Climatologic and hydrologic influences on the oxygen isotope ratio of tree cellulose in coastal southern California during the late 20th century

The oxygen isotope ratio ( δ 18 O) of precipitation in continental, midlatitude regions is a complex measure of atmospheric dynamics and regional climate variability and can be preserved in geologic archives. However, continuous modern observations of precipitation δ 18 O in many midlatitude regions, particularly in the coastal western United States, are sparse. Here, tree‐ring cellulose δ 18 O from southern California is used to assess the potential of this proxy as an indicator of long‐term hydroclimate and atmospheric variability. From 1954 to 2004, we observed that cellulose δ 18 O was well replicated within a single stand of blue oak ( Quercus douglasii ) on interannual and decadal time scales. By using a forward mechanistic model, we demonstrate that cellulose δ 18 O is not driven solely by the oxygen isotope composition of precipitation at the site nor any other single hydroclimate variable. Instead, the interannual variability in cellulose δ 18 O prior to 1979 is positively correlated with growing season soil water δ 18 O and after 1979 is negatively correlated with relative humidity. In addition, 2 years (1983 and 1998) of anomalously low cellulose δ 18 O coincided with the wettest years in California and the strongest El Niño events of the late 20th century. For these years, decreased near surface evaporation and/or increased upper‐level condensation could account for the more depleted cellulose δ 18 O values. While blue oak cellulose δ 18 O is sensitive to atmospheric and hydroclimate variability, the varying temporal correlation between the cellulose δ 18 O and different environmental variables complicates any attempt to use the cellulose oxygen isotopes for reconstructions of climate variability beyond the calibration period.