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"Ghost cypress"—standing trees killed by increased salinity—indicate sea-level rise (SLR) effects along lower reaches of many coastal plain rivers. Mature cypress can survive indefinitely in permanently flooded sites, but experience mortality at salinities as low as 2 to 3 ppt. Thus, ghost trees in permanently inundated sites can indicate mortality due to increased salinity. Ghost cypress were mapped along the margins of the Neuse River estuary and fluvial-estuarine transition zone (FETZ), along with co-indicators of salinity as a potential cause of death. The distribution was compared with other indicators of upstream propagation of SLR effects; all occurred within a 25 km river reach. Many ghost cypress are consistent with SLR-driven mortality, but in the upper FETZ the co-indicators argue against it, and throughout the study area some ghost cypress lack co-indicators of salinity effects and may have been killed by other factors. The upstream limit of ghost cypress with co-indicators suggesting possible SLR-driven mortality, and the downstream limit of Nyssa aquatica and N. biflora, whose habitats and niches overlap almost entirely with Taxodium except for less salinity tolerance, occur downstream of other indicators of the leading edge of SLR. The furthest upstream is the hydraulic impact of backwater effects on river flow. Downstream, other effects are encountered: a transition from occasionally to frequently flooded wetlands, sedimentary burial of Pleistocene alluvial terraces, and a shift from dominantly mineral floodplain soils to Histosols. The ecological indicators of cypress and tupelo are furthest downstream. Hydraulic (backwater) effects are the leading edge of SLR impacts on the Neuse, trailed by geomorphological, sedimentological, and pedological indicators. Though biota often respond more rapidly to changes than landforms and soils, ecological indicators such as ghost cypress and forest-to-marsh transitions that are salinity dependent are the downstream-most sentinels of sea-level encroachment in rivers. [ABSTRACT FROM AUTHOR]

Sea-level rise and climate change stresses pose increasing threats to coastal wetlands that are vital to wildlife habitats, carbon sequestration, water supply, and other ecosystem services with global significance. However, existing studies are limited in individual sites, and large-scale mapping of coastal wetland degradation patterns over a long period is rare. Our study developed a new framework to detect spatial and temporal patterns of coastal wetland degradation by analyzing fine-scale, long-term remotely sensed Normalized Difference Vegetation Index (NDVI) data. Then, this framework was tested to track the degradation of coastal wetlands at the Alligator River National Wildlife Refuge (ARNWR) in North Carolina, United States, during the period from 1995 to 2019. We identified six types of coastal wetland degradation in the study area. Most of the detected degradation was located within 2 km from the shoreline and occurred in the past five years. Further, we used a state-of-the-art coastal hydrologic model, PIHM-Wetland, to investigate key hydrologic processes/variables that control the coastal wetland degradation. The temporal and spatial distributions of simulated coastal flooding and saltwater intrusion confirmed the location and timing of wetland degradation detected by remote sensing. The combined method also quantified the possible critical thresholds of water tables for wetland degradation. The remote sensing–hydrologic model integrated scheme proposed in this study provides a new tool for detecting and understanding coastal wetland degradation mechanisms. Our study approach can also be extended to other coastal wetland regions to understand how climate change and sea-level rise impact wetland transformations. [ABSTRACT FROM AUTHOR]

The effects of sea level rise and coastal saltwater intrusion on wetland plants can extend well above the high-tide line due to drought, hurricanes, and groundwater intrusion. Research has examined how coastal salt marsh plant communities respond to increased flooding and salinity, but more inland coastal systems have received less attention. The aim of this study was to identify whether ground layer plants exhibit threshold responses to salinity exposure. We used two vegetation surveys throughout the Albemarle-Pamlico Peninsula (APP) of North Carolina, USA to assess vegetation in a low elevation landscape (≤ 3.8 m) experiencing high rates of sea level rise (3–4 mm/year). We examined the primary drivers of community composition change using Non-metric Multidimensional Scaling (NMDS) and used Threshold Indicator Taxa Analysis (TITAN) to detect thresholds of compositional change based on indicator taxa, in response to potential indicators of exposure to saltwater (Na, and the Σ Ca + Mg) and elevation. Salinity and elevation explained 64% of the variation in community composition, and we found two salinity thresholds for both soil Na+ (265 and 3843 g Na+/g) and Ca+ + Mg+ (42 and 126 µeq/g) where major changes in community composition occur on the APP. Similar sets of species showed sensitivity to these different metrics of salt exposure. Overall, our results showed that ground layer plants can be used as reliable indicators of salinity thresholds in coastal wetlands. These results can be used for monitoring salt exposure of ecosystems and for identifying areas at risk for undergoing future community shifts. [ABSTRACT FROM AUTHOR]

Salinization of freshwater ecosystems impacts carbon cycling, a particular concern for coastal wetlands, which are important agents of carbon sequestration. Previous experimental work using salt additions as a proxy for sea level rise, reveals widely divergent effects of salt on soil carbon processes. We performed a laboratory salt addition experiment on two different types of wetland soils (Ponzer muck and Hyde loam, both poorly drained organic soils) from the Coastal Plain of North Carolina. We used a commercial aquarium salt mix to make treatment solutions of 0, 2.5 and 10 ppt salinity and independently manipulated solution pH (5.5, 7.2, 8.8) for a full factorial experimental design. Our goal was to identify the effects of increasing ionic strength and increasing soil solution pH on soil carbon solubility and turnover. Microbial respiration and dissolved organic carbon solubility were depressed by marine salts, while pH manipulation alone had minimal effect. The addition of marine salts substantially reduced rates of carbon mineralization, reduced carbon solubility, and preferentially reduced the abundance of phenolic and aromatic organic molecules in solution. In the more acidic Ponzer muck, where salt additions dropped the pH from > 5 to < 4, we measured more substantial declines in DOC concentrations than in the base saturated Hyde loam. In contrast, in the base saturated Hyde loam, more marine salts remained in solution and the treatment effects on rates of carbon mineralization were more pronounced. Our results provide a clear demonstration of how ion exchange mechanisms result in indirect effects of salinization on the pH of soil solution and the solubility of organic matter. These indirect effects may explain much of the existing variation in reports of salt effects on soil carbon dynamics. [ABSTRACT FROM AUTHOR]

Climate change is driving ecological shifts in coastal regions of the world, where low topographic relief makes ecosystems particularly vulnerable to sea‐level rise, salinization, storm surge, and other effects of global climate change. The consequences of rising water tables and salinity can penetrate well inland, and lead to particularly dramatic changes in freshwater forested wetlands dominated by tree species with low salt tolerance. The resulting loss of coastal forests could have significant implications to the coastal carbon cycle. We quantified the rates of vegetation change including land loss, forest loss, and shrubland expansion in North Carolina's largest coastal wildlife refuge over 35 yr. Despite its protected status, and in the absence of any active forest management, 32% (31,600 hectares) of the refuge area has changed landcover classification during the study period. A total of 1,151 hectares of land was lost to the sea and ~19,300 hectares of coastal forest habitat was converted to shrubland or marsh habitat. As much as 11% of all forested cover in the refuge transitioned to a unique land cover type—"ghost forest"—characterized by standing dead trees and fallen tree trunks. The formation of this ghost forest transition state peaked prominently between 2011 and 2012, following Hurricane Irene and a 5‐yr drought, with 4,500 ± 990 hectares of ghost forest forming during that year alone. This is the first attempt to map and quantify coastal ghost forests using remote sensing. Forest losses were greatest in the eastern portion of the refuge closest to the Croatan and Pamlico Sounds, but also occurred much further inland in low‐elevation areas and alongside major canals. These unprecedented rates of deforestation and land cover change due to climate change may become the status quo for coastal regions worldwide, with implications for wetland function, wildlife habitat, and global carbon cycling. [ABSTRACT FROM AUTHOR]

Background and Aims Coastal plant communities globally are highly vulnerable to future sea-level rise and storm damage, but the extent to which these habitats are affected by the various environmental perturbations associated with chronic salinization remains unclear. In this study, we examine the relationship between North Carolina wetland tree community composition and basal area change and indicators of salinization such as soil salt ion content and elevation. Methods We surveyed 34 forest plots in forested, freshwater wetlands across the Albemarle–Pamlico Peninsula. A subset of our study sites had been sampled during the previous decade as part of the Carolina Vegetation Survey, enabling us to investigate the environmental effects on current community structure, and community change over time. Key Results Multi-variate (ordination) analysis and linear regression models of tree community composition revealed that elevation and soil salt content were correlated with changes in total site tree basal area. Shifts in tree community composition were, however, only weakly correlated with indicators of salinization, specifically elevation, soil sulphate and sodium, but not chloride. While the majority of plots experienced gains in basal area over the past decade, consistent with secondary succession, sites with high soil salt content or low elevation experienced basal area (biomass) loss during the same period. Conclusions The key factors associated with chronic saltwater intrusion (soil ion content) likely explain recent changes in tree biomass, and potential shifts in community composition in low-elevation sites along the North Carolina coast. Not only is it probable that other coastal forest ecosystems worldwide will experience similar stressors and shifts in community biomass and structure as sea levels rise, but the ability of these habitats to deliver key ecosystem services like carbon sequestration and flood defence will be compromised as a result. [ABSTRACT FROM AUTHOR]

Saltwater intrusion is an increasing problem for coastal aquifers due to rising sea levels, especially in areas of low hydraulic gradient, such as barrier-island aquifer systems. Large-scale studies are particularly useful for identifying trends in the effects of sea-level rise, especially for relatively understudied barrier-island aquifers. The barrier islands of coastal North Carolina, U.S.A., lie in a hot spot of sea level rise, where rates are projected to exceed global means throughout this century. Herein, we present a study of the effects of sea-level rise using four primary study sites, each with varying hydraulic properties, island width, and rates of sea-level rise. We use these sites to understand the viability of barrier-island aquifers in North Carolina, but also to understand in general the factors that are most important to the future viability of all barrier-island aquifers. We utilize a one-dimensional steady-state analytical model that allows the calculation of the position of the toe of the saltwater wedge at the base of the aquifer while limiting a rise in the water table. We determine the position of the toe for various sea-level projections up to the year 2100 and utilize this parameter as an assessment of the viability of the aquifer. Our findings suggest that higher island width and lower hydraulic conductivity are the most sensitive parameters and help to limit the effects of sea-level rise on aquifer viability. Aquifer risk maps calculated for the entire North Carolina coast, which equate risk with the position of the toe relative to island width, demonstrate that sea-level rise is projected to have increasing impact on aquifer viability over time, but that areas with narrower width and higher sea-level rise rates are the most vulnerable. • Saltwater intrusion will increase at an exponential rate as sea-level rises along the coast of North Carolina. • Greater island width and lower hydraulic conductivity are the greatest insulators to increased intrusion for barrier-island aquifers. • It is projected that many surficial barrier-island aquifers in North Carolina will lose the entirety of their freshwater lenses by 2080. • The northern section of the North Carolina coast will be at a higher risk than the southern section. [ABSTRACT FROM AUTHOR]