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

Abstract

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 13C discrimination in tree‐ring glucose (Δi′, i = C‐1 to C‐6) and metabolic deuterium fractionation at H1 and H2 (εmet) 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 εmet, 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 εmet variability.