Summary Understanding isotope fractionation mechanisms is fundamental for analyses of plant ecophysiology and paleoclimate based on tree-ring isotope data. To provide new insights into isotope fractionation, we analysed intramolecular 13 C discrimination in tree-ring glucose ( Δ i ’, i = C-1 to C-6) and metabolic deuterium fractionation at H 1 and H 2 ( ε met ) combinedly. This dual-isotope approach enabled deconvolution of isotope signals. We found evidence for physiological processes affecting Δ 1 ’ and Δ 3 ’ which respond to air vapour pressure deficit ( VPD ), and processes affecting Δ 1 ’, Δ 2 ’, and ε met which respond to precipitation but not VPD . These relationships exhibit change points dividing a period of homeostasis (1961-1980) from a period of metabolic adjustment (1983-1995). Homeostasis may result from sufficient groundwater availability. Additionally, we found Δ 5 ’ and Δ 6 ’ relationships with radiation and temperature which are temporally stable and consistent with previously proposed isotope fractionation mechanisms. Based on the multitude of climate covariables, intramolecular carbon isotope analysis has an extraordinary potential for climate reconstruction. While isotope fractionation beyond leaves is usually considered to be constant, we propose significant parts of the carbon and hydrogen isotope variation in tree-ring glucose originate in stems (precipitation-dependent signals). As basis for follow-up studies, we propose mechanisms introducing Δ 1 ’, Δ 2 ’, Δ 3 ’, and ε met variability.

New insights into the mechanisms of post-rubisco isotope fractionation from combined analysis of intramolecular<sup>13</sup>C and deuterium abundances in<i>Pinus nigra</i>tree-ring glucose

New insights into the mechanisms of post-rubisco isotope fractionation from combined analysis of intramolecular13C and deuterium abundances inPinus nigratree-ring glucose