New insights into the mechanisms of plant isotope fractionation from combined analysis of intramolecular <sup>13</sup><scp>C</scp> and deuterium abundances in <i>Pinus nigra</i> tree‐ring glucose
Thomas Wieloch, Meisha Holloway‐Phillips, Jun Yu, Totte Niittylä
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 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.