Stable carbon isotope ratios of Pinus sylvestris from northern Finland and the potential for extracting a climate signal from long Fennoscandian chronologies

Stable carbon isotope ratios were measured on the latewood cellulose of 36 Scots pine trees from four sites in northern Finland. δ¹³C values were corrected for changes in the δ¹³C of air and de-trended to remove the effect of tree age. Simple linear and multivariate correlations were used to determine the nature and strength of any climate signal. At three sites, the dominant controls are, in descending order, summer sunshine, temperature and antecedent precipitation. At the fourth site, the dominant controls are antecedent precipitation and air relative humidity, while summer sunshine and temperature appear unimportant. δ¹³C values record changes in the concentration of CO₂ in the stomatal chambers, which reflects the balance between stomatal conductance and photosynthetic rate. Photosynthetic rate is controlled primarily by photon flux (sunshine) and temperature, suggesting that this dominates at three of the sites, whereas stomatal conductance is controlled by air humidity and soil moisture status (antecedent precipitation), suggesting that it dominates at the fourth site. The balance between these controls varies spatially and it is likely to have varied locally during the Holocene. Without some independent estimate of either stomatal conductance or photosynthetic rate, there is limited potential for using δ¹³C alone to extract a clear and consistent climate signal from the long Fennoscandian pine chronologies.