Propagule Pages: The red mangrove

Species: Rhizophora mucronata

Common names: Red Mangrove, Tanga-lahy

Size: Generally up-to 25m tall; at Honko up-to approximately 7m.

Features: Prominent arching prop roots, very long propagules

Local uses: Building and firewood

DSCF8636

This is one of the seven species of mangrove present at the Honko site and displays
the ‘characteristic’ mangrove prop-root structures that grow out from the lower parts of
the trunk in an arch towards the substrate. As with all rooting adaptations in
mangrove species this unusual growth form helps the individual survive. One way
it helps is to anchor the tree securely in the shifting water logged soil of
the forest; as a result this also contributes to consolidating sediments which
in turn maintains ecosystem function….but that is for another ‘Propagule Pages’.
Upon closer inspection these roots appear to be suffering from a skin
affliction which has resulted in a large number of warts along their surface,
however these are lenticels. These specialised pores facilitate gas exchange
(importantly oxygen can enter the internal tissues) at the root level as
mangrove sediments are a harsh anoxic environment.

DSCF8637

Rhizophora mucronata is part of the Rhizophoraceae family of
mangroves and as is common in this family produces propagules shaped like long
pointed rods; in this particular species their propagules can grow to be 45 cm
long. This viviparous reproductive strategy is very successful in the unique
circumstances that mangroves find themselves in, what with daily tidal
flooding, waves, anoxic sediment, herbivorous crabs. Propagules are essential
the next generation of mangroves which start to grow whilst still attached to
the parent plant, this extra investment in energy by the parent gives the young
plants a head start. When the propagule is ready it drops from the tree to the
sediment, on occasion piercing the sediment providing the perfect start to
life, if not the extra investment by the parent helps sustain the seeding until
such a time that the propagule finds itself embedded in sediment correctly;
sometimes this may mean a journey out to sea to land on a foreign shore and
take root!

propagules r.m

This species has a few local uses here in south west Madagascar, these are mainly in
construction – from building houses where they act as structural posts or roof
supports to creating fences. There is also some use for firewood on occasions when dead dry wood of this species is available.

root organisms

The large arching prop-root structures of R.mucronata also serve to provide essential hard substrates on which many organisms rely; from a place to settle and grow – for example mussels or algae – to a perfect hiding place – for fish or crabs – to a suitable perch for a bird – such as kingfishers. This addition to habitat complexity helps to drive high diversity values as it provides extra niches – in the form of hard surfaces, cryptic spaces al at differing heights within the tidal range – for animals to utilise that would otherwise struggle to survive here; or even not be able to live at all.

Thanks for reading, we hope that you can now recognise your prop-root from your pneumatophore; but to be certain they in the next Propagule Pages focusing on Sonneratia alba.

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