The stand structure and regeneration dynamics of trees > 2.0 m in trunk height were studied during 1982-1998 in a 1-ha plot in a sub-boreal conifer-hardwood mixed forest, northern Japan, with a dense dwarf bamboo in the understory. Total density was low in 1982 (651 trees/ha), as compared with other forests in Japan. Quercus crispula was dominant in the upper canopy layer but their saplings were rare, while Acer mono, Acer japonicum and Abies sachalinensis were dominant in the sub-canopy and understory layers with many saplings. Mortality of each species was quite low during the census period (average 0.57%/yr), and there was no clear difference among the four species. The age structure of Quercus crispula was bell-shaped with a peak at ca. 200 years, while that of the other three species was weakly reverse-J shaped or a rough plateau. In addition, no recruits growing over a height of 2 m were observed during the census period in Quercus crispula, but many recruits of the other species were observed. These suggest that Quercus crispula depended on episodic disturbances for the persistence of its population. Recruits of the three species except for Quercus crispula did not concentrate in canopy gaps probably because of the dense dwarf bamboo cover there. They showed a negative spatial association with their own canopy trees, but a positive association with canopy trees of Quercus crispula. Most of the crowns of the three species (Abies sachalinensis and the two Acer spp.) were lower than that of overtopping Quercus crispula. These spatial associations between recruits and canopy trees brought about the competitive effect of Quercus crispula on the growth rates of other species and that of itself. However, the low mortality of trees taller than 2 m indicates that intra-and interspecific competition was not strong as a structuring force of the tree community. Our long-term study suggests that factors affecting recruitment (disturbances and dwarf bamboo in the forest floor) are more important for species coexistence than intra-and interspecific competition between trees taller than 2 m.

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