Introduction

Wetland Loss and Restoration

Wetland loss is the result of a combination of factors which can be summarized as the balance between accretion and submergence. A stable salt marsh is one that has a higher net accretion of soils via deposition of sediment organic matter on the marsh substrate. Anthropogenic impacts on wetlands are widespread, in particular, climate change, altered sediment supply, and coastal development has led to the loss of valuable salt marshes and marine wetlands . On the current trajectory, salt marshes are expected to be reduced by 20-45% by the end of the 21^st century. Die-off of salt marsh vegetation, even in short episodes, can lead to rapid subsidence, erosion, and diminished sediment deposition rates accelerating further wetland loss. Salt marsh vegetation is shown to be resilient to wetland loss under favorable conditions of sediment supply, and this is driven in part by vegetative growth rates in both the rhizosphere and canopy. As eustatic and relative SLR increases, it is likely that coastal habitats will become increasingly vulnerable to these changes, and there seems to be a threshold where the rate of relative SLR can be greater than that the wetland vegetation can sustain, resulting in future devastating wetland loss.

Construction of coastal wetlands in the United States began in the late 20^th century and has become even more prominent today as the state of coastal wetlands has gotten increasingly dire. Restoration in the northern Gulf of Mexico can include de-novo construction of lost marsh platforms using fill or dredged sediments, thin-layer placement in existing marsh platforms, or construction of soil islands and cheniers that form a localized sediment source over time. Depending on the desired habitat, restoration project managers can opt to either plant the material with target vegetation or allow the site to be naturally colonized in hopes that natural ecological succession will follow a desired trajectory. The success of constructed wetlands is determined by management’s specific goals for the project, but factors that contribute to this success have been studied heavily. To ensure adequate colonization of wetlands by planted vegetation, the factors that must be considered are: 1) elevation, 2) planting density, 3) planting material (e.g., seeds, transplanted plugs, rhizomes), 4) physical and chemical sediment characteristics, and 5) fertilizer usage. These factors have varying purposes in wetland construction, but their core necessity is that they are required to ensure an adequate cost:benefit ratio to project managers and funding sources. Planting density depends on how the risks of physical stress compare to stress from competition. The starting material for transplanting salt marsh vegetation varies among species, but bareroot plants are appropriate for transplanting most grasses, according to the United States Department of Agriculture. Physical sediment characteristics such as texture, porosity, and bulk density can affect the aeration of the rhizosphere and thereby influence rhizobacteria and root growth. Fertilizers which add inorganic N and P are often applied and can shorten the period between planting and establishment of transplanted vegetation.

Restoration projects that construct Spartina-dominated marshes have been well explored in the past, but Juncus-dominated marshes are not as common. Assessment of salt marsh restoration projects should be done to observe the site’s progress towards pre-established goals, however, specific and time-oriented goals are often missing from project proposals. In these cases, effective criteria for assessing constructed wetlands often include collecting data concerning plant community diversity, biomass, and soil organic content and comparing them to a natural reference site over time.

Development of restored/constructed sites

The rate at which community composition, biomass, and sediment characteristics change post-construction varies among projects. Biomass is often measured in both the canopy and rhizosphere. The canopy of restored marshes are typically comparable to a natural reference site within 2-5 years, while root biomass can take upwards of 15 years. Soil organic matter increases over time as accumulated detritus is exported into the soil and becomes buried; as anaerobic conditions increase with depth, organic matter decomposition is further reduced. This carbon pool typically develops to natural levels in restored sites after 3-5 years, but this can take more than 10 years in some cases.

Restoration of a wetland is intertwined with the succession of species and substrate change to a desired endpoint. Restoration can best achieve a desired community structure by manipulating the factors that control succession on a local scale such as seed supply, substrate changes, elevation, and nutrients. Restoration managers can aid plant community structure development by reducing the use of old soils which are low in phosphorus. If old soils must be used, they can be restored by finding a balance between sufficient fertilization that promotes succession and excessive fertilization that favors strong competitors, which could reduce biological diversity and inhibit further succession. Succession within salt marshes is driven by 1) competition among plant species within the middle and high marsh zones, 2) storm events which can displace diverse communities and enable invasive species to take hold, and 3) conversion of high marsh to low marsh due to SLR. Community composition in restored marshes should have similar stages of primary succession to a reference site, however, the path of succession can be delayed by disturbances such as tropical storms and hurricanes. The path of succession in a restored salt marsh is sometimes used as an indicator of the marshes progress towards a reference site, however, inclusion of other indicators (e.g., structural, functional, landscape) can add in the assessment of a restored site’s ecological function.

Restored marsh characteristics such as coverage, species richness, and biomass vary in developmental trajectory with geomorphic position, tidal range, salinity, and soil classification creating additional complexity in determining restoration trajectory. Due to the relative infancy of coastal marsh restoration and the rarity of long-term monitoring of restoration projects, there is a shortage of data concerning the development of a single site over a time-period greater than fifteen years. Plant coverage of restored marshes can develop to reference levels as quickly as one year when planted with vegetation, or can take up to five years if a site is left only to naturally recruit plant species. Development of belowground biomass, which plays a role in carbon sequestration and marsh sustainability, varies among species as there are species-specific adaptations to abiotic stressors such as salinity sulfide-toxicity. Metabolic demand in producing root biomass is also species-specific. Due to the lack of long-term data on the trajectory of constructed salt marshes, it is imperative that projects are monitored for their progress to inform future efforts to restore similar systems and in the long-term inform on the resilience of these systems to climate change and SLR.