Relationship between Diameter at Breast Height and Tree Age in Populations of a Rare and Endangered Plant, Davidia involucrata

The growth rate of the diameter of trees is significantly correlated with their age; therefore, a growth model can be established to predict the dynamics of the population. To reveal the relationship between individual age and diameters at breast height (DBH) of D. involucrata, five populations were chosen in the largest distribution area of D. involucrata in Sichuan province. Dendrochronology and regression model was used to evaluate the distribution of the DBH and its relationship with the age, respectively. All 130 D. involucrata trees recorded in the studied plots, with the minimum DBH of 3.8 cm, and the maximum of 45.5 cm, corresponding to the minimum age of 4 years and the maximum age of 98 years. The results showed that in the first 11 DBH classes (DBH < 22 cm), the tree DBH increases relatively slowly with the growth of the age. After the 11th DBH class, the DBH class increases slowly with increasing tree age. The tree age exceeds 20 years, the increase in the DBH growth rates with tree age is similar among the five populations. The linear models were fitted to determine the regression equation for the DBH growth curve of D. involucrata, which can be well described by a parabolic model. Under normal forest stand development and growth conditions, the model can be widely applied to study the ages of individual D. involucrata.