Abstract
Annual latewood δ18O values from oak (Quercus robur L.) are presented for the period 1895–1994 for four trees growing in eastern England. We demonstrate, using the δ18O values of precipitation from Wallingford (1982–1994) and the longer time-series from Groningen (1965–1994), that the δ18O values of winter precipitation had the strongest influence on the δ18O values of tree-ring cellulose. The influence of winter precipitation on the spring/summer formed latewood may be explained by several factors: most soil water is replenished during the winter, water may take several months to penetrate the unsaturated zone, and oaks may sample water from lower in the profile during times of water stress. The δ18O values of oak latewood cellulose were also significantly (P<0.01) correlated with summer relative humidity. Therefore, although precipitation δ18O values may be altered by partial isotopic exchange with xylem water and the influence of stomatal conductance upon evapotranspiration, some of the original δ18O signature remains.
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