European oak species have long been considered relatively resistant to different disturbances, including drought. However, several recent studies have reported their decline initiated by complex changes. Therefore, we compared mature sessile oak trees (Quercus petraea (Matt.), Liebl.) infested versus non-infested by hemiparasitic yellow mistletoe (Loranthus europaeus Jacq.) during the relatively dry vegetation season of 2019. We used broad arrays of ecophysiological (maximal assimilation rate Asat, chlorophyll a fluorescence, stomatal conductance gS, leaf morphological traits, mineral nutrition), growth (tree diameter, height, stem increment), and water status indicators (leaf water potential Ψ, leaf transpiration T, water-use efficiency WUE) to identify processes underlying vast oak decline. The presence of mistletoe significantly reduced the Ψ by 1 MPa, and the WUE by 14%. The T and gS of infested oaks were lower by 34% and 38%, respectively, compared to the non-infested oaks, whereas the Asat dropped to 55%. Less pronounced but significant changes were also observed at the level of photosystem II (PSII) photochemistry. Moreover, we identified the differences in C content, which probably reduced stem increment and leaf size of the infested trees. Generally, we can conclude that mistletoe could be a serious threat that jeopardizes the water status and growth of oak stands.

中文翻译：

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干旱还是严重干旱？半寄生黄槲寄生 (Loranthus europaeus) 通过改变水分状况和生理反应来放大无柄橡树 (Quercus petraea) 的干旱压力

长期以来，欧洲橡树树种一直被认为相对能抵抗不同的干扰，包括干旱。然而，最近的几项研究报告称，它们的下降是由复杂的变化引发的。因此，我们比较了 2019 年相对干燥的植被季节期间受半寄生黄色槲寄生 (Loranthus europaeus Jacq.) 侵染和未侵染的成熟无柄橡树（Quercus petraea (Matt.), Liebl.）。我们使用了广泛的生态生理学（最大同化率 Asat、叶绿素 a 荧光、气孔导度 gS、叶片形态性状、矿物质营养）、生长（树径、高度、茎增量）和水分状况指标（叶水势 Ψ、叶蒸腾 T、水分利用效率） WUE）来确定橡木大量衰退的潜在过程。槲寄生的存在使 Ψ 显着降低了 1 MPa，WUE 显着降低了 14%。与未受侵染的橡树相比，受侵染的橡树的 T 和 gS 分别降低了 34% 和 38%，而 Asat 则降至 55%。在光系统 II (PSII) 光化学水平上也观察到不太明显但显着的变化。此外，我们确定了 C 含量的差异，这可能会减少受感染树木的茎增量和叶子大小。一般来说，我们可以得出结论，槲寄生可能是一种严重的威胁，会危及橡树的水分状况和生长。在光系统 II (PSII) 光化学水平上也观察到不太明显但显着的变化。此外，我们确定了 C 含量的差异，这可能会减少受感染树木的茎增量和叶子大小。一般来说，我们可以得出结论，槲寄生可能是一种严重的威胁，危及橡树的水分状况和生长。在光系统 II (PSII) 光化学水平上也观察到不太明显但显着的变化。此外，我们确定了 C 含量的差异，这可能会减少受感染树木的茎增量和叶子大小。一般来说，我们可以得出结论，槲寄生可能是一种严重的威胁，危及橡树的水分状况和生长。