New insights into the mechanisms of plant isotope fractionation from combined analysis of intramolecular ¹³<scp>C</scp> and deuterium abundances in <i>Pinus nigra</i> tree‐ring glucose

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 ¹³C discrimination in tree-ring glucose (Δ_i', i = C-1 to C-6) and metabolic deuterium fractionation at H¹ and H² (ε_met) combinedly. This dual-isotope approach was used for isotope-signal deconvolution. We found evidence for metabolic processes affecting Δ₁' and Δ₃', which respond to air vapour pressure deficit (VPD), and processes affecting Δ₁', Δ₂', 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 Δ₅' and Δ₆' 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 Δ₁', Δ₂', Δ₃', and ε_met variability.