Quantifying Drought Stress in a Temperate Beech Forest Using the CropWater Stress Index derived from Thermal-Infrared Remote Sensing

To gain a better understanding of tree vulnerability to drought stress, we need to observe when and where stress occurs. Established techniques tend to be limited by technical shortcomings in monitoring environmental and plant conditions at appropriate temporal and spatial scales. New techniques to overcome limitations are becoming available, but they must be benchmarked and tested in a range of conditions.

Thermal infrared (TIR) remote sensing allows drought stress detection because down-regulated transpiration due to water shortage also reduces evaporative cooling of the foliage. The TIR-based crop water stress index (CWSI), which compares canopy temperature to expected temperatures in a well-watered and un-watered canopy, has been used to quantify drought stress in crops for many decades, but its utility in forests remains uncertain due to complex canopy thermal structure and narrow temperature ranges in humid environments. We used a combination of ground-based and drone-based data to detect drought stress in a young beech stand in Luxembourg, comparing continuous TIR data for individual trees using tower-based IR thermometers with dendrometer and sap flux measurements on the same trees. Our comparison reveals strong correspondence between dendrometer-derived tree water deficit (TWD) and the TIR-based CWSI computed for the same tree, confirming the utility of the CWSI as a stress detection tool.

We also put into perspective the CWSI computed based on continuous measurements with values obtained from two drone flights, in order to answer the following questions:

• At what spatial resolution (leaf, crown, stand) can meaningful CWSI values be derived?

• How to derive a suitable (unstressed) base line, either based on continuous data or the ensemble of data points in a set of images?

High resolution drone data captured substantial within-canopy variation and noise, but also non-physical results, compared to expectations derived from other data, established theoretical basis for crops, and a new theoretical basis for forests. Our analysis takes us another step towards the ability to quantify tree drought stress when and where it first occurs.