Effects of salvage logging and sanitation felling on bark beetle (Ips typographus L.) infestations.
The European spruce bark beetle (Ips typographus) is the most devastating biotic disturbance agent in Norway spruce (Picea abies) forests of Central Europe and Scandinavia. To reduce damage by bark beetles, foresters aim at (i) preventing outbreaks by salvage logging of storm-damaged timber, and (ii) lowering bark beetle damage by sanitation felling of beetle-infested spruce trees. The effectiveness of these measures are controversially discussed but has not yet been thoroughly analyzed on a quantitative basis. We analyzed a survey dataset with annual resolution that covers 9 years and 487 forest districts (82% of the forested area) all over Switzerland to quantify the drivers of bark beetle infestations, in particular salvage logging and sanitation felling. Poisson log-normal models were used to analyze the dynamics of bark beetle infestations at the forest district level. Bark beetle infestations increased with increasing storm damage, heat sum, volume of Norway spruce stock and the number of infestation spots in the previous year. In contrast, infestations decreased with increasing proportions of sanitation felling relative to the total volume of infested spruce, and with increasing proportions of salvaged windthrown spruce. Our study is the first to quantify the combined effects of salvage logging and sanitation felling on the infestation dynamics of I. typographus in subsequent years, thus allowing forest managers to improve management strategies of bark beetle damage. Sanitation felling and salvage logging reduce the emergence of new infestation spots. In regions with large scale storm damage, salvage logging is clearly more urgent than sanitation felling in the first year and it is therefore recommended to prioritize in the first year after storm events.