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