NOAA/WDS Paleoclimatology - Rydval - Rabštejn - PCAB - ITRDB CZEC021

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.

树木极端生长降低事件，指森林生产力突发下降及碳固存能力衰减的现象。其强度的提升，可作为树木即将死亡的早期预警信号。然而，目前学界对极端生长降低事件的长期变化趋势仍知之甚少。本研究分析了1901年至2018年间，两种中欧针叶树种——欧洲赤松（Pinus sylvestris, PISY）和挪威云杉（Picea abies, PCAB）——的极端生长降低个体占比变化趋势。本研究采用一种全新的极端生长降低量化方法：将事件强度与其平均重现期相关联。我们对捷克与斯洛伐克全境的28个样地展开调研，涵盖高低海拔森林的1120条年轮宽度序列，以识别重现期为15年和50年的极端生长降低事件，并探究其与气候驱动因子的关联。研究结果显示，低海拔挪威云杉样地的生长降低幅度最大，表明挪威云杉对干旱胁迫具有较高的脆弱性。低海拔挪威云杉样地中，极端生长降低个体占比随时间呈上升趋势；高海拔挪威云杉样地的该占比则在1970至1980年代突发升高后，于近年有所下降；而高低海拔的欧洲赤松样地均未出现显著变化趋势。不过，所有样地类型的极端生长降低事件的气候驱动因子均随时间发生改变：自1990年代以来，由低温诱导的极端生长降低事件占比逐渐下降，而同期由干旱引发的事件频率则持续升高。我们还发现，欧洲赤松与挪威云杉分布下限区域的生长波动幅度，高于其分布上限区域。随着气候变化持续推进，中欧地区的气温与干旱发生频率预计将进一步升高，这无疑将对树木生长与活力产生显著影响。