Analysis of Vegetation Structure Causing Top-Dying In Mangrove Forest Trees in the Sundarbans in Bangladesh

Abstract

Sundarbans is the largest, biologically richest, and most extensive mangrove forest in the world (Christensen, 1984; Seidensticker and Hai, 1983; Hussain and Karim, 1994). Heritiera fomes (Sundri) is the predominant tree species, supporting about 65% of the total merchantable timber (Chaffey et al., 1985, and Siddiqi, 2001). A unknown and serious-killer disease (top dying) of H. fomes in Sundarbans is affecting millions of the trees. An inventory by Chaffey et al., (1985) revealed that there were about 45.2 million top dying trees in the Sundarbans of which 20 million had more than 50% of their crown affected by top dying. The loss of H. fomes will have a major impact on the Sundarbans mangrove ecosystem, which leads to economic losses. The vegetation structure was assessed in terms of tree height, bole diameter, species present, and regeneration status; and the intensity of top-dying within the plots was recorded on a rank scale. Most of the elements studied had no significant correlation with the top dying of Heritiera fomes. Of the hypotheses previously put forward to explain top-dying, the present results do not support enhanced salinity as the cause. It is likely that several of the above-mentioned environmental factors interact with each other to induce top dying in Sundri. However, the present results have showed that Sn, Exchangeable K, soil pH, Pb , Zn and Ni could be directly linked with top-dying of Heritiera fomes (Sundri) in Sundarbans, probably particularly by weakening the vigour of the trees and allowing other factors such as pathological agents to attack the plants.