Leaf-scale physics and thermal infrared sensing provide a glimpse into drought and heat stress of beech trees

Thermal infrared remote sensing is widely used to estimate transpiration rates and drought stress in crops (e.g. the Crop Water Stress Index, CWSI). However, interpretation of surface temperature data in forests is more difficult due to more complex canopy structure and uncertainty in the canopy aerodynamic resistance. To better understand the utility of canopy temperature data for drought detection in a deciduous forest, we mounted three thermal infrared (TIR) sensors on a tower, pointing onto the crowns of three individual beech trees, which were at the same time equipped with sap flow sensors and dendrometers used to record reductions in sapflow and increase in tree water deficit during dry periods. The tower was also equipped with sensors for air temperature, relative humidity, horizontal wind speed and net radiation, and the site was equipped with a rain gauge and soil moisture sensors at different depths down to 1 m depth.

Observed crown temperatures were put into relation with simulated temperature variations of two single 3 cm wide leaves, one with 0 stomatal conductance and one with infinite stomatal conductance, representing the extreme cases of a non-transpiring dry leaf, and a wet leaf, respectively.

Simulated dry leaf temperatures exceeded critical temperatures of 50 oC on several summer days in 2023 and 2024, indicating that evaporative cooling is needed to avoid permanent heat damage. At the same time, measured canopy temperatures deviated upwards from the simulated wet leaf temperatures with declining soil moisture and increasing tree water deficit as deduced from high-resolution dendrometer data. This illustrates the crucial effect of combined heat and drought stress, when evaporative cooling is most needed, but hampered by inadequate water supply.

The striking consistency between observed crown temperatures and simulated single-leaf temperatures of dry and wet leaves suggests that meteorological conditions at the top of the canopy (net radiation, air temperature and humidity, wind speed) are decisive for the energy balance of the majority of the leaves seen by the TIR sensors and opens the path to spatially resolved assessments of tree drought and heat stress. Hereby, characteristic leaf sizes play an important role for the interpretation of canopy temperature data, and for the vulnerability of plants to thermal stress during heat and drought waves. This presentation highlights these roles quantitatively and points to common pitfalls and knowledge gaps when modelling and interpreting leaf and canopy temperature data.