Structure and dynamics of tree populations within unsuccessful conifer plantations near the Shirakami Mountains, a snowy region of Japan

Abstract

At plantations of the evergreen conifer Cryptomeria japonica (ca. 20 years old) damaged by heavy snow, the growth of planted trees, composition of colonizing native hardwoods, and successional trends of tree populations were investigated. The study site of ca. 2 km×2 km was established in 1998 within a river basin contiguous to the Shirakami Mountains which was registered as a World Natural Heritage site. This site was originally covered with old-growth forests of Fagus crenata. Twenty-one plots (each 400 m 2) were established (11 in the plantations and 10 in adjacent remnant natural forests) and a census of trees was conducted in 1998 and in 2000. During this period, the mean basal area slightly decreased from 39.8 to 38.8 m 2 ha −1 in the natural forests, In the plantations, the mean basal area increased from 15.3 to 20.3 m 2 ha −1. Only focusing on C. japonica, the value changed from 11.4 to 14.2 m 2 ha −1, which was smaller than expected in the management table for this district (14.5–22.5 m 2 ha −1 on average). The smaller basal area of the plantations had resulted from a high mortality of C. japonica. That was caused by damage from heavy snow falls during winter, especially on east-facing slopes. This was considered to have resulted from spatial variation in snow depth depending on slope aspect. Native hardwoods had colonized the resulting openings in the plantations, and increased their basal area more rapidly than the planted conifers. They presently occupy 29% of the total basal area of the plantations on average. The composition of native hardwoods differed between the natural forests and the plantations; climax species occupied a significantly larger proportion of the basal area in the natural forests (e.g., F. crenata) and pioneer species had greater importance in the plantations (e.g., Pterocarya rhoifolia, Swida controversa, Phellodendron amurense). Pioneers in the plantations tended to distribute on the east-facing slopes, probably reflecting higher number of openings created there. As the result of the natural succession, tree communities in the plantations are expected to become dominated by climax species, but it will take quite a long time.