Relationships between Crown Length and the Increment of Diameter at Breast Height Over the Last Five Years of Japanese Cedar Trees in Overcrowded Young and Mature Plantations

Abstract

The correlation between crown length and mean increment in diameter at breast height (DBH) over a period of five years was investigated in five overcrowded, young and middle-aged Sugi (Cryptomeria japonica D. Don) stands. The threshold value of DBH, at which the mean DBH increment of Sugi trees is nearly zero, differed among the stands, whereas the threshold value of crown length, at which the mean DBH increment of Sugi trees is nearly zero, was 4.0 m in all stands. For the trees with a crown length over 4.0 m, the relationship between length index (with 4.0 m subtracted from the whole crown length) and mean DBH increment was analyzed using analysis of covariance (ANCOVA), and it was compared with the ANCOVA model with DBH as a covariate. The adjusted coefficient of determination of the latter model was significantly higher than that of the former model. With respect to the selection of thinned trees in overcrowded Sugi stands, it is reasonable to assume that trees with a crown length less than 4.0 m show little to no growth and that the greater the DBH, the more vigorous the growth of the trees with a crown length of over 4.0 m.