Shifts in the δ ¹³ C composition of Salix herbacea L. leaves in response to spatial and temporal gradients of atmospheric CO ₂ concentration

We investigated the response of three carbon isotope related parameters ($\delta ^{13}$C, discrimination and p$_{i}$/p$_{a}$ ratio) determined from Salix herbacea leaves to an altitudinal gradient in Austria and to short- (200 years) and long-term (> 140 000 years) increases in CO$_{2}$ concentration. The altitudinal results showed leaf $\delta ^{13}$C declined with increasing altitude. This supports the view that local trends in leaf $\delta ^{13}$C may follow quite different patterns to those observed globally. The effects of short-term CO$_{2}$ increases were investigated by analysis of dated herbarium material collected over the past two centuries. Leaf $\delta ^{13}$C declined in response to CO$_{2}$ increases of the past 200 years, which probably reflects the anthropogenic increase of atmospheric $^{12}$C as a result of fossil fuel burning and deforestation. Quaternary leaf macrofossils were analysed to examine the response of leaf $\delta ^{13}$C to long-term alterations in CO$_{2}$ and climate represented by a glacial cycle. The results suggest leaf $\delta ^{13}$C reflects the palaeotemperature under which the leaves formed, low temperatures being associated with high $\delta ^{13}$C. This is consistent both with observations from a global latitudinal survey of leaf $\delta ^{13}$C ranging from equatorial to polar regions and with experimental measurements made on plants grown at low temperatures. Comparisons of the general temperature trends of the Devensian late-glacial inferred from fossil leaf $\delta ^{13}$C values with palaeotemperature reconstructions from beetle remains and pollen show a similar overall pattern of events. Quaternary fossil leaves may therefore provide evidence for atmospheric CO$_{2}$ change (from the stomatal density record) and palaeotemperatures (from leaf $\delta ^{13}$C content) which together offer the exciting possibility of determining the comparative timing of both events.