Abstract

Unlike the well-understood alpine treeline, the upper range limits of tree taxa that do not reach the alpine treeline are largely unexplained. In this study, we explored the causes of the exceptionally high elevation (4270 m) occurrence of broad-leaved evergreen oaks (Quercus pannosa) in the south-eastern Himalayas. We assessed the course of freezing resistance of buds and leaves from winter to summer at the upper elevational limit of this oak species. Linked to leaf phenology, we analyzed freezing resistance and assessed minimum crown temperature for the past 65 years. We also examined potential carbon limitation at the range limit of this species. Last season buds and leaves operated at a safety margin of 5.5 and 11 K in mid-winter. Once fully dehardened early in July, last season foliage is damaged at −5.9 and new foliage at −4.6 °C. Bud break is timed for late June to early July when low temperature extremes historically were never below −3.0 °C. The monsoon regime ensures a long remaining season (149 days), thus compensating for the late onset of shoot growth. Compared with a site at 3450 m, specific leaf area is reduced, foliar non-structural carbohydrate concentrations are similar and the δ¹³C signal is higher, jointly suggesting that carbon limitation is unlikely at the range limit of this species. We also show that these oaks enter the growing season with fully intact (not embolized) xylem. We conclude that the interaction between phenology and freezing tolerance results in safe flushing, while still facilitating shoot maturation before winter. These factors jointly determine the upper range limit of this oak species. Our study illuminates an exceptional case of broad-leaved evergreen tree performance near the treeline, and by exploring a suite of traits, we can underpin the central role of flushing phenology in such a stressful environment.

中文翻译：

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解释了喜马拉雅山东南部常绿橡树的4,270 m高海拔极限。

摘要

与众所周知的高山林线不同，未达到高山林线的树木分类单元的上限在很大程度上无法解释。在这项研究中，我们探讨了阔叶常绿橡树（栎栎）异常高海拔（4270 m）发生的原因）在喜马拉雅山脉东南部。我们评估了从冬季到夏季，在这种橡树种的海拔上限，芽和叶的抗冻性过程。与叶片物候相关，我们分析了抗冻性并评估了过去65年的最低树冠温度。我们还研究了该物种范围限制下的潜在碳限制。上个季节的芽叶在冬季中期的安全系数为5.5和11K。一旦在7月初完全解硬，上个季节的叶子在-5.9处受损，新叶子在-4.6°C处受损。断芽定于6月下旬至7月初，当时低温极限历来从未低于-3.0°C。季风机制确保了一个较长的剩余季节（149天），从而弥补了芽生的较晚发作。与3450 m处的位置相比，比叶面积减少了，¹³ C信号较高，共同表明在该物种的范围极限处碳限制不太可能发生。我们还表明，这些橡木进入完整的（未栓塞的）木质部木质的生长季节。我们得出的结论是，物候与耐寒性之间的相互作用导致安全冲洗，同时仍促进冬前的芽成熟。这些因素共同决定了该橡木树种的上限。我们的研究阐明了常绿阔叶树在林线附近表现的特殊情况，并且通过探索一系列特征，我们可以在这种压力环境中巩固潮红物候学的核心作用。