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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2018年欧洲热浪：德国西部橡树和云杉的树状生态响应

2018年的欧洲热浪的特征是许多中欧和北欧国家的春季和夏季异常干燥和炎热。2018年6月至2018年8月的平均温度是自1881年以来第二高的温度。因此，许多植物，特别是树木，被推至生理极限。然而，尽管在实验室实验中已经对田间作物和幼树的干旱和热响应进行了充分的研究，但对于成熟林木的干旱和热响应知之甚少。在这项研究中，我们比较了德国西部和中西部的针叶树和落叶树种对2018年欧洲热浪期间的极端环境条件的响应。将经典的树状生态学技术（树木年轮分析）与年内茎干膨胀（树木硬度计）和树木水分吸收（树液流量传感器）的测量相结合，我们发现了云杉和橡树的对比响应。尽管云杉树木在2018年形成了一个狭窄的树年轮，并结合了日液流和树木硬度计参数与气候参数的相关性降低，橡树却形成了一个年轮环宽度高于平均水平的环，并与日气候参数和日照参数之间的相关性增强了关联。由树液流量和树状仪传感器得出的参数。总而言之，云杉树木对2018年热浪的反应是其生长活动的尽早完成，而橡树似乎根据其茎中的水分含量增强了其活动。