The influence of climate on 13C/12C and 18O/16O ratios in tree ring cellulose of Pinus sylvestris L. growing in the central Scandinavian Mountains

The stable isotope composition of tree rings is known to contain valuable information of past climatic and environmental changes, which may be used as a complement to tree ring width and maximum latewood density in climate reconstructions. In this study we examine the character and strength of the climate signal captured in the δ18O and δ13C values of Scots pine (Pinus sylvestris L.) trees growing at the tree-line in the central Scandinavian Mountains. Between 4 and 14 trees were pooled to produce annual records of carbon and oxygen isotope ratios from tree ring cellulose, spanning the period AD 1736–2006. Weather conditions of the current growing season most strongly influenced the carbon and oxygen isotope ratios in a given tree ring. Both records showed positive correlations with temperature, sunshine and air pressure, and negative associations with precipitation. The overall climate signal in δ13C appeared to be slightly stronger than that of δ18O. Spatial correlation analysis with gridded instrumental data demonstrated that the carbon series captured the summer temperature signal in a broad region of mid-west Sweden and the eastern part of Norway, greatly exceeding the spatial coverage of the signal derived from pine ring widths growing in the central Scandinavian mountains. A weak relationship between δ13C and precipitation and a much stronger temperature and sunshine dependence may imply that photosynthetic rate rather than stomatal conductance is more important in controlling the inter-annual tree ring δ13C variability in the area.Moreover, it was shown that, the climate–δ18O and δ13C relationship was temporally unstable throughout the twentieth century, which was linked to large-scale shifts in climate that may have altered the isotope-climate dependence. Our results thus demonstrate that, stable isotopes in tree rings from maritime high-altitude Scandinavia can provide high-resolution regional climate information, especially regarding parameters associated with temperature. The non-stationary nature of the isotope–precipitation relationship may provide important information on past changes in the large-scale atmospheric circulation during summer. However, these issues needs to be further investigated before pine tree ring isotopes from this region can be confidently used in palaeoclimate reconstructions.

Research highlights
► Annual cellulose δ18O and δ13C values of Scots pine from the Scandinavian Mountains.
► Character and strength of the climate signal in the isotopic records is examined.
► Positive relation to temperature and sunshine, and negative to precipitation.
► Overall climate signal in δ13C is slightly stronger than that of δ18O.
► Temporally unstable associations with observed climate throughout the 20th century.