Temporal stability of climate‐isotope relationships in tree rings of oak and pine (Ticino, Switzerland)

Abstract

Climate reconstructions based on stable isotopes in tree rings commonly rely on the assumption that climate‐isotope relations are stable over time. However, studies of tree ring growth have revealed trends thought to result from either physiological changes or changes in the climate‐growth relationship. We investigated whether or not similar trends exist for tree ring stable isotopic ratios using a statistical approach. Correlations between climate (temperature and precipitation amount) and tree ring cellulose δ13C and δ18O of oak and pine from Ticino, Switzerland, were calculated for the period AD 1660–2000. Climate calibration of tree rings was enabled by long‐term monthly resolved temperature and precipitation data sets on the basis of instrumental and documentary proxy data. Overall, five findings have been identified: (1) Isotopic ratios in tree rings most strongly reflect conditions of the current growing season, (2) temporally stable climate signals are found in pine δ13C only, (3) all other correlations between tree ring isotopes and climate are temporally unstable and characterized by shifts in correlation sign and strength, (4) climate signals in oak are strongest in the 20th century, and (5) tree ring δ13C reflects local climatic conditions while δ18O is influenced by large‐scale synoptic circulation. The nonstationary relationships observed could reflect changes in the relationship between the climate variables or a physiological adaptation to warmer conditions. Our results provide a cautionary note for the calibration of long tree ring series with 20th century relationships, at least for trees located at ecologically nonextreme sites.