Environmental factors affecting European spruce bark beetle (Ips typographus) outbreaks in Sweden : Linking AI-detected dead spruce, soil moisture, nature protection and bark beetle outbreak (Ips typographus) in Sweden

Sammanfattning: Norway spruce (Picea abies) is a vital tree species in Sweden's extensive forested landscape. However, the planting of spruce beyond its natural range has made it vulnerable to pests like the spruce bark beetle (Ips typographus), which has caused substantial damage to Swedish forests. With the effects of global warming and climate change becoming increasingly apparent, understanding the factors influencing spruce mortality is crucial for effective forest management. This study aimed to investigate the impact of soil moisture and tree size on the susceptibility of Norway spruce to spruce bark beetle attacks. Using a combination of an Artificial Intelligence (AI) program detecting dead spruce trees and soil moisture data from the Swedish University of Agricultural Sciences (SLU), areas in Stockholm County with dead spruce trees were identified. Spatial analysis using Geographic Information System (GIS) tools and a field survey were conducted to evaluate the hypotheses: H1: Spruce mortality will be higher in areas occurring within mesic soil moisture class (1) than within mesic-moist soil moisture class (2) and moist-wet soil moisture class (3). H2: Large diameter spruce trees will have a higher mortality rate from spruce bark beetle than smaller diameter trees. H3: Spruce mortality will be higher in areas located inside nature reserves. The results confirmed that spruce mortality was higher in areas with dryer soil, Additionally, larger diameter spruce trees exhibited higher mortality rates from spruce bark beetle attacks than smaller trees. Contrary to the initial hypothesis, the study did not find a higher spruce mortality rate in areas located outside nature reserves. The conclusion of this study is that both tree size and soil moisture affect the Norway spruce’s ability to defend itself from spruce bark beetle attacks, but it does not confirm that spruce trees inside nature reserves have a greater mortality rate due to bark beetle attacks than the surrounding areas. The study has contributed to a better understanding of how spruce bark beetles choose trees. It has also provided more knowledge and validation of the Swedish Forest Agency AI program, which is trained to find dead spruce trees. The fact that this is a working tool opens a lot of possibilities for further use. The knowledge of soil moisture’s effect on spruce survival will also help forest managers make better choices when selecting areas for planting spruce. This will also benefit both the forest owners economically and help suppress the spread of spruce bark beetle.