Do plants really reduce salt marsh erosion?

The results of this study challenge the traditional paradigm that plant roots directly prevent erosion along the coast.  Previous studies have focused solely upon the ability of above-ground plant stems and leaves to reduce wave forces in the water column, yet these studies have ignored the physical mechanism that results in the majority of salt marsh erosion – the undercutting of the marsh edge by waves.  To investigate marsh edge erosion, we placed extracted marsh cores into a wave flume and sent waves at them.  We tested for differences in erosion rates between cores with plants and without plants, for differences among plant species, and for differences in soil types.  The results showed that the soil type was the master variable that determined the erosion rate.  The presence of plants or live plant roots made no significant difference upon the erosion rate.  Rather than living plants and roots, we suggest that plant detritus makes the soil more resistant to erosion by lending cohesiveness to the soil, along with the associated changes in bulk density, percent organic matter, and particle size.  Thus, plants do not directly reduce erosion, but do so indirectly through modification of the soil parameters.  Of all the extracted cores that we tested, the dense, coarse, inorganic, and sandy sediment from a restored salt marsh eroded the quickest.  This study is important because it suggests that salt marsh restoration efforts should place the highest priority upon getting the soil right.