To go with the flow: a field and modelling approach of hydrochorous mangrove propagule dispersal

Mangrove ecosystems thrive in (sub)tropical, intertidal areas where adaptationslike vivipary and the hydrochorous dispersal of propagules become an absolutenecessity. As propagule dispersal and early growth allow for the replenishment ofexisting stands and colonization of new habitats, many authors recognize theimportance of these stages in structuring mangrove populations and communities.However, when it comes to the actual propagule dispersal and recruitmentmechanisms, there is an apparent lacuna in the current understanding ofmangrove ecology. The period between the mature propagule falling from theparental mangrove tree and the early growth of the established seedling, undervarious possible circumstances, remains in the dark. In this study we focus on thisparticular period by investigating both the places where these propagules end upas the pathways their dispersal units follow. And we go one step further.Mangrove forests are being destroyed worldwide at a threatening pace despitetheir tremendous asset to coastal human communities and associated biologicalspecies. The effect of human-induced (cutting and mangrove conversion toaquaculture 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 andAcanthaceae (now including the Avicenniaceae) was studied in Gazi Bay (Kenya),Galle and the Pambala-Chilaw Lagoon Complex (Sri Lanka). The study sitesdiffer both in tidal regime and vegetation structure, covering an interesting varietyof ecological settings to examine propagule dispersal. Field data and experimentsranging from micro/ mesotopographical measurements and successive propagulecounts to hydrodynamic and propagule dispersal experiments were collected orexecuted in situ.Two main methodological approaches were employed. Firstly, the question onmechanisms of propagule recruitment was addressed by statistically investigatingthe effect of microtopography, top soil texture and above-ground-root complexes onthe stranding and self-planting of propagules (Chapter 2&3). Afterwards,suitability maps were created using Geographical Information Systems (GIS) toassess whether a particular mangrove stand has the ability to succesfullyrejuvenate. Furthermore, the effect of degradation (tree cutting) (Chapter 2&3),sea level rise (Chapter 2&4) and microtopography-altering burrowing activities ofthe mangrove mud lobster Thalassina anomala (Chapter 3), was incoporated in theGIS-analyses. Secondly, the combined set-up of hydrodynamic modelling andecological dispersal modelling was developed to simulate propagule dispersalpathways influenced by dispersal vectors (tidal flow, fresh water discharge, wind),trapping agents (retention by vegetation or aerial root complexes) and seedcharacteristics (buoyancy, obligated dispersal period) (Chapter 5&6). This type ofapproach provided the possibility to explore propagule dispersal within itsecological context, but was also applied to an implication of shrimp pond arearestoration (Pambala-Chilaw Lagoon Complex, Sri Lanka) (Chapter 5) and toevaluate changes in propagule dispersal when sea level rises (Gazi Bay, Kenya)(Chapter 6).The main findings regarding propagule recruitment indicate that propagules arenot distributed equally or randomly within a mangrove stand, yet species-specificdistribution for anchorage occurs. Characteristics of the environment(microtopography, top soil texture and above-ground root complex) influencepropagule recruitment in a way that complex root systems (e.g. pencil roots andprop roots) facilitate the entanglement of dispersal units and a more compact soiltexture (like clay and silt) and a predominant flat topography creates suitableareas for stranding and self-planting of propagules. This combines effects ofexisting vegetation and abiotic factors on mangrove propagule establishment.Since propagule dispersal is not solely determined by species-specific propagulecharacteristics (e.g. buoyancy, longevity, etc.), I emphasize that propagule sortingby hydrochory has to be viewed within its ecological context. Propagule retentionby vegetation and wind as a dispersal vector, deserve a prominent role in studieson propagule dispersal. The significance of dense vegetation obstructing longdistance dispersal (LDD in its definition of this work), mainly in inner mangrovezones, supports our main finding that propagule dispersal is largely a shortdistance phenomenon. ‘Largely’ is here understood as quantitatively, notexcluding 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 thanbigger, torpedo-shaped propagules. We can however not fully support the TSHbecause (1) these differences are no longer valid when comparing between torpedoshapedpropagules of different sizes and (2) propagule dispersal is not alwaysdirected towards areas more inland, but can be strongly concentrated towards theedges of lagoons and channelsAnthropogenic pressure on mangrove ecosystems, more specifically clear-felling ormangrove conversion to aquaculture ponds, imposes limitations on propagulerecruitment due to reduced propagule availability and a decrease in suitablestranding areas where the architecture of certain root complexes, like prop rootsand pencil roots, function as propagule traps. These types of pressure appear tohave more severe consequences on propagule dispersal than the effect of sea levelrise on mangroves. Mangrove forests, which are not situated in an obviouslyvulnerable setting, can be resilient to a relative rise in sea level if a landward shiftof vegetation assemblages and successful early colonization is not obstructed byhuman-induced pressures. Also, and this renders mangrove forests vulnerable inspite of their intrinsic resilience, when the ‘capital’ of forest is severely reduced orimpoverished as happens extensively worldwide, the ‘interest’ on this capital,understood as propagule availability, delivery and trapping, will not allow them toefficiently cope with sea level rise, putting sustainability of mangrove ecosystemservices and goods at risk.In a larger framework of mangrove vegetation dynamics, knowledge on propaguledispersal will benefit management strategies for the conservation of mangrovesworldwide, besides its fundamental interest to fully fathom the ecology of thisparticular marine-terrestrial ecotone formation.Doctorat en Sciences