抄録
The relative spacing index of a stand is expressed as a function of the average distance between trees and mean tree height. In the self-thinning law, mean and total plant weights are expressed as negative exponential functions of plant density in overcrowded pure stands. In this study, we analyzed the relationship between the self-thinning exponent and the relative spacing index for overcrowded pure stands of two species, Cryptomeria japonica D. Don and Chamaecyparis obtusa Endl. An allometric model that explains the difference in the self-thinning exponent among species was proposed based on allometric relationships between mean tree height and average distance between trees, and between mean tree height and mean stem volume. The resulting self-thinning exponent for total stem volume, obtained from the proposed model, was -0.938 for C. japonica and -0.560 for C. obtusa. These values were statistically identical to the exponents obtained from regressions of the relationships between stand density and total stem volume for the two species. Therefore, the proposed model was applicable to C. japonica and C. obtusa stands. The characteristics related to stem slenderness and relative spacing index determines self-thinning exponent based on the proposed model. The difference in allometric exponents of mean tree height to average distance between trees was found between the two species, but no difference in the exponents of mean tree height to mean stem volume was found. The results indicate that the difference in self-thinning exponents between two species is produced by characteristics related to relative spacing index, rather than by characteristics related to stem slenderness.
