Climate-driven tree growth and mortality in the Black Forest, Germany-Long-term observations.

Abstract

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.