The strength of the correlation between annually resolved tree-ring carbon isotope indices and climate was explored using a moving window technique. Significant variations in the correlation with fixed-date monthly temperature were observed using a 15-year moving window. To investigate the influence of phenological changes upon this relationship, a 20-day moving window was applied to daily temperature data to define the period of optimal tree response. For oaks growing in east England, the strongest association began 87 days after initial bud burst, which corresponds to the opening of the second flush of leaves. These results demonstrate that daily climatic data may be used to calibrate proxy-climate relationships. ¿ 2001 American Geophysical Union