Trends of Avicennia marina Productivity as Influenced by Climatic Seasons: A Case Study of Mbweni Mangrove Forest Ecosystem
Avicennia marina (Forsk.) Vierh. is a salt tolerant plant that grows in the saline environment along the coastline. A study was carried out at Mbweni Mangrove Forests aimed at evaluating the effects of different climatic seasons on the productivity, growth, survival and spread of A. marina. A. marina litter falls were collected in nine litter traps set randomly at different stands of the mangrove forest where A. marina occupied at a density of 0.10 individual/m2. Monthly collected litter were sorted into separate components, i.e. leaves, stems and seeds, weighed, recorded and mean weight values were calculated. From the litter, Sodium ions were determined by using Atomic Absorption Spectrophotometer method while Chlorine ions were determined by Ion Chromatography method. In leaf litter accumulation, the highest production was found in September with a mean weight of 78.53g/m2. For Sodium and Chlorine ions, the mean concentration started to rise during the dry season of June to September. Lowest leaf litter mean values were recorded in April with a mean weight of 7.23g/m2 and the concentration of Sodium and Chlorine ions was found lower during the wet season of April and May. For the propagule organs, data were recorded for only three months in which the highest accumulation was found in April with a mean value of 19.3g/m2. During wet season in the month of April, high rates of propagules accumulation with dispersal was accelerated by water runoff leading to establishment of new colonies. For the duration of dry season (June to August) the soils had extreme salinity due to low rainfall. The inorganic ions, sodium (Na) and Chlorine (Cl) started to accumulate in the plant in order to maintain the homeostatic control, turgor pressure and osmotic adjustment. Accumulation of these ions and salts resulted to high productivity of leaf litter in September as a means of removing excess ions and unwanted salts. This was facilitated by strong wind that finally brought short rains in the month of November that also resulted to new vegetation. From this study, it can be concluded that varying climatic seasons has effects to A. marina productivity, growth and survival in the salt stressed environment, indicating that mangroves have a salt tolerant characteristics.
Key words: Avicennia marina, propagule organs, productivity, leaf litter, salinity.