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?

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