ABSRACTIn order to predict accurately how elevated atmospheric CO 2 concentrations will affect the global carbon cycle, it is necessary to know how trees respond to increasing CO 2 concentrations. In this paper we examine the response over the period AD 1895 -1994 of three tree species growing across northern Europe to increases in atmospheric CO 2 concentrations using parameters derived from stable carbon isotope ratios of trunk cellulose. Using the isotope data we calculate values of intrinsic water-use efficiency (IWUE) and intercellular CO 2 concentrations in the leaf (c i ).Our results show that trees have responded to higher levels of atmospheric CO 2 by increasing IWUE whilst generally maintaining constant c i values. However, the IWUE of most of the trees in this study has not continued to rise in line with increasing atmospheric CO 2 . This behaviour has implications for estimations of future terrestrial carbon storage.
the dry site (Sanddngham) than at the wet site (Babingley), but the differences are not statistically significant. These results illustrate the need for routine signal quantification in isotope records and hence a requirement for between-tree replication of isotope series in future studies. High-frequency (year-to-year) interseries correlation is shown to be relatively strong, indicating that only small numbers of replicate series are needed to represent interannual isotope variability accurately. However, common signal variance is diminished at lower (decadal and longer period) frequencies. This implies a neexl for increased sample replication in order to achieve chronology confidence equivalent to that routinely produced for simple ring width data. This work demonstrates that significant high-frequency climate signals are contained in isotopic measurements of trees whose ring widths contain little or no such information.
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