Individual-level Diameter Growth Models for Japanese Cypress Trees:

Evaluation of Distance-dependent and -independent Competition Indices

Abstract

The prediction of yield plays a crucial role in sustainable forest management practices. Competition indices have been widely adapted for individual-level yield growth models, and many studies examined the effectiveness of distance-dependent model and distance-independent model. In Japan, previous studies evaluated the efficacy of both models specifically for Japanese cedar (Cryptomeria japonica) trees and showed that the distance-independent model had the same accuracy as distance-dependent model. However, studies that have applied these indices for Japanese cypress (Chamaecyparis obtusa) trees are scarce. This study evaluated the prediction accuracies for the four distance-dependent competition indices and five distance-independent competition indices for an individual-level diameter model of Japanese cypress trees across varying thinning intensities within the Shikoku region. Additionally, the effectiveness of this model as a yield prediction technique were discussed. The findings suggest that the MD (Mean distance) and BR (Basal area ratio) had high interpretability for the diameter model of Japanese cypress trees, with distance-independent competition indices exhibiting higher interpretability than distance-dependent competition indices. Moreover, the distance-independent competition indices demonstrated satisfactory accuracy and efficiency for the diameter growth model of Japanese cypress across various thinning intensities, thereby supporting yield prediction.