Improving a tree-ring reconstruction from west-central Scandinavia: 900 years of warm-season temperatures

Abstract

Dendroclimatological sampling of Scots pine (Pinus sylvestris L.) has been made in the province of Jamtland, in the west-central Scandinavian mountains, since the 1970s. The tree-ring width (TRW) chronology spans several thousand years and has been used to reconstruct June–August temperatures back to 1632 bc. A maximum latewood density (MXD) dataset, covering the period ad 1107–1827 (with gap 1292–1315) was presented in the 1980s by Fritz Schweingruber. Here we combine these historical MXD data with recently collected MXD data covering ad 1292–2006 into a single reconstruction of April–September temperatures for the period ad 1107–2006. Regional curve standardization (RCS) provides more low-frequency variability than “non-RCS” and stronger correlation with local seasonal temperatures (51% variance explained). The MXD chronology shows a stronger relationship with temperatures than the TRW data, but the two chronologies show similar multi-decadal variations back to ad 1500. According to the MXD chronology, the period since ad 1930 and around ad 1150–1200 were the warmest during the last 900 years. Due to large uncertainties in the early part of the combined MXD chronology, it is not possible to conclude which period was the warmest. More sampling of trees growing near the tree-line is needed to further improve the MXD chronology.