The climatic significance of δ¹³C in subalpine spruces (Lötschental, Swiss Alps)

Abstract

Few stable carbon isotope studies exist from high mountain regions which consider both climatological and ecological influences. This study is the first presenting δ¹³C tree ring records from the subalpine vegetation belt of the European Alps (Lötschental, Switzerland). Pooled late wood samples from several trees (Picea abies) per site were used for studies of spatial site comparisons with respect to altitude (upper timberline/valley floor), exposure (N/S) and soil moisture (dry/moist). This investigation aims to assess how much these site conditions influence the climatic signal of δ¹³C. The δ¹³C site records (1946–1995 AD, late wood cellulose) show a decreasing long-term trend reflecting the atmospheric δ¹³C decrease during this period. We apply a new method for the correction of this anthropogenically induced CO₂ trend which considers changes in the atmospheric δ¹³C source value and plant physiological reaction due to changes in the partial pressure of atmospheric CO₂. The δ¹³C relationship to all investigated months' climatic parameters (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_PPT=−0.75, r_RH=−0.79). In contrast to tree ring width and density studies the observed temperature signal is not related to the altitude of the sample sites. The precipitation signal extracted from the carbon isotope time series increases with decreasing altitude and it remains strong at the upper timber line. This indicates the suitability of this isotope proxy for reconstruction of atmospheric humidity. Single extreme events (pointer years) provide stronger and more uniform reactions for dry—warm than for cool—humid summer conditions. Furthermore, the sites with moderately dry or moist soil conditions react more strongly and consistently than the extremely dry and moist sites at high elevation. Site exposure influences the absolute δ¹³C values (S-exposure high versus N-exposure low), but does not necessarily obscure the climatic signal of the stable isotope records.