Development and implementation of a spruce bark beetle susceptibility index: A framework to compare bark beetle susceptibility on stand level

The spruce bark beetle (spruce bark beetle) (Ips typographus) is one of the major disturbance agents in European forests. Damage by spruce bark beetle is expected to increase in the future, as a result of e.g. increased temperatures. However, not all forest stands are equally vulnerable. Therefore, describing the relative difference in susceptibility of different forest stands to spruce bark beetle infestation and to try to estimate changes in susceptibility under different management or climate scenarios is necessary to support decision making on forest management. We present a spruce bark beetle susceptibility index, which describes the relative susceptibility of forest stands to spruce bark beetle infestation. The index is based on empirical findings and expert opinion, and takes both climatic and stand variables into account. The index can be implemented in forest simulation programs. The susceptibility index was implemented in Heureka, a forest decision support system. To demonstrate the use of the index, simulations were run for three management scenarios: baseline; even-aged management focused on conifers, longer rotation: same as baseline but with longer rotation periods and mixed forest: same as baseline but retaining a higher share of broadleaves. For this purpose, an area of 2451 ha consisting of 751 stands was used. The index value per stand per five-year time period was obtained from the simulations. The index was calculated individually, per management strategy, for all 751 stands and thereafter mean index and harvest volume was obtained for the whole area. Mean susceptibility was higher, and harvest slightly lower, in the longer rotation scenario, compared with the baseline, while there were no differences between baseline and mixed. At individual stand level, the differences are more nuanced and, for example, certain stands have lower susceptibility in the mixed compared with the baseline scenario. The ability to simulate forest development and simultaneously get a measure of spruce bark beetle susceptibility will enable forest owners to identify vulnerable stands and evaluate effects of different management decisions to reduce the risk for future economic losses.