Projecting future wave attenuation by vegetation from native and invasive saltmarsh species in the United States

Abstract

Saltmarshes are naturally found along gently sloped coastlines of temperate zones including low-energy intertidal estuaries, shallow bays, and on the landward side of barrier islands. Often referred to as one of the most productive, dynamic, and valuable ecosystems on Earth, saltmarshes have been increasingly used as natural and nature-based features (NNBFs) for coastal defense. In preserved conditions, low-lying, regularly flooded marshes in the United States are naturally dominated by a single native species, Spartina alterniflora. However, over the last century the invasive Common Reed (i.e., Phragmites australis), which is directly associated with decreases in plant biodiversity and disruption of natural biochemical cycles, has become the dominant marsh species in North America. Our study quantified the influence of Phragmites australis invasion on future wave attenuation by vegetation when compared to native Spartina alterniflora marshes. We combined the coupled ADCIRC+SWAN with the point-based landscape model SLAMM and field-work-based explicit representation of vegetation to develop a series of modeling simulations based on a combination of low and high-intensity storms, SLR projections, and marsh migration scenarios. Wave attenuation by different marsh species was investigated using average vegetation characteristics (stem height, density, and diameter) for both Phragmites australis-dominated and Spartina alterniflora-dominated bay-wide modeling scenarios. Results are presented as spatially distributed differences in wave attenuation and average wave attenuation curves. Our results show that under current SLR conditions Phragmites australis-dominated marshes can provide significantly more wave attenuation than native Spartina alterniflora marshes during extreme hurricane events. On the contrary, under high-frequency and low-intensity storm events, Spartina alterniflora-dominated marshes are slightly more efficient than invasive Phragmites australis marshes. With the addition of SLR and the increase in the incoming wave heights Phragmites australis marshes can support more wave attenuation than Spartina alterniflora.