The distribution of carbon (C) within a salt marsh may vary among vegetation zones depending on production and decomposition dynamics and organic and mineral depositional history. We examined spatial and temporal variation of plant and soil C pools within a salt marsh fringing a coastal lagoon along the mid-Atlantic coast of the U.S. The total plant C pool increased from high marsh shrub to low marsh grass dominated areas. Much of the spatial variation in plant C pool was due to fine roots and small organic matter (dlm) that could not be identified by species, which averaged 2398 gC m −2 in Spartina patens-dominated, 2215 gC m −2 in Spartina alternifloradominated, and 676 gC m −2 in Juncus roemerianusdominated areas. Belowground C pool loss was 36% less for S.patens than S.alterniflora and was similar between S. alterniflora and J.roemerianus. Accretion and C accumulation rates were greater in the S.alterniflora-dominated stand than in the J.roemerianus-dominated stand. Our results suggest that landward migration onto terrestrial soils can lead to an estimated 80% increase in belowground plant C composed primarily of fine roots and dlm and 36-70% increase in soil carbon between 15 and 30 cm depths.
Wetland seed banks comprise the propagules of plant species that have species-specific germination requirements for germination in either flooded or dry conditions. At the community level, wetland structure and succession during and after a seasonal flooding event depends upon the early lifehistory requirements of species, including germination under flooded and dry conditions. We examined the effects of simulated flood and post-flood scenarios on seedling emergence from a seed bank of seasonally flooded grassland in the Pantanal, Brazil. Field samplings were conducted in both wet and dry seasons, both of which were subject to flood and post-flood conditions. A total of 70 species emerged from the seed bank, dominated by Poaceae and Cyperaceae. Sixteen species were exclusive to the wet and one exclusive to the dry season. The richness of perennial species was higher under flood conditions, while the richness of annuals was greater postflood. In general, the aquatic and amphibious species exhibited a significant germination response to flooding. Terrestrial species only germinated in post-flood conditions, with higher richness in the dry season. Four species had high seedling abundance in both treatments. The capacity of regeneration by seeds is high in these grasslands and can be increased by
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.
Salt marshes in two contrasting estuaries of the U.S. Mid-Atlantic coast, Barnegat Bay and Delaware Bay, were investigated to identify relationships between rates of sedimentation and marsh hydrogeomorphology. Barnegat Bay is a microtidal lagoon estuary with back-barrier and mainland coastal marshes, whereas Delaware Bay is a micro-mesotidal coastal plain estuary with sediment-rich estuarine marshes. Salt marshes of both estuaries are dominated by Spartina alterniflora. An analysis was performed to characterize marsh hypsometry and tidal flooding characteristics, and a coring study was conducted to measure rates of mineral sediment accumulation, organic matter accumulation, and vertical accretion using 137 Cs and 210 Pb chronology at nine sites in both estuaries. Mineral sediment and organic matter accumulation rates were significantly higher in Delaware Bay marshes (sediment mean and 1: 2.57±2.03 kg m-2 y-1 ; organic: 0.65±0.26 kg m-2 y-1) than in Barnegat Bay (sediment: 0.31±0.27 kg m-2 y-1 ; organic: 0.29±0.08 kg m-2 y-1), as were rates of accretion (Delaware Bay: 0.79±0.06 cm y-1 ; Barnegat Bay: 0.28±0.06 cm y-1). Regression analysis indicated that marsh accretion rates were positively correlated with rates of sediment and organic accumulation, but the upper limit of accretion was governed by sediment accumulation. Tidal flooding frequency and duration did not correlate with marsh accumulation or accretion rates in either estuary, suggesting that hydroperiod is subordinate to sediment availability in governing rates on 50−100 y time scales. If true, natural and (or) human influences on suspended-sediment production and transport in these estuaries has potential to impact marsh accretionary status and stability, independent of sea-level rise.
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.
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