STABLE CARBON ISOTOPES IN TREE-RINGS OF BEECH - CLIMATIC VERSUS SITE-RELATED INFLUENCES

Stable carbon isotopes in tree rings are a promising tool in palaeocli mate research, provided attempts are made to disentangle climatic from local effects (e.g. soil properties, competition, light). The C-13/C- 12 variations in cellulose of tree rings of beech (Fagus sylvatica) we re determined at several sites in the Swiss Central Plateau covering t he last 50 years. We chose sites which differ in moisture conditions a nd sampled cores from four to six trees per site. The mean C-13/C-12 s eries from the different dry sites (distant by up to 40 km) are closel y interrelated suggesting a common external cause. Correlation analysi s with climate data proved the total precipitation in the months May, June and July to have the strongest effect on the carbon isotopes (r = -0.73). This result is in agreement with the commonly used model whic h relates the isotope discrimination to the water use efficiency. On t he other hand, the isotope series of the wet sites are not as well cor related to the climate. At two of the sites (a dry and a humid) tree r ing width suddenly increased. We used this effect as a test-case to st udy the influence of local growth conditions on the climate-isotope re lationship.