It is important to predict the growth of Sugi forest plantations in old age. When predictions about the growth of Sugi forest plantations are made and there is a lack of growth data from older trees, it is possible that the accuracy of these predictions becomes worse. For example, it is known that the growth of Sugi does not get slower at older ages as expected from past growth predictions based on growth data from young to middle-aged trees. This study investigated the changes in extrapolated values of diameter at breast height (DBH) in old Sugi forest plantations with changes in the observation period of training data for model calibration. The study sites were long-term growth observation sites of Sugi forest plantations in the University of Tokyo Chiba Forest and Chichibu Forest. In this study, both DBH of individual trees and mean DBH of stands were analyzed by fitting Richards growth functions. The results showed that the accuracy of growth predictions in old ages was improved by including growth data from a sufficient number of older trees. From another point of view, growth prediction in old ages tended to underestimate actual growth if growth data did not include enough older trees.

重要的是预测老年杉杉林的生长。当做出关于杉木人工林生长的预测并且缺乏来自老树的生长数据时，这些预测的准确性可能会变差。例如，众所周知，杉树的生长不会像从幼龄到中年树木的生长数据基于过去的生长预测所预期的那样，在高龄时变慢。这项研究调查了Sugi老人工林的胸高直径（DBH）的外推值的变化，以及训练数据的观测期的变化，以进行模型校准。研究地点是东京大学千叶森林和秩父森林的杉木人工林的长期生长观察点。在这个研究中，通过拟合Richards生长函数来分析单个树木的DBH和林分的平均DBH。结果表明，通过包括足够数量的老树的生长数据，提高了老年生长预测的准确性。从另一个角度来看，如果增长数据中没有足够的老树，则老年人的增长预测往往会低估实际增长。