Résumé : 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.