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Abstract The frequency of salt marsh dieback events has increased over the last 25 years with unknown consequences to the resilience of the ecosystem to accelerated sea level rise (SLR). Salt marsh ecosystems impacted by sudden vegetation dieback events were previously thought to recover naturally within a few months to years. In this study, we used a 13‐year collection of remotely sensed imagery to provide evidence that approximately 14% of total marsh area has not revegetated 10 years after a dieback event in Charleston, SC. Dieback onset coincided with a severe drought in 2012, as indicated by the Palmer drought stress index. A second dieback event occurred in 2016 after a historic flood influenced by Hurricane Joaquin in 2015. Unvegetated zones reached nearly 30% of the total marsh area in 2017. We used a light detection and ranging‐derived digital elevation model to determine that most affected areas were associated with lower elevation zones in the interior of the marsh. Further, restoration by grass planting was effective, with pilot‐scale restored plots having greater aboveground biomass than reference sites after two years of transplanting. A positive outcome indicated that the stressors that caused the dieback are no longer present. Despite that, many affected areas have not recovered naturally, even though they are located within the typical elevation range of healthy marshes. A mechanistic modeling approach was used to assess the effects of vegetation dieback on salt marsh resilience to SLR. Predictions indicate that a highly productive restored marsh (2000 g m−2 year−1) would persist at a moderate SLR rate of 60 cm in 100 years, whereas a nonrestored mudflat would lose all its elevation capital after 100 years. Thus, rapid restoration of marsh dieback is critical to avoid further degradation. Also, failure to incorporate the increasing frequency and intensity of extreme climatic events that trigger irreversible marsh diebacks underestimates salt marsh vulnerability to climate change. Finally, at an elevated SLR rate of 122 cm in 100 years, which is most likely an extreme climate change scenario, even highly productive ecosystems augmented by sediment placement would not keep pace with SLR. Thus, climate change mitigation actions are also urgently needed to preserve present‐day marsh ecosystems.
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The Effect of Runnels on Salt Marsh Sediment Dynamics, Vegetation, and Nitrogen Cycling
Abstract Runnels, a climate adaptation technique that drains surface water to restore marsh vegetation and habitat, are increasingly being used to prevent the formation of shallow water impoundments or pannes in salt marshes that result in the loss of important ecosystem services. However, we know little about the effect of runnels on salt marsh biogeochemistry. This study measured how sediment characteristics and rates of nitrogen cycle processes were altered by impounded water and vegetation loss, and whether runnels can restore these marsh attributes to reference conditions. Impounded areas were 52 ± 4% less vegetated than nearby intact marsh, with 11 ± 2% less organic matter and 24 ± 5% higher bulk density. Additionally, impoundments removed 32 ± 32 µmol N m−2d−1less than reference marsh areas via denitrification. At six of the 11 runneled sites, vegetation percent cover increased by 40 ± 5%, accompanied by a 7 ± 3% recovery of organic matter and a 9 ± 6% reduction of bulk density. At sites where vegetation recovered to within 70% of reference plots at a site, runneled plots removed 97 ± 31 µmol more N m−2d−1than impoundments, which was also 82 ± 31 µmol more N m−2d−1than reference areas. The driver of recovery is related to initial site conditions, including higher redox potentials and lower porewater salinities, compared with sites where revegetation was unsuccessful. The extent of runnel effectiveness and the recovery of vegetation, sediment characteristics, and nitrogen cycle processes was variable among runneled marshes, and the effectiveness of runnels may depend on initial site-specific characteristics and degree of initial degradation.
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- Award ID(s):
- 2203322
- PAR ID:
- 10573661
- Publisher / Repository:
- Springer Science + Business Media
- Date Published:
- Journal Name:
- Estuaries and Coasts
- Volume:
- 48
- Issue:
- 3
- ISSN:
- 1559-2723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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