Early warning signals of Norway spruce decline in Eastern Europe

Climate change is affecting forest ecosystems all around the globe, in particular through warming as well as increases in drought frequency and intensity. Possible impacts range from effects on the provisioning of ecosystem services such as carbon sequestration to tree mortality.

Norway spruce (Picea abies (L.) H. Karst.) is one of the most important coniferous species at the European level. In the actual context of climate change, especially with the increase in drought severity and frequency Norway spruce is likely to be at risk. Severe droughts during the vegetation period may, for example, negatively affect the resilience of Norway spruce and its’ ability to resist bark-beetle attacks. In recent extremely dry years in Central Europe, this has been observed through the large dieback of Norway spruce forests. In Eastern Europe, however, no extensive Norway spruce decline has been reported so far, posing the question how these forests will develop in the future?

To address this question, we present and analyze a tree-ring network consisting of 155 Norway spruce chronologies from Eastern Europe (Romania). As sites were selected along elevational transects in the Carpathians, our network allows to assess future impacts of climate change using a space for time substitution. The focus of our analysis is on the early warning signals of climate-change induced stress: negative trends in basal area increment and increased sensitivity of tree growth, assessed over the statistics first-order autocorrelation and standard deviation. A clear decrease in basal area increment was observed over the last two decades in the northern part of the Eastern Carpathians, which was more pronounced for younger stands and at lower elevations. At the same time, the first-order autocorrelation showed a sharp decrease at lower elevations. Our results highlighted increasing stress conditions of Norway spruce-based forests in Eastern Europe. In the current climatic scenarios, we may expect high mortality and forest diebacks also in the eastern part of Europe. Mitigation solutions are required as soon as possible.