Water stress and acclimation of Pinus sylvestris detected through UAV-acquired leaf thermal images in a long-term irrigation experiment

Background/Question/Methods

Extreme climatic events such as droughts and heat waves are predicted to intensify with climate change. Drought-induced hydraulic failure and increased vapor pressure deficit (VPD) 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 evaluate the potential of UAV thermal imaging to detect drought stress in Pinus sylvestris in a long-term irrigation experiment in southern Switzerland. We investigated the seasonal and diurnal relationships between ecophysiological traits, such as gas exchange, water potentials, and leaf temperature.

Results/Conclusions

We found no differences in photosynthesis or leaf temperature between control and irrigated trees during winter. In summer (higher air temperature and evapotranspiration demand), we found that control trees reached higher leaf temperatures and sooner in the day compared to irrigated trees. Higher leaf temperature resulted from a premature stomatal closure to avoid xylem cavitation. This was supported by the steep decrease of photosynthesis and water potentials observed in control trees early in the afternoon. The maximum daily photosynthesis and xylem conductivities were similar for both treatments, suggesting that trees acclimated to their respective environments. However, trees exposed to drier conditions (i.e. control trees) were more sensitive to increases in VPD. Overall, canopy temperature was a good proxy for assessing trees' vulnerability to drought. UAVs thermal imaging is an important tool for evaluating forest stress before critical mortality and detecting the onset of water stress over large forest areas.