Restoration of Climate Change-Induced Retreat of Tidally Influenced Freshwater Forested Wetlands

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Wetlands in river deltas - like the Mississippi River Delta Plain - may be more vulnerable to sea-level rise. Historically, coastal wetlands responded to these changes by increasing surface elevation or migrating up-slope. USGS conducts research to identify the biogeochemical influences on sediment addition in coastal wetland areas. 

Tidally Influenced Freshwater Forested Wetlands
Tidally influenced freshwater forested wetlands

Science Issue and Relevance: Sea-level rise, which is projected to increase from 0.8 m to 1.5 m by 2100, can result in the excessive inundation of coastal wetlands, anoxic sediment conditions and limited wetland productivity. The vulnerability of wetlands to sea-level rise may be even greater in river deltaic settings, like the Mississippi River Delta Plain, which experience higher rates of relative sea-level rise (1.2-1.65 cm·y-1) due to geological subsidence. Historically, coastal wetlands responded to sea-level rise through in situ increases in relative elevation or up-slope migration. However, anthropogenic alteration of local hydrology restricts fluvial exchange and sediment deposition, which are needed for elevation gain. Additionally, human development prevents upland migration of wetland communities, effectively forcing a net loss in wetland area.

Restoration techniques that focus on increasing wetland surface elevation and restoring natural hydrology may prove suitable in mitigating impacts of sea-level rise and promoting wetland productivity. While research on tidally influenced forested wetlands in the Southeast has documented changes in soil processes along salinity gradients, very little research has been directed toward attempts to restore the edaphic characteristics of healthy stands. More information is needed on how these particular systems will respond to the addition of sediment during periods of sea-level rise.

USGS greenhouse facility located in Lafayette, Louisiana
USGS greenhouse facility located in Lafayette, Louisiana

Methodologies for Addressing the Issue: This study will be conducted in a USGS greenhouse facility located in Lafayette, Louisiana, within the geographical area of our simulated coastal wetland environments. The mesocosm design will provide a controlled setting in which to identify the biogeochemical influences of sediment addition treatments separately from the hydro-edaphic influences of salinity intrusion. Response variables will be measured in three plant communities under non-tidal regimes: intermediate marsh (Schoenoplectus americanus), forested wetland (Nyssa biflora) and a transitional mixed community (S. americanus + N. biflora). Nyssa biflora was chosen because there has been very little focus on this species compared with baldcypress, (Taxodium distichum), yet N. biflora often remain in coastal swamp forests deteriorating due to subsidence – the major focus of this project – in lieu of combinations of salinity intrusion and subsidence. Yet, photosynthesis, leaf xylem pressure potential, height growth, stem biomass, and root biomass of N. biflora seedlings are fairly sensitive to flooding, such that different rates of sediment-slurry addition may prove beneficial to survival and the maintenance of root biomass.

Response variables will be measured in three plant communities
Response variables will be measured in three plant communities

The resulting experimental design is a split-plot design. In this design the whole-plot is a randomized complete block design with two blocks each containing sixteen mesocosms. The CO2 treatment is applied at the whole plot level. The split-plot occurs on the block level, and the main effects of salinity, sediment addition and community type will be applied randomly to the sixteen individual mesocosms within the CO2 treatment blocks. 

Future Steps: Future studies funded by the Climate and Land Use program will investigate the impact of elevated CO2 on carbon storage potential in systems transitioning from tidal freshwater forested wetland to oligohaline marsh.