Self-thinning and size variation in a sugi ( Cryptomeria japonica D. Don) plantation

Abstract

Density and stem volume in a sugi ( Cryptomeria japonica D. Don) plantation were monitored for 15 years from 1983 to 1997. The tree density decreased year after year from 5002 to 3108 ha −1. The time-trajectory of mean stem volume and density provided evidence in favor of the −3/2 power law of self-thinning. The skewness of the frequency distribution of stem volume showed positive values, which means that the distribution is more or less L-shaped, and the skewness decreased with time, which indicates that smaller trees died as the stand grew. This trend is consistent with the asymmetric or one-sided competition hypothesis that self-thinning is driven by competition for light. Dead trees tended to distribute randomly with the stand growth. The relationship between standard deviation of stem volume and mean stem volume was formulated by a power function, whose exponent was significantly less than unity. This shows that coefficient of variation, ranging from 61.7 to 82.5%, decreased with increasing mean stem volume. That is, the relative size variation becomes small with stand growth.