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N-EWN publications from our partners at the University of New Hampshire

Our partners at the University of New Hampshire recently published three papers in line with N-EWN research. Read about them below!

Short-Term Recovery of Pilot Living Shoreline Projects for Salt Marsh Habitat in New Hampshire

Over the past 6 years, the New Hampshire (NH) Department of Environmental Services has shifted its preference for shoreline stabilization from traditional engineered shorelines (e.g., seawalls, concrete armoring) to nature-based living shoreline (LS) solutions. To improve the expectations and outcomes of future projects, we monitored three LS pilot projects in the Great Bay Estuary of NH from 2019 to 2022, estimated short-term recovery of the soil biogeochemistry, plant community, and habitat use by fauna, and documented adaptive management needs. Each LS was paired with a nearby (< 200 m) reference salt marsh and a degraded shoreline. After 4 years, halophyte cover had recovered 25–75% in the low marsh and 26–70% in the high marsh. Creation of salt marsh habitat supported similar mummichog population abundances (10–24 indiv. per trap) similar to reference marshes and substantially greater than no action control shorelines (0–1 indiv. per trap). Aerobic or mildly anaerobic reduction–oxidation potentials in the low marsh (14–302 mV) and high marsh (243–266 V) were attributed to rapid drainage of the marsh in two of the LS projects. A novel planting technique of spreading a turf with pre-installed graminoid plugs across the marsh surface at one of the sites may have jumpstarted development of anerobic soil conditions within 3 years (Low: − 192.7 ± 14.9 mV, High: − 119.0 ± 25.3 mV). Opportunistic foraging waterfowl and burial by wrack led to annual replacement replanting and seasonal wrack removal as adaptive management needs of these and future projects. LS projects in the Great Bay Estuary provide functional salt marsh habitat and improved shoreline resilience that can serve as a valuable strategy for coastal restoration.

Runnels Reverse Mega-pool Expansion and Improve Marsh Resiliency in the Great Marsh, Massachusetts (USA)

One of the main mechanisms for salt marsh decline across the United States is the inability of the marsh surface to keep pace with sea level rise. The interior platform is especially vulnerable, leading to the encroachment of short form Spartina alterniflora pannes, pool formation, and ultimately runaway pool expansion if recovery is not possible. Coastal ecologists in New England have been implementing a restoration strategy of runnels, or shallow channels, to enhance drainage of oversaturated and ponding interior marshes. In 2015, runnels were constructed to drain two large and expanding pools in the Great Marsh System of Massachusetts, USA. Vegetation, elevation, and hydrology were monitored using field sampling and remote sensing analysis pre- and post-restoration over seven growing seasons to document the trajectory of the pools and adjacent salt marsh platforms. Pool drainage improved reflecting tidal cycles after three years. Substantial colonization of S. alterniflora and S. patens into the previously unvegetated pools required three growing seasons. In the adjacent platform, S. patens and Distichlis spicata increased in abundance with substantial declines in S. alterniflora. The runnel for one pool became blocked by vegetation after three years and inhibited drainage and recovery of the vegetation in the pool yet not the platform. Runnels may be a viable solution for restoring interior marshes following vegetation loss yet substantial improvements in vegetation and hydrology may require 3 – 5 years and complete recovery of the vegetation community in the regularly drained portion of the system for at least a decade.

Evaluating Thin-Layer Sediment Placement as a Tool for Enhancing Tidal Marsh Resilience: a Coordinated Experiment Across Eight US National Estuarine Research Reserves

Thin-layer sediment placement (TLP) is a promising management tool for enhancing tidal marsh resilience to rising seas. We conducted a 3-year experiment at eight US National Estuarine Research Reserves using a standardized implementation protocol and subsequent monitoring to evaluate effects of sediment placement on vegetation in low and high marsh, and compared this to control and reference plots. Sediments added to experimental plots were sourced from nearby quarries, were sandier than ambient marsh soils, and had more crab burrowing, but proved effective, suggesting that terrestrial sources can be used for tidal marsh restoration. We found strong differences among sites but detected general trends across the eight contrasting systems. Colonization by marsh plants was generally rapid following sediment addition, such that TLP plot cover was similar to control plots. While we found that 14-cm TLP plots were initially colonized more slowly than 7-cm plots, this difference largely disappeared after three years. In the face of accelerated sea-level rise, we thus recommend adding thicker sediment layers. Despite rapid revegetation, TLP plots did not approximate vegetation characteristics of higher elevation reference plots. Thus, while managers can expect fairly fast revegetation at TLP sites, the ultimate goal of achieving reference marsh conditions may be achieved slowly if at all. Vegetation recovered rapidly in both high and low marsh; thus, TLP can serve as a climate adaptation strategy across the marsh landscape. Our study illustrates the value of conducting experiments across disparate geographies and provides restoration practitioners with guidance for conducting future TLP projects.

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