Shade-tolerant temperate broad-leaved trees are more sensitive to thermal stress than light-demanding species during a moderate heatwave

Abstract

With climate warming, the frequency and severity of extreme climatic events such as heat waves increase the risk of temperature-induced leaf damage. Severe damage can significantly weaken forest trees and lead to accelerated forest mortality. Cross-species studies investigating the thermal sensitivity of temperate tree species are still rare. Here, we aim to elucidate the thermal sensitivity of twelve tree species, of the genera Acer, Carpinus, Fagus, Fraxinus, Ostrya, Quercus, and Sorbus growing in the Vienna Woods, Austria. Thermal sensitivity, defined here as a decline of the maximum quantum yield of photosystem II (Fv/Fm) with increasing temperature, was measured on sun-exposed branches under varying levels of heat stress and compared with the turgor loss point (πtlp) as a drought resistance trait. We further included Ellenberg values for shade-tolerance to classify species into either shade-tolerant or light-demanding species. We calculated six different leaf thermotolerance traits: the temperature at the onset (5%) of the Fv/Fm decline (T5), the temperature at which Fv/Fm was half the maximum value (T50), the temperature at which only 5% Fv/Fm remained (T95), the decline width between T5 and T50 (DWT50-T5), between T50 and T95 (DWT95-T50), and between T5 and T95 (DWT95-T5). T5 ranged from 38.0 ± 0.2°C to 49.1 ± 0.5°C across all species and was close to the maximum air temperature of 37.1°C recorded in 2021. T50 values of all species were at least 11.1°C to 21.2°C above the maximum air temperature. πtlp did not clearly explain any differences in thermal sensitivity. DWT50-T5 had the strongest explanatory power to indicate thermal sensitivity depending on a species’ shade-tolerance. We conclude that the inclusion of light-demanding broad-leaved tree species into planting schemes contributes to increasing stand stability under climate change, in particular, it augments the resistance of forest stands to heatwaves.