Heat tolerance of temperate tree species from Central Europe

• Applicant: Professor Dr. Markus Hauck
• Subject Area: Ecology and Biodiversity of Plants and Ecosystems
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 566425873

Climate-change induced tree mortality or productivity declines have (if not mediated by herbivory, pathogens or fire) mostly been attributed to drought stress. Although the importance of drought is undisputed, evidence is accumulating that the role of heat stress as a separate factor has been underestimated. The photosystem II (PS II) is known to be particularly sensitive to heat and its functionality can be easily tracked with chlorophyll fluorescence analysis. Several recent studies have substantiated species-specific differences in heat tolerance based on reductions of the maximum quantum yield (Fv/Fm) of PS II. However, these seminal studies were based on short-term heat exposures for 15 or 30 min, which often resulted in unrealistically high critical temperatures compared to ambient conditions even under climate change. In recent work, we tested therefore the feasibility of heat treatments of up to 4 h, which comes closer to the duration of daily heat maxima in the noon and afternoon hours in 14 temperate tree species native or introduced to Central Europe. Critical reductions in Fv/Fm occurred after 4 h of heat at lower temperatures compared to short-term experiments in that study. While all species coped well with 35 °C, reductions in Fv/Fm started to occur in some species at leaf temperatures of 40 °C and were widespread at 45 °C. After establishing the measurement protocol and principle results of the heat tolerance of temperate tree species in our previous work, the objective of this project is to deepen our studies on the heat tolerance of temperate tree species as a distinct factor that has to be treated separately from drought tolerance. This is necessary to better understand the biological response of trees to climate change and also provides important information for forest management. The work proposed here includes the analysis of the seasonal and site-dependent variability of heat tolerance to understand the trees’ potential for heat acclimation during the growing season and at different sites differing in slope aspect and elevation. In addition to foliar heat tolerance, leaf temperatures in forest canopies are survey with thermal imaging in order to better relate heat tolerance mechanisms from the laboratory to field conditions.

• DFG Programme: Research Grants