Climate-driven Avicennia germinans expansion reduces marsh edge erosion in coastal Louisiana (USA)

Subtropical black mangrove (Avicennia germinans) is expanding into higher latitudes and encroaching on existing temperate salt marshes with the potential to alter a variety of ecological processes. In the Mississippi River Delta (Louisiana, U.S.A.), wave-driven marsh edge erosion is on the order of meters per year and accounts for half of the total marsh loss. Here, we show that edge erosion is reduced by 40–60% when colonized by Avicennia as compared to salt marsh dominated by Spartina alterniflora. This reduction is associated with a greater soil shear strength, which in turn is driven by more live belowground biomass and deeper roots. Crucially, Avicennia reduces edge erosion only when well established, i.e., marshes with sparse Avicennia cover had similar edge erosion rates to those with Spartina, likely due to a shallower root biomass. Overall, climate-driven Avicennia expansion will not prevent marsh edge erosion, but can significantly slow it down. Avicennia could be used as a nature-based solution to slow the rate of edge erosion if it is planted inland allowing it 5–10 years to mature before being exposed along the marsh edge.