| Summary: | 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 (<I>Fagus silvaatica</I> L.), pedunculate oak (<I>Quercus robur</I> L.) and scots pine (<I>Pinus sylvestris</I> L.), for the period 1895 to 1994, to examine the responses of these trees to recent changes in climate and atmospheric CO<SUB>2</SUB>. The high frequency δ<SUP>13</SUP>C 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 δ<SUP>13</SUP>C 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 δ<SUP>13</SUP>C series is the most responsive of the species to climate fluctuations, it is also noted that the combined δ<SUP>13</SUP>C 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 δ<SUP>13</SUP>C 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 δ<SUP>13</SUP>C data to indicators of leaf gas exchange, it is evident that the rapid increasing trend in atmospheric CO<SUB>2</SUB> concentration ([CO<SUB>2</SUB>]), 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 CO<SUB>2</SUB> in the leaf (c<SUB>i</SUB>). The results suggest that the increasing [CO<SUB>2</SUB>] induced all three tree species to increase assimilation rate and reduce stomatal conductance to such an extent that c<SUB>i</SUB> 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 C<SUB>3</SUB> plants to increased [CO<SUB>2</SUB>] and it is more likely that both assimilation rate and stomatal conductance have changed.
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