Climate variation and the stable carbon isotope composition of tree ring cellulose: an intercomparison of Quercus robur, Fagus sylvatica and Pinus silvestris

The relationship between climate parameters and the carbon stable isotope composition (δ¹³C), of annual tree ring cellulose is examined for three native British tree species; Common beech (Fagus sylvatica L.), Pedunculate oak (Quercus robur L.) and Scots pine (Pinus sylvestris L.). The last 100 annual tree rings of six trees, two of each species, were cut into slivers and the a-cellulose extracted. Annual δ¹³C values of each species were averaged to produce three species δ¹³C chronologies. These were compared with climate parameters from a nearby meteorological station. The carbon stable isotope discrimination (Δ¹³C) of pine is consistently lower, by approximately 2.5‰, than that of beech and oak. Although the exact cause of this offset cannot be identified, similar differences in carbon isotope ratios have been noted between other gymnosperm and angiosperm species and attributed to inherent physiological differences. As this offset is consistent, once centred around the same mean δ¹³C and Δ¹³C chronologies from these 3 species can be combined. Δ¹³C chronologies of the three species demonstrate strong cross-correlations in both high and low frequency fluctuations. Low frequency fluctuations, although consistent between species, show no direct climate relationship, and may be linked with physiological responses to increasing CO₂ concentrations. Significant correlations do exist between the high frequency δ¹³C fluctuations and climate parameters. The high frequency δ¹³C series of all three species are most significantly correlated with the same two climate parameters and have the same seasonal timing; July — October average maximum temperature and June — September average relative humidity. Pine δ¹³C is the most responsive species to climate changes and displays the most significant correlations with all the climate parameters studied. However, an average series of all three high frequency species δ¹³C series shows the most significant correlations with climate. Assuming these relationships are consistent spatially and temporally,high frequency δ¹³C chronologies from the three species studied are climatically comparable and can be combined to reconstruct the same climatic information.