Mangrove forests need help to survive
MANGROVE forests are a common sight in tropical and subtropical areas of the world like Indonesia, Florida in the US, parts of Brazil and Australia.
They can also be found on African coasts, including in KwaZulu-Natal and the Eastern Cape.
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 sea grasses, 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’ 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 South Africa don’t get freeze events, mangroves still stop occurring at a certain latitude.
We wanted to know why this is the case and to determine if there are other areas on South Africa’s coast that are climatically suitable for mangroves but where the forests don’t grow.
We also wondered, since climate change will make some parts of the world warmer in the coming years, whether mangroves might be able to grow in parts of South Africa where they’re not found now. Finally, we wanted to understand whether climate change will make areas in South Africa where mangroves exist unsuitable for the forests in future.
We discovered it is South Africa’s high wave energy that keeps mangroves from spreading: their seeds are pushed around the ocean without being 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 thrive.
It’s therefore essential to safeguard existing mangrove forests. Whatever is done must be done soon to preserve these important wetlands.
As the first step in our research, we created a species distribution model.
The model identified good candidate areas. But mangroves did not occur in those places. This meant there must be another limitation.
Then we ran the same model but took climate change into account..
Mangroves spread to new locations through floating seeds, which fall from the trees and are carried out to sea.
The ocean model showed that although the Agulhas Current transports mangrove seeds rapidly south to those suitable estuaries, the coast is exposed. These conditions make it difficult and increasingly unlikely for mangrove seeds to reach and enter relatively small estuary mouths.
The 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.
We recommend that estuaries 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.