Assessing the impacts of forest stand structure and landscape on the formation of Ips typographus damage hotspots in Finland

Abstract

Damage caused by the European spruce bark beetle (SBB; Ips typographus) to Norway spruce (Picea abies) is expected to increase under climate change, particularly in Finland where Norway spruce is one of the most ecologically and economically important tree species. SBB damage often occurs in spatial hotspots (locations where high numbers of SBB damage cluster), with similar damage patterns in nearby locations. Evidence-informed decision-making requires an understanding of how stand and landscape attributes contribute to the formation of SBB damage hotspots. In this study, we analysed the impacts of stand and landscape attributes on SBB damage hotspots in the southern half of Finland, comprising 11.4 M ha of forestry land. We used data on SBB damage based on the spatial distribution of salvage logging operations due to SBB damage (a proxy for SBB damage) during 2012–20. The spatial autocorrelation of SBB damage was assessed using the global Moran’s I, and we identified hotspots using local Moran’s I. According to these, we classified the SBB-damaged stands as not forming hotspots, forming hotspots, or forming recurrent hotspots. Generalized linear mixed-effect models were used to examine how stand and landscape attributes influenced the probability of an infested area turning into a hotspot and the probability of a hotspot turning into a recurrent hotspot. Overall, SBB-damaged stands presented a low global spatial autocorrelation but still formed hotspots. Hotspots first aggregated in southeastern Finland, and thereafter also in the southwest. The low recurrence of hotspots over time suggests that the conditions necessary for sustained aggregation were not consistently met. While hotspot formation was associated with stand age and proximity to SBB damage during the previous two years, recurrent hotspots were more linked to stand development class and past damage proximity. The timely identification and removal of SBB-infested trees are crucial to reduce damage hotspot formation.