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.

lake blue

From maritima

Glacial lakes (fed primarily by glacial meltwater) are often a milky blue-green. These surreal lakes captivated me when I visited Torres del Paine National Park in Chile in October 2008. A park ranger explained to me that the unique color comes from suspended sediment, or "rock flour," reflecting sunlight. When I visited Mendoza, in the foothills of the Argentine Andes, in November, I noticed the same color quality in a lake there (see images of Pehoe Lake in Torres del Paine and the lake in Mendoza above). I think that the blue color in the Mendoza lake is probably also related to suspended sediment - there must be impressive erosion from the Andes each year. The lake was fed by a turbid, muddy, braided river. Water color can be affected by a number of factors, such as algal growth, bottom color, and depth, so I can't be sure, but it seems highly likely that the same process of erosion and sediment suspension caused this lovely scene in dry, sunny Mendoza as in the famously harsh landscape of Torres del Paine.

From maritima