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.

树木极端生长衰退指森林生产力骤降、碳汇能力减弱的事件。其强度增大可作为树木即将死亡的早期预警信号。但目前学界对极端生长衰退的长期变化趋势仍知之甚少。我们分析了1901年至2018年间，中欧两种针叶树种——欧洲赤松（Pinus sylvestris, PISY）和挪威云杉（Picea abies, PCAB）的极端生长衰退树木占比变化趋势。我们采用了一种将衰退幅度与生长平均重现期相关联的极端生长衰退量化新方法。我们对捷克与斯洛伐克境内的28处样地开展了调查，共获取覆盖高、低海拔森林的1120条树轮宽度序列，针对重现期为15年和50年的极端生长衰退事件进行识别，并分析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衰退程度最为显著，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，出现极端生长衰退的树木占比随时间持续上升；高海拔挪威云杉样地的该占比则在1970-1980年代出现骤增后，近年呈现下降趋势；而高、低海拔的欧洲赤松样地均未出现显著的变化趋势。不过，所有样地类别的极端生长衰退气候驱动因子均随时间发生了转变：自1990年代以来，由低温诱导的极端生长衰退事件占比逐步下降，而同期由干旱引发的极端生长衰退事件频率则持续升高。我们还观察到，欧洲赤松与挪威云杉在分布下限区域的生长波动幅度，显著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。