Data from: No role for xylem embolism or carbohydrate shortage in temperate trees during the severe 2015 drought

1. Temperate forests are predicted to experience an increased frequency and intensity of climate change-induced summer droughts and heat waves in the near future. Yet, while previous studies clearly showed a high drought sensitivity of different temperate tree species, the vulnerability of the physiological integrity of these trees remains unclear. 2. Here, we assessed the sensitivity of six temperate tree species to severe water limitation during three consecutive growing seasons including the exceptional 2015 central European summer drought and heat wave. Specifically, we assessed stem increment growth, sap flow, water potentials, hydraulic traits, and non-structural carbohydrate contents in leaves and branches to determine how mature temperate trees responded to this exceptional weather event and how the observed responses relate to variation in xylem embolism and carbohydrate economy. 3. We found that the trees’ pre-dawn water potentials reached their minimum values during the 2015 summer drought and most species reduced their sap flow by up to 80%. Also, increment growth was strongly impaired with the onset of the drought in all species. Despite the strong responses in the trees’ growth and water relations, all species exhibited minimum midday leaf water potentials well away from values associated with severe embolism (P50). In addition, we detected no distinct decrease in non-structural carbohydrate contents in leaves, bark and stems throughout the drought event. 4. Synthesis: This study shows that mature individuals of six common central European forest tree species strongly reacted to a severe summer drought by reducing their water consumption and stopping growth. We found, however, no indications for xylem embolism or strong carbohydrate depletion in these trees. This suggests, that xylem embolism formation and carbohydrate reserve depletion are not routine in temperate trees during seasonal strong drought and reveals a low vulnerability of the physiological integrity of temperate trees during drought events as we describe here.

1. 温带森林在不久的将来预计将面临气候变化引发的夏季干旱与热浪在发生频率与强度上的双重提升。尽管既往研究已明确证实不同温带树种对干旱具有较高敏感性，但此类树木生理完整性的脆弱性仍有待阐明。
2. 本研究针对包括2015年中欧罕见夏季干旱与热浪在内的三个连续生长季，评估了6种温带树种对极端水分限制的敏感性。具体而言，我们测定了成熟温带树木的茎干生长增量、液流、水势、水力性状以及叶片与枝条中的非结构性碳水化合物（non-structural carbohydrate）含量，以明确成熟温带树木对该极端气象事件的响应模式，以及观测到的响应与木质部栓塞（xylem embolism）和碳水化合物收支变化的关联。
3. 研究结果显示，树木的黎明前水势在2015年夏季干旱期间达到最低值，多数树种的液流速率最高下降达80%。同时，所有树种的茎干生长增量均在干旱发生初期受到显著抑制。尽管树木的生长与水分关系均出现强烈响应，但所有树种的正午叶水势最小值仍远低于引发严重木质部栓塞的阈值（P50）。此外，在整个干旱事件期间，我们未检测到叶片、树皮与茎干中的非结构性碳水化合物含量出现显著下降。
4. 综合分析：本研究表明，6种中欧常见森林树种的成熟个体通过降低水分消耗、停止生长来响应极端夏季干旱。然而，我们未在这些树木中发现木质部栓塞或严重碳水化合物耗竭的迹象。这提示，在季节性强干旱事件中，木质部栓塞形成与碳水化合物储备耗竭并非温带树木的普遍响应模式，同时也证实了如本研究所述，温带树木在干旱事件中的生理完整性具有较低的脆弱性。