SHIFTS IN THE DELTA-C-13 COMPOSITION OF SALIX-HERBACEA L LEAVES IN RESPONSE TO SPATIAL AND TEMPORAL GRADIENTS OF ATMOSPHERIC CO2 CONCENTRATION

We investigated the response of three carbon isotope related parameters (deltaC-13, 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 (> 140000 years) increases in CO2 concentration. The altitudinal results showed leaf deltaC-13 declined with increasing altitude. This supports the view that local trends in leaf deltaC-13 may follow quite different patterns to those observed globally. The effects of short-term CO2 increases were investigated by analysis of dated herbarium material collected over the past two centuries. Leaf deltaC-13 declined in response to CO2 increases of the past 200 years, which probably reflects the anthropogenic increase of atmospheric C-12 as a result of fossil fuel burning and deforestation. Quaternary leaf macrofossils were analysed to examine the response of leaf deltaC-13 to long-term alterations in CO2 and climate represented by a glacial cycle. The results suggest leaf deltaC-13 reflects the palaeotemperature under which the leaves formed, low temperatures being associated with high deltaC-13. This is consistent both with observations from a global latitudinal survey of leaf deltaC-13 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 deltaC-13 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 CO2 change (from the stomatal density record) and palaeotemperatures (from leaf deltaC-13 content) which together offer the exciting possibility of determining the comparative timing of both events.