Abstract
The delay of thinning in planted forests has become one of the major problems for Japanese forest management. Using the proportional hazard model, this study aimed at the estimation of the probability distribution of elapsed time to first thinning after Sugi planted stands become subject to subsidized thinning, and of the effect of real stumpage price to the distribution. The data of thinning history in the period of 1980 to 1999 was gathered for 250 ha of privately owned Sugi stands in Misaki, Kaminaka Town, Tokushima Prefecture, planted betwen 1964 and 1989. Two findings were obtained. Firstly, the probability that thinning occurs dropped after a peak at 7 to 8 years of elapsed time, resulting in a significant ratio of stands remaining without thinning. Secondly, the decision of first thinning was affected by the real stumpage price, defined as the ratio of stumpage price to forestry labor wage. The drop of real stumpage price was thought to be a cause of a sharp decline in vigorousness of thinning during the 20 years of the observation period.
