Extreme tree growth reductions represent events of abrupt forest productivity decline and carbon sequestration reduction. An increase in their magnitude can represent an early warning signal of impending tree mortality. Yet the long-term trends in extreme growth reductions remain largely unknown. We analyzed the trends in the proportion of trees exhibiting extreme growth reductions in two Central-European conifer species—Pinus sylvestris (PISY) and Picea abies (PCAB)—between 1901 and 2018. We used a novel approach for extreme growth reduction quantification by relating their size to their mean recurrence interval. Twenty-eight sites throughout Czechia and Slovakia with 1120 ring width series representing high- and low-elevation forests were inspected for extreme growth reductions with recurrence intervals of 15 and 50 years along with their link to climatic drivers. Our results show the greatest growth reductions at low-elevation PCAB sites, indicating high vulnerability of PCAB to drought. The proportions of trees exhibiting extreme growth reductions increased over time at low-elevation PCAB, decreased recently following an abrupt increase in the 1970–1980s at high-elevation PCAB, and showed nonsignificant trends in high- and low-elevation PISY. Climatic drivers of extreme growth reductions, however, shifted over time for all site categories as the proportion of low-temperature-induced extreme growth reductions declined since the 1990s, whereas events caused by drought consistently increased in frequency during the same period. We observed higher growth volatility at the lower range of distribution compared with the upper range margin of PISY and PCAB. This will undoubtedly considerably impact tree growth and vitality as temperatures and incidence of drought in Central Europe are expected to further increase with ongoing climate change.

中文翻译：

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气候驱动的中欧冷杉和樟子松极端生长减少的趋势

树木生长极度减少代表森林生产力突然下降和碳固存减少的事件。它们幅度的增加可以代表即将到来的树木死亡的预警信号。然而，极端增长放缓的长期趋势在很大程度上仍然未知。我们分析了两种中欧针叶树树种——樟子松(PISY) 和冷杉云杉中表现出极端生长减少的树木比例的趋势(PCAB)——1901 年至 2018 年之间。我们通过将它们的大小与其平均复发间隔相关联，使用了一种新方法来进行极端生长减少的量化。在捷克和斯洛伐克的 1120 个环宽系列代表高海拔和低海拔森林的 28 个地点接受了检查，检查是否出现了极端的生长减少，重复间隔为 15 年和 50 年，以及它们与气候驱动因素的联系。我们的结果显示，低海拔 PCAB 站点的生长下降幅度最大，表明 PCAB 对干旱的脆弱性很高。在低海拔 PCAB 中表现出极端生长减少的树木比例随着时间的推移而增加，在 1970-1980 年代在高海拔 PCAB 中突然增加后最近有所下降，并且在高海拔和低海拔 PISY 中显示出不显着的趋势。然而，极端增长放缓的气候驱动因素，随着自 1990 年代以来低温引起的极端生长减少的比例下降，所有地点类别的变化随着时间的推移而发生变化，而同期干旱引起的事件发生的频率一直在增加。与 PISY 和 PCAB 的范围上限相比，我们观察到分布范围下限的增长波动性更高。这无疑将显着影响树木的生长和活力，因为随着持续的气候变化，预计中欧的气温和干旱发生率将进一步增加。与 PISY 和 PCAB 的范围上限相比，我们观察到分布范围下限的增长波动性更高。这无疑将显着影响树木的生长和活力，因为随着持续的气候变化，预计中欧的气温和干旱发生率将进一步增加。与 PISY 和 PCAB 的范围上限相比，我们观察到分布范围下限的增长波动性更高。这无疑将显着影响树木的生长和活力，因为随着持续的气候变化，预计中欧的气温和干旱发生率将进一步增加。