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

Summary Understanding isotope fractionation mechanisms is fundamental for analyses of plant ecophysiology and paleoclimate based on tree-ring isotope data. To gain 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 was used for isotope-signal deconvolution. We found evidence for metabolic 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 a remarkable potential for climate reconstruction. While isotope fractionation beyond leaves is currently 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.