A team of scientists from the Vrije Universiteit Brussel, Nelson Mandela University, the Moss Landing Marine Laboratories and the University of the Western Cape has been able to show for the first time that ocean currents and coastal geomorphology may continue to hold back the spread of mangrove forests along the South African coast, even as global warming creates a more ideal climate for them. Their study was published in the renowned Journal of Ecology.
The study suggests that by 2050, nine of the 31 estuaries where mangroves grow today will become unsuitable for these plants and six additional estuaries could be added where mangroves do not currently grow.
Tom Van der Stocken of VUB/NASA Jet Propulsion Laboratory (JPL/Caltech): “This study shows for the first time that although climate change will make places in South Africa climatically suitable for mangroves, they may not be able to settle there naturally. This is because coastal geomorphology itself and regional ocean currents prevent the supply of seedlings.”
The study offers new insight into how the distribution of mangroves may evolve globally as our world warms. Moreover, it provides evidence of a previously unknown mangrove forest.
An ideal climate doesn’t mean mangroves can grow there
Mangrove forests are found along the coastlines of mainly tropical and subtropical regions. They cannot thrive at higher latitudes because it is too cold there. But climate change is causing changes in temperature and precipitation patterns. A VUB study in South America has already shown that mangroves are expanding towards the poles.
However, even though factors such as temperature, precipitation and tides are ideal, that does not mean mangroves will grow there. The site must also be able to be colonised by mangrove seedlings. Mangroves are spread by seeds and fruits, collectively called propagules, transported by river, coastal and open-ocean currents. Many propagules end up locally, but some may travel up to 100s to 1,000s of kilometres.
Van der Stocken: “To predict how the distribution of mangroves may change in the future, we need to understand all the factors that determine their distribution. In previous research, factors such as climatic conditions, coastal geomorphology and dispersal via water currents were studied separately, but technological developments in recent years have now allowed us to integrate all the factors for the first time and make predictions about the distribution of mangroves in a warming climate.”
South Africa and global warming
The study focused on the mangrove forests of South Africa. South Africa’s coastline is characterised by strong wave action; mangroves, which are not particularly fond of waves, thrive there only in sheltered estuaries. At low river flows, these estuaries often remain cut off from the sea, so mangrove seedlings spread only locally. In some cases, prolonged (more than three months) closure of estuaries here leads to mangrove mortality. Due to these particular dynamics, mangroves in South Africa have a very patchy distribution even under ideal climate, with presence in only 16% of the 192 estuaries.
The study investigated whether seedling distribution can explain this specific distribution of mangrove forests, which estuaries along this coastline are suitable for mangroves, and how their distribution would be affected under different scenarios of climate change.
“Our results indicate that today there are eight estuaries within the current range and another 14 beyond the current range where mangroves could grow. However, they do not grow here because ocean currents and the particular coastal geomorphology prevent the dispersal of seeds and fruits to these locations,” explains Dr Jacqueline Raw, postdoctoral researcher at Nelson Mandela University and lead author of the study. “But we know the mangroves could thrive here. An experiment from 50 years ago also substantiates this. Back then, a mangrove forest was planted 60km south of its natural range boundary. This mangrove forest still exists and has even naturally propagated to adjacent areas.”
A surprising find: an unprecedented forest
She adds: “Another surprising result is that we found a mangrove forest in an estuary within its known range that was not yet included in the national estuarine botanical database.”
Van der Stocken: “Our species distribution model showed that the estuary where the forest was identified would be suitable for mangrove. Colleagues in South Africa were able to confirm via Google Earth and a site visit that mangrove does indeed exist. We know that we should always interpret model results with caution and in recognition of the limitations of the model. But this observation illustrates that models can also have their utility in informing and guiding research agendas and fieldwork.”
The various members of the research team combined their expertise: Jacqueline Raw and colleagues from Nelson Mandela University and the University of the Western Cape had documented climatic and geomorphological aspects in an earlier study, and Raw integrated these into species distribution models that predict whether sites are suitable for mangroves under different climatic conditions. This has been linked to a dispersal model developed at JPL, and published in an earlier study led by Van der Stocken and Dr Dustin Carroll, a physical oceanographer at Moss Landing Marine Laboratories affiliated with JPL.