AbstractIn the past decades, the general public got more and more aware of the threats posed on ecosystems
throughout the world. Despite the increasing media attention and conservation efforts, the degradation
of ecosystems through over-exploitation and pollution continues or even worsens. Especially local
people, who are dependent on local products for their livelihoods, are immediately affected by the
degradation of these ecosystems through reduction of much needed resources and services. The
mangrove forest is one of those sensitive ecosystems under immense anthropogenic pressure and
because this muddy, dense forest is often seen as 'wasteland', vast stretches of forest are clear-felled
and/or converted for agricultural, industrial and housing purposes. Since mangroves can not reproduce
vegetatively, they solely depend on dispersal of their seeds (propagules) to regenerate and (re)colonise
areas. Knowledge about the dispersal abilities of the mangrove propagules is therefore crucial in
understanding regeneration of these forests and to draw up appropriate conservation plans. Though the
hydrochory of mangrove propagules has been known since a long time, it may come as a surprise that
very little research has been done on the dispersal process and its determining factors.
The aim of this study is to gather information on dispersal dynamics of mangrove propagules to
resolve the current knowledge gap on the moment between release of a propagule from the parent tree
and its establishment after a certain dispersal period through water (hydrochory).
To study these dispersal dynamics, we did a tracking experiment where we followed the propagule
dispersal of two mangrove species (Ceriops tagal and Rhizophora mucronata) by releasing marked
propagules in different vegetation assemblages along the intertidal gradient and by, subsequently,
tracking them in the following weeks. The experimental setup was kept as simple as possible to allow us
to infiltrate, at least partly, into the the complexity of dispersal dynamics. In the tracking experiment we
found significant differences between the different plots:
- propagules released in the most landward and seaward plots dispersed over large distances1,
because of the more open structure of the forest and the adjacent tidal creek, resp.
- propagules released in the inner mangrove plots (high root density and muddy substrates) were
subjected to high amounts of predation by crabs and dispersed over smaller distances.
Although propagules dispersed mostly in landward directions or along the shore, no clear overall
directional pattern is observed. This shows the importance of local topography in dispersal, since it
directs the flow of tidal currents and thus the dispersal directions of propagules.
Based on the tracking experiment and experiments on the relative speed of propagules in along
shore water currents, we conclude that C. tagal propagules are faster dispersers and are subjected to
higher predation pressure than R. mucronata propagules, due to differences in several morphological
(e.g. size) and anatomical characteristics (e.g. propagule tissue density and width of propagule cuticle).
The hypothesis resulting from a preceding study (De Ryck et al., 2007) stating that the reduced
buoyancy of desiccated propagules was due to shrinkage of air pockets was rejected. Studying the
propagules' anatomy revealed that the higher density of desiccated propagules resulted from
parenchyma cells that shrunk rather than a reduction of air spaces in the aerenchyma.
Looking at the propagules of Ceriops tagal and Rhizophora mucronata and the low survival rates
due to predation and desiccation, we wondered what a large energy investment it must be for a
mangrove tree to invest in such large propagules. Therefore, an experiment was put up to see whether
these green propagules could reduce the energy input of the parent plant through autonomous
photosynthesis. After wrapping C. tagal propagules in fabric, lowering the light intensity about 24
times, we compared the length growth between wrapped and non-wrapped propagules. A significant
reduction in growth was observed for propagules wrapped in fabric underscoring our hypothesis of
autonomous photosynthetic activity.
As a main conclusion, we can say that tracking experiments have a large potential of explaining
dispersal dynamics in the field, if an adequate number of dispersing propagules can be recovered each
day and a sufficiently large searching perimeter2 is reached. Therefore, results of propagule tracking
experiments in situ can be used to insert in predictive models (Di Nitto et al., 2008), opening
possibilities of upscaling results.
We can conclude that propagule dispersal is strongly linked to local topography and that predationby
crabs and retention by stilt, knee and dense pencil roots are major factors in reducing dispersal distances
of C. tagal and R. mucronata propagules. Although most subjected to predation, C. tagal propagules
seem to dispose of better characteristics (e.g. lower tissue density) for Long Dispersal Distance than R.
1 To give an idea of what we mean with large distances: landward plots (> 50m up to several hundred meters) and seaward
plots (>50m to possibly several kilometers)
|Date of Award||Aug 2009|
|Supervisor||Nico Koedam (Promotor), Nele Schmitz (Co-promotor), Farid Dahdouh Guebas (Co-promotor), Elisabeth Robert (Advisor) & James Gitundu Kairo (Co-promotor)|