Abstract: Extreme climatic events such as droughts and heat waves are predicted to intensify with climate change. Drought-induced hydraulic failure and increased VPD (Vapor Pressure Deficit) are some of the main drivers of forest mortality globally. Unmanned aerial vehicles (UAVs) can assess the onset of drought stress through canopy temperature over large forest areas. Under water-stressed conditions, trees close their stomata, which results in lower transpiration rate and heat dissipation. Thus, high leaf temperature is often the signal of drought stress in trees. Mapping leaf thermal signature in forest ecosystems can provide important insight into management strategies under global warming.
In this study, we determine how three European oak species (Q. petraea, Q. ilex and Q. coccifera) distributed across a large environmental gradient going from temperate to semi-arid conditions (1) diurnally regulate their canopy temperature and net gas exchange rates (photosynthesis and stomatal conductance) over the summer, and (2) acclimate their temperature tolerance (Tcrit for the photosynthetic apparatus and cell vitality) and thermal safety margins along the summer.
We found differences in net diurnal dynamic of photosynthesis, hydraulic traits, and leaf temperatures between the three oak species suggesting a strong acclimation to local climatic environment. However, we found that the thermal thresholds were similar across species, indicating that neither temperature nor soil water availability were driving thermal sensitivity. Overall, these results show the importance of leaf thermoregulation through evapotranspiration, especially for species growing under high temperatures.