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.