Developing tree ring position-specific isotope analysis to improve reconstructions of climate and carbon allocation

The isotopic composition of tree ring cellulose is an important proxy for reconstructing paleoenvironmental conditions and their effects on tree carbon allocation. However, cellulose’s isotopic composition is the combined result of climate-related fractionations and fractionations related to tree metabolic fluxes. We are developing a new method to deconvolve these climate and metabolic signals using position-specific isotope analysis (PSIA) on the glucose monomers that make up cellulose. Previous work has shown that some intramolecular sites record isotopic signatures of water use efficiency more faithfully than the molecular average. We further hypothesize that the isotopic composition of other intramolecular sites will be sensitive to tree metabolic changes, including shifts in carbon allocation. We have demonstrated that gluconate (an oxidation product of glucose) is an excellent candidate for PSIA via Electrospray Ionization Orbitrap. Using positionally labeled standards, we have confirmed that δ13C measurements of 6 fragment ions can be used to independently constrain the carbon isotope composition of 5 unique atomic positions. In a measurement that consumes only 40 nmol of gluconate, we can observe these fragment ions with a precision of 0.3-0.4‰, sufficient to distinguish between trends recorded by different atomic positions as expected based on existing studies of tree rings and plant sugars. Additionally, we have shown that δ13C measurements of gluconate standards on the ESI-Orbitrap accurately match δ13C values obtained via EA-IRMS. We prepare tree ring samples for PSIA by hydrolyzing whole wood or cellulose to glucose, and then oxidizing to gluconate. Upcoming measurements of trees grown in climate chambers under a variety of vapor pressure deficit, relative humidity, and temperature conditions will clarify the climate and metabolic signals recorded at each atomic position. Our new method for PSIA of tree ring cellulose will support more nuanced reconstructions of paleoclimate and its effects on tree growth.