Species Distribution in Different Ecological Zones and Conservation Strategy of Halophytes of Sundarbans Mangrove Forest of Bangladesh

Abstract

Species distribution of halophytes and their conditions in different ecological zones of Sundarbans mangrove forest were evaluated. Water flow within the rivers of the Sundarbans mangrove forest and seasonal variation of water salinity of the rivers were discussed. Highest salinity (electrical conductivity) 27,500 micromhos/cm (Polyhaline zone) and 5800 micromhos/cm (Oligohaline zone) was observed in April; and lowest salinity was 10,150 micromhos/cm (Polyhaline zone, July) and 500 micromhos/cm (Oligohaline zone, July). Environmental constraints and ecophysiological response in relation to soil environment and plant and plant roots in relation to salinity were highlighted. Surface soil of this halophytic forest is silty clay loam; soil pH ranges from 6.9 to 7.5; calcium is the dominant cation and is highest in the Oligohaline zone followed by Mesohaline and Polyhaline zones. The respiratory roots (pneumatophores) of various forms (such as peg like, knee root, buttress roots, stilt roots) in different species were described. Measurement of redox potential in soils from selected places showed low values particularly in the Oligohaline zone (+60 mV, where the Heritiera fomes plant shows top dying) followed by Mesohaline and Polyhaline zone. Low oxidation-reduction potentials (anaerobiosis) may be one of the factors responsible for the cause of top dying, and these properties influence plant growth and distribution. Highest soil salinity (6950 micromhos/cm) was in the Polyhaline zone, and in the Oligohaline zone, highest salinity was 650 micromhos/cm. The vegetation pattern is given based on field observation, with the dominant species; and associated species are placed in relation to density and abundance. Comparison of species composition (large and small trees, shrubs, herbs, palms, fern, lichen and climbers) and their condition in the three ecological zones showed the nature of their distribution pattern. Plant association and distribution of halophyte species in relation to salinity in different ecological zones indicated their nature of distribution. Anthropogenic and natural threats to the halophytic mangrove species and their conservation are discussed. Research for mangrove conservation in relation to stress ecology and rehabilitation of mangrove halophytes, linkage between the structure of mangroves and ecosystem services and future of mangrove halophytes were highlighted.