Jonathan M. Willis

Interactive effects of salinity, flooding, and soil type on Panicum hemitomon

It is well documented that Louisiana is experiencing wetland loss at rates greater than any other locale in the world. High rates of relative sea-level rise, a combination of eustatic sea-level rise and subsidence, is anticipated to compound this problem further in the future through increased flooding and encroachment of saline water into freshwater wetlands. The research presented in this paper examines the interactive effect of increased salinity level, flooding depth, and soil type on the growth responses of a dominant Louisiana fresh-water marsh plant, Panicum hemitomon, whose prevalence in Louisiana is currently in decline. This study was conducted under greenhouse conditions and employed a factorial design consisting of three salinity levels (0, 1.5, 3.0 ppt), three hydrologic regimes (0, 10, 20 cm), and two soil types (high organic content, low organic content). Panicum hemitomon productivity was significantly reduced even under the relatively small increases in salinity level (1.5 and 3.0 ppt) imposed in this study. Interestingly, moderate flooding tended to increase productivity, although this relationship was not statistically significant. Significantly greater productivity was observed for plants grown in mineral soil compared with organic soil. These results indicate that any degree of saline influx into P. hemitomon-dominated wetlands will result in decreased vigor and localized decline of this species. Moderate increases in the degree of freshwater inundation may not be as damaging as originally expected and, in fact, may actually stimulate production. However, if increased flooding is accompanied by increased salinity levels, which is anticipated to occur, then the overall effect on this species will be detrimental.

Salt marsh‐mangrove ecotones: using structural gradients to investigate the effects of woody plant encroachment on plant–soil interactions and ecosystem carbon pools

Summary 1.Changing winter climate extremes are expected to result in the poleward migration of mangrove forests at the expense of salt marshes. Although mangroves and marshes are both highly valued ecosystems, the ecological implications of mangrove expansion have not been fully investigated. 2.Here we examined the effects of mangrove expansion on below-ground properties related to peat development and carbon storage. We investigated plant-soil interactions in marshes and across mangrove forest structural gradients in three locations in the northern Gulf of Mexico (USA). We compared our results to those from terrestrial grasslands where the effects of woody plant encroachment are often influenced by rainfall and plant traits. 3.Abiotic conditions at our study locations differed, particularly in terms of physicochemical properties related to precipitation. Marsh species composition, marsh above-ground biomass, and mangrove forest structural complexity also varied across these locations. Marshes in the driest location (Central Texas) had higher salinities and were dominated by low biomass succulent plants and lower soil carbon pools. Marshes in the wetter, less saline locations (Louisiana and North Florida) contained high biomass grasses and higher soil carbon pools. 4.At all locations, above-ground biomass and above-ground carbon pools were higher in mangroves than marshes; however, below-ground soil carbon pools were only higher in mangroves than marshes in the driest location. In the wetter locations, the linkages between mangrove forest structure and soil properties were minimal or not significant. However, in the driest location, there was a significant increase in soil properties related to peat development and carbon storage with increased mangrove forest structural development. 5.Synthesis: Our results indicate that the ecological implications of woody plant encroachment in tidal saline wetlands are dependent upon precipitation controls of plant-soil interactions. Although the above-ground effects of mangrove expansion are consistently large, below-ground influences of mangrove expansion appear to be greatest along low-rainfall coasts where salinities are high and marshes being replaced are carbon poor and dominated by succulent plants. Collectively, these findings complement those from terrestrial ecosystems and reinforce the importance of considering rainfall and plant-soil interactions within predictions of the ecological effects of woody plant encroachment. This article is protected by copyright. All rights reserved.

Impacts of the Deepwater Horizon oil spill on the salt marsh vegetation of Louisiana

The coastal wetland vegetation component of the Deepwater Horizon oil spill Natural Resource Damage Assessment documented significant injury to the plant production and health of Louisiana salt marshes exposed to oiling. Specifically, marsh sites experiencing trace or greater vertical oiling of plant tissues displayed reductions in cover and peak standing crop relative to reference (no oiling), particularly in the marsh edge zone, for the majority of this four year study. Similarly, elevated chlorosis of plant tissue, as estimated by a vegetation health index, was detected for marsh sites with trace or greater vertical oiling in the first two years of the study. Key environmental factors, such as hydrologic regime, elevation, and soil characteristics, were generally similar across plant oiling classes (including reference), indicating that the observed injury to plant production and health was the result of plant oiling and not potential differences in environmental setting. Although fewer significant impacts to plant production and health were detected in the latter years of the study, this is due in part to decreased sample size occurring as a result of erosion (shoreline retreat) and resultant loss of plots, and should not be misconstrued as indicating full recovery of the ecosystem.

Growth Response and Tissue Accumulation Trends of Herbaceous Wetland Plant Species Exposed to Elevated Aqueous Mercury Levels

The impacts of elevated aqueous mercury levels (0, 2, and 4 ppm) on the growth status and mercury tissue concentrations of Eleocharis parvula, Saururus cernuus, Juncus effuses, Typha latifolia, and Panicum hemitomon were determined. Both short-term (net CO2 assimilation) and long-term (biomass) indicators of plant growth status suggest that Eleocharis parvula, Saururus cernuus, and Juncus effuses were relatively unimpacted by elevated mercury levels, whereas Typha latifolia and Panicum hemitomon were somewhat impacted at elevated mercury levels. Eleocharis parvula, Panicum hemitomon, and Typha latifolia generally had the greatest overall belowground tissue concentrations of mercury (2 ppm treatment: 7.21, 7.32, and 9.64 ppm respectively; 4 ppm treatment: 16.23, 18.23, and 13.98 ppm, respectively) and aboveground tissue concentrations of mercury (2 ppm treatment: 0.01, 0.04, 0.02; 4 ppm treatment: 0.26; 0.11; 0.17 ppm, respectively). However, the species investigated in this study demonstrated lower levels of mercury accumulation into tissues when compared with similar investigations of other aquatic plants, suggesting that the above species are not optimal for phytoremediation efforts.

Thresholds in marsh resilience to the Deepwater Horizon oil spill

Ecosystem boundary retreat due to human-induced pressure is a generally observed phenomenon. However, studies that document thresholds beyond which internal resistance mechanisms are overwhelmed are uncommon. Following the Deepwater Horizon (DWH) oil spill, field studies from a few sites suggested that oiling of salt marshes could lead to a biogeomorphic feedback where plant death resulted in increased marsh erosion. We tested for spatial generality of and thresholds in this effect across 103 salt marsh sites spanning ~430 kilometers of shoreline in coastal Louisiana, Alabama, and Mississippi, using data collected as part of the natural resource damage assessment (NRDA). Our analyses revealed a threshold for oil impacts on marsh edge erosion, with higher erosion rates occurring for ~1–2 years after the spill at sites with the highest amounts of plant stem oiling (90–100%). These results provide compelling evidence showing large-scale ecosystem loss following the Deepwater Horizon oil spill. More broadly, these findings provide rare empirical evidence identifying a geomorphologic threshold in the resistance of an ecosystem to increasing intensity of human-induced disturbance.

Evaluation of Enhanced Panicum amarum Establishment through Fragment Plantings and Humic Acid Amendment

Abstract The ability of the dune grass Panicum amarum to resprout from vegetative fragments as well as the response of this species to a commercially available soil conditioner (humic acid) were evaluated using a factorial design in a greenhouse setting. A general trend of greater P. amarum biomass production with humic acid amendment was noted. Panicum amarum growth did not demonstrate either an asymptote or decline with the highest level of humic acid addition, indicating that its growth may potentially be further enhanced by humic acid levels greater than those used in this study. The survivorship of P. amarum fragments (belowground fragment material only and aboveground material fragment only) were comparable with whole-plant planting success with high survivorship noted for all three planting approaches. As anticipated, the whole-plant planting treatments resulted in the greatest total biomass, followed by the belowground-only plantings, which had greater biomass production than the aboveground-only plantings. Overall, the addition of commercial humic acid to the soil during the planting of P. amarum appears to have the potential for enhancing establishment and growth. Importantly, this research demonstrates that the use of P. amarum fragments may be a successful alternative approach in projects where it may be desirable to double planting densities or in situations where whole plant transportation is logistically difficult. Further evaluations of these restoration methods under more realistic field scenarios with longer monitoring of effects are required to determine fully the benefits of these techniques.

Field assessment of the impacts of Deepwater Horizon oiling on coastal marsh vegetation of Mississippi and Alabama

The Deepwater Horizon incident, which occurred in April 2010, resulted in significant oiling of coastal habitats throughout the northern Gulf of Mexico. Although the most substantial oiling of coastal salt marshes occurred in Louisiana, oiling of salt marshes in Mississippi and Alabama was documented as well. A field study conducted in Mississippi and Alabama salt marshes as a component of the Deepwater Horizon Natural Resource Damage Assessment determined that >10% vertical oiling of plant tissues reduced live vegetation cover and aboveground biomass (live standing crop) relative to reference sites in this region through fall 2012. This reduction of live vegetation cover and aboveground biomass appears to have largely resulted from diminished health and vigor of Juncus roemerianus, a key salt marsh species in Mississippi and Alabama. Fewer significant reductions in live vegetation cover and aboveground biomass were detected by the fall 2013 sampling, suggesting that vegetation in oiled salt marshes in this region may have begun to recover. This is corroborated by low levels of Deepwater Horizon oil contamination in these salt marsh soils. However, these findings should be interpreted in the context of the restricted sampling intensity of the present study. Environ Toxicol Chem 2016;35:2791-2797.

Mercury concentrations in oligohaline wetland vegetation and associated soil biogeochemistry

Concentrations of mercury were determined in above- and below-ground tissues of dominant plant species, as well as soils, in the wetlands of Lake Maurepas, Louisiana. Indicators of wetland soil biogeochemical status, such as soil redox potential, pore-water nutrient concentrations, and pore-water total sulfides, were also determined. Total mercury concentrations in plant tissues were within the typical range for vegetation not exposed to mercury contamination. Similarly, total mercury concentrations in soils were typical of uncontaminated wetlands within this geographic region. Soil methyl mercury levels in this study are slightly lower than those reported in other studies of nearby wetlands. This may reflect the less extensive geographic sampling in this study, or the low water levels in the Lake Maurepas system immediately prior to and during this study, which would have altered soil biogeochemical status. This is corroborated by measurements of soil redox potential and soil pore-water nitrogen and sulfur constituents conducted during this study that suggest minimal sulfate reduction was occurring in surficial soils. This study indicates that the wetlands surrounding Lake Maurepas are typical of many uncontaminated oligohaline wetlands in the southeastern U.S. in regard to mercury concentrations.

Importance of Site History and Environmental Setting on Soil Properties in Restored Louisiana Back-Barrier Island Salt Marshes

ABSTRACT Feher, L.C.; Willis, J.M., and Hester, M.W., 2018. Importance of site history and environmental setting on soil properties in restored Louisiana back-barrier island salt marshes. Although previous studies have identified significant differences between the soils of salt marshes restored with dredged material and natural marshes, it is unclear how these differences might affect plant community composition, soil development, and the overall stability of the marsh through time. Using soil shear strength as a metric of soil integrity and resistance to erosion, this study utilized field surveys of one natural and two restored back-barrier island salt marshes in coastal Louisiana to examine differences in shear strength and other soil properties in relation to site history and environmental setting. Soil properties at both restored sites were largely dependent on the source of the fill material in addition to site-specific restoration approaches. Soils at the younger restored marsh were significantly more finely grained than soils at either the older restored marsh or the natural marsh. High soil conductivity resulting from limited tidal exchange likely hindered plant establishment at the younger restored marsh, whereas the soil properties and vegetation cover of the older restored marsh were relatively similar to that of the natural marsh. Soil shear strength was comparable between the natural and restored marshes but was significantly higher in areas with finely grained soils. Therefore, differences in soil properties between the natural and restored marshes may reflect site-specific restoration approaches. As such, sediment source and hydrologic regime are important drivers of back-barrier marsh restoration success that can exert strong influences on soil development and likely the resilience and sustainability of these habitats.

Assessing coastal wetland vulnerability to sea-level rise along the northern Gulf of Mexico coast: Gaps and opportunities for developing a coordinated regional sampling network

Coastal wetland responses to sea-level rise are greatly influenced by biogeomorphic processes that affect wetland surface elevation. Small changes in elevation relative to sea level can lead to comparatively large changes in ecosystem structure, function, and stability. The surface elevation table-marker horizon (SET-MH) approach is being used globally to quantify the relative contributions of processes affecting wetland elevation change. Historically, SET-MH measurements have been obtained at local scales to address site-specific research questions. However, in the face of accelerated sea-level rise, there is an increasing need for elevation change network data that can be incorporated into regional ecological models and vulnerability assessments. In particular, there is a need for long-term, high-temporal resolution data that are strategically distributed across ecologically-relevant abiotic gradients. Here, we quantify the distribution of SET-MH stations along the northern Gulf of Mexico coast (USA) across political boundaries (states), wetland habitats, and ecologically-relevant abiotic gradients (i.e., gradients in temperature, precipitation, elevation, and relative sea-level rise). Our analyses identify areas with high SET-MH station densities as well as areas with notable gaps. Salt marshes, intermediate elevations, and colder areas with high rainfall have a high number of stations, while salt flat ecosystems, certain elevation zones, the mangrove-marsh ecotone, and hypersaline coastal areas with low rainfall have fewer stations. Due to rapid rates of wetland loss and relative sea-level rise, the state of Louisiana has the most extensive SET-MH station network in the region, and we provide several recent examples where data from Louisiana’s network have been used to assess and compare wetland vulnerability to sea-level rise. Our findings represent the first attempt to examine spatial gaps in SET-MH coverage across abiotic gradients. Our analyses can be used to transform a broadly disseminated and unplanned collection of SET-MH stations into a coordinated and strategic regional network. This regional network would provide data for predicting and preparing for the responses of coastal wetlands to accelerated sea-level rise and other aspects of global change.