Shifts in radial growth responses of coastal Picea abies induced by climatic change during the 20th century, central Norway

Climate-growth relationships since the end of the 19(th) century were analysed by using regional climate data and North Atlantic Oscillation index data, together with spruce (Picea abies (L) Karst.) tree-ring data from an altitudinal gradient close to the coast in central Norway (64degreesN). Correlation and response function analyses were used to decipher both spatial and temporal diameter growth responses. A positive response for May and June temperatures dominated along the entire gradient, but the importance of individual months shifted through time. In periods when May and June temperatures were above their long-term means, the importance of other climate factors increased. Winter climate was significantly related to radial tree growth mainly for periods up to the 1940s, but not during the second half of the 20(th) century. This pattern was principally caused by changes in the response to winter precipitation. The summer climate acquired increased importance in periods with warm and moist winters. The climate-growth relationship in this study changed continuously and dynamically during the studied period: this might be a general phenomenon that should be carefully considered in dendroclimatological studies. In scenarios for forest development, it is of vital importance to build on detailed knowledge of growth responses to multiple climate variables for all seasons, particularly where the oceanic influence is predicted to be spatially extended.