Landscape patterns of the spruce bark beetle (Ips typographus) and associations with conservation areas and forest conditions

Sammanfattning

The European spruce bark beetle (Ips typographus) is a destructive pest species of Norway spruce (Picea abies). A recent drought-induced outbreak in Sweden that began in 2018 killed 34 million cubic meters (m3) of Norway spruce. There has been debate as to whether conservation areas, which typically do not allow any pest-management interventions, may be to blame for infestations because they harbor suitable host material. On the other hand, managed Norway spruce monocultures are highly susceptible to infestations. Other forest characteristics also influence infestation dynamics. From satellite data, I modeled infestation probability (occurrence) and intensity in 1-hectare (ha) pixels and at spatial scales of 100 m, 500 m, and 2000 m across 3.55 million ha in southeastern Sweden. Five predictor variables were used: conservation areas with spruce, average birch volume, soil moisture, clearcut edges, and spruce stem diameter. Infested pixels were further categorized by infestation size—small, medium, or large—depending on the proportion of the pixel infested and modeled using the same predictors. Occurrence and severity models were also run for pixels in conservation areas and those in managed land separately, without conservation areas as a predictor variable. Results indicated that conservation areas did seem to have an association with the occurrence and intensity of infestations. At larger scales, they hindered infestation severity, but at smaller scales, they appeared to be positively associated with infestation occurrence. However, other forest characteristics, namely birch volume and soil moisture, seemed to play larger roles. Most infestations were small, and the predictor variables behaved differently across size classes, especially in large infestations. There was some variation in the associations between predictor variables and response variables in the separated managed and conservation area models. Moreover, conservation areas did seem to play a role in the probability and severity of infestations, though magnitude and direction of the apparent effects were scale-dependent. These results suggest that conservation areas may not be the main driver of infestations, which are more likely influenced by a complex interaction of forest characteristics. Understanding these nuanced relationships is critical for developing more balanced forest management strategies that reconcile biodiversity conservation with climate-resilient pest control.