Influence of precipitation seasonality on pinon pine cellulose δD values

The influence of seasonal to interannual climate variations on cellulose hydrogen isotopic composition (delta D) was assessed by analysing tree rings and needles of pinon pine (Pinus edulis and P. monophylla). Sites spanned a gradient of decreasing summer precipitation, from New Mexico to Arizona to Nevada. Tree rings were divided into earlywood, latewood and whole-year increments, and annual cohorts of needles were collected. The study period (1989-96) included two La Nina events (1989, 1996) and a prolonged El Nino event (1991-95). Winter and spring moisture conditions were strongly related to October-March Southern Oscillation Index (SOI) in New Mexico and Arizona, with above-average precipitation occurring in El Nino years. Wood delta D values at these sites were correlated with winter and spring moisture conditions. Needle delta D values were correlated with summer moisture conditions in New Mexico and with winter moisture and SOI in Arizona. Low cellulose delta D values observed from 1991 to 1993 in both wood and needles occurred during wet El Nino years, whereas high delta D values in needles were present during the dry, La Nina years of 1989 and 1996. North-eastern Nevada does not receive precipitation anomalies related to ENSO, and thus cellulose delta D values did not reflect the ENSO pattern observed at the other sites. Cellulose delta D values were strongly, inversely correlated with relative humidity variations at all sites, as predicted by a mechanistic model. Contrary to predictions from the same model and observations from more mesic areas, time series of cellulose delta D values were not directly correlated with interannual or seasonal variations in precipitation delta D values or temperature at any of the sites. On a regional basis, however, mean delta D values in needles and wood were correlated with mean annual temperature and delta D values of precipitation. This suggests that temporal averaging may bias relationships between biological systems and climate.