Seasonal stable-carbon isotope variability in tree rings: possible paleoenvironmental signals

Plant stable-carbon isotope fractionation models indicate that δ¹³C of atmospheric CO₂, CO₂ concentration, light and moisture stress, among other factors, may potentially affect the δ¹³C of fixed carbon. Seasonal δ¹³C variations in tree rings may therefore represent a new tool for paleoenvironmental reconstruction. The seasonal δ¹³C patterns in growth rings exist in trees (conifer and hardwood) from tropical and temperate localities, and isotopic variation is even seen in trees which are lacking or have poorly-defined rings. The patterns in different rings from a single tree are usually similar, although differences in amplitude and timing of maxima and/or minima are common. Some of the differences may be attributable to radial variation of the δ¹³C patterns which was found to be substantial in a severely water-stressed tree. Correlations of δ¹³C patterns and corresponding seasonal environmental variation for one well-monitored tree showed greater response of the δ¹³C change to measured soil moisture and precipitation than to temperature, calculated soil moisture, solar radiation, or net photosynthesis (as estimated from CO₂ release).