Remotely sensed surface temperatures for the analysis of evapotranspiration, drought and heat stress in Central European deciduous forests

Recurrent drought between 2018 and 2021 resulted in decreased vitality and productivity of Central European forest ecosystems and increased mortality rates across many tree species. With progressing climatic change, drought and heat stress events are projected to further increase, highlighting the need for monitoring approaches that allow early detection and regular interpretation. Therein, changes in evapotranspiration (ET) may indicate plant water stress earlier than visible symptoms such as canopy browning or dieback. The ongoing temp-2-stress project across six broadleaved forest regions in Germany integrates space-borne satellite observations with high-resolution uncrewed aerial vehicle (UAV) measurements to assess ET, vegetation health and derived drought and heat stress indices from stand to regional scales. For larger-scale satellite analyses, the focus lies on ET (and, e.g., ESI, Evaporative Stress Index) products from the ECOSTRESS and MODIS missions and on multispectral indices (e.g., NDVI, Normalized Difference Vegetation Index) derived from a harmonized LANDSAT/Sentinel dataset; the data coverage spans several years encompassing the 2018 to 2021 period. For high-resolution imagery over long-term forest monitoring plots located within each of the studied forest regions, we employ a UAV-based multispectral and thermal camera system (Micasense Altum PT) combined with an on-board four-component net radiation system (Apogee), which provides quality in-situ data for modelling ET with energy-balance based approaches such as the DATTUTDUT model. For a validation of derived stress indices, independent ground measurements of meteorological key variables, soil moisture and tree growth are available at the long-term forest monitoring plots, of which one is additionally equipped with an eddy covariance tower for further methodological scrutiny. Here, we present preliminary results from a unique forest irrigation experiment that is covered in the temp-2-stress UAV missions in addition to the mentioned long-term monitoring sites. Three irrigated and three non-irrigated mixed broadleaf forest plots were surveyed repeatedly during the 2025 growing season, allowing to assess temporal dynamics in surface temperature, ET and vegetation vitality and their sensitivity to irrigation and meteorological conditions. We expect increasing differences among irrigated and non-irrigated plots with increasing summer temperatures (analyses in progress). These results will serve to evaluate the potential of the UAV-based approach to detect differences in water availability and evaporative response in forests at the (sub-) stand level.