THE CLIMATE-CARBON ISOTOPE RELATIONSHIP IN TREE-RINGS AND THE SIGNIFICANCE OF SITE CONDITIONS

Considerable complexity exists regarding the relationship between tree ring delta(13)C and climate. We proceed from the theoretically derive d and experimentally confirmed finding that the relative humidity of t he atmosphere but also the soil water content can influence the stomat al opening of plant leaves and consequently the delta(13)C of photosyn thetically fixed carbon. Therefore, the potential to reconstruct humid ity variations by delta(13)C in tree rings should depend on the water conditions at the site where the trees are growing. We analysed delta( 13)C series (3-year ring samples) of cellulose of beech trees (Fagus s ilvatica) covering the time period from 1934 to 1989 for 3 sites in Sw itzerland: a relatively dry and a humid site close together and anothe r dry site 30 km distant. The delta(13)C series from the two dry sites are closely interrelated and are inversely correlated to the total pr ecipitation amount of the months May + June + July. This is most expre ssed for first differences, i.e., differences of consecutive values, w hile the long-term trends are more strongly influenced by biological e ffects. A multivariate analysis shows that at the dry sites. delta(13) C is relatively high for dry/warm summers and low for cool/wet summers . The respective correlations for the humid site are less distinct. In addition, we compared average delta(13)C values for beech, pine and s pruce trees from sites differing in soil moisture conditions. For all species we found that the drier the sites the more positive the tree d elta(13)C values are. We conclude that at relatively dry sites in temp erate-moist climatic conditions. short-term precipitation variations c an be reconstructed by delta(13)C measurement on tree rings of beech a nd probably also of the conifers pine and spruce.