Can Mangroves Adapt to Rising Seas?

mangroves

Along the coastal waters, land and sea intertwine in the tangle of twisted mangrove forest that encircles the islands of Micronesia. Photo by Ami Vitale.

If you’re the gambling type who likes to beat the odds, here’s a tip: In the race against climate change, place your money on mangroves.

According to a new report from The Nature Conservancy, Wetlands International and the Cambridge Coastal Research Unit, mangroves could be able to keep pace with sea level rise in some places. The authors reviewed a broad range of existing evidence and found that mangroves can build up soil at rates of 1 to 10 mm per year. In many places that rate is well within the range of the current 3 mm per year rise in sea levels, potentially allowing mangroves to remain in place even as rising seas threaten to engulf them.

Mangrove forests are trees and shrubs that thrive in the tidal waters of tropical or sub-tropical coastal areas — in the United States, they are mainly found around Florida and up into the Gulf Coast. If you’ve never seen a mangrove, picture a motley chorus line of tangled tree legs rising up from brackish water.

But they’ve seen better days: Scientists estimate that 35% or more of the world’s mangroves have been lost in recent decades, decimated by coastal development. Yet mangroves are also increasingly being recognized for their value as natural defenses against storm waves, as carbon storage and as nurseries for many marine creatures such as shrimp, crabs, fish and more.

How Do Mangroves Build Up Soils?

The report aims to present a picture of what science knows about soil build-up currently and what still needs to be known. It’s an area that has received little research attention to date.

“Mangroves have complex roots that help to trap and bind the sediments on the soil surface, while the unseen growth of roots beneath the soil surface helps build up the soil from below,” explains Dr. Anna McIvor, lead author of the report and a scientist at the Cambridge Coastal Research Unit. (See diagram below.)

mangrove soil diagram

Diagram of a mangrove tree and the soil beneath it, illustrating how surface elevation change (soil build-up) may occur over time.

The processes that influence soil build-up — such as sediment deposition, erosion, root growth, decomposition, the burrowing of crabs and other animals, and more — are complicated, and how they interact is not widely understood. Moreover, a multitude of variables can influence the rate at which these processes occur in any given location. What’s most needed, say the authors, is more data on soil elevation changes, over longer time periods and from more varied locations.

Knowing more about how the soil build-up process works and where its not working well will be crucial in helping scientists address mangrove restoration. The authors found that some mangrove forests have historically built up soil at pace or faster than sea level rise. For instance, mangroves in Twin Cays, Belize, have created a layer of old roots and sediments that is 8 meters thick in some places. But in other areas, soils are likely not building up at high enough rates — and these are the areas where science will need to focus.

“We are just beginning to develop this picture. It seems that mangroves won’t keep up in all locations, but there’s also tantalizing evidence that we might be able to manage mangroves to help this process,” says Mark Spalding, senior marine scientist with The Nature Conservancy and co-author of the report. “That might mean restoration where mangroves have been degraded or lost, but it could also mean taking a wider view, to restoring natural river flows and sediment movements along  coasts.”

The authors also caution that while the build-up of soils in some mangroves is keeping up with sea level rise now, there could be a threshold point at which they cannot continue growing at the same rate. But even when mangroves cannot fully keep up, their ability to hold soils together and to make fractional increases in elevation could help protect coastal areas. Maintaining a wide strip of mangroves as the front line between the sea and the land is a potential solution for coastal engineers and could be less costly than building and extending ever-higher sea walls.

Projections of rising sea levels had scientists worried that mangroves would start to disappear even faster than in recent decades. But as scientists race to better understand how nature will adapt and respond to climate change, this report offers a note of hope and an important lesson: Root for the underdog, because all it takes is a few millimeters to win.

Read the full report.

Darci Palmquist is a senior science writer for The Nature Conservancy. Previously she served as editorial manager for nature.org, the website of The Nature Conservancy, as well as for the Conservancy's e-newsletter. She is based in Amherst, Massachusetts.




Comments: Can Mangroves Adapt to Rising Seas?

  •  Comment from Karen L. McKee

    You’ve written a nice post about the report by McIvor et al. However, you give the impression that the authors conducted the research and collected the data they reviewed. In fact, all of the data and insights about how mangroves keep pace with sea-level was the work of scientists not mentioned in your post. My comment is not meant to criticize this report (which properly referenced contributors to these insights), but to point out that there is a big difference between scientists who carry out research and make discoveries and those who write summaries based on other’s research.

    For more information about the mangroves in Belize and how scientists conducted some of the research mentioned in your post, your readers might want to watch this video: http://youtu.be/1o4nz0hbR8U

    K.L. McKee
    Scientist Emeritus
    U.S. Geological Survey

    •  Comment from Darci Palmquist

      Hi Karen, Thanks very much for reading our blog and submitting a comment. As you stated, the report is a review of existing research by others and we certainly did not intend to give the impression otherwise. Language in the introduction has been amended in an attempt to clarify that point.

      Best,
      Darci

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