tropical rainforest. Remote Sensing 12 (2020): 651. 
• Bulusu, M., Ellsäßer, F., Stiegler, C., Ahongshangbam, J., Marques, I., Hendrayanto, Röll, A., Hölscher, D.: UAV-based thermography reveals spatial and temporal variability of evapotranspiration from a tropical rainforest. Frontiers in Forests and Global Change 6 (2023): 1232410.
• Cortes-Molino, A., Valdes-Uribe, A., Ellsäßer, F., Bulusu, M., Ahongshangbam, J., Hendrayanto, Hölscher, D., Röll, A.: Combining UAV thermography, point cloud analysis and machine learning for assessing small-scale evapotranspiration patterns in a tropical rainforest. Ecohydrology 17 (2023): e2604. 
• Dash JP, Pearse GD, Watt MS. UAV multispectral imagery can complement satellite data for monitoring forest health. Remote Sensing. 2018 Aug 3;10(8):1216.
• Ellsäßer, F., Röll, A., Ahongshangbam, J., Waite, P.A., Schuldt, B., Hölscher, D.: Predicting tree sap flux and stomatal conductance from drone-recorded surface temperatures in a mixed agroforestry system - A machine learning approach. Remote Sensing 12(2020):4070.
• Ellsäßer, F., Röll, A., Stiegler, C., Hendrayanto, Hölscher, D.: Introducing QWaterModel, a QGIS plugin for predicting evapotranspiration from land surface temperatures. Environmental Modelling and Software 130 (2020): 104739.
• Ellsäßer, F., Stiegler, C., Röll, A., June, T., Hendrayanto, Knohl, A., Hölscher, D.: Predicting evapotranspiration from drone-based thermography – a method comparison in a tropical oil palm plantation. Biogeosciences 18 (2021): 861-872. 
• Fisher JB, Lee B, Purdy AJ, Halverson GH, Dohlen MB, Cawse‐Nicholson K, Wang A, Anderson RG, Aragon B, Arain MA, Baldocchi DD. ECOSTRESS: NASA's next generation mission to measure evapotranspiration from the international space station. Water Resources Research. 2020 Apr;56(4):e2019WR026058.
• Röll, A., Kang, T., Hahn, P., Ahongshangbam, J., Ellsäßer, F., Hendrayanto, Sharma, P., Wintz, T., Hölscher, D.: Complex canopy structures control tree transpiration: A study based on 3D modelling in a tropical rainforest. Hydrological Processes 37 (2023): e15045. 
• Valdés-Uribe, A., Hölscher, D., Röll, A.: ECOSTRESS Reveals the Importance of Topography and Forest Structure for Evapotranspiration from a Tropical Forest Region of the Andes. Remote Sensing 15 (2023): 2985. 

If you are interested in temp-2-stress, e.g., for theses, project cooperation, potential follow-up projects, or simply for fun, please feel free to contact: