Regional differences in climatic responses of Norway spruce (Picea abies L. Karst) growth in Norway

We examined growth responses of Norway spruce using tree-ring series from increment cores and monthly climate variables over the period 1900–1998. The 1398 cores were selected from 588 plots scattered all over Norway. We correlated tree-ring indices with temperature, precipitation, Palmer drought severity index and length of the growing season. The weather in June had the largest influence on ring widths. However, two different, and almost opposite, response types were found: Tree growth was restricted by June precipitation in the lowlands in southeastern Norway, but by the June temperature in other regions and at high altitudes. In order to define the shift between these two main response types, we correlated response functions with various 30-year mean climatic variables, including humidity and aridity indices. The 30-year mean June temperature was the variable most clearly showing this shift in response, with a threshold at 12–13 °C. At sites with normal temperature below this threshold, spruce responded positively to unusually warm and dry June months, and vice versa.