Tree mortality of European beech and Norway spruce induced by 2018-2019 hot droughts in central Germany

Anthropogenic climate change pushes forest ecosystems globally beyond their limits. Widespread events of forest die-off have been attributed to direct and indirect impacts of increasingly frequent and intense droughts. Here, we focus on an extensive mortality event in Norway spruce (Picea abies (L.) H. Karst.) and European beech (Fagus sylvatica (L.)) forests in Germany, following the successive 2018 - 2019 hot droughts. To examine whether this die-off indeed attributed to observed trends in drought occurrence and intensity, we sampled 143 beech and 186 spruce trees at three low-elevation sites (Spessart, Hassberge, Fichtelberg) with different edaphic properties in northern Bavaria. We analysed long-term hydroclimatic sensitivity and growth responses to extreme events of five site- and species-specific tree-ring width chronologies, including a reference site for each species. Growth of beech was sensitive to drought in April to June, whereas spruce growth was strongly related to drought during June to August, except at slightly higher elevations at the Fichtelberg site, where a summer temperature signal was observed. Trees at the Spessart and Hassberge sites showed an increased response to hydroclimatic conditions in April following the extreme drought in 1976 and from the 1990s onwards at the Fichtelberg site. Spruce bark beetle (Ips typographus L.) outbreaks during the 2018 drought accelerated the high mortality rates in around 50% of the trees at the Spessart and Hassberge site. In 2018, around 7% of all beech trees died at the Hassberge site, the site with the highest clay content. Our results suggest that these widespread mortality events can be attributed to an increasing drought sensitivity and were accelerated by the consecutive recent drought years. Sustainable forest management practices for these ecologically and economically important tree species are required to mitigate the effects of global warming in the future.