Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Matthew L Kirwan

Committee Member

Elizabeth A Canuel

Committee Member

Christopher J Hein

Committee Member

David S Johnson

Committee Member

J Patrick Megonigal


The coastal landscape is a naturally shifting mosaic of distinct ecosystems that are rapidly migrating with climate change. While directional changes in climate, such as warming and sea level rise, are fundamentally reorganizing the coastal landscape, ecosystem function, especially carbon storage, is affected to an unknown degree. This dissertation presents four chapters that examine the role of ecosystem transitions in coastal carbon dynamics across a range of spatial scales – within individual ecosystems, between two ecosystems, and at the landscape between an array of ecosystems. Ghost forests, or the marsh-forest ecotone, serves as an ideal example of a migratory ecotone. As sea levels rise, terrestrial forests die-off from salt water intrusion and are replaced by salt-tolerant marsh species. While this transition is widely seen and studied, we present the first field study that quantifies carbon loss during this transition (Chapter 1). Significantly, we find that the loss of carbon during marsh migration can be replaced by the accumulating marsh soils, but the timescale for this replacement is at the scale of centuries. Warming, a co-occurring climate stressor, is expected to affect carbon storage to an unknown degree as it affects both antagonistic properties to soil carbon storage: production and decomposition. In a whole-ecosystem soil warming experiment, we find that moderate amounts of warming consistently maximized root growth, marsh elevation gain, and belowground carbon accumulation (Chapter 2). However, our work indicates nonpermanent benefits as global temperatures continue to rise and elevated temperatures exacerbate marsh elevation and carbon loss. At the landscape scale, we see that while climate change can drastically reduce or increase the extent of coastal habitats, compensatory mechanisms largely maintain individual ecosystem extents (Chapter 3). However, coastal squeeze in some environments still reduce extents of ecosystem critical to regional carbon storage. Blue carbon habitats that comprise the coastal zone are able to compensate this loss in less time than it takes to accrue that loss. These findings reveal unique functional compensatory mechanisms at the landscape scale that quickly absorb carbon losses and could facilitate increased regional carbon storage in the face of accelerating climate change. Finally, we concentrate on ecosystem vulnerability of salt marshes, an ecosystem with a critical role in global and local carbon dynamics. By leveraging decadal SET data, we are able to identify early warning signals of marsh collapse in the changing microtopography of the marsh surface (Chapter 4). Increasing microtopographic heterogeneity in degrading salt marshes mirrored trends in a diverse array of systems with alternative stable states – indicating that early warning signals of marsh drowning and ecosystem transition are observable at small-spatial scales prior to runaway ecosystem degradation. Congruence between traditional and novel metrics of marsh vulnerability indicate that microtopographic metrics can be easily applied to existing SET records to identify hidden vulnerability before widespread marsh degradation.




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