Plant diversity and anthropogenic disturbances in the Sal (Shorea robusta C.F. Gaertn) forests of Bangladesh

Sal (Shorea robusta C.F. Gaertn) is a dipterocarpus tree species in Central Asia native to the southern slopes of Himalaya, and gregariously distributed in India, Nepal, Bangladesh, Bhutan and southern China. These tropical moist deciduous forests of Bangladesh are locally known as Sal or Gajari forests which cover 32% of the forested land. The uncontrolled growths of the population in Bangladesh lead to massive deforestation, a degradation of forests, wildlife and biodiversity. Both the Madhupur and Bhawal National Park occupy most of Sal forests of Bangladesh. They have a high socio-economical and ecological importance for the central part of Bangladesh but they are mostly unexplored and research is not well documented. Few studies on floral diversity (incomplete list of plants) can be found, but no systematic studies on the role of anthropogenic disturbances, their impact on plant diversity, structure and spatial diversity have been done so far. The aim of this research was to measure the biodiversity, the level of human disturbances, the effect of human disturbances on the plant diversity, and to find out the spatial characteristics of Sal. The study area in the Madhupur National Park was classified according to different levels of disturbances and protection. The core area of the Bhawal National Park was analysed on the basis of a changing intensity of recreational activities. Mature trees, seedlings, saplings, climbers and herbs were measured on 300 plots; beside descriptive statistics, biodiversity indices and neighbour-based variables were calculated to assess the spatial characteristics of the Sal forests. In total 134 plant species were identified in the Madhupur National Park, out of them 129 species were found in the core zone. A total of 43 plant species were enumerated in the core area of the Bhawal National Park. The diversity index and evenness decreased, and the concentration of dominance increased with rising disturbances. In most cases, the plant density and basal area of the mature trees showed a declining trend with the increase of disturbances. Sal grows comparatively faster than other associate species and tended to be more dominant over its natural associates from the low disturbed to the highly disturbed forests. The general dispersion of Sal is correlated to species mixture and its density. The core zone of the Bhawal National Park is a pure Sal forest where mostly mixed forests occur in the Madhupur National Park. The density of shrubs and climbers increased with the decreasing V level of human disturbances. No coarse woody debris was found in the Bhawal National Park, whereas very few snags (2.2ha-1) were found in the Madhupur National Park. The diameter and height class distributions indicate a mixture of very young to giant trees in the Madhupur National Park compared to the less diverse forests in the Bhawal National Park. In the highly disturbed and low protected forests of the buffer and peripheral zone the natural regeneration is in general endangered due to the cultivation of agricultural crops and introduction of exotic tree species. Additionally the picnic activities hamper the tree vitality substantially. Finally recommendations for further research activities about forest dynamics, the rate of deforestation and loss of biodiversity as well as measures for conservation management are given. Key words: Diversity index, neighbour-based variables, evenness, spatial structure, coarse woody debris, deciduous forests, dominance, deforestation, exotic specie