Mangrove ecosystems thrive in (sub)tropical, intertidal areas where adaptations<p>like vivipary and the hydrochorous dispersal of propagules become an absolute<p>necessity. As propagule dispersal and early growth allow for the replenishment of<p>existing stands and colonization of new habitats, many authors recognize the<p>importance of these stages in structuring mangrove populations and communities.<p>However, when it comes to the actual propagule dispersal and recruitment<p>mechanisms, there is an apparent lacuna in the current understanding of<p>mangrove ecology. The period between the mature propagule falling from the<p>parental mangrove tree and the early growth of the established seedling, under<p>various possible circumstances, remains in the dark. In this study we focus on this<p>particular period by investigating both the places where these propagules end up<p>as the pathways their dispersal units follow. And we go one step further.<p><p>Mangrove forests are being destroyed worldwide at a threatening pace despite<p>their tremendous asset to coastal human communities and associated biological<p>species. The effect of human-induced (cutting and mangrove conversion to<p>aquaculture ponds) as well as indirectly and/or ‘naturally’ evolving disturbances<p>(sea level rise) on propagule hydrochory occupies an important place in this study.<p><p>Dispersal of water-buoyant propagules of the family Rhizophoraceae and<p>Acanthaceae (now including the Avicenniaceae) was studied in Gazi Bay (Kenya),<p>Galle and the Pambala-Chilaw Lagoon Complex (Sri Lanka). The study sites<p>differ both in tidal regime and vegetation structure, covering an interesting variety<p>of ecological settings to examine propagule dispersal. Field data and experiments<p>ranging from micro/ mesotopographical measurements and successive propagule<p>counts to hydrodynamic and propagule dispersal experiments were collected or<p>executed in situ.<p><p>Two main methodological approaches were employed. Firstly, the question on<p>mechanisms of propagule recruitment was addressed by statistically investigating<p>the effect of microtopography, top soil texture and above-ground-root complexes on<p>the stranding and self-planting of propagules (Chapter 2&3). Afterwards,<p>suitability maps were created using Geographical Information Systems (GIS) to<p>assess whether a particular mangrove stand has the ability to succesfully<p>rejuvenate. Furthermore, the effect of degradation (tree cutting) (Chapter 2&3),<p>sea level rise (Chapter 2&4) and microtopography-altering burrowing activities of<p>the mangrove mud lobster Thalassina anomala (Chapter 3), was incoporated in the<p>GIS-analyses. Secondly, the combined set-up of hydrodynamic modelling and<p>ecological dispersal modelling was developed to simulate propagule dispersal<p>pathways influenced by dispersal vectors (tidal flow, fresh water discharge, wind),<p>trapping agents (retention by vegetation or aerial root complexes) and seed<p>characteristics (buoyancy, obligated dispersal period) (Chapter 5&6). This type of<p>approach provided the possibility to explore propagule dispersal within its<p>ecological context, but was also applied to an implication of shrimp pond area<p>restoration (Pambala-Chilaw Lagoon Complex, Sri Lanka) (Chapter 5) and to<p>evaluate changes in propagule dispersal when sea level rises (Gazi Bay, Kenya)<p>(Chapter 6).<p><p>The main findings regarding propagule recruitment indicate that propagules are<p>not distributed equally or randomly within a mangrove stand, yet species-specific<p>distribution for anchorage occurs. Characteristics of the environment<p>(microtopography, top soil texture and above-ground root complex) influence<p>propagule recruitment in a way that complex root systems (e.g. pencil roots and<p>prop roots) facilitate the entanglement of dispersal units and a more compact soil<p>texture (like clay and silt) and a predominant flat topography creates suitable<p>areas for stranding and self-planting of propagules. This combines effects of<p>existing vegetation and abiotic factors on mangrove propagule establishment.<p>Since propagule dispersal is not solely determined by species-specific propagule<p>characteristics (e.g. buoyancy, longevity, etc.), I emphasize that propagule sorting<p>by hydrochory has to be viewed within its ecological context. Propagule retention<p>by vegetation and wind as a dispersal vector, deserve a prominent role in studies<p>on propagule dispersal. The significance of dense vegetation obstructing long<p>distance dispersal (LDD in its definition of this work), mainly in inner mangrove<p>zones, supports our main finding that propagule dispersal is largely a short<p>distance phenomenon. ‘Largely’ is here understood as quantitatively, not<p>excluding epic colonization events of rare but important nature.<p>In accordance with the Tidal Sorting Hypothesis (TSH) of Rabinowitz (1978a),<p>smaller, oval-shaped propagules were found to disperse over larger distances than<p>bigger, torpedo-shaped propagules. We can however not fully support the TSH<p>because (1) these differences are no longer valid when comparing between torpedoshaped<p>propagules of different sizes and (2) propagule dispersal is not always<p>directed towards areas more inland, but can be strongly concentrated towards the<p>edges of lagoons and channels<p><p>Anthropogenic pressure on mangrove ecosystems, more specifically clear-felling or<p>mangrove conversion to aquaculture ponds, imposes limitations on propagule<p>recruitment due to reduced propagule availability and a decrease in suitable<p>stranding areas where the architecture of certain root complexes, like prop roots<p>and pencil roots, function as propagule traps. These types of pressure appear to<p>have more severe consequences on propagule dispersal than the effect of sea level<p>rise on mangroves. Mangrove forests, which are not situated in an obviously<p>vulnerable setting, can be resilient to a relative rise in sea level if a landward shift<p>of vegetation assemblages and successful early colonization is not obstructed by<p>human-induced pressures. Also, and this renders mangrove forests vulnerable in<p>spite of their intrinsic resilience, when the ‘capital’ of forest is severely reduced or<p>impoverished as happens extensively worldwide, the ‘interest’ on this capital,<p>understood as propagule availability, delivery and trapping, will not allow them to<p>efficiently cope with sea level rise, putting sustainability of mangrove ecosystem<p>services and goods at risk.<p><p>In a larger framework of mangrove vegetation dynamics, knowledge on propagule<p>dispersal will benefit management strategies for the conservation of mangroves<p>worldwide, besides its fundamental interest to fully fathom the ecology of this<p>particular marine-terrestrial ecotone formation. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished