A B S T R A C T Dealing with tree mortality caused by forest pests is one of the most demanding challenges in forest protection and management. The current spruce bark beetle infestation in the Polish part of the Białowieża Forest (BF), which started in 2012, is considered the largest in history and has nearly eliminated Norway spruce (Picea abies (L.) Karst) as a major forest tree species there. This research analyzed the dynamics and spatial extent of the BF outbreak from 2015 to 2019. The

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... ensive beetle epidemic caused a more than six times higher number of infested trees in 2019 as compared to 2015. Under the conditions of the unprecedented ongoing I. typographus outbreak, the dynamics of spruce stand mortality varied across the study area. A lower spruce mortality was observed in Białowieża National Park as compared to other forest districts. Host and environmental factors influenced the timing of spruce mortality. Spruce height, stand age, the proportion of spruce, the share of area covered by tree crowns, excluding spruce and dominant species in the stands, were the key factors of bark beetle infestation. The most intensive outbreak progressed in stands older than 100 years dominated by spruce, and areas with the lowest dynamics of spruce dieback were localized in mixed young forests or pine stands. The methods and results presented in this study serve as baseline information, supporting the efforts to model the spread of bark beetles and future decision-making.