The climate-carbon isotope relationship in tree rings and the significance of site conditions

Considerable complexity exists regarding the relationship between tree ring δ3C and climate. We proceed from the theoretically derived and experimentally confirmed finding that the relative humidity of the atmosphere but also the soil water content can influence the stomatal opening of plant leaves and consequently the δ3C of photosynthetically fixed carbon. Therefore, the potential to reconstruct humidity variations by δ3C in tree rings should depend on the water conditions at the site where the trees are growing. We analysed δ3C series (3-year ring samples) of cellulose of beech trees (Fagus silvatica) covering the time period from 1934 to 1989 for 3 sites in Switzerland: a relatively dry and a humid site close together and another dry site 30 km distant. The δ3C series from the two dry sites are closely interrelated and are inversely correlated to the total precipitation amount of the months May + June + July This is most expressed for first differences, i.e., differences of consecutive values, while the long-term trends are more strongly influenced by biological effects. A multivariate analysis shows that at the dry sites, δ3C 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 δ3C values for beech, pine and spruce trees from sites differing in soil moisture conditions. For all species we found that the drier the sites the more positive the tree δ3C values are. We conclude that at relatively dry sites in temperate-moist climatic conditions, short-term precipitation variations can be reconstructed by δ3C measurement on tree rings of beech and probably also of the conifers pine and spruce.