Structural and dynamic properties of the dry dipterocarp forest in central highland of Vietnam

In Vietnam, the dry dipterocarp forest accounts some 500,000 ha and concentrates in lowland parts of Dak Lak and southern Gia Lai in central highland (CH). This forest type is commonly found where is a monsoon climate with a long marked dry season (5-6 months) and a total amount of rainfall ranges from 1200 to 1600 mm/year. The objectives of this study are: (i) to determine community structure and distribution of the dry dipterocarp forest in CH; (ii) to describe the diameter distribution of forest stands; (iii) to classify the site indexes as yield capacity; and (iv) to simulate the diameter increment and the basal area-density (G-N) diagram as an expression of the stand dynamic. Density (N), diameter at breast height (DBH) and basal area (G), frequency of tree species are collected in 40 sample plots of 1ha each. Data for height and diameter growth are obtained by stem analysing 100 trees of 4 main dominant tree species. Simulation method has been applied based on mathematical models developing for each research property of the forest stand. Based on the accumulate entropy of tree species, the dry dipterocarp forest can be divided into five main dominance-types according to the five main dominant species: Shorea obtusa, Dipterocarpus tuberculatus, Diptercarpus obtusifolius, Shorea siamensis and Terminalia spp. which can be divided further into 17 sub-types. The diameter distribution structure can be described by using the Weibull distribution with two parameters. Through the explanation the meaning of the parameters and their relationship, 7 types of stand structure have been identified. As yield capacity of the dry dipterocarp forest in different sites, four site indexes have been classified based on a height growth function. The diameter increment of interval 10 years for each diameter class is predicted by each site index. The point of dbh size at which the diameter increment reaches its maximum has been calculated. It is a basis for determination of minimum harvestable diameter. A G-N diagram was devised and a method of predicting the tree number and basal area of each diameter class of a stand was developed. Present and optimal stocking of the dry dipterocarp forest in CH were discussed.