This study consists of two parts, both important to the problematics concerning regeneration and reforestation of mangrove areas. Firstly, the hypothesis was investigated whether soil factors may be responsible for mangrove zonation. Secondly, an attempt was made to improve genetic diversity analysis on mangroves. 1. The hypothesis that mangrove zonation and more generally vegetation structure is caused by physicochemical soil factors was examined in two study sites, Galle and Rekawa, on the southwest coast of Sri Lanka. This hypothesis is supported by McKee (1993) and Matthijs (1995) who performed similar investigations in, respectively Florida and Kenya, in areas with a clear zonation pattern. The study sites considered in this investigation showed many differences compared with those mentioned above. Tidal action in Sri Lanka is nearly absent (maximum lm), the mangrove species showed no clear zonation pattern, the sites were characterized by a complex hydrology and human interference was important and difficult to estimate (not documented). With the results obtained in this study no data can be provided to support the hypothesis. Firstly, the fluctuations of the soil parameters along the transects did not show clear correlations with changes in vegetation structure (floristic composition). Secondly, the stress conditions in Galle and Rekawa were different from those observed by McKee (1993) and Matthijs (1995), who found salinity levels higher than seawater (up to 35%o) and sulphide concentrations in the millimolar range. The sites of this study were lagoons without direct and permanent access to the sea and with essentially a freshwater supply through which the salinity stayed in the range of 10 to 35%o. The sulphide concentrations never exceeded 100 pM. The hypothesis cannot be rejected but the results show once again the complexity of the problem. The physicochemical soil parameters, influenced by many external factors specific for each mangrove area, determine the spatial organisation of the mangrove species and, vice versa, the mangrove vegetation modifies the environment. The relationship between soil factors and the mangrove vegetation structure shall be determined only if the correlations between all ecosystem variables (vegetation, soil factors, hydrology, human impact, etc.) can be established. This requires a quantitative and qualitative study of those variables on longer term and in different mangrove areas. 2. The methodology used to improve genetic diversity analysis on mangroves, showed that major problems occur in the application of native gel electrophoresis, primarily due to the presence of quantities of phenols, quinones, tannins and phenol oxidases in mangrove tissues (Goodall & Stoddart, 1989). However, further investigations on extraction, by changing pH o f or adding tannase to the extraction buffer and by ultrafiltration of the extracts, could lead to satisfying results. Isoelectric focusing, which was tested rather than improved, will probably represent a valuable, alternative technique after further investigation. Randomly amplified polymophic DNA analysis, of which the utility in genetic variation analysis of many species has been proven, can be considered as an efficient technique for such analysis on mangrove tissues. |
