Diurnal variations of needle water isotopic ratios in two pine species

Diurnal fluctuations of leaf water isotope ratios (delta O-18 and delta D) were measured for Jeffrey (Pinus jeffreyi Balf.) and lodgepole (Pinus contorta Douglas ex Louden) pine. Two trees per species were sampled every few hours on 15-16 October 2005 and 19-20 June 2006. Diurnal gas exchange was measured during the summer sampling. In fall 2005, leaf water delta O-18 ranged from 0.7 to 9.0aEuro degrees, and leaf water delta D ranged from -70 to -50aEuro degrees. In summer 2006, leaf water delta O-18 ranged from 7.7 to 20.7aEuro degrees, and leaf water delta D ranged from -61 to -24aEuro degrees. Diurnal variation of leaf water isotope values typically reached a maximum in early afternoon, began decreasing around midnight, and reached a minimum in mid-morning. Both periods showed a high degree of enrichment relative to source water, with leaf water-source water enrichments ranging up to 37.8aEuro degrees for delta O-18, and up to 95aEuro degrees for delta D. Leaf water enrichment varied by season with summer enrichment being greater than fall enrichment. A steady-state model (i.e., modified Craig-Gordon modeling) for leaf water isotope compositions did not provide a good fit to measured values of leaf water. In summer, a non-steady state model provided a better fit to the measured data than the steady-state model. Our findings demonstrate substantial leaf water enrichment above source water and diurnal variations in the isotopic composition of leaf water, which has application to understanding short-term variability of atmospheric gases (water vapor, CO2, O-2), climate studies based on the isotopic composition of tree rings, and ecosystem water fluxes.