Through a combination of species modelling and ocean
modelling, Jacqueline L Raw discovered it is South
Africa’s high wave-energy that keeps mangroves from
spreading: their seeds are pushed around the ocean
without being easily able to get washed into the
estuaries where they can take root and grow.
Mangrove forests are a common sight in some tropical and
sub-tropical areas of the world like Indonesia, Florida in the
US, parts of Brazil and Australia. They can also be found on
African coasts, including South Africa’s KwaZulu-Natal and
Eastern Cape provinces.
These tidal forests of trees and shrubs are often talked about
in the context of climate change. Along with other coastal
wetlands like salt marshes and seagrasses, they are able to
store more carbon than terrestrial ecosystems. Waterlogged
soils preserve the organic carbon and prevent decomposition – and
if they’re left undisturbed, this “blue carbon” is locked up
over thousands of years. This means they can play a key role
in the oceans’s carbon cycle.
Mangroves are also valuable assets as ecosystems because
they support a significant amount of biodiversity.
Mangroves won’t grow in cool climates. In the northern
hemisphere, their range ends at areas where it snows in
winter. But, even though the coasts of places like Brazil,
Australia and South Africa don’t get freeze events,
mangroves still stop occurring at a certain latitude in
the southern hemisphere.
DST-NRF Research Chair Jacqueline L Raw in Shallow Water
Ecosystems Ocean Sciences Campus, Nelson Mandela University
wanted to know why this is the case and to determine whether
there are other areas along South Africa’s coast that are
climatically suitable for mangroves but where the forests
They also wondered, since climate change will make some parts
of the world warmer in the coming years and decades, whether
mangroves might in future be able to grow in parts of South
Africa where they’re not found now. Finally, they wanted to
understand whether climate change will make areas in South
Africa where mangroves currently exist unsuitable for the
forests in future.
Their new study reveals the answers. Through a combination of
species modelling and ocean modelling, they discovered it is
South Africa’s high wave-energy that keeps mangroves from
spreading: their seeds are pushed around the ocean without
being easily able to get washed into the estuaries where they
can take root and grow.
And existing mangrove forests are at risk from changes in
rainfall, which are predicted in climate change scenarios for
the region. Increased rainfall can lead to more flooding.
Less rainfall can result in estuary mouths closing off from
the sea. Extreme or repeated occurrences of either change
can make it difficult for mangroves to survive and thrive.
It’s therefore essential to safeguard existing mangrove forests.
Different approaches will be needed for mangrove forests in
rural and urban areas, but whatever is done must be done
soon to preserve these important wetlands.
Mangrove forests don’t cover a lot of ground in South Africa.
Their total range in the country is only about 2,000 hectares
across 32 of the 214 estuaries along about 1,000km of the country’s
east coast. Mangroves are classified as an indigenous forest
type and are therefore included in the country’s National
Forests Act. This requires that all natural forests on private,
communal or state-owned land are protected. Most mangrove
forests in South Africa, however, are not in formally protected areas.
As the first step in their research, they created a species
distribution model. They collated everything they knew about
where mangroves grow in South Africa. They ran the model to
see where else in South Africa those conditions were met and
whether mangroves grew there.
The model identified good candidate areas. But mangroves did
not occur in those places. This meant there must be another
process creating this limitation.
Then they ran the same model but took climate change into account
by feeding in data about areas that are predicted to become warmer
(and so may be more hospitable for mangrove forests in future).
This also showed that estuaries further south would be suitable
for mangroves, but that conditions in some estuaries that currently
support mangroves could become unfavourable – and this could lead
to mangrove loss.
Mangroves spread to new locations through floating seeds (known as propagules),
which fall from the trees and are carried out to sea. Recent research
has shown how important ocean currents are for transporting mangrove
propagules in different part of the world. They wondered whether this
could be the process limiting mangroves from occurring in those other
Through a collaboration with colleagues at the University of Brussels
and the San José State University in California in the US, they were
able to include some of this ocean modelling in their study. This was
a way to simulate how mangrove propagules would float offshore in
the southern African region.
The ocean model showed that although the Agulhas Current transports
mangrove propagules rapidly south (about 600km in three weeks) to
those suitable estuaries, the coast is very exposed, with lots of
waves and sandy beaches. These conditions make it difficult and
increasingly unlikely for mangrove propagules to reach and enter
relatively small estuary mouths.
Many seeds wash up on beaches or rocky shores where they cannot
establish new mangrove forests or become part of existing forests.
The ocean modelling confirmed that propagules can float for weeks
or months without reaching an estuary.
These findings suggest that mangroves are not going to become
more widespread in South Africa as temperatures rise with climate
change. This is contrary to what has been predicted at the global
scale for mangrove forests, and what is already occurring in other regions.
They are however not advocating for mangroves to be manually planted
further south because the places where this would happen are already
occupied by salt marsh vegetation. Salt marshes support different
species to mangroves; for example they provide habitat for certain
birds to nest that don’t use mangroves. Replacing one natural
ecosystem with another is not recommended.
Value what they have
They recommend that estuaries currently supporting mangroves be
safeguarded through appropriate conservation, restoration, and
management measures. This would give mangroves the best possible
chance of naturally responding to climate change (as they have
done through millennia).
The protection and management of mangroves in South Africa needs
to be integrated into coastal management practices and biodiversity
conservation, as well as national and provincial climate adaptation strategies.
For example, stewardship programmes are likely to help reduce human
impacts on these ecosystems. In rural areas where mangrove wood is
sometimes used as building material and for building fish traps,
approaches such as community-based monitoring, or payment for ecosystem
services programmes, are viable options. In contrast, in urban areas
mangroves can be afforded protection by reducing pollution and limiting
activities through zoning in estuary management plans.
Provided by the IKCEST Disaster Risk Reduction Knowledge Service System