Oxygen and carbon isotopic ratios of tree-ring cellulose in a conifer-hardwood mixed forest in northern Japan

Abstract

Oxygen and carbon isotopic ratios (δ¹⁸O and δ¹³C) were analyzed for cellulose extracted from tree rings of 5 oak trees (Quercus crispula) and 4 fir trees (Abies sachalinensis) standing in a 1 ha plot of a sub-boreal conifer-hardwood mixed forest, northern Japan. The δ¹⁸O variations were well correlated between individual trees of Q. crispula (canopy trees) and A. sachalinensis (recently grown-up sub-canopy trees), although A. sachalinensis had about 1 ‰ higher δ¹⁸O values than Q. crispula on average and there was an apparent one-year phase lag between δ¹⁸O variations of the two species. The similar inter-annual variation in δ¹⁸O among different individuals and species suggests a common environmental control. Contrary to δ¹⁸O, the inter-annual variations in δ¹³C did not possess any common trends among individual trees for either Q. crispula or A. sachalinesis, suggesting that the ecological effects, such as spatial heterogeneities in δ¹³C and/or concentration of CO₂ in canopy air and/or competition for light with neighboring trees, regulate the δ¹³C of photosynthetic products in each tree. Seasonal variations of the δ¹⁸O and δ¹³C within annual tree rings of Q. crispula showed random and cyclic characteristics, respectively. The difference between the annual patterns of δ¹⁸O and δ¹³C supports the idea that δ¹⁸O is controlled by some environmental factors, which change from year to year, but δ¹³C is primarily governed by physiological conditions of the tree itself, which repeat regularly in every growing season. The historical variation in δ¹⁸O of tree-ring cellulose in Q. crispula has negative correlations with those in both of winter and summer precipitation amounts, whereas it does not show any relationship with temperature, probably due to multiple source areas of water vapor for the precipitation at the studied area. Because the δ¹⁸O of precipitation in northern Japan is positively correlated with air temperature, the correlation between δ¹⁸O and winter precipitation suggests that, in a year of heavy snowfall, the soil in this forest retains larger amount of lower δ¹⁸O water derived from snowmelt, which is taken by roots of Q. crispula in summer. On the other hand, the negative correlation with summer precipitation cannot be elucidated by the δ¹⁸O of rainfall, but must be explained by a higher relative humidity in the growing season in a year of larger summer rainfall. Our results confirm the potential of δ¹⁸O of tree-ring cellulose to reconstruct past climate in a forest with a heavy snowfall, and suggest the importance of the hydrological knowledge in an atmosphere-soil-plant system for the utilization of tree-ring δ¹⁸O in paleoenvironmental purposes.