Impacts of Environmental and Spatio-Temporal Drivers on Ips Typographus -Induced Mortality of Norway Spruce in South-Eastern Finland

Large-scale outbreaks of the European spruce bark beetle (SBB, Ips typographus L.) have severely disturbed Norway spruce-dominated forests across Northern Europe in recent decades. In Finland, SBB outbreaks and the subsequent mortality of Norway spruce have increased over the last two decades, particularly raising concerns after the outbreaks in 2012-2013. In this study, we developed generalized additive models to assess how various environmental drivers (stand structure, landscape, topography, weather and their interactions) and spatio-temporal drivers contribute to the variability in the SBB-induced mortality of Norway spruce during an SBB outbreak in south-eastern Finland. The study data covered managed areas where salvage-sanitation loggings and clear-cuttings were implemented, as well as conserved areas where wind-damaged trees were left unsalvaged. Among the grouped environmental drivers, the stand variables had the highest impact on SBB-induced spruce mortality, followed by the landscape and topography variables. The SBB-induced spruce mortality was higher in forest stands, which had a high proportion (> 65%) of spruce with a diameter larger than 20 cm, the mean stand diameter larger than 25 cm, were growing on east-facing slopes, were situated at less than 200 m from the managed or unsalvaged gaps, and had growing season temperature sum of more than 1300 degree days. The SBB-induced spruce mortality was spatially clustered in the study locations and forest compartments. Our findings improve understanding of the contribution of environmental and spatio-temporal drivers to the SBB-induced spruce mortality. They provide support for identifying high-risk areas for SBB outbreaks in the boreal region of northern Europe and highlight timely preventive risk management measures to mitigate SBB outbreaks after severe wind damage.