Stand structure and dynamics during a 16-year period in a sub-boreal conifer–hardwood mixed forest, northern Japan

Abstract

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% per year), and there was no clear difference among the four species. The age structure of Q. 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 Q. crispula, but many recruits of the other species were observed. These suggest that Q. crispula depended on episodic disturbances for the persistence of its population. Recruits of the three species except for Q. 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 Q. crispula. Most of the crowns of the three species ( A. sachalinensis and the two Acer spp.) were lower than that of overtopping Q. crispula. These spatial associations between recruits and canopy trees brought about the competitive effect of Q. 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.