Drivers of annual variation in tree growth and forest sensitivity to storm damage in Finland

In this thesis, my aim is to study the drivers of tree growth variation and forest predisposition to storm disturbance in Finland. More specifically, the thesis aims to answer the following research questions: (1) What is the role of tree provenance in the climatic control of radial growth variation in Norway spruce (Picea abies (L.) Karst. )? (2) How do weather conditions outside of growing season affect radial growth variation in Norway spruce and Scots pine (Pinus sylvestris L.)? (3) How are forest properties, forest management and abiotic environmental factors connected to the storm damage probability of forest stands and individual trees? (4) Do the same factors affect stand-level damage probability in different storm types: autumn extra-tropical cyclones and summer thunder storms? (5) Is fine-scale topographic information connected to tree-level storm damage probability? The thesis addresses these questions by analyzing extensive empirical data sets. The different climatic drivers of Norway spruce provenances were studied using a tree-ring data from seven Norway spruce provenance experiments in Finland, established already in the 1930s and located in different climatic conditions and containing a large variety of provenances. The effects of non-growing season climatic conditions on tree-growth were studied by comparing tree-ring data from unmanaged forests with variables describing winter conditions and modelled tree frost hardiness levels. Storm damage probability on stand and tree levels was examined with storm damage data sets collected at Finnish National Forest Inventory plots after major storms. A statistical modeling approach was used throughout the thesis, utilizing methods such as generalized mixed effects models and statistical distributions of extreme values. The results revealed provenance differences in radial growth variation in Norway spruce. Provenances differed most in their growth response to winter temperature, as adaptation to low winter temperatures was weaker in Central European than in Northern European provenances. While cold winter temperatures were associated with frost damage and declined radial growth in Central European spruce provenances transferred north, simple temperature variables were not sufficient in studying the responses of trees to conditions outside of the growing season in natural forests. Instead, the results showed signs of reduced growth after events of insufficient frost hardiness levels and winters with high frost sum of snowless days. This indicates that accounting for a complexity of factors, such as frost hardiness of trees, snow cover and soil frost, is needed to understand the implications of weather conditions outside of growing season to tree growth. Stand-level damage probability was affected by stand characteristics and previous management operations. On tree-level, damage probability was connected to type of the tree species (conifer or broad-leaved) and tree height as well as recent changes in wind exposure and wood decay in the stand. Storm damage probability in autumn storms and summer