Estimation of Crown Suppression Rate and Stem Growth Reduction Rate of Young Sugi Trees Under the Four-Level Competition Status

Abstract

This study aimed to quantify the influence of competition statuses with surrounding vegetation on crown suppression and stem growth reduction in planted trees. To this end, the four-level competition classification commonly used in previous studies (C1-C4) was converted into a numerical competition index (1-4). Based on this index, we estimated the crown suppression rate, expressed as the proportion of suppressed crown length and branch volume, and the associated reduction in stem growth. Two models were used for estimation: one model explained stem growth using exposed branch volume with the proportion of suppressed crown length as a parameter, while the other assessed the influence of the average competition index from four directions on stem growth. Using parameters from each model, we calculated the crown suppression rate and the stem growth reduction rate corresponding to each competition level. Note that C3 was excluded from analysis in this study due to the small sample size. The results showed that the crown suppression rates for C1, C2, and C4 were approximately 0％, 50％, and 80％, respectively. These values were lower than the visual criteria used to define these competitive statuses. In terms of stem growth, C2 and C4 were associated with 34％ and 86％ reductions, respectively, compared to C1, which experienced virtually no crown suppression. The discrepancy between the crown suppression rate and growth reduction rate in C2 suggests that the intensity of suppression by competitive vegetation is not uniform in the vertical direction, which likely affects the degree of growth reduction.