Two unprecedented outbreaks of the European spruce bark beetle, Ips typographus L. (Col., Scolytinae) in Austria since 2015: Different causes and different impacts on forests

Austria has been facing two outbreaks of the European spruce bark beetle, Ips typographus L., in the last ten years. In this study, we compile data of damage caused by I. typographus L. as well as storm and snow breakage from 2002 to 2022 in two regions affected by these bark beetle outbreaks based on the Documentation of Forest Damaging Factors in Austria and analyze them in context of climatic factors. The first outbreak started in 2015 and affected Norway spruce forests at low elevation (< 600 m) in the north of Austria. Annual damage peaked in 2018 at 3.3 million m³ (representing 2.6% of total growing stock in the area). The second outbreak started in 2021 and affected mountainous spruce forests in the south of Austria. In this case extensive damage by storm in 2018 and snow breakage in the two following winters preceded the bark beetle outbreak. Annual damage by I. typographus L. reached 1.7 million m³ (2.4% of growing stock) in 2022. Most forests are located on steep mountain slopes and have important protective function against natural hazards. High temperatures allowed completion of two generations up to 1,400 m elevation. Linear regression models showed that damage by I. typographus L. was significantly affected by climatic water balance. This effect was stronger in the north than in the south. We discuss the different patterns of the outbreaks and challenges for bark beetle management in the context of climate change.