Ecology and Evolutionary Biology, University of California, Santa Cruz, Santa Cruz, CA 95060, USA.
Tree Physiol. 2023 May 12;43(5):769-780. doi: 10.1093/treephys/tpad006.
Injury to the xylem and vascular cambium is proposed to explain mortality following low severity fires. These tissues have been assessed independently, but the relative significance of the xylem and cambium is still uncertain. The goal of this study is to evaluate the xylem dysfunction hypothesis and cambium necrosis hypothesis simultaneously. The hot dry conditions of a low severity fire were simulated in a drying oven, exposing Sequoia sempervirens (Lamb. ex D. Don) shoots to 70 and 100 °C for 6-60 min. Cambial viability was measured with Neutral Red stain and water transport capacity was assessed by calculating the loss of hydraulic conductivity. Vulnerability curves were also constructed to determine susceptibility to drought-induced embolism following heat exposure. The vascular cambium died completely at 100 °C after only 6 min of heat exposure, while cells remained viable at 70 °C temperatures for up to 15 min. Sixty minutes of exposure to 70 °C reduced stem hydraulic conductivity by 40%, while 45 min at 100 °C caused complete loss of conductivity. The heat treatments dropped hydraulic conductivity irrecoverably but did not significantly impact post-fire vulnerability to embolism. Overall, the damaging effects of high temperature occurred more rapidly in the vascular cambium than xylem following heat exposure. Importantly, the xylem remained functional until the most extreme treatments, long after the vascular cambium had died. Our results suggest that the viability of the vascular cambium may be more critical to post-fire survival than xylem function in S. sempervirens. Given the complexity of fire, we recommend ground-truthing the cambial and xylem post-fire response on a diverse range of species.
木质部和形成层损伤被认为可以解释低严重度火灾后树木的死亡。这些组织已经被独立评估过，但木质部和形成层的相对重要性仍不确定。本研究的目的是同时评估木质部功能障碍假说和形成层坏死假说。在干燥箱中模拟了低严重度火灾的干热条件，将红杉（Sequoiadendron sempervirens (Lamb. ex D. Don)）嫩枝暴露于 70 和 100°C 下 6-60 分钟。使用中性红染色测量形成层活力，并通过计算水力传导率损失来评估水分传输能力。还构建了脆弱性曲线，以确定暴露于热后对干旱诱导栓塞的敏感性。在 100°C 下，仅 6 分钟的热暴露就会使维管束完全死亡，而在 70°C 的温度下，细胞仍能存活长达 15 分钟。60 分钟的 70°C 暴露会使茎的水力传导率降低 40%，而 100°C 下 45 分钟则会导致传导率完全丧失。热处理会不可恢复地降低水力传导率，但对火灾后栓塞的敏感性没有显著影响。总的来说，在暴露于热后，高温对维管束的破坏作用比木质部更快。重要的是，木质部在形成层死亡很久之后，仍保持功能直到最极端的处理。我们的结果表明，在红杉中，形成层的活力可能比木质部功能对火灾后存活更关键。鉴于火灾的复杂性，我们建议在广泛的物种中对火灾后形成层和木质部的响应进行实地验证。
