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