Growth performance of Dipterocarpus alatus and Hopea odorata in degraded secondary forest land in Southern Vietnam

The two dipterocarp species, Dipterocarpus alatus and Hopea odorata, have been widely planted in degraded forest land in Southern Vietnam in the last decades. However, study on growth characteristics of these species and their associated factors is still limited. Therefore, the present study aimed to examine growth performance in different stand densities, and classify tree quality in 28-year old D. alatus and H. odorata plantations. Our results of analysis of covariance (ANCOVA) indicated that in pure stand aged 28 years, D. alatus outperformed H. odorata in tree growth, biomass and volume. In addition, except for four growth variables including tree height at the first branch (Hb), crown length (CL), crown ratio (CR) and linear crown index (LCI), the remaining variables were negatively affected by stand density. We observed that medium quality trees occupied the greatest proportion in both D. alatus (47%; n = 425) and H. odorata (50%; n = 400). Except for CR and LCI, the class of good quality trees had the greatest values in the remaining examined growth variables. In linear discriminant analysis (LDA) model, the classification accuracy of testing set was relatively high in both D. alatus (85%) and H. odorata (91%). The most important variables for tree quality classification in D. alatus were crown diameter (CD) and diameter at breast height (DBH). Meanwhile, the most important variables in H. odorata were CD and tree height (H). These obtained results suggest that controlling crown size is important for shaping individual tree quality. Our data evidenced that D. alatus and H. odorata planted in Southern Vietnam with density equal or less than 500 trees per ha could yield high proportion of good and medium quality trees.