Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2010
Publication Date: 3/14/2010
Citation: Snyder, K.A., Monnar, R., Poulson, S.R., Hartsough, P., Biondi, F. 2010. Diel Variations in Needle Water Isotopic Composition in Two Pine Species. Tree: Structure and Function. 24:585-595. Interpretive Summary: The focus of the current study was to understand variations in the stable isotopic ratios of leaf water in different species of pine trees. The goal is to understand how the stable isotopic composition of water in pine needles is recorded by these different pine species because these isotopic values are often used as long-term indicators that integrate complex ecological processes. Since these isotope values are often difficult and costly to measure, leaf water isotope models based on a mechanistic understanding are essential. This study provides empirical data to determine which theoretical model is appropriate for these pine forests. Additionally tree rings from pine trees have been used to model past climates, in order to determine how accurate these models are, we need to understand how pine trees actively assimilate water inputs and the stable isotopic values in leaf water are a core part of this type of tree ring analyses. Therefore developing an understanding of the factors that affect leaf water isotope values in actively growing trees will contribute to our ability to model past climates.
Technical Abstract: The diurnal fluctuations of leaf water stable isotopes (d18O and dD) were measured for Jeffrey (Pinus jeffreyi) and lodgepole (Pinus contorta) pine trees. Two trees per species were sampled every few hours on October 15-16, 2005 and June 19-20, 2006. Diurnal gas exchange (stomatal conductance, transpiration, and net CO2 assimilation) was measured during the summer diurnal cycle. In fall 2005, leaf water d18O ranged from 0.7 to 9.0‰, and leaf water dD ranged from –70 to –50‰. In summer 2006, leaf water d18O ranged from 7.7 to 20.7‰, and leaf water dD ranged from –61 to –24‰. Diurnal variation of leaf water isotope values typically reached a maximum in the early afternoon, began decreasing around midnight, and reached a minimum in mid-morning. Both sampling periods showed a high degree of enrichment relative to source water, with leaf water-source water enrichments ranging up to 37.8‰ for d18O, and up to 95‰ for dD. However, leaf water isotopic 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 the measured values of leaf water. In the 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 isotopic composition of leaf water, which has application to understanding short-term variability of atmospheric gases (water vapor, CO2, O2), climate studies based on the isotopic composition of plant tissues, and ecosystem water fluxes.