The European heat wave of 2018 was characterized by extraordinarily dry and hot spring and summer conditions in many central and northern European countries. The average temperatures from June to August 2018 were the second highest since 1881. Accordingly, many plants, especially trees, were pushed to their physiological limits. However, while the drought and heat response of field crops and younger trees have been well investigated in laboratory experiments, little is known regarding the drought and heat response of mature forest trees. In this study, we compared the response of a coniferous and a deciduous tree species, located in western and central–western Germany, to the extreme environmental conditions during the European heat wave of 2018. Combining classic dendroecological techniques (tree–ring analysis) with measurements of the intra–annual stem expansion (dendrometers) and tree water uptake (sap flow sensors), we found contrasting responses of spruce and oak trees. While spruce trees developed a narrow tree ring in 2018 combined with decreasing correlations of daily sap flow and dendrometer parameters to the climatic parameters, oak trees developed a ring with above–average tree–ring width combined with increasing correlations between the daily climatic parameters and the parameters derived from sap flow and the dendrometer sensors. In conclusion, spruce trees reacted to the 2018 heat wave with the early completion of their growth activities, whereas oaks appeared to intensify their activities based on the water content in their tree stems.
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