Recent warming is affecting species composition and species areal distribution of many regions. However, although most treeline studies have estimated the rates of forest expansion into tundra, still little is known about the long-term dynamic of stand productivity at the forest-tundra intersection. Here, we make use of tree-ring data from 350 larch (Larix sibirica Ledeb.) and spruce (Picea obovata Ledeb.) sampled along the singular altitudinal treeline ecotone at the Polar Urals to assess the dynamic of stand establishment and productivity, and link the results with meteorological observations to identify the main environmental drivers. The analysis of stand instalment indicated that more than 90% of the living trees appeared after 1900. During this period, the stand became denser and moved 50 m upward, while in recent decades the trees of both species grew faster. The maximum afforestation occurred in the last decades of the twentieth century, and the large number of encountered saplings indicates that the forest is still expanding. The upward shift coincided with a slight increase of May–August and nearly doubling of September–April precipitation while the increase in growth matched with an early growth season warming (June + 0.27 °C per decade since 1901). This increase in radial growth combined with the stand densification led to a 6–90 times increase of biomass since 1950. Tree-ring based twentieth century reconstruction at the treeline ecotone shows an ongoing forest densification and expansion accompanied by an increased growth. These changes are driven by climate change mechanism, whereby the leading factors are the significant increase in May–June temperatures and precipitation during the dormant period. Exploring of phytomass accumulation mechanisms within treeline ecotone is valuable for improving our understanding of carbon dynamics and the overall climate balance in current treeline ecosystems and for predicting how these will be altered by global change.

中文翻译：

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极地乌拉尔山脉上游林线过渡带树木生长和林分生产力的气候变化证据

最近的变暖正在影响许多地区的物种组成和物种面积分布。但是，尽管大多数树线研究都估计了森林向苔原扩展的速度，但是对于森林-苔原交叉点林分生产力的长期动态知之甚少。在这里，我们利用极地乌拉尔山脉沿单一奇数海拔林线过渡带采样的350个落叶松（Larix sibirica Ledeb。）和云杉（Picea obovata Ledeb。）的年轮数据来评估林分建立和生产力的动态，并链接结合气象观测结果确定主要的环境驱动因素。对林分安装的分析表明，1900年以后出现了90％以上的活树。在此期间，林分变得更密并向上移动50 m，而在最近的几十年中，两种树的生长速度都更快。最大的造林发生在二十世纪的最后几十年，并且遇到的大量树苗表明森林仍在扩大。上升趋势与五月至八月的降水量略有增加和九月至四月的降水量几乎翻了一番，而生长量的增加与生长季节的早期变暖相吻合（自1901年以来，每十年的六月+ 0.27°C）。自1950年以来，这种径向生长的增加与林分的致密化相结合，导致生物量增加了6–90倍。在树线过渡带进行的基于树轮的20世纪重建表明，森林的不断致密化和扩张伴随着生长的增加。这些变化是由气候变化机制驱动的，因此，主要因素是在休眠期间5月至6月的温度和降水显着增加。探索林线过渡带内植物气藻的积累机制，对于增进我们对当前林线生态系统中碳动态和总体气候平衡的理解，以及预测全球变化将如何改变这些认识，具有重要意义。