How can we build a mangrove green belt in Demak?

Most of the mangrove forest of Demak, in North Java (Indonesia), has been removed for aquaculture over the last century. Shrimp farming required deforesting mangroves areas, and excavating the soil to create ponds where shrimps could be raised.  Some of those ponds were abandoned and re-colonised by mangroves. But some abandoned locations have remained empty over time.

Drone picture of aquaculture ponds by Silke Tas, edited by Alejandra Gijón. (c) S. Tas.

The mangrove habitat is also being lost along the coastline. The shoreline of Demak has been moving landwards during the last decades due to a combination of coastal erosion (i.e. coastal sediment being taken away by waves and currents), and subsidence (i.e. sinking of the ground in the region due to groundwater extraction). In spite of the general loss of habitat, mangroves have episodically expanded at some locations along the coastline.

Our colleague Celine wondered why mangrove establishment was successful at some locations and not at others, and she designed a number of field experiments to answer this question.Her full study was published by the beginning of this year 🙂 (the link is included at the end of the post!), and we have summarised some of her main findings below.

Schematised picture of a mangrove coastline (left). Pictures of locations with and without mangrove recruitment in aquaculture ponds and along the coastline of Demak, from van Bijsterveldt et al. (2020). 

Why were mangroves colonising some abandoned ponds, and not others?

Mangrove expansion often took place in abandoned ponds with relatively higher bed elevations and high pond drainage, which resulted in a more consolidated ground in which seedlings could establish.  Ponds where the sediment was too soft were detrimental for mangrove recruitment, since they provided less stability for seedlings.

Why were mangroves expanding seaward at some locations?

Seaward expansion of existing mangroves was strongly associated to elevated mudflats, with smaller water depths. High waves become unstable and break further seawards at shallow areas, which reduces the wave height at the coastline. This results in a relatively calmer area where small mangroves seedlings are less disturbed by waves.  Inversely, more hollow (and deeper) profiles were linked to mangrove retreat.

How can we apply this for future mangrove restoration schemes?

The findings of Celine et al. (2020) suggest that restoration of abandoned ponds can be stimulated by improving pond drainage (i.e. removing water of the ponds, for instance using channels). This enhances sediment stability (making the ground drier and stiffer), and allows ponds to accrete through the drainage channels.

Seaward expansion can be induced by changing the morphology of the foreshore, so that is relatively wider and shallower. This requires promoting sediment accumulation at the coastline.  Future posts will discuss how this option has been explored in Demak.

Diagram showing which factors drive mangrove expansion and retreat, from van Bijsterveldt et al. (2020). 

Related links:

How to restore mangroves for greenbelt creation, by Celine van Bijsterveld et al., (2020)

Why are mangroves degrading at Timbulsloko, and why is this worrying?

How do small mangroves find a spot to grow?

Windows of opportunity: how do small mangroves find a spot to grow?

Mangroves seeds drift along the coastline, transported by waves and currents, until they reach land. If the conditions are right at their destination, the seeds will develop roots to fix themselves into the ground and they will grow over time. But if the conditions are not right, the seedlings may be uprooted, and mangrove colonisation will fail. Understanding what makes a spot suitable for mangroves is thus key to restore them

Picture of a mangrove seedlings in a sand bank (chenier) near Timbulsloko. (c) Silke Tas.

What is the right spot for a mangrove seed?

When a mangrove seed reaches an emerged location (like the beach), it will be exposed to sea waves and currents. Thorsten Balke and his research team wondered how the tide and waves influence the settlement of small mangroves, and designed laboratory experiments to understand such relationship. Based on their experiments they concluded that:

  1. Mangrove seedlings need to be emerged for a sufficiently long time, so that they have enough time to develop their roots and fix themselves into the ground.
  2. While the mangroves are still small, the local waves should also be small. Otherwise mangroves could be toppled over by relatively bigger waves.
  3. In the long term, their roots should be long enough to withstand extreme weather events like storms, and the bed level changes they may cause.

Diagram showing the windows of opportunity suggested by Balke et al (2011). (c) Alejandra Gijón.

Do we have conditions suitable for mangrove establishment in Timbulsloko?

The area of Timbulsloko is subsiding, which means that the ground is sinking and that the coastline is increasingly flooded over time. This could have adverse effects for mangrove establishment in multiple ways. For instance, subsidence reduces the time available for seedlings to establish during the tidal cycle. Larger water depths also allow bigger waves to reach the coastline, which may uproot the small trees.

If a location is too deep for mangroves to grow, any natural or artificial mangrove colonisation (for instance planting trees) will fail, because the habitat is not suitable for them. This leads to the question of identifying which spots have the right conditions to be restored. We’ll discuss this topic in the following post :).

Related links:

Full article by Balke et al. (2011), where they investigated the windows of opportunity:

Why are mangroves degrading in Timbulsloko, and why is it worrying?

How can we build a mangrove belt?