Use of thermal imaging to detect evaporative cooling in coniferous and broadleaved tree species of the Mediterranean maquis

Abstract Among forest types, the Mediterranean maquis is specifically exposed to fluctuations in water availability. Therefore, monitoring the water-use patterns of its major tree species is key in quantifying the local and regional water balance. However, the traditional measurement methods of tree water-use at high spatial scales are difficult and labor-intensive, thus indirect methods become useful. Evaporation of water from the stomatal pores on the leaf surface involves evaporative cooling, and hence the differences between the leaf temperature and its surrounding air temperature (ΔT leaf-air ) can serve as a reliable estimator for tree water-use. Here, we used direct measurement of transpiration rate (E) with a gas exchange system, simultaneously with ground Thermal Infra-Red (TIR) imaging to study the relationship between ΔT leaf-air and E in dominant tree species of the Mediterranean maquis. Controlled experiments were conducted in parallel with measurements in the forest, on five tree species of contrasting leaf shapes (Conifers: Pinus halepensis ; Cupressus sempervirens ; Broadleaf: simple: Quercus calliprinos ; Ceratonia siliqua ; compound: Pistacia lentiscus ). Next, we used a quantitative approach, applying a leaf energy balance model to estimate E from the TIR images and compared it to the direct measurement of the gas exchange system. We report evaporative cooling across the five species, replicated in tree saplings and in mature trees in the forest. The conifers were significantly cooler than broadleaves by up to ∼4 °C and produced narrower ΔT leaf-air ranges. Estimations of E from ΔT leaf-air were relatively close to the observed E, with some overestimations. Our observations show that TIR imaging can detect transpiration-related differences in ΔT leaf-air among species and can be used to estimate E in natural environments. Yet, the dependence of ΔT leaf-air on E is species-specific and thus, empirical associations must be developed separately for each of the species.