The Himalayas are experiencing dramatic warming and drought events, which occur at a faster rate than the global average. How will the high-elevation ecosystems cope with such changing conditions? This study aims to combine cambial phenology with quantitative wood anatomy to understand how the growth dynamics affect the trade-off between safety and efficiency of water transport in the Himalayan forests. We investigated the timing of xylogenesis and its relationship with hydraulic wood anatomical traits in Himalayan birch (Betula utilis D. Don) at two altitudes in the central Himalayas. Xylogenesis was monitored weekly from April to September 2014 by collecting microcores. We measured vessel area on histological sections, calculated the hydraulic diameter, and assessed the effects of climate on xylem production rate. Xylogenesis occurred from June to September in both study sites. Moisture availability influenced positively xylem growth besides thermal conditions. The onset and duration of vessel enlargement were correlated with the hydraulic diameter. Trees with earlier onsets and longer durations of vessel enlargement exhibited larger hydraulic diameters. The close relationship between cambial phenology and hydraulic diameter suggests that earlier growth resumption induced by climate changes may result in larger vessels, which are more efficient in water transport, but vulnerable to drought-induced cavitation. The ongoing warming and drying climate conditions may menace the survival of birch forest in the central Himalayas.

喜马拉雅山正在经历剧烈的变暖和干旱事件，其发生速度快于全球平均水平。高海拔生态系统将如何应对这种不断变化的条件？本研究旨在将形成层物候学与定量木材解剖学相结合，以了解生长动态如何影响喜马拉雅森林中水运安全与效率之间的权衡。我们研究了喜马拉雅桦木 ( Betula utilisD. Don）在喜马拉雅中部的两个高度。从 2014 年 4 月到 2014 年 9 月，每周通过收集微核监测木糖发生。我们测量了组织切片上的血管面积，计算了水力直径，并评估了气候对木质部生产率的影响。两个研究地点的木糖生成均发生在 6 月至 9 月。除热条件外，水分可用性对木质部生长产生积极影响。血管扩大的发生和持续时间与水力直径相关。发病较早和血管扩大持续时间较长的树木表现出较大的水力直径。形成层物候与水力直径之间的密切关系表明，气候变化引起的较早恢复生长可能会导致更大的船只，从而更有效地进行水运，但易受干旱引起的气蚀。持续变暖和干燥的气候条件可能威胁到喜马拉雅山脉中部白桦林的生存。