Severe freezing events could be one of the crucial causes preventing the advance of alpine timberline.However,studies on the characteristics of freezing events across altitudes and vegetation types at high elevations are scarce.Based on the measurements of air and soil temperature for forests and shrubs across an Abies georgei var.Smithii timberline ecotone along a north-facing slope in the Sergyemla Mountains,southeast Tibet,we analyzed altitudinal variations of freezing events for different vegetation types during growing season based on two different deftnitions-canopy temperature and soil temperature.Results are as follows:1) compared with canopy temperature of the growing season,soil temperature lagged behind nearly one month,and the length of growing season based on canopy temperature was significantly longer than that calculated by soil temperature for subalpine and timberline forests.2) Although the growing season length varied between different definitions,there were more freezing events in Rhododendron shrubs in the growing season than in the subalpine and timberline forests,and the frequency for the former was 1 to 3 times more than the latter.Besides,the intensity was stronger and the duration was longer in Rhododendron shrubs than in the subalpine and/or timberline forests.Our results indicated that there were more growing-season freezing events in Rhododendron shrubs at high elevations,with their frequency,intensity,and duration all tending to strengthen with rising altitudes.This might result in the difficulty of seedling establishment above the timberline.%低温事件的加剧可能是限制高山林线向更高海拔爬升的一个关键原因,然而有关高海拔地区不同海拔、不同植被覆盖下的低温事件分布特征的研究十分缺乏.通过对西藏东南部色季拉山急尖长苞冷杉林线过渡带同一坡面不同海拔森林和灌丛冠层温度和土壤温度的监测,并基于冠层温度和土壤温度计算植物的生长季长度,分析生长季期间低温事件在不同海拔、不同植被的分异特征,得到如下结果:1.土壤温度定义的生长季比冠层温度定义的生长季滞后近一个月,且对于亚高山森林和林线,冠层温度定义的生长季长度明显大于土壤温度定义的生长季长度.2.无论以哪种方法定义生长季,期间出现的低温事件均表现为高山杜鹃灌丛＞亚高山针叶林和林线,前者低温事件出现频率达后者的2～4倍,且总体而言高山杜鹃灌丛中低温事件的强度更大,持续时间相对更长.研究结果表明高山杜鹃灌丛存在更多的低温事件,而且随海拔增加,其频率、强度和持续时间不断加剧,这可能直接限制了乔木树种幼苗在林线之上低矮灌丛中的存活和生长,进而限制了林线乔木树种向更高海拔的分布.
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