- Department of Ecology, Swedish University of Agricultural Sciences
Lindman, Ly; Ranius, Thomas; Schroeder, Martin
In recent years, the outbreaks of the Eurasian spruce bark beetle (Ips typographus) have become more frequent and severe, while temperatures have been rising. Our understanding of the consequences of climate warming can be improved by comparing the performance of I. typographus between geographical regions with different climates. We applied that approach by placing out Norway spruce logs at sites selected along a 1300 km north -south gradient in Sweden to study the effect of regional climate and microclimate on I. typographus. To obtain a microclimatic gradient at each site, we placed the logs at sun-exposed edges and in the shaded inner forest (10 m or 50 m from stand edges). We assessed the probability of colonisation of I. typographus and the thermal sums required for complete development.The species' habitat preferences differed along the climate gradient including six geographical regions: in cooler regions, a higher proportion of sun-exposed logs was colonised in comparison to the shaded ones, while that difference was much smaller in warmer regions. The thermal sum needed for development tended to increase with warmer regional climate. Seasonal thermal sum exceeded the requirements for development of two generations per year in all three shade levels in the warmest region while in the coldest region two generations were only possible in some of the sun-exposed logs.Outbreaks of I. typographus are more frequent and severe in regions with a warmer climate, and our study reveals two explanations for this: first, I. typographus has more suitable breeding sites in warmer regions, as shaded logs inside the forest offer additional suitable habitat; second, the potential population growth increases with the probability of completing two fully established generations per year. We recommend that after stormfellings, wind-felled trees are removed before they are colonised by I. typographus to decrease the outbreak risk. The observed difference in habitat preferences between geographical regions implies that, in cooler sites in the north, it is enough to remove recently wind-felled spruces that are sun-exposed, such as along edges, while in the south, also trees in more shaded conditions inside forests are a suitable substrate for I. typographus. In a warming climate, the increased risk and severity of I. typographus outbreaks is a factor making it less attractive to use Norway spruce in forestry.
Bark beetle; Deadwood; Log colonisation; Microclimate; Sun-exposure; Temperature logger; Thermal sum
Forest Ecology and Management
2023, Volume: 544, article number: 121216