Impact of climatic variation on growth of Fagus sylvatica and Picea abies in Southern Sweden

Growth and its relationship with recent competitive performance of Norway spruce and European beech were investigated in semi-natural mixed forests in southern Sweden. Dendroclimatological methods revealed the main influencing climate variables. The radial growth of beech is mainly determined by the late summer conditions from the previous year. The July temperature has become increasingly more important since the 1950s and even more significant since the 1990s. Before the 1950s, the main factor for spruce growth was the summer precipitation. This influence has nearly vanished, and the spruce trees react more distinctly to temperature of the previous summer. This shift in the climate signal indicates a sensitive response of spruce growth to changed conditions and points to a distinctly increased sensitiveness of spruce under future conditions. The competition interactions between mainly dominating spruce and mostly subordinate beech are already changing. European beech growth potential could benefit from warm summers and prolonged vegetation periods and increase its mean diameter in relation to spruce. In contrast, Norway spruce is more threatened by increased abiotic and biotic disturbances. The higher competitive vigour and changed vitality of beech and the lower vitality of spruce may alter the distribution boundaries of both species in Sweden.