Over the last years, Silke has investigated how cheniers, a special kind of sandbanks, travel along the muddy mangrove coast, since they function like “sand barriers” that protect mangrove forests from storms. Her fieldwork adventures involved crawling in the mud to get measurements, sometimes renouncing to them if a poisonous snake chose her instrument as their holiday spot, or searching for stolen equipment in a scooter. You can learn more about her story in the following article of Stories of Science, written by science communication student Emma Blanken.
Trans7 travelled to Timbulsloko and made a documentary about the flooding problems of the area, and the research work being done to help mangrove restoration.
We’re working on the subtitles, but there’s a segment in English after 9:18, where Celine explains her field experiments.
Why are mangroves degrading at Timbulsloko, and why is this worrying?
How can we grow a mangrove belt along the coastline?
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).
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?
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
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:
- 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.
- While the mangroves are still small, the local waves should also be small. Otherwise mangroves could be toppled over by relatively bigger waves.
- 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 :).
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?
Timbulsloko is a small village in the north coast of the Indonesian island of Java (Demak), one hour away from the city of Semarang. There’s only one way to get to the village from land: a narrow road diverging from the national highway, that eventually enters a small community formed by brick houses and small stands selling nasi goreng and krupuk.
Picture of the road towards Timbulsloko. (c) Silke Tas.
As the trip continues, the landscape becomes clearer, and wide extensions of water surround the path on both sides, transforming the road into an artificial peninsula. Unfinished concrete houses line up along the margins of the road, which are the only emergent ground. Some of the buildings have been abandoned, but others have been rebuilt on concrete platforms that elevate the houses a meter from the ground.
Timbulsloko wasn’t always flooded by water. Over the last century, a large part of its mangrove forest was removed to build shrimp farming ponds. This means that the area was deforested and excavated to build large ponds where shrimps could grow.
Not only the soil has been excavated, but the whole coastal region is also sinking. Groundwater extraction in the nearby city of Semarang has caused the land to subside, meaning that the coastal region is sinking deeper with respect to mean sea level every year.
Effect of groundwater extraction over time. (c) Alejandra Gijón.
Road from Semarang to Timbulksloko. (c) Alejandra Gijón.
The mangrove loss does not help improve this situation. Mangrove forests behave like living wood fences, which shelter coastal areas from storms. Waves and currents slow down inside the mangrove forest. This means that any sand or mud particles being carried by the water can deposit and accumulate between the trees, in the same way that sugar deposits at the bottom of a still cup of coffee.
The accumulation of sediment can at least partly compensate for local subsidence, and sea level rise. By losing large part of its mangrove forest, Timbulsloko has thus lost a natural mechanism to protect itself against rising sea levels. But this also means that restoring the mangroves could be a way to improve the situation. This requires answering several questions:
- What are the conditions that small mangroves need to grow healthy? Are those conditions satisfied in Timbulsloko? And if not, can we recreate them artificially?
- Will the mangroves, once grown, be able to survive the subsidence rates in Timbulsloko?
- And to what extent could the restored mangroves compensate subsidence, and protect the village?
Wetlands International started a mangrove restoration project in Timbulsloko in 2014, in collaboration with the local communities and Indonesian and Dutch institutions. The BioManCO research project builds on the knowledge collected during the pilot, and aims to help giving answers to the previous questions.
During the next posts we’ll explain the experiments we’ve done over the last years, and how they helped us build new knowledge for mangrove restoration. Keep checking our website if you want to read more 🙂
Windows of opportunity: how do small mangroves find a spot to grow?
How can we build a green belt in Demak?
Building with Nature Indonesia website:
Hi and welcome to our website!
We are Celine, Silke and Alejandra, three PhD researchers investigating how to restore mangrove forests. During the next posts we’ll share our experiences of the last years; from our anecdotes in the field, to the lessons learnt during our research.
You can also follow our fieldwork experiences via Instagram (@biomancoresearch)
If you have any questions, or want to share your ideas with us, do not hesitate to contact us via info@BioManCO.org :).