The climatic significance of delta C-13 in subalpine spruces (Lotschental,Swiss Alps) - A case study with respect to altitude, exposure and soil moisture

Few stable carbon isotope studies exist from high mountain regions which co nsider both climatological and ecological influences. This study is the fir st presenting delta C-13 tree ring records from the subalpine vegetation be lt of the European Alps (Lotschental. Switzerland), Pooled late wood sample s from several trees (Picea abies) per site were used for studies of spatia l site comparisons with respect to altitude (upper timberline valley floor) , exposure (NS) and soil Moisture (dry . moist). This investigation aims to assess how much these site conditions influence the climatic signal of del ta C-13. The delta C-13 site records (1946-1995 AD, late wood cellulose) sh ow a decreasing long-term trend reflecting the atmospheric delta C-13 decre ase during this period. We apply a new method for the correction of this an thropogenically induced CO2 trend which considers changes in the atmospheri c WC source value and plant physiological reaction due to changes in the pa rtial pressure of atmospheric CO2. The delta C-13 relationship to all inves tigated months' climatic parameters I temperature, precipitation, relative air humidity was found to be very strong with highest correlations in July, August, the time of late wood development (maximum r(T) = 0.74, r(PPJ) = - 0.75, r(RH) = -0.79). In contrast to tree ring width and density Studies th e observed temperature signal is not related to the altitude of the sample sites, The precipitation signal extracted from the carbon isotope time seri es increases with decreasing altitude and it remains strong at the upper ti mber line. This indicates the suitability of this isotope proxy for reconst ruction of atmospheric humidity. Single extreme events (pointer years) prov ide stronger and more uniform reactions for dry warm than for cool-humid su mmer conditions. Furthermore. the sites with moderately dry or moist soil c onditions react more strongly and consistently than the extremely dry and m oist sites at high elevation. Site exposure influences the absolute delta C -13 values (S-exposure high versus N-exposure low), but does not necessaril y obscure the climatic signal of the stable isotope records.