Tran Van Con

Silviculture Division

Forest Science Institute of Vietnam

Dong Ngac, Tu Liem, Hanoi, Vietnam

E-mail: tranvancon@fsiv.org.vn or tranvan_con@yahoo.com

ABTRACT

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.

Key words:

Dry dipterocap forest, Central highland, dominance-structure, diameter distribution, height growth, site index, diameter increment, basal area- density diagram.

INDRODUCTION

The dry dipterocarp forest occurs naturally only on dry sites of the mainland of southeast Asia: Vietnam, Laos, Cambodia, Thailand, Mianmar, and a small area of northeast India. The characteristic species of this forest type are Dipterocarpus obtusifolius, Dipterocarpus tuberculatus, Shorea obtusa, Shorea siamensis. The other common species are Simdora cochinchinenesis, Xylia dolabriformis, Terminalia sp., Pterocarpus pedatus… This type of forest has an open canopy and is composed of small to medium-size xeric species. Most of the tree species shed their leaves during the dry season and form new leaves before the beginning of the raining season. These deciduous trees occasionally occur with evergreen species.

In Vietnam, the dry dipterocarp forest accounts some 500,000 ha and concentrates in lowland parts of Dak Lak and southern Gia Lai (from 300-600m over the sea). It is commonly found where there is a monsoon climate with a long marked dry season (5-6 months) and the total amount of rainfall ranges from 1200 to 1600mm/yr.

The central highland (CH) of Vietnam can broadly be divided up into three geographic zones, (i) a northern zone formed by the Kotum plateau and southern Annamite mountains; (ii) the flat central plateau of Gia Lai and Dak Lak and (iii) the Dalat plateau and southern mountains. Within each of these areas there are several sub landscapes, composed mainly of smaller subsystems of mountain ranges at various elevations.

Due to the changes in species composition, time-series, growth relations, tree-sizes, quality, yield and also density change, the forest structure may be very different. This difference is not only very important for planning, implementing and monitoring silvicultural measures but also essential for ecological investigation of forest ecosystems.

The overall relationship between topography and forest cover (Figure 1 and 2) is characterized by (i) a continuous extent of lowland and lower montane forest stretching from south west Lam Dong to the eastern coast at Phu Yen, (ii) dry zone deciduous forest in lowland parts of Dak Lak and Gia Lai, (iii) upper montane forest in Kontum and Lam Dong in areas over 1500 m in elevation.

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. Which are necessary for the practical treatments of forest stand.

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