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.