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SUMMARY
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.