Willows (Salix) are some of the most abundant shrubs in cold alpine and tundra biomes. In alpine regions, seed dispersal is not limiting upwards willow expansion, so the upslope shift of willow shrublines is assumed to be a response to climatic warming. Very little, however, is known about the recent spatiotemporal dynamics of alpine willow shrublines. The world's highest willow shrublines (ca 4900 m a.s.l.) are located on the Tibetan Plateau (TP) and provide a rare opportunity to test their sensitivity and responses to rapid warming and the associated increase in the demand for water in ecosystems. We used a new data set comprising 24 Salix shrubline plots along a 900-km latitudinal gradient (30‒38°N) to reconstruct the rates of annual shrub recruitment and shifting shrubline positions since 1939. Shrub densification and shrubline advances were promoted by pronounced summer warming before 2010, contributing to widespread greening on the TP. These trends, however, reversed due to warming-induced moisture limitation after 2010, which thus represented a tipping point of warming/drying trade-offs. Climatic warming and drying are predicted to accelerate in the following decades, so alpine plant communities may be at an increasing risk of population decline or even range contraction.

中文翻译：

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青藏高原水分限制使变暖引起的灌木系前进停滞

柳树（Salix）是寒冷的高山和苔原生物群落中最丰富的灌木。在高山地区，种子传播不限制向上的柳树扩张，因此假设柳树灌木的上坡移动是对气候变暖的反应。然而，人们对高山柳灌木丛的近期时空动态知之甚少。世界上最高的柳树灌木丛（海拔 4900 米）位于青藏高原（TP），提供了一个难得的机会来测试它们对快速变暖的敏感性和响应以及生态系统对水需求的相关增加。我们使用了一个包含 24 个Salix的新数据集沿 900 公里纬度梯度（30-38°N）绘制灌木线地块，以重建自 1939 年以来每年灌木补充和移动灌木线位置的速率。 2010 年之前的夏季明显变暖促进了灌木致密化和灌木线前进，从而促进了大面积绿化TP。然而，由于 2010 年后变暖引起的水分限制，这些趋势发生了逆转，因此代表了变暖/干燥权衡的临界点。预计未来几十年气候变暖和干燥将加速，因此高山植物群落可能面临越来越大的种群下降甚至范围缩小的风险。