Why it’s crucial to protect our mangrove forests

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 South Africa’s KwazuluNatal 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’ 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 whether there are other areas along 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 and decades, whether mangroves might in future 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 currently 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 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. Whatever is done must be done soon to preserve these important wetlands.

Mangrove forests’ total range in South Africa 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 however, are not in formally protected areas.

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 process creating this limitation.

Then we ran the same model but took climate change into account..

Mangroves spread to new locations through floating seeds (known as propagules), which fall from the trees and are carried out to sea.

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.

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, and what is already occurring in other regions.

We 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.

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.