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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.

The predicted increase in temperature, dry summers, and extreme meteorological events will affect many harmful species both by directly accelerating their propagation rates as well as by weakening their host plants. There might also be shifts in the interrelations in species communities that can hardly be predicted. Some fungal species, that had previously been inconspicuous, caused notable damages in the drought periods of the last two decades. Higher winter temperatures often increase the survival rates of many fungal and insect species. Because hot and dry summers are likely to become more frequent and heavy storms tend to occur more often, an increase in massive outbreaks of bark beetles with corresponding damage is to be expected. As a result of global trade, more invasive alien fungi and insects are introduced into Europe where the climatic conditions become more favorable for them to establish themselves on native or introduced host trees. Insects and diseases can have a profound impact on forest dynamics. Therefore, these disturbances must be included in the discussion of future tree species composition and of forest development scenarios.

The dynamics of a recent outbreak of the spruce bark beetle (Ips typographus) in Switzerland was ruled by a devastating winter storm in 1999 and the drought and heat of the summer 2003. Starting from a similar level of population sizes, estimated as the rate of infested growing stock, beetle populations increased differently in magnitude and time among different regions in Switzerland. Accordingly, we expected local or regional genetic differentiation as a result of such repeated population expansion/breakdown dynamics. We analyzed 5 nuclear microsatellites of spruce bark beetles sampled from pheromone traps at 30 locations distributed over Switzerland. Our genetic results did not indicate any sign of population differentiation, structure, isolation by distance, or recent bottlenecks. This complete lack of genetic structure suggests that spruce bark beetles are highly mobile, precluding the formation of a spatial structure at neutral molecular markers. Thus, this molecular–genetic approach does not allow us to discriminate among regional gene pools and to identify the origin of expanding beetle populations.

Using an extensive dataset from the Canton Berne spatial patterns were analysed of the first Ips typographus- infestations of standing trees after the storm Lothar on December 1999. Our results suggest that these first infesta- tions were not caused by bark beetles indiscriminately spreading out of windthrow areas. Rather, we found rela- tionships between levels of attacked standing trees and cleaned, scattered windthrown trees. We therefore conclude that our results point to reduced host tree resistance as a key factor. Abstract: Ein umfangreiches Datenmaterial aus dem Kanton Bern erlaubte die Analyse des raumlichen Verteilungsmusters des ersten Stehendbefalls durch den Buchdrucker Ips typo- graphus, der als Folge des Sturmes Lothar entstand. Unsere Resultate deuten darauf hin, dass der erste Befall stehender Baume nicht massgeblich durch eine ungerichtete Dispersion der Kafer aus den Windwurfflachen hervorgerufen wurde. Hingegen fanden wir raumliche Beziehungen zwischen dem Stehendbefall und geraumten Sturm-Streuschaden im Rest- bestand. Wir gehen deshalb davon aus, dass unsere Resultate eine reduzierte Widerstandskraft der Restbestande als Schlusselfaktor anzeigen.