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

Abstract

SummaryUnderstanding 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 intramolecular13C discrimination in tree-ring glucose (Δi’,i= C-1 to C-6) and metabolic deuterium fractionation at H1and H2(εmet) combinedly. This dual-isotope approach was used for isotope-signal deconvolution.We found evidence for metabolic processes affectingΔ1’ andΔ3’ which respond to air vapour pressure deficit (VPD), and processes affectingΔ1’,Δ2’, andεmetwhich respond to precipitation but notVPD. 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 foundΔ5’ andΔ6’ 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 introducingΔ1’,Δ2’,Δ3’, andεmetvariability.