Heat tolerance of temperate tree species from Central Europe

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 ( F v / F m ) 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. Therefore, we tested 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 F v / F m occurred after 4 h of heat at lower temperatures compared to short-term experiments. While all species coped well with 35 °C, reductions in F v / F m started to occur in some species at leaf temperatures of 40 °C and were widespread at 45 °C. Broadleaved trees were generally more tolerant to heat than conifers. Though heat tolerance was correlated with drought tolerance, deviations from the regression line showed that these factors have to be regarded as independent factors when assessing the climate change sensitivity of tree species. • Hot droughts have become more frequent and more intense due to climate change. • Climate change-induced tree mortality has mostly been attributed to drought. • The photosystem II is highly sensitive to heat independent of tree water relations. • We studied heat tolerance at temperatures from 35 – 50 °C for up to 4 h. • Heat and drought tolerance were correlated, but not in all tree species.