Effects of residual trees on tree height of 18- and 19-year-old Cryptomeria japonica planted in group selection openings

This study examined the effects of residual trees on the height of 18- and 19-year-old Cryptomeria japonica planted in group selection openings of about 0.1 ha at Yufuin in Oita Prefecture in southern Japan. We first developed a general model expressing variation in the height for all of the planted trees within the openings from indices of both between-cohort competition (the effects of residual trees) and within-cohort competition (among the even-aged planted trees). The between-cohort competition index was calculated both with and without taking into account the direction that residual trees were positioned from subject trees. The two models suggested that residual trees located near the northern edge of the opening were not effective competitors for light for the planted trees within the opening. In addition, our results indicated that both between-cohort competition and within-cohort competition are important for explaining tree height in closed stands within group selection openings. Using the general model, we then estimated the potential loss of height growth for locally dominant trees within the openings that could be attributed only to the influence of residual trees. These results indicated that in addition to the effect of opening size, both the degree of crowding and the direction of residual trees from the planted trees are important factors affecting the height of individual trees planted in group selection openings.