By Han Winterwerp and Thorsten Balke
If you ever visit a mangrove-mud coast, you will see that the mangroves grow more or less between the waterlines at mean high water and the waterline at the highest tidal level occurring in a year. Understanding the relation between tides and mangroves is therefore essential to rehabilitation efforts.
The moon and the sun rule the tide. Because the moon circles around the earth, and earth and moon circle around the sun, the tide is not constant. We find higher tidal ranges one or two days after full and new moon, and smaller tidal ranges in between.
The high tides around full and new moon are called “spring-tide”, the lower tidal ranges are known as “neap-tide”. The time between one spring-tide and the next is about two weeks. Because the distance from the earth to the sun varies, not all spring-tides are equally large. In fact, the larger spring-tides occur around spring and autumn. The high water at this extreme spring-tide forms the highest water levels at the head of a mangrove forest.
Picture source: Wikipedia Commons
So why are tides important for our mangrove story? Our guest author Thorsten Balke explains:
Mangrove seeds, also called “propagules”, drop from a tree and float around in the water. The tide brings them to the mudflats. Once the tide deposits the seeds on the mudflat a race against the tide begins. If the propagules win, a new forest may be born.
Because the propagules can easily float away, mangrove seeds need to take root as soon as possible before the next tide comes to get them. Depending on the growth conditions this can take several days. The spring and neap cycle of the tides will determine when and where the mangrove seeds are given such a window of opportunity.
In short, mangrove seeds can be transported to new places and take root only when the next high water is lower than the water level at which the propagules were deposited. This occurs during the days after full or new moon. Unfortunately, even if they made it that far, waves can rip out the newly anchored propagules and they have to start again another time. The mud on which the mangroves grow is very soft, and only gives little support to the young tress. Grown-up trees strengthen the soft soil, thereby creating their own more stable conditions.
So now we also understand why mangroves can only establish above mean high water. Thus, next time you look at the moon, you can tell what is happening in the mangroves!
Mangrove (Avicennia) propagules.
Photo by Thorsten Balke.
Han Winterwerp works for the Dutch research institute Deltares and is a professor at the Delft University of Technology, where he teaches and develops instruments to measure and predict the behavior and properties of mud. He specializes in the behavior of mud in open waters.
Thorsten Balke has recently finished his PhD, studying the conditions in which mangrove seeds (propagules) can take root, the so-called window of opportunity. You can read more about his research here. Thorsten now also works at Deltares as an ecologist. He will tell us another time on his work on Blue Carbon in relation to the restoration of mangrove-mud coasts.
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