Browsing by Author "Turunen, Tarja"

Norway spruce (Picea abies (L.) Karst.) is one of the economically most important tree species in Finland. It is known to be drought-sensitive species and expected to suffer from the warming climate. In addition, warmer temperatures benefit pest insect Eurasian spruce bark beetle (Ips typographus L.) and pathogen Heterobasidion parviporum, which both use Norway spruce as their host and can make the future of Norway spuce in Finland even more difficult. In this thesis, adult Norway spruce mortality was studied from false colour aerial photographs taken in years between 2010 and 2021. Dead trees were detected from the photos by visual inspection, and mortality was calculated based on the difference in the number of dead trees in the photos from different years. The aim was to find out if Norway spruce mortality in Finland had increased over time, and what were the factors that had been driving tree mortality. The results indicate that tree mortality was the highest in the last third of the studied 10-year period, so it was concluded that tree mortality had increased over time. Various possible tree mortality drivers were analysed and found to be connected to tree mortality. Each driver was analysed individually by testing correlation with tree mortality. In addition, linear regression analysis and segmented linear regression with one breakpoint were used with the continuous variables. Increased tree mortality correlated with higher stand mean age, mean height, mean diamater, and mean volume, supporting the findings in earlier research. Mortality was connected to the proportion of different tree species in the stand: the higher the proportion of spruce, the higher the mortality, and the higher the proportion of deciduous trees, the lower the mortality. Of different fertility classes, tree mortality was the highest in the second most fertile class, herb-rich heat forest, and mortality decreased with decreasing fertility. Dead trees were also found to be located closer to stand edges than the stand centroid. Increased temperature resulted in increased mortality. Increased vapour pressure deficit (VPD) and drought, which was analysed with Standardized Precipitation Evapotranspiration Index (SPEI) of different time scales, were also connected with increased tree mortality. Further research is required for understanding and quantifying the joint effect of all the interacting mortality drivers. Nevertheless, it seems that for Norway spruce, the warmer future with increased mortality is already here, and it should be taken into consideration in forest management. Favouring mixed stands could be one of the solutions to help Norway spruce survive in the warming climate.