Browsing by Author "Wahlman, Werna"

Norway spruce (Picea abies) is one of the most economically important tree species in northern and central Europe. Root rot caused by Heterobasidion annosum s.l. and European spruce bark beetle (Ips typographus) are major disturbance agents of Norway spruce, and likely to have even greater impact on spruce-dominated forests as climate warms. This thesis investigated the direct interaction between Heterobasidion root rot and I. typographus. The aim was to examine if presence of root rot and the stress it poses to a tree increases the risk for subsequent bark beetle attack. 442 Norway spruce trees from nine different stands were measured and included in this study. Before the final felling, symptoms caused by I. typographus were evaluated from each tree based on visual assessments of crown and stem conditions. After the final felling, the sample plots were relocated from the clearcut areas, and the stumps of the sampled trees were re-assessed for Heterobasidion root rot. Exploratory analysis and binomial Generalized Linear Mixed Model (GLMM) were used to analyze relationships between different explanatory variables and their affect to I. typographus infestation. The best predictors for I. typographus infestation were incidence of root rot and to a lesser extent, decreased 5-year diameter growth and larger diameter at breast height. 75% of root rot infected trees were also infested with I. typographus, and the amount of root rot was considerably higher within dead and severely infested trees compared to alive and moderately infested trees. Results suggest that root rot is one of the significant factors making trees susceptible to I. typographus infestation, especially when the population density of bark beetles is low and stable. Thus, protecting trees from root rot infection may help to protect trees from I. typographus infestation as well.