Marsh Migration Research Articles

Assessing the potential long-term effects of sea-level rise on salt marsh’s coastal protective capacity under different climate pathway scenarios

Salt marshes act as natural barriers that reduce wave energy during storm events and help protect coastal communities located in low-lying areas. This ecosystem can be an important asset for climate adaptation due to its particular capability of vertically accrete to adjust to long-term changes in water levels. Therefore, understanding marsh protection benefits thresholds in the face of sea-level rise (SLR) is important for planning future climate adaptation. In this context, the main goal of this manuscript is to examine how the storm protection benefits provided by salt marshes might evolve under SLR projections with different probability levels and emission pathways. In this study, a modeling framework that employs marsh migration predictions from the Sea Level Affecting Marshes Model (SLAMM) as parameterization into a hydrodynamic and wave model (ADCIRC + SWAN) was utilized to explicitly represent wave attenuation by vegetation under storm surge conditions. SLAMM predictions indicate that the SLR scenario, a combination of probability level and emission pathways, plays a substantial role in determining future marsh migration or marsh area loss. For example, results based on the 50% probability, stabilized emissions scenario show an increase of 45% in the marsh area on Maryland’s Lower Eastern Shore by 2100, whereas Dorchester County alone could experience a 75% reduction in total salt marsh areas by 2100 under the 1% probability, growing emissions scenario. ADCIRC + SWAN results using SLAMM land cover and elevation outputs indicate that distinct temporal thresholds emerge where marsh extent sharply decreases and wave heights increase, especially after 2050, and exacerbates further after 2080. These findings can be utilized for guiding environmental policies and to aid informed decisions and actions in response to SLR-driven environmental changes.

Upland vegetation removal as a potential tool for facilitating landward salt marsh migration

To increase the resilience of salt marshes subject to sea‐level rise impacts, managers can focus on interventions within current marsh footprints or in adjacent uplands to facilitate landward marsh migration. The latter approach may be more appropriate when degradation is severe and in situ intervention options are limited. Strategies for facilitating marsh migration include removing artificial barriers, soil grading to reduce steep topography, and manipulating adjacent upland vegetation that can hinder migration, but experiments testing the effectiveness of these activities are limited. We therefore conducted a field experiment to determine if physically removing three upland vegetation types (forest, shrub, and Phragmites australis) adjacent to a Rhode Island salt marsh facilitates short‐term marsh migration. Upland vegetation removal led to increased ambient light in all habitats, significantly enhanced marsh plant cover, extent, and elevation in shrub habitat, and declines in total bird abundance in forest and shrub habitats. Enhanced migration did not occur in forest or Phragmites habitats, and in shrubs, marsh plants only colonized where Baccharis halimifolia, common in upper marsh borders, had been removed. Five years after removal, all upland habitats and associated vegetation were indistinguishable from initial conditions. Our study suggests that upland plant removal might provide a limited window for facilitating salt marsh migration and that more intensive methods may be needed for sustained, longer‐term benefits. It also demonstrates that there may be ecological trade‐offs to consider when altering upland habitats to enhance landward marsh migration.

Massive Sea-Level-Driven Marsh Migration and Implications for Coastal Resilience along the Texas Coast

Massive Sea-Level-Driven Marsh Migration and Implications for Coastal Resilience along the Texas Coast

Characterizing low‐lying coastal upland forests to predict future landward marsh expansion

AbstractSea level rise (SLR) is causing vegetation regime shifts on both the seaward and landward sides of many coastal ecosystems, with the eastern coast of North America experiencing accelerated impacts due to land subsidence and the weakening of the Gulf Stream. Tidal wetland ecosystems, known for their significant carbon storage capacity, are crucial but vulnerable blue carbon habitats. Recent observations suggest that SLR rates may exceed the threshold for elevation gain primarily through vertical accretion in many coastal regions. Therefore, research has focused on mapping the upslope migration of marshes into suitable adjacent lands, as this landward gain may be the most salient process for estimating future wetland resiliency to accelerated rates of SLR. However, our understanding of coastal vegetation characteristics and dynamics in response to SLR is limited due to a lack of in situ data and effective mapping strategies for delineating the boundaries, or ecotones, of these complex coastal ecosystems. In order to effectively study these transitioning ecosystems, it is necessary to employ reliable and scalable landscape metrics that can differentiate between marsh and coastal forests. As such, integrating vegetation structure metrics from light detection and ranging (lidar) could enhance traditional mapping strategies compared to using optical data alone. Here, we used terrestrial laser scanning (TLS) to measure changes in forest structure along elevation gradients that may be indicative of degradation associated with increased inundation in the Delaware Bay estuary. We analyzed a set of TLS‐derived forest structure metrics to investigate their relationships with elevation, specifically seeking those that showed consistent change from the forest edge to the interior. Our findings revealed a consistent pattern between elevation and the plant area index (PAI), a metric that holds potential for enhancing the delineation of complex coastal ecosystem boundaries, particularly in relation to landward marsh migration. This work provides support for utilizing lidar‐derived forest structural metrics to enable a more accurate assessment of future marsh landscapes and the overall coastal carbon sink under accelerated SLR conditions.

Horizontal Rates of Wetland Migration Appear Unlikely to Keep Pace with Shoreline Transgression under Conditions of 21st Century Accelerating Sea Level Rise along the Mid-Atlantic and Southeastern USA

This investigation evaluated two fundamental assumptions of wetland inundation models designed to emulate landscape evolution and resiliency under conditions of sea level rise: that they can (1) migrate landward at the same rate as the transgressing shoreline and (2) immediately replace the plant community into which they are onlapping. Rates of wetland (e.g., marsh, mangrove) migration were culled from 11 study areas located in five regions of focus: Delaware Bay, Chesapeake Bay, Pamlico Sound, South Florida, and Northwest Florida. The average rate of marsh migration (n = 14) was 3.7 m yr−1. The average rate of South Florida mangrove migration (n = 4) was 38.0 m yr−1. The average rate of upland forest retreat (n = 4) was 3.4 m yr−1. Theoretical rates of shoreline transgression were calculated using site-specific landscape slope and scenario-based NOAA sea level rise elevations in 2050. Rates of shoreline transgression over the marsh landscape averaged 94 m yr−1. The average rate of shoreline transgression in the mangrove-dominated areas of South Florida was 153.2 m yr−1. The calculated rates of shoreline transgression were much faster than the observed horizontal marsh migration, and by 2050, the offset or gap between them averaged 2700 m and ranged between 292 and 5531 m. In South Florida, the gap average was 3516 m and ranged between 2766 m and 4563 m. At sites where both horizontal marsh migration and forest retreat rates were available, the distance or gap between them in 2050 averaged 47 m. Therefore, the results of this study are inconsistent with the two fundamental assumptions of many wetland inundation models and suggest that they may overestimate their resilience under conditions of 21st century accelerating sea level rise.

Retreating coastal forest supports saltmarsh invertebrates

AbstractAs sea‐level rise converts coastal forest to salt marsh, marsh arthropods may migrate inland; however, the resulting changes in arthropod communities, including the stage of forest retreat that first supports saltmarsh species, remain unknown. Furthermore, the ghost forest that forms in the wake of rapid forest retreat offers an unknown quality of habitat to marsh arthropods. In a migrating marsh in Virginia, USA, ground‐dwelling arthropod communities were assessed across the forest‐to‐marsh gradient, and functional use of ghost forest and high marsh habitats was evaluated to determine whether marsh arthropods utilized expanded marsh in the same way as existing marsh. Diet and body condition were compared for two marsh species found in both high marsh and ghost forest (the detritivore amphipod, Orchestia grillus, and the hunting spider, Pardosa littoralis). Community composition differed among zones along the gradient, driven largely by retreating forest taxa (e.g., Collembola), marsh taxa migrating into the forest (e.g., O. grillus), and unique taxa (e.g., Hydrophilinae beetles) at the ecotone. The low forest was the most inland zone to accommodate the saltmarsh species O. grillus, suggesting that inland migration of certain saltmarsh arthropods may co‐occur with early saltmarsh plant migration and precede complete tree canopy die‐off. Functionally, O. grillus occupied a larger trophic niche in the ghost forest than the high marsh, likely by consuming both marsh and terrestrial material. Despite this, both observed marsh species primarily consumed from the marsh grass food web in both habitats, and no lasting differences in body condition were observed. For the species and functional traits assessed, the ghost forest and high marsh did not show major differences at this site. Forest retreat and marsh migration may thus provide an important opportunity for generalist saltmarsh arthropods to maintain their habitat extent in the face of marsh loss due to sea‐level rise.

This land is your land, this could be marsh land: Property parcel characteristics of marsh migration corridors in Rhode Island, USA

Salt marshes, critical habitats offering many ecosystem services, are threatened by development, accelerated sea level rise (SLR) and other anthropogenic stressors that are projected to worsen. As seas rise, some salt marshes can migrate inland if there is adjacent, permeable, undeveloped land available. Facilitating marsh migration is necessary for coastal resilience efforts, but extensive coastal development can make finding suitable migration corridors challenging. This work seeks to characterize changes in land use, ownership, and economic value at the property parcel level within current versus future marsh areas for the state of Rhode Island, USA. We find that most parcels currently containing salt marsh are publicly owned, whereas most adjacent parcels projected to contain new salt marsh in 2050 are privately owned. Additionally, parcels containing new marsh in 2050 have 47% higher per-hectare assessed values than parcels containing current marsh. We describe the locations and characteristics of parcels within migration corridors with the lowest per-hectare values that may be the most cost-effective for marsh conservation practitioners to protect. This study highlights the expanding land use types and landowner sets that will be involved in marsh conservation decisions, and the economic value of potential migration corridors where costly tradeoffs may be necessary to promote coastal resilience.

Upland forest retreat lags behind sea-level rise in the mid-Atlantic coast.

Ghost forests consisting of dead trees adjacent to marshes are striking indicators of climate change, and marsh migration into retreating coastal forests is a primary mechanism for marsh survival in the face of global sea-level rise. Models of coastal transgression typically assume inundation of a static topography and instantaneous conversion of forest to marsh with rising seas. In contrast, here we use four decades of satellite observations to show that many low-elevation forests along the US mid-Atlantic coast have survived despite undergoing relative sea-level rise rates (RSLRR) that are among the fastest on Earth. Lateral forest retreat rates were strongly mediated by topography and seawater salinity, but not directly explained by spatial variability in RSLRR, climate, or disturbance. The elevation of coastal tree lines shifted upslope at rates correlated with, but far less than, contemporary RSLRR. Together, these findings suggest a multi-decadal lag between RSLRR and land conversion that implies coastal ecosystem resistance. Predictions based on instantaneous conversion of uplands to wetlands may therefore overestimate future land conversion in ways that challenge the timing of greenhouse gas fluxes and marsh creation, but also imply that the full effects of historical sea-level rise have yet to be realized.

Quantifying the effects of sea level rise driven marsh migration on wave attenuation.

Sea level rise (SLR) is the most significant climate change-related threat to coastal wetlands, driving major transformations in coastal regions through marsh migration. Landscape transformations due to marsh migration are manifested in terms of horizontal and vertical changes in land cover and elevation, respectively. These processes will have an impact on saltmarsh wave attenuation that is yet to be explored. This study stands as a comprehensive analysis of spatially distributed wave attenuation by vegetation in the context of a changing climate. Our results show that: i) changes in saltmarsh cover have little to no effect on the attenuation of floods, while ii) changes in elevation can significantly reduce flood extents and water depths; iii) overland wave heights are directly influenced by marsh migration, although iv) being indirectly attenuated by the water depth limiting effects of water depth attenuation driven by changes in elevation; v) the influence of saltmarsh accretion on wave attenuation is largely evident near the marsh edge, where the increasing elevations can drive major wave energy losses via wave breaking. Lastly, vi) considering the synergy between SLR, marsh migration, and changes in elevation results in significantly more wave attenuation than considering the eustatic effects of SLR and/or horizontal marsh migration alone, and therefore should be adopted in future studies.

Marsh migration and beyond: A scalable framework to assess tidal wetland resilience and support strategic management

Tidal wetlands are critical but highly threatened ecosystems that provide vital services. Efficient stewardship of tidal wetlands requires robust comparative assessments of different marshes to understand their resilience to stressors, particularly in the face of relative sea level rise. Existing assessment frameworks aim to address tidal marsh resilience, but many are either too localized or too general, and few directly translate resilience evaluations to recommendations for management strategies. In response to the deficiencies in existing frameworks, we identified a set of metrics that influence overall marsh resilience that can be assessed at any spatial scale. We then developed a new comprehensive assessment framework to rank relative marsh resilience using these metrics, which are nested within three categories. We represent resilience as the sum of results across the three metric categories: current condition, adaptive capacity, and vulnerability. Users of this framework can add scores from each category to generate a total resilience score to compare across marshes or take the score from each category and refer to recommended management actions we developed based on expert elicitation for each combination of category results. We then applied the framework across the contiguous United States using publicly available data, and summarized results at multiple spatial scales, from regions to coastal states to National Estuarine Research Reserves to finer scale marsh units, to demonstrate the framework’s value across these scales. Our national analysis allowed for comparison of tidal marsh resilience across geographies, which is valuable for determining where to prioritize management actions for desired future marsh conditions. In combination, the assessment framework and recommended management actions function as a broadly applicable decision-support tool that will enable resource managers to evaluate tidal marshes and select appropriate strategies for conservation, restoration, and other stewardship goals.

Projecting future wave attenuation by vegetation from native and invasive saltmarsh species in the United States

Saltmarshes are naturally found along gently sloped coastlines of temperate zones including low-energy intertidal estuaries, shallow bays, and on the landward side of barrier islands. Often referred to as one of the most productive, dynamic, and valuable ecosystems on Earth, saltmarshes have been increasingly used as natural and nature-based features (NNBFs) for coastal defense. In preserved conditions, low-lying, regularly flooded marshes in the United States are naturally dominated by a single native species, Spartina alterniflora. However, over the last century the invasive Common Reed (i.e., Phragmites australis), which is directly associated with decreases in plant biodiversity and disruption of natural biochemical cycles, has become the dominant marsh species in North America. Our study quantified the influence of Phragmites australis invasion on future wave attenuation by vegetation when compared to native Spartina alterniflora marshes. We combined the coupled ADCIRC+SWAN with the point-based landscape model SLAMM and field-work-based explicit representation of vegetation to develop a series of modeling simulations based on a combination of low and high-intensity storms, SLR projections, and marsh migration scenarios. Wave attenuation by different marsh species was investigated using average vegetation characteristics (stem height, density, and diameter) for both Phragmites australis-dominated and Spartina alterniflora-dominated bay-wide modeling scenarios. Results are presented as spatially distributed differences in wave attenuation and average wave attenuation curves. Our results show that under current SLR conditions Phragmites australis-dominated marshes can provide significantly more wave attenuation than native Spartina alterniflora marshes during extreme hurricane events. On the contrary, under high-frequency and low-intensity storm events, Spartina alterniflora-dominated marshes are slightly more efficient than invasive Phragmites australis marshes. With the addition of SLR and the increase in the incoming wave heights Phragmites australis marshes can support more wave attenuation than Spartina alterniflora.

Can the marsh migrate? Factors influencing the growth of Spartina patens under upland conditions

The high elevation salt marsh plant Spartina patens can potentially cope with accelerated sea level rise by migrating inland, but the ability to do so may differ among plant ecotypes. We compared performance among ecotypes collected from three different sites within mesocosms in which we manipulated soil type, plant litter and salinity. Half of our treatment levels simulated conditions plants would encounter when expanding into terrestrial environments (i.e., upland soil, litter present and low salinity); the other half expansion into tidal creeks (i.e., marsh soil, litter absent, and high salinity). Plant litter and salinity did not significantly affect aboveground biomass or rhizome growth and only affected flowering in a three-way interaction with site. However, all three parameters were significantly affected by soil conditions and the site × soil interaction. Upland soil conditions reduced aboveground biomass, rhizome growth and flowering, as compared to marsh soil conditions, for ecotypes from some sites but not others. When just comparing plant performance in the upland soil treatment, ecotypes from some collection sites did better than others. One plausible explanation for this ecotypic variation is pre-adaptation to differences we found in organic matter content among our collection sites, with the ecotype collected from the site with the lowest organic matter content generally being least impacted by upland soil conditions. Our results indicate that S. patens ecotypes can vary in their capacity to successfully expand into uplands, and thus we suggest prioritizing conservation of such ecotypes, as well as their use in restoration efforts. Consideration of ecotypic variation might also prove useful in deciding where to focus conservation efforts for marsh migration.

A marsh multimodel approach to inform future marsh management under accelerating sea‐level rise

Abstract Accelerating sea‐level rise combined with the stresses of human land use threatens the persistence of tidal marshes. The proper management of existing marshes and the conservation of lands for marsh migration require a synthesis of factors affecting future marsh evolution. There are a number of existing marsh models, with different parameters and run at different scales, that can assist in this type of assessment. However, as with many models forecasting future conditions, there is no clearly identified ‘best’ model and they all perform slightly differently across different scenarios and with different suites of available data inputs. In this paper, we worked with local and regional managers to inform the development of an ensemble methodology that uses results from multiple marsh models in conjunction with social, land use and environmental data to inform marsh management, conservation and restoration under sea‐level rise. The methodology was developed and tested in three locations in the Virginia portion of the Chesapeake Bay, United States, using existing marsh migration models already run throughout the Bay. Stakeholder groups of local decision makers and a steering committee composed of regional managers were engaged in the process to ensure that the resulting methodology met current management needs. The need for a multimodel approach to identifying marsh migration pathways was supported by the marsh migration comparison done during methodology development which showed disparate results from multiple marsh migration models. Five existing marsh model outputs were combined into a single Marsh Migration Corridor Envelope (MMCE), which encompasses the potential area of current upland expected to become marsh under a selected sea‐level rise scenario. Within the identified MMCE, land covers were assessed for suitability for marsh support and the socio‐economic context of the parcels of land was considered. Last, the current condition of existing marsh on the properties was assessed to determine preservation activities that can increase their longevity. Together, these pieces of information inform a physical and sociological understanding of tidal marshes that can allow for a management framework that incorporates both current and future concerns.

Using Geospatial Analysis to Guide Marsh Restoration in Chesapeake Bay and Beyond

Coastal managers are facing imminent decisions regarding the fate of coastal wetlands, given ongoing threats to their persistence. There is a need for objective methods to identify which wetland parcels are candidates for restoration, monitoring, protection, or acquisition due to limited resources and restoration techniques. Here, we describe a new spatially comprehensive data set for Chesapeake Bay salt marshes, which includes the unvegetated-vegetated marsh ratio, elevation metrics, and sediment-based lifespan. Spatial aggregation across regions of the Bay shows a trend of increasing deterioration with proximity to the seaward boundary, coherent with conceptual models of coastal landscape response to sea-level rise. On a smaller scale, the signature of deterioration is highly variable within subsections of the Bay: fringing, peninsular, and tidal river marsh complexes each exhibit different spatial patterns with regards to proximity to the seaward edge. We then demonstrate objective methods to use these data for mapping potential management options on to the landscape, and then provide methods to estimate lifespan and potential changes in lifespan in response to restoration actions as well as future sea level rise. We account for actions that aim to increase sediment inventories, revegetate barren areas, restore hydrology, and facilitate salt marsh migration into upland areas. The distillation of robust geospatial data into simple decision-making metrics, as well as the use of those metrics to map decisions on the landscape, represents an important step towards science-based coastal management.

Effect of Seashore Distance on potential distribution, floristic Diversity, and salt marsh Migration of Mareotic halophytes in Egypt

Effect of Seashore Distance on potential distribution, floristic Diversity, and salt marsh Migration of Mareotic halophytes in Egypt

Multi-temporal high-resolution marsh vegetation mapping using unoccupied aircraft system remote sensing and machine learning

Coastal wetlands are among the most productive ecosystems in the world and provide important ecosystem services related to improved water quality, carbon sequestration, and biodiversity. In many locations, wetlands are threatened by coastal development and rising sea levels, prompting an era of tidal wetland restoration. The creation and restoration of tidal marshes necessitate the need for ecosystem monitoring. While satellite remote sensing is a valuable monitoring tool; the spatial and temporal resolution of imagery often places operational constraints, especially in small or spatially complex environments. Unoccupied aircraft systems (UAS) are an emerging remote sensing platform that collects data with flexible on-demand capabilities at much greater spatial resolution than sensors on aircraft and satellites, and resultant imagery can be readily rendered in three dimensions through Structure from Motion (SfM) photogrammetric processing. In this study, UAS data at 5 cm resolution was collected at an engineered wetland at Poplar Island, located in Chesapeake Bay, MD United States five times throughout 2019 to 2022. The wetland is dominated by two vegetation species: Spartina alterniflora and Spartina patens that were originally planted in 2005 in low and high marsh elevation zones respectively. During each survey, UAS multispectral reflectance, canopy elevation, and texture were derived and used as input into supervised random forest classification models to classify species-specific marsh vegetation. Overall accuracy ranged from 97% to 99%, with texture and canopy elevation variables being the most important across all datasets. Random forest classifications were also applied to down-sampled UAS data which resulted in a decline in classification accuracy as spatial resolution decreased (pixels became larger), indicating the benefit of using ultra-high resolution imagery to accurately and precisely distinguish between wetland vegetation. High resolution vegetation classification maps were compared to the 2005 as-built planting plans, demonstrating significant changes in vegetation and potential instances of marsh migration. The amount of vegetation change in the high marsh zone positively correlated with interannual variations in local sea level, suggesting a feedback between vegetation and tidal inundation. This study demonstrates that UAS remote sensing has great potential to assist in large-scale estimates of vegetation changes and can improve restoration monitoring success.

Assessing the Impact of Future Sea Level Rise on Blue Carbon Ecosystem Services on Long Island, New York

Salt marsh ecosystems provide critical climate mitigation ecosystem services through carbon sequestration. Sea level rise (SLR) has variable effects on these ecosystems, both driving marsh migration into upland areas and causing inundation and erosion that reduces marsh extent. How salt marsh carbon sequestration responds to SLR thus represents an important carbon cycle feedback to climate change. Here, we examine the consequences of one meter (1 m) of SLR for salt marsh ecosystem carbon sequestration for Long Island, New York and for the North Fork peninsula in far northeastern Long Island using three different assumptions for salt marsh carbon sequestration rates. For the entirety of Long Island, SLR will reduce future carbon sequestration by 22 million tons of carbon dioxide (CO2) by 2100 under the medium sequestration rate assumption compared to a no-SLR scenario. This represents a net loss of $137.5 billion in carbon sequestration ecosystem service value due to SLR. On the North Fork peninsula, however, SLR increases sequestration by 370,000 tons of CO2 with a medium sequestration rate assumption relative to a no-SLR scenario. However, the magnitude of uncertainty in future carbon sequestration due to different assumptions of carbon sequestration rates is greater than the impact of SLR on carbon sequestration, pointing to the need for the use of field-based measurement of sequestration rates in managing coastal ecosystem response to climate change.

Hazardous and contaminated sites within salt marsh migration corridors in Rhode Island, USA

As salt marshes attempt to migrate upland due to sea level rise, they will encounter many kinds of land development and infrastructure in highly populated, urbanized coastal communities. Hazardous and contaminated sites (HCSs) -- facilities and infrastructure that store, use, or release harmful substances -- are particularly concerning obstacles to salt marsh migration because of their potential to release contaminants if their structural integrity is compromised. Inventorying HCSs within migration pathways can inform coastal resilience planning. To understand what kinds of HCSs migrating marsh may encounter in Rhode Island, USA, we inventoried sites from federal and state sources, assigned contaminant hazard rankings to most sites, and overlayed them with projected marsh migration corridors. We found that HCSs are extensive across marsh migration corridors in the state, especially in urban areas. Among the most common HCSs in and around Rhode Island salt marshes are stormwater outfalls, underground storage tanks, and facilities registered with EPA's Resource Conservation and Recovery Act (RCRA) or EPA's National Pollutant Discharge Elimination System (NPDES). These sites pose varying hazards to human and aquatic life if breached, with some sites representing little or no threat but most posing some degree of hazard to their surroundings. This coastal HCSs inventory can inform prioritization and management of coastal salt marshes subject to accelerated sea level rise. Management decisions such as allowing marsh migration, implementing adaptation actions to build salt marsh elevation, or erecting physical barriers at marsh sites will influence future salt marsh extent, marshes' ability to provide ecosystem services, and public health exposures to toxic releases. In addition, as Rhode Island and other coastal states work to promote coastal resiliency, this type of inventory can inform decisions about which HCSs to prioritize for remediation and other climate adaptation actions. Marsh migration is just one potential consequence of sea level rise, so many of the considerations outlined here are widely applicable to the broader goal of preparing coastal communities for rising seas.

Lidar-Imagery Fusion Reveals Rapid Coastal Forest Loss in Delaware Bay Consistent with Marsh Migration

Tidal wetland ecosystems and their vegetation communities are broadly controlled by tidal range and inundation frequency. Sea-level rise combined with episodic flooding events are causing shifts in thresholds of vegetation species which reconstructs the plant zonation of the coastal landscape. More frequent inundation events in the upland forest are causing the forest to convert into tidal marshes, and what is left behind are swaths of dead-standing trees along the marsh–forest boundary. Upland forest dieback has been well documented in the mid-Atlantic; however, reliable methods to accurately identify this dieback over large scales are still being developed. Here, we use multitemporal Lidar and imagery from the National Agricultural Imagery Program to classify areas of forest loss in the coastal regions of Delaware. We found that 1197 ± 405 hectares of forest transitioned to non-forest over nine years, and these losses were likely driven by major coastal storms and severe drought during the study period. In addition, we report decreases in Lidar-derived canopy height in forest loss areas, suggesting forest structure changes associated with the conversion from forest to marsh. Our results highlight the potential value of integrating Lidar-derived metrics to determine specific forest characteristics that may help predict future marsh migration pathways.

Integrated Modeling of Dynamic Marsh Feedbacks and Evolution Under Sea‐Level Rise in a Mesotidal Estuary (Plum Island, MA, USA)

AbstractAround the world, wetland vulnerability to sea‐level rise (SLR) depends on different factors including tidal regimes, topography, creeks and estuary geometry, sediment availability, vegetation type, etc. The Plum Island estuary (PIE) is a mesotidal wetland system on the east coast of the United States. This research applied a newly updated Hydro‐MEM (integrated hydrodynamic‐marsh) model to assess the impacts of intermediate‐low (50 cm), intermediate (1 m), and intermediate‐high (1.5 m) SLR on marsh evolution by the year 2100. Model advancements include capturing vegetation change, inorganic and below and aboveground organic matter portion of marsh platform accretion, and mudflat creation. Although the results indicate a low vulnerability marsh at the PIE, the vegetation changes from high to low marsh under all SLR scenarios (2%–22%), with the higher bounds belonging to higher rise scenarios. Lower SLR produces more productive marsh (13% gain in high productivity regions), whereas the highest SLR scenario causes increased tidal inundation, which leads to loss in productivity (12% change from high to low productivity regions), generation of mudflats (17% of the domain land), and marsh migration to higher lands. Sensitive nonlinear tidal flow changes, which may be increased or decreased with SLR as a result of mudflat creation, marsh migration, and bottom friction change, emphasize the importance of integrated modeling approaches that include dynamic marsh feedbacks in hydrodynamic modeling and varying hydrodynamic effects on the marsh system.