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

Oxygen and carbon isotopic ratios (delta(18)O and delta(13)C) 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 delta(18)O 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 1parts per thousand higher delta(18)O values than Q. crispula on average and there was an apparent one-year phase lag between delta(18)O variations of the two species. The similar inter-annual variation in delta(18)O among different individuals and species suggests a common environmental control. Contrary to delta(18)O, the inter-annual variations in delta(13)C 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 delta(13)C and/or concentration of CO2 in canopy air and/or competition for light with neighboring trees, regulate the delta(13)C of photosynthetic products in each tree. Seasonal variations of the delta(18)O and delta(13)C Within annual tree rings of Q. crispula showed random and cyclic characteristics, respectively. The difference between the annual patterns of delta(18)O and delta(13)C supports the idea that delta(18)O is controlled by some environmental factors, which change from year to year, but delta(13)C is primarily governed by physiological conditions of the tree itself, which repeat regularly in every growing season. The historical variation in delta(18)O 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 delta(18)O of precipitation in northern Japan is positively correlated with air temperature, the correlation between delta(18)O and winter precipitation suggests that, in a year of heavy snowfall, the soil in this forest retains larger amount of lower delta(18)O 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 delta(18)O 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 delta(18)O 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 delta(18)O in paleoenvironmental purposes.