Drivers of primary bark beetle (Coleoptera: Curculionidae, Scolytinae) infestations in even-aged, uneven-aged, and unmanaged Norway spruce forests in central Sweden

Abstract

Bark beetle outbreaks are an increasingly important disturbance in boreal forests, yet empirical evidence is limited on how management systems influence stand- and tree-level susceptibility to bark beetle infestation. We quantified infestation patterns of Ips typographus, Pityogenes chalcographus, and Polygraphus spp. across 29 spruce-dominated forest stands in central Sweden representing mature even-aged, uneven-aged, and unmanaged forests. Using GLMs and GLMMs, we evaluated how management type, stand structure, and tree characteristics influenced stand- and tree-level infestation probability. Stand-level predictors provided limited support for explaining infestation probability. For I. typographus and Polygraphus spp. the measured stand-level variables did not explain infestation probability better than intercept-only models. In contrast, P. chalcographus showed a more consistent stand-level response, with infestation probability decreasing with increasing spruce density. At the tree level, both I. typographus and P. chalcographus showed higher infestation probability in even-aged stands, independent of diameter, sun exposure, and tree condition. Ips typographus preferentially colonized larger and more sun-exposed trees, whereas P. chalcographus favoured smaller trees. Polygraphus spp. showed no detectable response to tree size or sun-exposure. Our results suggest that, under endemic conditions, bark beetle infestation could not be inferred from management category alone. Instead, infestation probability depended on beetle species and was more clearly related to individual host-tree characteristics than to broad stand-level differences among management types.