Climate‐driven tree growth and mortality in the Black Forest, Germany: Long‐term observations

Episodic tree mortality can be caused by various reasons. This study describes climate‐driven tree mortality and tree growth in the Black Forest mountain range in Germany. It is based on a 68‐year consistent data series describing the annual mortality of all trees growing in a forest area of almost 250 thousand ha. The study excludes mortality caused by storm, snow and ice, and fire. The sequence of the remaining mortality, the so‐called "desiccated trees," is analyzed and compared with the sequence of the climatic water balance during the growing season and the annual radial growth of Norway spruce in the Black Forest. The annual radial growth series covers 121 years and the climatic water balance series 140 years. These unique time series enable a quantitative assessment of multidecadal drought and heat impacts on growth and mortality of forest trees on a regional spatial scale. Data compiled here suggest that the mortality of desiccated trees in the Black Forest during the last 68 years is driven by the climatic water balance. Decreasing climatic water balance coincided with an increase in tree mortality and growth decline. Consecutive hot and dry summers enhance mortality and growth decline as a consequence of drought legacies lasting several years. The sensitivity of tree growth and mortality to changes in the climatic water balance increases with the decreasing trend of the climatic water balance. The findings identify the climatic water balance as the main driver of mortality and growth variation during the 68‐year observation period on a landscape‐scale including a variety of different sites. They suggest that bark beetle population dynamics modify mortality rates. They as well provide evidence that the mortality during the last 140 years never was as high as in the most recent years.

间歇性树木死亡可由多种诱因引发。本研究聚焦德国黑山林区由气候驱动的树木死亡与林木生长状况，其依托一套时长68年的连续数据集，记录了该林区内近25万公顷林地中所有树木的年死亡情况。本研究剔除了风暴、雪灾、冰害及火灾引发的树木死亡数据，针对剩余的、即所谓「枯干树木」的死亡序列展开分析，并将其与黑山林区生长季气候水平衡（climatic water balance）序列以及挪威云杉（Norway spruce）的年径向生长序列进行对比。
年径向生长数据集的时长为121年，气候水平衡数据集则长达140年。这套独一无二的长时间序列数据集，可实现在区域空间尺度上对数十年尺度干旱与热胁迫对林木生长及死亡的影响进行定量评估。
本研究汇编的数据表明，过去68年间黑山林区枯干树木的死亡率由气候水平衡主导：气候水平衡的下降与树木死亡率上升及生长量下降同步发生。连续的炎热干旱夏季会加剧树木死亡与生长衰退，这是由于干旱遗留效应可持续数年之久。林木生长与死亡对气候水平衡变化的敏感性，随气候水平衡的下降趋势而增强。
研究结果证实，在涵盖多种立地类型的68年观测周期内，气候水平衡是驱动景观尺度下树木死亡与生长变化的核心因子。研究同时表明，小蠹虫种群动态会改变树木死亡率。此外，本研究还提供证据显示，过去140年间的树木死亡率从未达到近年的峰值水平。