Marsh Vulnerability to Sea Level Rise Beyond 2100

As the global rate of sea level rise accelerates, salt marshes unable to keep pace vertically or laterally become vulnerable to drowning. We evaluated the vulnerability of a 40 km   salt marsh in Georgia, USA, to historical and accelerated rates of sea level rise. We expected the marsh to be resilient because it receives high sediment inputs and has room to migrate landward. However, sediment cores showed marsh accretion rates were lower than the historical rate of sea level rise and independent of elevation. Using a vertical accretion model, we found the extent of the Georgia marsh was stable through 2100 under historical and accelerated sea level rise scenarios because it relied on elevation capital rather than sufficient rates of accretion or migration. Beyond 2100, however, the marsh rapidly lost area as it depleted its elevation reserve under an accelerated rate of sea level rise; by 2160, only 12% of the initial marsh area remained. 

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This work highlights the role of elevation capital in short-term salt marsh stability and the need to look beyond 2100 to capture long-term marsh vulnerability. This work is part of my postdoctoral research conducted at VIMS. Paper forthcoming in Estuarine, Coastal and Shelf Science.

Marsh Vulnerability to Sea Level Rise Beyond 2100

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Elevation Capital Extends Life of Marshes in the Sediment-deficient Plum Island Estuary, MA

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Coastal salt marshes naturally maintain their elevation relative to sea level through feedbacks that influence vertical accretion and landward migration. However, when a marsh is limited by how much it can grow vertically or migrate laterally, it becomes vulnerable to drowning. The Plum Island Estuary in MA is a sediment-deficient system bordered by steep uplands. As such it is a classic example of a marsh system we expect to drown under an accelerated rate of sea level rise.

We applied a new vertical accretion model to Plum Island marshes and found that even though the marshes can't keep pace with accelerated rates of sea level rise, they maintain their areal extent through 2100. Why don't they drown? Because Plum Island marshes have high elevation capital. They can afford to lose elevation and still support marsh vegetation. However, we predict widespread conversion of high to low marsh and eventual drowning beyond 2100. Plum Island highlights how marshes with high elevation capital can survive for decades to centuries even under conditions in which they will inevitably drown. This work is part of my postdoctoral research conducted at VIMS. Check out my paper in Limnology and Oceanography to learn more.

Elevation Capital Extends Life of Marshes in the Sediment-deficient Plum Island Estuary, MA

New Marsh Accretion Model: Spatially-explicit Model Accounts for Elevation and Landscape Position

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The vulnerability of a salt marsh to drowning due to sea level rise depends on 2 main processes: vertical accretion and lateral migration. Most point-based vertical accretion models predict accretion and migration based solely on elevation. We developed a new spatially-explicit, point-based vertical accretion model that calculates accretion rates depending on both elevation and landscape position relative to the nearest channel. We do so by incorporating variable concentrations of suspended sediment based on distance from the nearest sediment source (ie, a channel).

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By considering elevation and landscape position, we can more accurately capture changes in accretion and landward migration driven by feedbacks between sediment deposition, organic matter accumulation, and sea level. This new model is easily parameterized for multiple sites. To date we have applied it to marshes in MA, VA, and GA to predict their responses to accelerated rates of sea level rise through 2100. This work is part of my postdoctoral research conducted at VIMS.

New Marsh Accretion Model:Spatially-explicit Model Accounts for Elevation and Landscape Position

Modeling the Effects of Climate, Predation, & Dispersal on Black Mangrove Range Expansion in FL

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Environmental conditions, biotic interactions, and dispersal mechanisms play critical roles in determining species ranges. For black mangrove (Avicennia germinans), warming winter temperatures at its poleward range limit facilitate expansion into coastal salt marshes. Propagules can be transported by tides and currents to marshes far from established forests where they encounter biotic interactions (eg, predation) that affect survival and subsequent forest development.

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I developed a stage-based population model informed by field experiments to investigate the relative roles of freeze regime, predation by purple marsh crab (Sesarma reticulatum), and dispersal events on forest expansion into salt marsh. Model results show all measures of mangrove stand development and regeneration decreased with increased freeze and predation intensities, modulated by propagule dispersal. Furthermore, we find that interactions between freezes, predation, and dispersal affect local mangrove establishment, which informs patterns and rates of regional mangrove expansion. Check out my paper in Ecological Modelling to learn more. 

Modeling the Effects of Climate, Predation, & Dispersal on Black Mangrove Range Exansion in FL

Temperature Thresholds for Black Mangrove Freeze Damage, Mortality, and Recovery in North America

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Data from a regional monitoring network of 38 sites (Mangrove Migration Network) led by Michael Osland at the USGS were used to refine temperature thresholds for black mangrove (Avicennia germinans) freeze damage, mortality, and recovery across the southeastern US.

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Temperature and vegetation data collected before and after an extreme freeze event in 2018 show that the temperature thresholds for leaf damage for mangroves at their northern range limit are near -4 °C; for mortality, the thresholds are near -7 °C. Due to resprouting and recruitment from propagules, full recovery from the 2018 freeze is expected at most sites within 1-3 years.

Refined temperature thresholds resulting from this study can improve predictions of range expansion and ecological transformations driven by climate change. 

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Check out our paper in Journal of Ecology to learn more about this project and the Mangrove Migration Network.

Temperature Thresholds for Black Mangrove Freeze Damage, Mortality and Recovery in North America

Grazing by Marsh Periwinkle Facilitates Mangrove Encroachment in Cordgrass-dominated Salt Marshes

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Warming winter temperatures at poleward range limits facilitate mangrove expansion into coastal salt marshes that support grazing fauna. Fauna such as marsh periwinkle (Littoraria irrorata) can exert strong top-down biotic pressure on dominant salt marsh plants like smooth cordgrass (Spartina alterniflora). To evaluate whether grazing by marsh periwinkle affects mangrove encroachment into cordgrass-dominated marshes, we conducted a field survey and greenhouse experiment comparing periwinkle density, grazing preference, and grazing damage on black and red mangrove seedlings and cordgrass culms. We found that regardless of periwinkle density, black and red mangrove seedlings incurred little to no grazing damage, whereas damage to cordgrass led to mortality within 6 weeks. Results suggest grazing by marsh periwinkle may facilitate mangrove encroachment into salt marshes at poleward range limits.

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This project was carried out by an undergraduate student I mentored during Summer 2017 while I was a PhD student at UF. We are preparing a manuscript for publication.

Grazing by Marsh Periwinkle Facilitates Mangrove Encroachment in Cordgrass-dominated Salt Marshes

Predation Limits Black Mangrove Encroachment

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Climate change-drive range expansion of black mangrove (Avicennia germinans) is predicted along the northern Gulf of Mexico, where sea level rise is also driving the retreat of freshwater forest and landward expansion of salt marsh. However, top-down control by marsh fauna may inhibit mangrove encroachment. To investigate the effects of bottom-up controls (flooding frequency, soil depth, soil salinity) and top-down controls (propagule predation and herbivory) on seedling establishment, I compared fates of caged and non-caged mangrove propagules across a landscape of marsh and freshwater forest.

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Within 12 days, purple marsh crab (Sesarma reticulatum) consumed 99% of non-caged propagules across the landscape. When protected from predation (ie, caged), seedling establishment depended on flooding frequency; when cages were removed from older seedlings, they experienced mild to severe herbivory. This work revealed that predation and herbivory strongly suppress colonization, suggesting that range expansion studies should incorporate biotic controls.

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This work was part of my PhD dissertation. Check out my paper in Hydrobiologia to learn more.

Predation Limits Black Mangrove Encroachment

Community Reassembly in Coastal Freshwater Forest

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Sea level rise elicits changes in coastal plant communities by altering the physical conditions that affect ecosystem processes and species distributions. A prime example is found along the Big Bend coast of Florida where islands of freshwater forest transition to relict stands as salt stress and tidal flooding prevent regeneration and coastal storms exacerbate tree mortality.

I investigated community turnover on a set of forest islands in various stages of decline, extending a seminal study on freshwater forest die off initiated in 1992. Since the original study the local rate of SLR increased 0.43 mm/y and tidal flooding increased 22-117% across islands. Looking across the >20-year period of record, tree mortality in more frequently flooded islands approached 100% and a reassembly trajectory emerged in which forest vegetation was replaced by halophytic shrubs, which were replaced by herbaceous salt marsh and along tidal flooding gradient. Looking ahead, other reassembly trajectories may develop coastal plants respond to changing environmental conditions.

This work was part of my PhD dissertation. Check out my paper in Global Change Biology to learn more.

Community Reassembly in Coastal Freshwater Forest

Sea Level Rise Adaptation in Yankeetown, FL

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Sea level rise is a growing threat to rural coastal communities with economies dependent on natural resources. For Yankeetown, FL, sea level rise jeopardizes marine and terrestrial habitats that make up 86% of its area and consequently impacts recreational fishing and ecotourism that drive the local economy.

As a graduate student I collaborated with Yankeetown and UF Levin College of Law to establish a 17-acre Natural Resource Adaptation Action Area and corresponding Sea Level Rise Adaptation Action Plan. The plan incorporates citizen science, SLAMM-based sea level rise simulations, and planning strategies modeled after state recommendations to increase ecological and economic resilience to environmental change.

In 2016, Yankeetown amended its comprehensive plan to formally adopt the Adaptation Action Plan and serves as a pilot for other rural communities facing the challenges of a changing climate.

Sea Level Rise Adaptation in Yankeetown, FL