The present PhD dissertation is composed of fourteen papers, preceded by a general introduction and followed by an epilogue that explains the entire research framework, including the links between the different case-studies, the local and global applicability of the results and the relevance of the work in the framework of development co-operation. The bulk of the research was carried out in four sites in Kenya and Sri Lanka: Gazi Bay and Mida Creek in Kenya, and Unawatuna-Galle and Pambala-Chilaw in Sri Lanka. After a revision of the often controversial issue of the floral composition of mangroves in Sri Lanka (SW coast), followed by a synthesis of image attributes in the interpretation of aerial photographs in Kenya and Sri Lanka, a series of case-studies are presented which cover the study of mangrove vegetation structure dynamics (incl. zonation issues), mangrove regeneration and regenerative constraints (in particular propagule predation). Gazi and Galle appeared to be more disturbed than Mida and Pambala. The studies of these disturbed sites show different types of structural changes in vegetation over time, as well as the existence of interactions between the mangrove and terrestrial vegetation (sedges and coconut plantation). Parallel investigation of the adult, young and juvenile vegetation layers in combination with sequential aerial photography from the past allow an understanding of the dynamic processes and a prediction of the future forest structure, the latter subject to different scenarios (no change, mangrove cutting, natural hazards, etc.). This approach allows for the decision if a certain mangrove stand has the potential to successfully renew and rejuvenate or if anthropogenic pressure renders human interference such as restoration imperative. In the framework of re-afforestation programs the study of the fate of mangrove juveniles in rehabilitation plots is essential. Whereas the monitoring of mangrove regeneration provides information on the general status of mangrove juveniles, the study of propagule predation in particular has shown that crabs and gastropods may constitute a problem for the successful regeneration of mangroves in Kenya and Sri Lanka. Results also show how propagule predation is one link in the chain of events that leads to a particular vegetation structure (zoned or unzoned), and how hydrology and anthropogenic pressure may possibly influence it. The outcome therefore gives direct information for forest management issues by identifying the species, the areas and the processes that are likely to display higher propagule predation rates. Another aspect of mangrove regeneration is more directed towards natural regeneration conditions, more specifically in biogeographical limit situations such as in the Parc National du Banc d' Arguin (Mauritania), a World Heritage Site where the mangroves of the West-African coast reach their northernmost distribution. As a biogeographically marginal, monospecific mangal in an area where freshwater availability has decreased over the last decades, concern about the survival of the mangroves has been expressed. However, except for inappropriate topographical conditions (mangroves growing in terrestrial locations, with little chance for propagule survival) the mangrove did not show signs of reduced vitality at its biogeographical limit, but even displayed a diversity of vegetation structure types (high trees, wide trees, shrub formations and 'sebkha' formations). An empirical revisit of the Point-Centred Quarter Method (PCQM) prompted by fieldwork experience, tackles the problems associated with this method, such as ambiguous tree architectural settings for measuring the tree diameter, aggregation of trees or the over- or under-estimation of the density and the basal area. The factor by which the latter occurs, apparently is influenced by the vegetation structure of the forest. The final paper provides an integrated review of mangrove vegetation structure dynamics based on existing literature as well as on the previous case-studies and summarises and defines the different types of vegetationstructure dynamics. |
