Norway spruce is economically the most important tree species in Northern Europe; therefore, its productivity and vitality have key role in the forest management of this region. Tree height is directly linked to the productivity of the stand and is affected by growing conditions and tree genetics. Height increment in young stands determines the further development of the stand therefore, it is important to evaluate the effect of various factors on tree height growth during this stage and assess

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... the possibilities to improve it with silvicultural measures and tree breeding. This doctoral thesis aims to assess the effect of microenvironmental conditions and genetics on the height increment of Norway spruce at a juvenile age in planted hemiboreal forests in Latvia. The doctoral thesis summarizes seven thematically linked scientific publications, and their results indicate the possibilities for substantial improvement of the growth of Norway spruce at a juvenile age. Tree height is considerably and significantly affected by the type of soil preparation and soil fertilization with wood ash a year before planting or by applying additional fertilizer during the planting process. Furthermore, the effect of these forest management measures lasts for at least the next 10 years. Providing favorable microenvironmental conditions considerably increases the proportion of spruces with lammas shoots, which in turn, increases the total height increment of these trees. A positive feedback loop exists between the presence of apical lammas shoots and the tree height increment, leading to increased tree height in young stands. The occurrence of lammas shoots and the intra-annual development of the height increment is genetically determined. Growth intensity (mm per day) for progenies from open-pollinated families of plus-trees with larger height increment culminated later in the season, had higher growth intensity at the active growth phase and remained high for a longer period.