LOW- AND HIGH FREQUENCY MIDSUMMER TEMPERATURE CHANGES FROM NORTHERN FENNOSCANDIAN FOREST-LIMIT SCOTS PINE TREE-RINGS

Mid-summer (July) temperatures were inferred from the supra-long ring-width chronology of Scots pine (Pinus sylvestris L.) for northern forest-limit of Fennoscandia (between 66°-70° N and 21°-30° E), covering the last seven and a half millennia. The chronology was built using Regional Curve Standardization (RCS), which allows both, short- and long-term (high and low frequency) variability to be extracted from this annually resolved record. Annual values of ring-widths from 1087 samples (trees) in all including living trees, snags and subfossil timber collected from small lakes were averaged into this chronology. Homogeneity of tree-ring data over the geographical distribution has been demonstrated. Highly consistent tree-ring chronologies were built from diverse sites as well as from various age-classes of pine trees.

Many (but not all) of the negatively anomalous years in the pine chronology correlate with major volcanic eruptions. This can be understood as an impact of decreased summer temperatures due to volcanic forcing over a single or various years after the eruption.

It has been occasionally argued that low-frequency variation is not preserved in tree-ring chronologies. Here we compared the centennial tree-ring index width previously published reconstructed ground surface temperature (GST) variations from the same region for the last two millennia. GST record is supposed to reflect summer time temperatures in line with tree-rings. It can be observed, that centennial tree-ring index and GST variations fluctuate rather synchronously thought the time, and, one could claim that century-long variation in tree-rings is preset and actual.

The present pine chronology is the longest annual, absolutely dated summer-temperature proxy so far published. It was shown here that using the modern methodology of chronology building, tree-ring chronologies reveal not only high-frequency but also very low-frequency variation of temperatures.