Abstract: Management of the invasive Phragmites australis haplotype has focused on controlling its abundance in wetlands where it reduces biodiversity. However, little information is available on establishment of native communities and reinvasion by seed following removal using herbicides. The potential for reinvasion and development of native vegetation were evaluated using a seedbank assay and a vegetation survey along gradients from the channel edge to the marsh interior (0, 5 and 20 m distance) in three tidal freshwater marsh sites - Natural, treatment 6 months prior (Treated), and untreated (Phragmites). Recolonization potential from the seedbank was high with >18,500 seedlings m−2 in Treated samples. Richness and density of native species were low in the interior of Treated and Phragmites sites as compared to the Natural marsh. Few species were present in Treated site vegetation 11 months following treatment where P. australis litter comprised a large proportion of the cover. Results indicate that planting native vegetation to outcompete P. australis seedlings and total removal of P. australis to cut off the seed supply may be necessary for successful longer-term restoration and establishment of native species.
Abstract: In situ plant production is often assumed to be the major contributor to organic matter
(OM) accumulation and vertical accretion in tidal marshes. Here, we evaluate the contribution
of mud-associated OM in salt and brackish marshes in Louisiana. Based on 14 soil cores,
the OM content of the mud fraction—i.e., any material smaller than 64 μm—was 17% -
7% for the salt marshes and 28% - 14% for the brackish marshes. This remains nearly
uniform over the top 35 cm depth, suggesting that this material is deposited contemporaneously
with the mud. The dry bulk density of the mud (300–450 kgm−3) is also much lower than
what was estimated using a previously proposed two-constituent mixing model (1990
kgm−3). To reconcile this discrepancy, we developed a modified mixing model that includes
mud OM and differentiates sand as a separate constituent with its high dry bulk density.
The model estimates that mud contributes to ~ 60% of the total marsh vertical accretion
in Louisiana, considerably higher than the ~ 14% estimated with the two-constituent
mixing model. The result, which is a direct consequence of the relatively high porosity
of mud, highlights that mud deposition is crucial for the accretion of microtidal
marshes. Further, the model estimates that the mud OM constitutes ~ 60% of the total
soil OM, emphasizing that in situ plant production is not the only—and, in minerogenic
marshes, not the major—contributor to OM accumulation.
Abstract: Natural disturbances have the potential to limit exotic species invasions. The exotic grass, Urochloa humidicola, has high propagation potential following seasonal flooding, but, flooding tends to reduce the cover. Neither the potential for U. humidicola forma seed bank for recruitment following seasonal flooding, nor themechanism responsible for reducing adult plant cover during floods is currently known. The soil seed bank of a U. humidicola was sampled for 2 years. To examine the potential role of competition with aquatic macrophytes in reducing its cover during floods, cuttings of U. humidicola were flooded in tanks in presence and absence of aquaticmacrophytes for 3 months. Seedling density of U. humidicola was higher post-flood (245 seedlings/m−2) than the post-dry (130 seedlings/m−2). Germination occurred throughout the year, with highest seedling densities at the five to 7 months following soil collection. Competition with aquatic plants significantly increased mortality (c. 70%) of inundated U. humidicola. Our findings suggest that the survival potential of U. humidicola under flood is high, but is limited by shading of aquatic macrophytes. Nonetheless, its seed bank germinates throughout the year, though delayed by flooding. Control of invasive species in grasslands of the Pantanal depend on natural hydrological and biological drives.
Elsey-Quirk T, Graham SA, Mendelssohn IA, Snedden G, Day JW, Twilley RR, Sha G, Sharp LA, Pahl J, Lane RR. 2019. Mississippi River sediment diversions and coastal wetland sustainability: Synthesis of responses to freshwater, sediment, and nutrient inputs. Estuarine, Coastal and Shelf Science 221: 170–183.
Abstract: Management and restoration of coastal wetlands require insight into how inundation, salinity, and the availability of mineral sediment and nutrients interact to influence ecosystem functions that control sustainability. The Mississippi River Delta, which ranks among the world's largest and most productive coastal wetland complexes, has experienced extensive deterioration over the last century due, in large part, to enhanced vulnerability to relative sea-level rise and lateral erosion caused by a combination of natural processes and anthropogenic modifications of hydrology. This land loss crisis has prompted the State of Louisiana to develop a comprehensive restoration plan that includes constructing and implementing a series of large-scale sediment diversions that will reconnect sediment- and nutrient-rich Mississippi River water to adjacent bays, estuaries, and wetlands. Sediment loading through diversions is predicted to enhance the long-term sustainability of coastal wetlands; however, the additive effects of increased inundation, abrubt changes in the salinity regime, and high nutrient loads on wetland plant growth and organic matter (SOM) decomposition rates, which help regulate accretion and elevation change, is uncertain. Therefore, this review attempts to synthesize existing information to inform predictions of the interactive effects of diversions on these drivers of coastal wetland sustainability. The data suggest that sediment deposition within an optimal elevation range will increase the overall productivity of existing wetlands where prolonged flooding does not counter this effect by limiting plant growth. A reduction in salinity may increase plant productivity and cause vegetation shifts to less salt tolerant species, but seasonal swings in salinity may have unforeseen consequences. Nutrient-loading is predicted to lead to greater above- ground productivity, which, in turn, can facilitate additional sediment trapping; however, belowground productivity may decline, particularly in areas where sediment deposition is limited. In areas experiencing net deposition, nutrient-enrichment is predicted to enhance belowground growth into new sediment and contribute to positive effects on soil organic matter accumulation, accretion, and elevation change. Thus, we contend that sediment input is essential for limiting the negative effects of flooding and nutrient-enrichment on wetland processes. These conclusions are generally supported by the biophysical feedbacks occurring in existing pro-grading deltas of the Mississippi River Delta complex.
Abstract: Vegetation plays a key role in influencing the morphodynamics of river deltas, yet channelization of most of the world's rivers limits delta movement and resulting vegetation patterns. Thus, our understanding of vegetation dynamics in newly formed and abandoned deltaic wetlands is still poor. The artificial channel diversion of the mouth of the YellowRiver in 1996 created conditions that mimic a natural delta lobe shift by increasing freshwater, sediment, and nutrient supply to wetlands along the new Yellow River course (NYR) and allowing seawater encroachment in the abandoned YellowRiver course (OYR). To examine the effects of this river channel shift on the vegetation and seed bank structure, above-ground vegetation and seed bank species richness and diversity were examined fromthe channel to the marsh interior in wetlands of both OYR and NYR. A total of 17 plant species were found growing across both sites, 9 species were in OYR and 16 species in NYR. Soil depth did not influence seed bank density in OYR, but the seed bank density in the 0–5 cmsoil layerwas significantly greater than in the 5–10 cm soil layer in NYR. Species diversity of the vegetation and soil seed bank was strongly influenced by soil salinity and hydrology, which varied along the gradient from seaside to river bank. There was a greater separation in species composition between seed bank and vegetation in the OYR than in the NYR. The findings suggest that channel diversion of the Yellow River had a significant effect to the above-ground vegetation. However, the species richness and diversity of soil seed banks in the OYR was similar to that of the NYR, indicating that seed banks had a greater tolerance to external disturbance compared with vegetation.
Abstract: Robust assessments of ecosystem stability are critical for informing conservation and management decisions. Tidal marsh ecosystems provide vital services, yet are globally threatened by anthropogenic alterations to physical and biological processes. A variety of monitoring and modeling approaches have been undertaken to determine which tidal marshes are likely to persist into the future. Here, we conduct the most robust comparison of marsh metrics to date, building on two foundational studies that had previously and independently developed metrics for marsh condition. We characterized pairs of marshes with contrasting trajectories of marsh cover across six regions of the United States, using a combination of remote-sensing and field-based metrics. We also quantified decadal trends in marsh conversion to mudflat/open water at these twelve marshes. Our results suggest that metrics quantifying the distribution of vegetation across an elevational gradient represent the best indicators of marsh trajectories. The unvegetated to vegetated ratio and flood-ebb sediment differential also served as valuable indicators. No single metric universally predicted marsh trajectories, and therefore a more robust approach includes a suite of spatially-integrated, landscape-scale metrics that are mostly obtainable from remote sensing. Data from surface elevation tables and marker horizons revealed that degrading marshes can have higher rates of vertical accretion and elevation gain than more intact counterparts, likely due to longer inundation times potentially combined with internal recycling of material. A high rate of elevation gain relative to local sea-level rise has been considered critical to marsh persistence, but our results suggest that it also may serve as a signature of degradation in marshes that have already begun to deteriorate. This investigation, with rigorous comparison and integration of metrics initially developed independently, tested at a broad geographic scale, provides a model for collaborative science to develop management tools for improving conservation outcomes.
Abstract: Understanding plant species interactions along successional trajectories is critical for managing and restoring ecosystems, as both resource availability and abiotic stresses change over time to affect competitive outcomes and species distributions. Newly created ecosystems experience a succession of plant species and rapid changes in resource availability, which may influence the outcome of biotic interactions. How these biotic interactions vary along abiotic gradients in early successional systems is not well understood. Here, we tested the hypothesis that species-specific tolerances to flooding would influence their relative ability to capture resources (i.e. nutrients) and affect com- petition intensity between pioneer and secondary successional species in an early successional created tidal marsh. We transplanted a competitively dominant higher marsh species, saltmeadow cordgrass (Spartina patens), across an elevation gradient within and outside of clones of a pioneer stress-tolerant low marsh species, smooth cordgrass (S. alterniflora). Within 6 months, S. alterniflora had suppressed the stature and growth of S. patens; however, the magnitude of this competitive effect increased at lower marsh elevations where S. alterniflora was more efficient at capturing available nitrogen (N). In unvegetated areas, where S. patens vigour was high, the amount of available N was approximately 40 times greater than within S. alterniflora clones. Synthesis and applications. Our results demonstrate that competition intensity of the stress-tolerant species over the competitive species depended on relative re- source capture in response to abiotic stress. Managing for specific vegetation communities in marsh restoration, therefore, requires insight into these relationships and interactions. Specifically, marsh restoration in high nutrient environments will limit the succession to high density competitive species due to competition with stress-tolerant pioneer species, particularly at lower elevations.
Abstract: Saline coastal marshes are blue carbon ecosystems with relatively high soil carbon (C) stocks and high rates of soil C accumulation. Loss of saline wetlands due to relative sea‐level rise, land‐use change, and hydrologic alterations liberates previously stored C and reduces the capacity for future C sequestration. Widespread wetland loss has prompted marsh restoration and creation projects around the world; however, little is known about the timescale and capacity for created marshes to function as blue C sinks and the role of environmental conditions in mediating soil C accumulation in restoration sites. Using a chronosequence of five created saline marshes ranging in age from 5 to 32 yr and two adjacent natural reference marshes in southwest Louisiana, USA, short‐ and longer‐term C accumulation rates (SCAR and LCAR, respectively) were determined using feldspar marker horizons and peat depth in cores at six locations in each marsh. Created marshes ranged in elevation from −12 to 41 cm (NAVD88) and supported assorted plant community compositions driven by local environmental conditions. SCAR ranged from 75 to 430 g C·m−2·yr−1, which were comparable in the two youngest and two oldest marshes. Longer‐term CAR ranged from 18 to 99 g C·m−2·yr−1 but did not significantly differ among marshes of different ages. Our findings indicate that LCAR in these created marshes were influenced by site‐specific environmental conditions (i.e., stem density and mineral sediment) rather than marsh age. Results suggest that conditions appropriate for the establishment of vegetation with high stem densities, such as Distichlis spicata and Spartina patens, may facilitate higher LCAR in created marshes, which may be useful for restoration project planning and mitigation of climate change.
Abstract: Tidal wetland fluxes of particulate organic matter and carbon (POM, POC) are important terms in global budgets but remain poorly constrained. Given the link between sediment fluxes and wetland stability, POM and POC fluxes should also be related to stability. We measured POM and POC fluxes in eight microtidal salt marsh channels, with net POM fluxes ranging between −121 ± 33 (export) and 102 ± 28 (import) g OM·m ⁻² ·year ⁻¹ and net POC fluxes ranging between −52 ± 14 and 43 ± 12 g C·m ⁻² ·year ⁻¹ . A regression employing two measures of stability, the unvegetated-vegetated marsh ratio (UVVR) and elevation, explained >95% of the variation in net fluxes. The regression indicates that marshes with lower elevation and UVVR import POM and POC while higher elevation marshes with high UVVR export POM and POC. We applied these relationships to marsh units within Barnegat Bay, New Jersey, USA, finding a net POM import of 2,355 ± 1,570 Mg OM/year (15 ± 10 g OM·m ⁻² ·year ⁻¹ ) and a net POC import of 1,263 ± 632 Mg C/year (8 ± 4 g C·m ⁻² ·year ⁻¹ ). The magnitude of this import was similar to an estimate of POM and POC export due to edge erosion (−2,535 Mg OM/year and − 1,291 Mg C/year), suggesting that this system may be neutral from a POM and POC perspective. In terms of a net budget, a disintegrating wetland should release organic material, while a stable wetland should trap material. This study quantifies that concept and demonstrates a linkage between POM/POC flux and geomorphic stability.
Abstract: Coastal wetlands can serve as a considerable sink for carbon (C) gases. However, the capacity for wetlands to serve as more permanent C and N sinks over the long term is less clear given the time dependence of sediment deposition, organic matter decomposition, and anthropogenic land use change. In this study, we compare the short-term (decadal scale) and long-term (millennial scale) C and N accumulation rates estimated using ¹³⁷Cs and radiocarbon dating of vibracores collected from a freshwater coastal wetland in the Louisiana Mississippi River deltaic plain (Atchafalaya River delta). The mean short-term (60 yrs) sediment accumulation rate was 1.4 cm/yr while the mean rate of long-term (100–1000 yrs) sediment accumulation was an order of magnitude lower at 0.12 cm/yr. Annual rates of C and N accumulated over the past several thousand years were approximately 10% of that over the past 60 years after correcting for bulk density. These results are similar to other coastal wetlands and suggest that time scale must be considered in determining the relative permanence of C and N storage in coastal wetland soils. This difference is especially important for assessing the role of C cycling in relation to global change models and N cycling related to water quality in accurately quantifying the role of coastal deltaic fresh water wetlands in regulating these biogeochemical cycles.
Abstract: Many coastal wetlands are subject to the combined effects of reduced sediment input and increased nutrient loads from watersheds. Restoration strategies focused on increasing marsh elevation and acreage can involve adding sediment through dredge sediment deposition or diversion of river water. It is unclear, however, how sediment inputs influence plant productivity in areas also receiving high nutrient loads. We tested the hypothesis that productivity of Spartina patens is greater with both nutrient and sediment addition than either or neither in a greenhouse experiment. Soil organic matter and nitrogen concentrations were predicted to increase with nutrient addition, but decrease with sediment addition. Plants experienced one of two levels of sediment deposition (control and 4 applications of 2 cm river silt) within either nutrient-enriched (6.96 N, 1.82 P, 1.82 K mg/L) or control tanks. Spartina patens exhibited nearly double the height, stem density, and aboveground biomass in nutrient treatments as compared to controls. Belowground biomass was also stimulated by nutrient-enrichment. Sedimentation reduced the emergence of new stems, but increased fine root biomass. Nutrient enrichment further stimulated root and rhizome growth into added surface sediment. Despite a large plant response to added nutrients, soil properties were unaffected by nutrient-enrichment.
Abstract: In seasonally flooded wetlands, inundation and associated organic debris deposition followed by a drawdown period can promote plant community diversity across space and time. Post-flood regeneration might be influenced by the direct effect of flooding on seed dispersal and seedling emergence, as well as the indirect effect of organic debris on seed trapping and germination. Our objective was to examine the influence of seasonal flooding, topography, and organic debris cover on seedling distribution in a seasonally flooded grassland. We measured species richness, seedling abundance, and organic debris cover for 3 yr in a seasonally flooded grassland in the Pantanal, Brazil, at three topographic levels at the end of the flood season and during the dry season when there was no debris deposition. A total of 43 species were recorded, with no difference in species richness detected between seasons. However, the abundance of some species was higher post-flood than during the dry period. The greatest seedling abundance and richness were found post-flood at intermediate elevations, followed by high and the lowest elevations. Seed germination and seedling establishment were likely suppressed at low topographic positions due to shading from organic debris and poor drainage. Therefore, areas with predictable annual floods promote diversity by creating spatial and temporal variations in environmental conditions.