Boreal forests have faced increasingly severe disturbances caused by insect pests. These disturbances are primarily due to human-induced loss in structural and functional diversity, forest heterogeneity, and changes in temperature and precipitation caused by climate change. In Finland, the intensity of forest insect outbreaks has increased, and many pest insect species have spread further north. More information is needed to understand the specific factors that make forest stands more vulnerable to pest outbreaks. This thesis aims to explore the challenges of forest pest dynamics by examining the interactions between biotic and abiotic factors and their ecological and economic impacts. It focuses on two key pest species: the common pine sawfly (Diprion pini L.) and the European spruce bark beetle (Ips typographus L.), investigating the triggers of pest outbreaks and their effects on forest ecosystems.
The first study examined the long-term effect of natural enemies on the mortality of D. pini during the cocoon stage, finding that forest characteristics, especially nutrient-poor sites, can reduce the effectiveness of natural enemies. The second study identifies soil and topographic factors that increase the severity of defoliation by D. pini, with more severe damage occurring on flatter, more fertile sites. In the third study, the cumulative effects of prolonged D. pini outbreak on Scots pine (Pinus sylvestris L.) radial growth and associated economic losses are quantified, showing significant effects based on defoliation severity. Furthermore, the trees that suffered severe defoliation did not recover to the pre-outbreak increment level during the 10-year study period. The fourth study focuses on environmental factors predisposing Norway spruce (Picea abies (L.) Karst.) forests to I. typographus. The highest probabilities of severe infestation were linked to eastern aspects, moderately steep slopes, and rich soil fertility. This study showed the specific environmental drivers intensifying the susceptibility of forest stands to I. typographus outbreaks.
Overall, these studies show that natural enemy regulation depends on specific stand factors, benefiting the performance of enemy species. Further, certain types of factor combinations could predispose stands to insect-induced disturbances. The key findings of my thesis will help to increase knowledge of factors that can regulate or predispose forest stands to insect-induced forest disturbance. These main outcomes should be considered for future forest management planning.