Tuesday, June 9, 2009

plants vs. soil - a meaningless fight

Salt marshes and mangroves are reported to protect coasts from erosion and reduce storm surges. Salt marsh area has been correlated with reduced property damage from hurricanes along the Gulf Coast (Costanza et al. 2008, Ambio), and mangrove forest purportedly protected villages and reduced the death toll of a 1999 Indian cyclone (Das and Vincent 2009, PNAS).

However, a new study about to be published in Proceedings of the National Academy of Sciences (PNAS) reports that the soil, not the vegetation, in marshes is the important the feature preventing coastal erosion. The study, led by Rusty Feagin of Texas A&M, put large cores of marsh with and without vegetation in a wave tank and observed erosion patterns. Vegetation did not affect the rate of erosion, but sandy soils eroded more quickly than organic soils. The report questions the efficacy of salt marsh restorations that aim to prevent coastal erosion.

I look forward to reading the full publication. While I am sure it is a worthy scientific contribution, I think drawing conclusions about restoration efforts from these findings is misguided. The key about marshes is that they are biogenic - the organisms (in this case, plants) build the habitat. Were the plants not present, there would not be bare soil, but rather open water. Therefore, the idea of erosion protection from marsh soils without plants is nonsensical. Additionally, the key to maintaining the habitat is not erosion resistance, but sediment balance: how quickly sediment is added compared to the rate it is eroded. The plants likely act on the sediment addition end, even if they don't always contribute to the prevention of erosion (although they likely also do this in peaty soils, like in New England marshes). Finally, the coastal protection value of marshes comes from erosion prevention AND wave attenuation. In models, it is mostly the wave attenuation process that affords coastal protection from storms.

You can read a brief about the article published online at Nature.

1 comment:

  1. Thanks for the opportunity to keep this discussion going, it is fun to think about these things.

    I agree that it does not make much sense to do a salt marsh restoration project without plants. As you state, the ecological engineering (accretion) will be critical to maintaining the marsh elevation with respect to sea level rise. I see this as a long term process. It also goes hand in hand with the plants altering the soil over the long term, such that it becomes more resistant to erosion. But, that does not mean that roots hold the soil together during a short term wave event, as shown in our study.

    A related story, which plays on these two different processes, is that there are a lot of large salt marsh restoration projects going on around here, but the problem has been that they erode away, especially in the areas exposed to wave attack. Our work lends explanation to the idea that the problem was that these areas were constructed with sediment that was too sandy to withstand such erosive forces, even as it was vegetated. So, it is easy for me to say that next time, they need to get the soil right, irrespective of the plants, as that is the key problem. However, in reality, it is very difficult to find a good marsh soil somewhere else without destroying another marsh, these soils take decades to produce around here. And, as others have presented, the cost of trucking sediment can become prohibitive. So, what can we do? Well, in the last ten years, they have been constructing geotubes, large socks filled with sand, and placing them in front of the marsh edge as breakwaters. This sort of works. A problem, though, is that if you barricade the marsh too much, then it will drown from sea level rise in the long term, because even though tidal flow can get in there, there just is too much barricading to get the sediments in very well. So, it's short term wave protection at the expense of long term sea level rise. How can you do both? Since our experiment a few years ago, they actually started building much more permeable barriers, in fact at the location where we took some of our samples. This is nice, because it breaks up the waves, and still allows the tidal flow to carry plenty of sediments. Still, it stunk because it was a perfect marsh edge to do field research on, but at least it has stopped eroding now. It does look ugly, though.


    Dr. Rusty Feagin
    Spatial Sciences Laboratory
    Dept. Ecosystem Science & Management
    Texas A&M University