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VEGETATION SUCCESSION IN THE MANGROVES OF UNAWATUNA, GALLE, SRI LANKA: COMPARISONS OF CURRENT AND HISTORIC VEGETATION STRUCTURE AND STRATIFICATION.

par Nyandwi, Alphonse
Promoteur Dahdouh-Guebas, Farid
Co-Promoteur Satyanarayana, Behara;Jayatissa, Loku Pulukkutige
Publication Non publié, 2022-08-30

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Résumé :

Galle-Unawatuna is a dynamic mangrove ecosystem in Sri Lanka that is under threat from bothnatural and anthropogenic factors. This ecosystem is well-known for its socio-economic andecological significance in providing ecosystem processes, functions, goods, and services. Longtermmonitoring is thus necessary to assess species succession and to understand underlyingfactors. A pioneering study predicted that Bruguiera gymnorrhiza would eventually dominatethe mangrove forest's surface cover. A follow-up study revealed that the dominant layer ofyoung species and went on to predict that a decade later, B. gymnorrhiza would supplant otherspecies and dominate the mangrove adult forest in sector two. This decade later is “now” andthe main objective of this study is to therefore test these predictions made ten years ago. Thus,the current comparative research was designed to: 1) assess the structure and stratification ofmangrove species composition, 2) investigate the potential link between vegetationcomposition and selected environmental factors. The ground truth survey, using a plot-basedmethod, was used to revisit historical transects and collect vegetation structural parametersfrom a total of 116 plots. Finally within a 20 × 50 m² section of the forest a detailedtriangulation of all trees was covered in an area that was already investigated about 20 yearsago to document vegetation dynamics on tree level. Therefore, we gained insights in thecurrent succession, and stratification of the forest by calculating stem density, basal area,frequency, and their relative values, as well as the importance value and complexity index. Ourfindings show that the current forest parameters; basal area, density, frequency, Importancevalue are relatively high compared to previous campaigns. Excoecaria agallocha has declined,whereas Bruguiera gymnorrhiza and Rhizophora apiculata have increased. Bruguieragymnorrhiza, on the other hand, does not dominate sector two. In sector two, R. apiculata hassurpassed E. agallocha's dominance. We found changes in vegetation composition andstructure from the past, thus, we can confirm the presence of the 'moving mosaic' in the Galle-Unawatuna mangrove area.