Stable isotopes in tree rings : biosensors of climate and atmospheric carbon-dioxide variations

The objective of this dissertation is to use the annual tree ring width and cellulose carbon and hydrogen stable compositions off three native British tree species: common beech (Fagus silvaatica L.), pedunculate oak (Quercus robur L.) and scots pine (Pinus sylvestris L.), for the period 1895 to 1994, to examine the responses of these trees to recent changes in climate and atmospheric CO₂. The high frequency δ¹³C series of all three species display the most significant correlations with the climate parameters and, using simple regression models, it is also evident that this tree ring measure provides the most accurate climate reconstructions. Vapour pressure deficit, averaged for the months July to September, is the climate parameter most significantly correlated with the δ¹³C data. However, it is shown that this may not necssarity mean that the trees are responding dominantly to this parameter. Although it is apparent that the pine species δ¹³C series is the most responsive of the species to climate fluctuations, it is also noted that the combined δ¹³C signal from all three tree species displays an enhanced climate signal. Superimposed on the common climate signals are long term trends that are comparable for the δ¹³C series of all three species and the δD of the pine trees, but do not show any common signal for the other δD series or any of the ring width series. Converting the δ¹³C data to indicators of leaf gas exchange, it is evident that the rapid increasing trend in atmospheric CO₂ concentration ([CO₂]), that began around 1930, coincides with a rising trend in leaf intrinsic water use efficiency but does not coincide with a change in the internal concentration of CO₂ in the leaf (c_i). The results suggest that the increasing [CO₂] induced all three tree species to increase assimilation rate and reduce stomatal conductance to such an extent that c_i remained initially constant. As there are no consistent increases in ring widths during this period, the dominant response could be inferred to be stomatal conductance. However, preferential partitioning of photosynthates to other plant organs, especially roots, is a common response of C₃ plants to increased [CO₂] and it is more likely that both assimilation rate and stomatal conductance have changed.