【研究紹介】

Tree ring isotope composition to inform models assessing the sensitivity of the forest carbon budget to increased atmospheric CO₂ and climate change

EPRON  Daniel　　Kyoto University／Professor (tree physiology)

　　The objective of the project is to demonstrate that the isotope composition of tree rings provides invaluable information on the physiological response of forests to changes in atmospheric CO₂ and climate.

　　Evapotranspiration, canopy photosynthesis and stand-level water use efficiency were calculated from the 20-year series of eddy-covariance measurements of CO₂ and water fluxes. Watershed level evapotranspiration was calculated from the 50-year series of hydrological data.

　　Wood cores were sampled on 14 hinoki cypress trees in the Kiryu Experimental Watershed (Shiga) and crossed-dated. Thin cross sections containing tree rings from the last 50 years were cut to extract a-cellulose which is currently being analysed for δ¹³C, δ¹⁸O, δ²H using a continuous-flow system with a pyrolysis elemental analyser and an isotope ratio mass spectrometer.

　　In addition, to ascertain the relation between xylem water and source water, sapwood from 4 additional hinoki trunks was sampled monthly from March to August with soil cores to clarify the relationship between xylem water and soil water. Water was extracted by cryogenic vacuum distillation and is currently being analysed for ¹⁸O and ²H with a cavity ring-down spectrometer.

　　The isotope composition of atmospheric water vapour and leaf water extracted from 4 trees is also currently being analysed for ¹⁸O and ²H to estimate the Péclet effect on leaf water enrichment in ²H and ¹⁸O, which will be used to evaluate the relationship between the isotope composition of the α-cellulose extracted the outer growth ring and the xylem water.

　　The results will be evaluated statistically when all results will be available (expected at the end of December) and evapotranspiration, canopy photosynthesis and stand-level water use efficiency will be simulated over 50 years, accounting for the long-term changes in stomatal conductance and water use efficiency derived from the isotope composition of tree and compared to eddy-covariance and hydrological data.

Keywords：Tree rings, stable isotopes, forest carbon budget, atmospheric CO₂, climate change, watershed