Impact of management strategies on forest susceptibility to spruce bark beetle damage and potential trade-offs with timber production and biodiversity

The European spruce bark beetle (SBB) is an important insect pest in many countries such as Sweden and has caused the loss of millions of trees over the past few decades. Forest management targeting key variables in the forest can be a potential tool to decrease SBB susceptibility. In this paper, we simulated forest development over a 70-year planning horizon and evaluated the effect of different forest management strategies on spruce bark beetle susceptibility, timber production and biodiversity indicators. We used national forest inventory plots located in Kronoberg county, Southern Sweden, from 2016 to 2020 to perform the analyses. A reference strategy mimicking current management practices was simulated and compared with four other management strategies that can be an alternative to decrease spruce bark beetle damage. The four management strategies were (1) mixed forest stands, (2) shorter rotations and no thinnings, (3) prolonged rotations and (4) continuous cover forestry. The strategies differed in how and when regeneration, pre-commercial thinning, thinnings and final fellings were performed. The optimization of each of the strategies was aimed at reducing spruce bark beetle susceptibility while simultaneously investigating trade-offs with a range of timber production demands. In addition, we simulated a combined strategy where any of the strategies could be chosen with the objective of reducing spruce bark beetle susceptibility. Also, we evaluated each strategy with respect to biodiversity indicators described in the Swedish environmental quality objective Living Forests. The results show that a combination of all strategies is the most effective option to manage the forest to achieve the lowest average susceptibility in the analysed forest area. Shorter rotation management also resulted in low susceptibility. In addition, management strategies leading to large reductions in the abundance of large stem diameter Norway spruce trees in the landscape achieve lower susceptibility values. Our results suggest that various management strategies, alone or in combination with others, can be successfully employed to decrease forest susceptibility to spruce bark beetle damage. However, achieving multiple management objectives simultaneously, such as timber production and promotion of biodiversity, may require additional constraints in the mathematical models in addition to the settings used to describe each of the strategies. Future work should explore incorporating these additional constraints to better optimize management decisions.