Ecogeomorphological consequences of climate and human-driven mangrove expansion into salt marshes

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Mangroves – woody plants generally found on tropical coasts – are expanding northward due to warmer winter temperatures. Along the northern Gulf of Mexico coast, and especially in southern Louisiana, mangroves are starting to replace coastal marsh grasses. This could have a positive outcome as mangroves may be more effective than marsh grasses at stabilizing sediment and reducing the extremely rapid rate of marsh loss in the region (known as Louisiana’s Coastal Crisis). Because of the potential positive impact of mangrove colonization, some stakeholders in Louisiana have started planting mangroves through a grass-roots initiative called the Saint Bernard Parish Black Mangrove Planting Project. For this project the investigators will i) quantify the effectiveness of mangroves at reducing marsh edge erosion by wave attack, ii) identify techniques that promote the survival of planted mangroves, and iii) predict future mangrove distribution. This information will directly benefit coastal managers in Saint Bernard Parish, in coastal Louisiana, and in the northern Gulf of Mexico. Stakeholders will be involved through a formalized advisory board, and the results of this project will be communicated through townhall meetings. The investigators will work with the local high school and expand their environmental science curriculum. They will also start a citizen-science mangrove-expansion-monitor-program through a mobile phone application, aimed at informing the general public about plant ecology and coastal erosion. Through a combination of remote sensing analysis and field-based research, the investigators propose to quantify the unconstrained ecological engineering potential of mangroves to reduce marsh edge erosion rates, specifically, and change sediment and soil building dynamics along the shoreline, more broadly. They will identify the conditions that promote mangrove expansion at the fine scale, which is hypothesized to be linked to sediment released by marsh edge retreat, and quantify the ecogeomorphic changes that occur with a vegetation shift from marsh grass to mangrove. Field manipulation experiments will test the best strategies for planted mangrove survivorship and growth (e.g., sediment addition and vegetation removal). A numerical model will be developed to test hypothesis on feedbacks between marsh edge erosion, vertical accretion, and mangrove colonization. They will use a newly developed model as a decision-support tool to produce probabilistic predictions of marsh erosion and mangrove distribution under various climatic (SLR and winter freezes) and management (planting frequency and location) scenarios. Despite the project being focused on Louisiana, the issue of mangrove expansion is relevant for low latitude coastal areas globally.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.