Quantitative knowledge of xylem physical tolerance limits to dehydration is essential to understanding plant drought tolerance but is lacking in many long-vessel angiosperms. We examine the hypothesis that a fundamental association between sustained xylem water transport and downstream tissue function should select for xylem that avoids embolism in long-vessel trees by quantifying xylem capacity to withstand air entry of western North American oaks (Quercus spp.). Optical visualization showed that 50% of embolism occurs at water potentials below −2.7 MPa in all 19 species, and −6.6 MPa in the most resistant species. By mapping the evolution of xylem vulnerability to embolism onto a fossil-dated phylogeny of the western North American oaks, we found large differences between clades (sections) while closely related species within each clade vary little in their capacity to withstand air entry. Phylogenetic conservatism in xylem physical tolerance, together with a significant correlation between species distributions along rainfall gradients and their dehydration tolerance, suggests that closely related species occupy similar climatic niches and that species' geographic ranges may have shifted along aridity gradients in accordance with their physical tolerance. Such trends, coupled with evolutionary associations between capacity to withstand xylem embolism and other hydraulic-related traits, yield wide margins of safety against embolism in oaks from diverse habitats. Evolved responses of the vascular system to aridity support the embolism avoidance hypothesis and reveal the importance of quantifying plant capacity to withstand xylem embolism for understanding function and biogeography of some of the Northern Hemisphere’s most ecologically and economically important plants.

木质部对脱水的物理耐受性限制的定量知识对于了解植物的耐旱性至关重要，但许多长导管被子植物缺乏这种知识。我们研究了这样的假设：持续的木质部水运输和下游组织功能之间的基本关联应该通过量化北美西部橡树（栎属）的木质部承受空气进入的能力来选择避免长导管树栓塞的木质部。光学可视化显示，在所有 19 个物种中，50% 的栓塞发生在水势低于 -2.7 MPa 的情况下，而在抵抗力最强的物种中，50% 的栓塞发生在水势低于 -6.6 MPa 的情况下。通过将木质部对栓塞脆弱性的演变映射到北美西部橡树化石年代的系统发育上，我们发现进化枝（部分）之间存在巨大差异，而每个进化枝内密切相关的物种在承受空气进入的能力方面差异不大。木质部物理耐受性的系统发育保守性，以及沿降雨梯度的物种分布与其脱水耐受性之间的显着相关性，表明密切相关的物种占据相似的气候生态位，并且物种的地理范围可能根据其物理耐受性沿着干旱梯度发生变化。这种趋势，加上抵御木质部栓塞的能力和其他水力相关性状之间的进化关联，为来自不同栖息地的橡树带来了广泛的栓塞安全边际。 维管系统对干旱的进化反应支持了栓塞避免假说，并揭示了量化植物抵御木质部栓塞的能力对于了解北半球一些在生态和经济上最重要的植物的功能和生物地理学的重要性。