Increasing dryness challenges trees' ability to maintain water transport to the leaves. Most plant hydraulics models use a static xylem response to water stress. Yet, in reality, lower soil moisture and warmer temperatures during growing seasons feed back onto xylem development. In turn, adjustments to water stress in the newly built xylem influence future physiological responses to droughts. In this study, we investigate the annual variation of anatomical traits in branch xylem in response to different soil and atmospheric moisture conditions and tree stress levels, as indicated by seasonal predawn leaf water potential (ΨL,pd). We used a 6-year field experiment in southwestern USA with three soil water treatments applied to Pinus edulis Engelm trees-ambient, drought (45% rain reduction) and irrigation (15-35% annual water addition). All trees were also subject to a natural 1-year acute drought (soil and atmospheric) that occurred during the experiment. The irrigated trees showed only moderate changes in anatomy-derived hydraulic traits compared with the ambient trees, suggesting a generally stable, well-balanced xylem structure under unstressed conditions. The artificial prolonged soil drought increased hydraulic efficiency but lowered xylem construction costs and decreased tracheid implosion safety ((t/b)2), suggesting that annual adjustments of xylem structure follow a safety-efficiency trade-off. The acute drought plunged hydraulic efficiency across all treatments. The combination of acute and prolonged drought resulted in vulnerable and inefficient new xylem, disrupting the stability of the anatomical trade-off observed in the rest of the years. The xylem hydraulic traits showed no consistent direct link to ΨL,pd. In the future, changes in seasonality of soil and atmospheric moisture are likely to have a critical impact on the ability of P. edulis to acclimate its xylem to warmer climate. Furthermore, the increasing frequency of acute droughts might reduce hydraulic resilience of P. edulis by repeatedly creating vulnerable and less efficient anatomical structure.

中文翻译：

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松树对急性干旱和长期干旱的不同木质部反应。

不断增加的干燥度挑战了树木保持水分向叶片输送的能力。大多数工厂水力学模型对水压力使用静态木质部响应。然而，实际上，生长季节中较低的土壤湿度和较暖的温度会反馈到木质部发育中。反过来，对新建木质部中水分胁迫的调整会影响未来对干旱的生理反应。在这项研究中，我们调查了分支木质部在不同土壤和大气湿度条件和树木胁迫水平下的年度性状变化，如季节性的黎明前叶片水势（ΨL，pd）所示。我们在美国西南部进行了为期6年的田间试验，对松果树进行了三种土壤水处理：环境，干旱（减少雨水量为45％）和灌溉（每年加水15-35％）。在实验过程中，所有树木都遭受了自然的一年自然干旱（土壤和大气）。与周围树木相比，灌溉树木只显示出适度的解剖结构水力性状变化，这表明在无压力条件下，木质部结构总体稳定，平衡。人工长时间的土壤干旱提高了水力效率，但降低了木质部的建设成本并降低了气管内爆安全性（（t / b）2），这表明木质部结构的年度调整遵循安全性与效率的权衡。严重干旱使所有处理的水力效率均下降。急性和长期干旱的结合导致了新木质部的脆弱和低效，破坏了其余年份所观察到的解剖结构权衡的稳定性。木质部水力性状与showedL，pd没有直接一致的联系。将来，土壤季节变化和大气湿度可能会对食用紫菜使木质部适应气候变暖的能力产生重大影响。此外，急性干旱的频率不断增加，可能会通过反复创建脆弱且效率较低的解剖结构来降低可食假单胞菌的水力弹性。