Applying age-based mortality analysis to a natural forest stand in Japan

Mortality analysis of tree populations is widely applied to forest science studies. When mortality analysis is applied to forest inventories, the use of the variable of tree age is not common due to the difficulty of measuring age data censored in successive observations. The purpose of this study is to apply age-based mortality analysis, i.e., survival analysis, to the individual tree populations in a natural forest. The study site was the secondary natural stand of 0.26 ha dominated by fir (Abies firma), hemlock (Tsuga sieboldii), and oak (Quercus serrata) in the Boso peninsula, Japan. First, we measured the ages of the trees with diameter at breast height greater than or equal to 5 cm using a RESISTOGRAPH. Then, tree mortality probabilities of both non-parametric Kaplan–Meier estimates and parametric probability distributions of Gamma and Weibull were estimated by applying survival analysis techniques to the tree age data. The results implied that the survival analyses could be implemented not only by the non-parametric estimates in any cases but also by the common parametric distributions of Gamma and Weibull in cases in which the tree mortality probability distribution had a monotonously decreasing shape as observed in the immature natural stand in the study site.