Tree-ring sequences from 65 living and subfossil pines (Pinus sylvestris) near the polar timberline in Scandinavia and from 202 spruces (Picea abies) either living or found in historic buildings and stemming from the timberline area of the Swiss Alps, which is cold-moist in summer, were radiodensitometrically analysed and used to construct chronologies. The Scandinavian curve stretches back to A.D. 481, that from the Alps to A.D. 982. Summer temperatures can be reconstructed from maximum density. The sequences from the boreal zone reflect warm and cold summers clearly, while, for biological reasons, those from the Alps only reflect warm summers with certainty. Climatic extremes in Scandinavia do not coincide with those in the Alps, but decennial fluctuations display some synchroneities, the most marked occurring in the fifteenth to sixteenth centuries.
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Schweingruber et al. (1988) studied this question.