Summary The carbon and oxygen isotopic composition of tree-ring cellulose was examined in ponderosa pine (Pinus ponderosa Dougl.) trees in the western USA to study seasonal patterns of precipitation

tree-ring cellulose was examined in ponderosa pine (Pinus ponderosa Dougl.) trees in the western USA to study seasonal patterns of precipitation inputs. Two sites (California and Oregon) had minimal summer rainfall inputs, whereas a third site (Arizona) received as much as 70% of its annual precipitation during the summer months (North American monsoon). For the Arizona site, both the δO and δC values of latewood cellulose increased as the fraction of annual precipitation occurring in the summer (July through September) increased. There were no trends in latewood cellulose δO with the absolute amount of summer rain at any site. The δC composition of latewood cellulose declined with increasing total water year precipitation for all sites. Years with below-average total precipitation tended to have a higher proportion of their annual water inputs during the summer months. Relative humidity was negatively correlated with latewood cellulose δC at all sites. Trees at the Arizona site produced latewood cellulose that was significantly more enriched in O compared with trees at the Oregon or California site, implying a greater reliance on an O-enriched water source. Thus, tree-ring records of cellulose δO and δC may provide useful proxy information about seasonal precipitation inputs and the variability and intensity of the North American monsoon.