Comparing Scots pine tree-ring proxies and detrending methods among sites in Jämtland, west-central Scandinavia

Scots pine tree-ring width (TRW) data from Jämtland in the Central Scandinavian Mountains has been used to reconstruct summer temperatures back to 1630 BC. However, it was recently shown that this reconstruction was of limited spatial importance. In this paper, we aim to explain this limitation in the TRW data as a temperature proxy, as well as assess the temperature information from new maximum latewood density (MXD) data. Furthermore, the effect of two standardization methods is evaluated: regional curve standardization (RCS) and a more traditional standardization, termed “non-RCS” standardization. Three TRW and two MXD sites were analyzed. Our results showed that despite the proximity to the Norwegian Sea, the MXD data is a powerful temperature proxy. Difference among sites in TRW data, especially on decadal timescales, together with a lower temperature association, suggests that other factors, such as changes in the local climate regimes, weakens the temperature signal. In general the RCS method overestimates pine growth trends in the latter half of the twentieth century, a feature not seen when using “non-RCS” standardization. This is likely due to an age-bias of older trees in most recent parts of the tree-ring chronologies. This effect will have consequences when reconstructing climate with tree-ring data. To overcome this problem, all age-classes should be represented throughout a chronology. If this is not possible, the use of “non-RCS” standardization is recommended, although this method results in a loss of low-frequency variability.