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Tree Species Diversity and Regeneration Traits of the Dominant Species in a Dipterocarp Forest in Bangladesh: Implications for Conservation

Shekhar R. Biswas
Department of Biology, Lakehead University, Thunder Bay, Ontario, Canada

Khaled Misbahuzzaman
Institute of Forestry and Environmental Sciences, Chittagong University, Bangladesh

Abstract/ Executive Summary:

Forests dominated by dipterocarps are one of the few remaining terrestrial areas with high biodiversity in Bangladesh. This study was conducted in a remnant dipterocarp forest reserve to identify tree species diversity in the reserve and key regeneration traits of the dominant species. Vegetation in transects and quadrats was measured, and regeneration traits (weight, length, diameter and dispersal distance) were investigated using seed trap experiments. We identified 66 tree species, belonging to 27 families and 45 genera with four clearly defined strata. Tree density was 384.05 ± 12.76 stems/ha and Dipterocarpus turbinatus alone contributed 23.45% to the total forest area. Trees in the intermediate diameter class (30.10–40.00 cm dbh) have high regeneration potential. We developed regression models to predict seed dispersal for different diameter classes. We found problems in recruitment for the dominant species. We suggest planting of native species to aid natural regeneration and for better management and conservation of biodiversity in the reserve. Similar management approaches may be applicable for other dipterocarp dominated forests in Bangladesh.

Keywords: Dipterocarpus turbinatus, Foristic composition, Seed dispersal, Diameter class distribution, Phytosociology

Location:

Start Date:

End Date:

Program Area: Conservation and Biodiversity

Commodity: Plant

Objectives:

The specific objectives of the study were: i) to record tree species diversity, floristic structure and composition; and ii) to investigate the key regeneration traits of the dominant species, Dipterocarpus turbinatus. In-depth understanding of these factors might help to provide guidance for scientific management and conservation planning of this forest.

Methodology:

The study was conducted in the Idgaon Forest Reserve (3941.7 ha) of Cox’s Bazar, Bangladesh (20°51′ and 21°47′N and 91°54′ and 92°19′E). The topography is irregular and consists of a series of ridges (Das 1980). The soil varies from clay to clay loam on level ground, and from sandy to coarse sand on hilly ground (Chowdhury 1969). The climate is tropical moist (Canonizado and Mabud 1998) with high rainfall (700–1200 mm), concentrated in the monsoon period from June to September (Hossain et al. 1997). The mean monthly temperature ranges from 15.75°C in January to 42.6°C in May. The mean annual maximum and minimum temperatures are 29.7°C and 21.8°C, respectively. The government has imposed a ban on commercial felling in this forest. However, until recently, the silvicultural system was clear felling followed by artificial regeneration, with a gradual reduction in felling rotation. Initially, the rotation was 120 years (Banerjee 1943); thereafter it was reduced to 100, 80 (Zahiruddin 1954), 60 (Chowdhury 1969) and then 40 years (Blamforth and Howlader 1988). The references given coincide with the time when the rotation reduction occurred. METHODS Quadrat size determination and randomisation Vegetation was analysed using 50 quadrats, each of 20 m × 20 m. A species–area curve (Misra 1968) was used to determine quadrat size. Controlled randomisation was used to select ten sites for sampling (see Figure 1), taking two factors into consideration: i) accessibility of the site and ii) level of disturbance. Inaccessible and highly disturbed sites were avoided. Within each of the selected ten sites, five quadrats were randomly placed, using a random number generator, to define the duration of the walk (number of minutes) inside the forest. Walks were up to 30 min (approximately 1 km), so that within each site the maximum distance between quadrats was approximately 1 km. Vegetation study The spatial architecture of the forest was studied using 45 line transects, each 100-m long and 10-m wide, placed randomly throughout the forest. In each transect, the presence of any clearly defined stratum and height (from ground to treetop) was estimated. In each quadrat, the presence of a tree more than 10 cm dbh (diameter at breast height) was recorded, and the number of individuals present was counted. Seedlings and saplings below 10 cm dbh were not counted. Tree species were identified following Heining (1925), Brandis (1906) and Prain (1903) and the taxonomic literature was consulted for the correct nomenclature of the taxa. Herbarium specimens were collected and identified at Bangladesh National Herbarium. Measurements of dbh were grouped into nine diameter classes: D1 (10–20 cm), D2 (20.1–30 cm), D3 (30.1–40 cm), D4 (40.1–50 cm), D6 (50.1– 60 cm), D7 (60.1–70 cm), D8 (70.1–80 cm) and D9 (> 80 cm). Floristic data were analysed for relative frequency and relative density following Shukla and Chandal (1993); for basal area following Phillips (1959); and for relative dominance following Shukla and Chandal (1993). Key regeneration traits Key regeneration traits of the dominant tree species (Dipterocarpus turbinatus) were studied in terms of seed quality and dispersal behaviour. A total of 15 trees from nine different diameter classes (D1 to D9) were selected for this study. Sixteen traps (each 1 m2 ) were placed on the ground around each tree at a distance of 5–40 m from the tree base. As emphasis was placed on natural conditions, no special measures were taken to restrict seed predators. We counted the total seed within each trap after 1 month. Seed length and diameter were measured using slide calipers with a Vernier constant of 0.002. Seed weight was measured using an electric balance with an accuracy of 0.001 g.

Source of Information: International Journal of Biodiversity Science and Management 4 (2008) 81–91

Article Link (Online Journal): DOI 10.3843/Biodiv.4.2:2

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Achievement/Findings:

Therefore, the preferred management system for our forest may be a massive programme of planting with native species while maintaining natural plant colonisation. Finally, this study underlines the lack of available basic information. Thus, permanent sampling plots should be established in these forests to facilitate periodic evaluation of the ecological parameters.

Knowledge Management: Journal