Photos by Barend van Maanen November 13, 2020
Study shows mangrove forests are threatened by sea level rise, lack of sediment and human activity
Mangrove forests, found around tropical and subtropical shorelines, provide a multitude of environmental and economic benefits to coastal systems around the world. In addition to providing habitats for a variety of species and serving an important role in carbon sequestration, mangrove forests also act as a natural coastal protection, stabilizing the coastline and serving as one of the first lines of defense against storms for coastal communities. Because mangrove trees cannot survive if they are underwater for too long, however, the combination of sea-level rise and the decreasing mud supply from rivers poses a serious threat.
A new study published in Environmental Research Letters utilizes computer simulations to show how coastal forests retreat landward under sea-level rise, especially in coastal areas with reduced mud content in the water. Mangroves can withstand flooding by tides and capture sediment to raise the soil in which they grow, but the trees cannot survive if they are underwater for too long, which is why the combination of sea level rise and the lowering sediment supply from rivers pose a serious threat. The simulations include interactions among tides, mud transport and, for the first time, multiple mangrove species.
Christian Schwarz, an assistant professor at the University of Delaware, is a part of the international research team together with Utrecht University and the University of Exeter which developed the computer model. He stresses this model was developed to investigate how the interaction between multiple mangrove species and different environmental conditions shape diversity and vulnerability under different sea level rise and sediment conditions.
Looking at interactions between assemblages of mangrove species that live in the lower, middle and upper intertidal area, as well as water and sedimentary interactions and how the coastal profile evolves over time, the researchers chose three specific species of mangroves for their study: Rhizophora mangle, Avicennia germinans and Laguncularia racemose. These three species represent the lower, middle and upper intertidal mangroves and thrive at different elevations.
One of the novelties of this study is that there hasn’t been much research on mangrove diversity and how the different species react to different environmental conditions brought on by sea level rise.
The researchers ran simulations with a variety of scenarios of which three showed the most distinct behaviors: high sea level rise with low sediment supplies; low sea level rise with high sediment supplies; and low sea level rise and low sediment inputs. These were run both in the absence of and the presence of anthropogenic, or human-made, barriers that would impede landward migration.
Each scenario indicated some problems, but also some hope, for the mangroves.
In the extreme scenario with fast rising sea levels and low sediment supplies, mangroves cannot catch sediment fast enough, which makes them increasingly inundated and consequently migrate upland or inland, what is known as coastal squeeze.
As sea levels rise and mangroves want to retreat landward, their retreat can be blocked by urbanization and human-made structures, such as dams or other constructed flood protection works.
Thus, human-made structures are predicted to be detrimental to mangrove forests under the scenario. This leads to a reduction in forest extent and diversity, since low living mangroves come to outcompete middle and upper mangroves — or those mangroves that live closer to the shore.
In the absence of a human-made tidal barrier, however, mangrove species can remain stable if sufficient landward habitat is available.
“In these endangered areas where the worst conditions for mangroves exist, you only have two options to preserve them. You can either try to restore the sediment sources, for example, removing a dam upstream, or you need to make sure mangroves have enough space to transgress upland/landward with rising sea levels if you want to conserve these systems,” said Schwarz. “Coastal squeeze drastically removes the area covered by mangroves and consequently their benefits as coastal protection or carbon sequestration.”
A positive scenario for the mangroves occurred when sea level rise was relatively slow and a high amount of sediment was available in the water. Under these conditions, the mangrove forests were able to expand seaward, colonizing elevated mudflats while experiencing little stress from human-made barriers, which was also true when the sea level rise was low and the sediment inputs were low.
This is important because it shows that mangrove coverage can increase despite sea level rise if the sediment supply is sufficient and landward accommodation space is available.
However, this was not all positive news. This mangrove forest expansion also led to a loss in mangrove diversity, showing that mangrove expansion doesn’t always mean the conservation of mangrove diversity.
“The most seaward mangroves have dense aerial roots, sticking out of the sediment surface, which are crucial to supply the mangrove tree with oxygen during the intertidal submersion times,” said Schwarz. “These roots allow them to catch most of the sediment, which can even lead to the landward mangrove portion being cut off from sediment supply and diversity loss. The implications of species-specific sediment capture on forest diversity was quite unexpected for us.”
This reduction in sediment delivery to the more landward mangrove species makes them incapable to capture sediment, build up with sea levels rise and makes them vulnerable to be replaced by species who are adapted to being underwater for longer periods of time — as the roots of the more seaward mangroves catch the sediment but still let the water through, flooding the higher intertidal mangroves.
It was also shown that if sediment supply is low, even slow rates of sea level rise could lead to mangrove forest retreat underlying the importance of sediment supply.
Overall, Schwarz said that it will be important in future studies to examine the role of the different species-specific responses to mangrove forest loss in order to evaluate the future extent and diversity of mangrove forests and to help protect these vulnerable ecosystems.
“The loss of mangrove species will have dramatic ecological and economic implications,” said Schwarz. “Future management strategies need to weigh the loss of coastal mangroves and their inherent functions such as coastal protection and biodiversity conservation with the costs of mitigation strategies as restoring sediment delivery to coasts or providing upland accommodation space, which needs to be addressed sooner rather than later.”