Matthew L Kirwan

Virginia Institute of Marine Science · Department of Physical Sciences

Plain Language Summary Uncertain adjustments in how plants allocate new biomass between roots and shoots cause disagreement about the degree to which global plants will increase carbon sequestration under climate change. These biomass allocation adjustments are especially important in coastal marshes, among the strongest carbon sinks on Earth and v...

The impact of saltwater intrusion on coastal forests and farmland is typically understood as sea-level-driven inundation of a static terrestrial landscape, where ecosystems neither adapt to nor influence saltwater intrusion. Yet recent observations of tree mortality and reduced crop yields have inspired new process-based research into the hydrologi...

Consumers can directly (e.g., consumption) and indirectly (e.g., trophic cascades) influence carbon cycling in blue carbon ecosystems. Previous work found that large grazers have nuanced effects on carbon stocks, yet, small, bioturbating‐grazers, which remove plant biomass and alter sediment properties, remain an understudied driver of carbon cycli...

As global climate change alters the magnitude and rates of environmental stressors, predicting the extent of ecosystem degradation driven by these rapidly changing conditions becomes increasingly urgent. At the landscape scale, disturbances and stressors can increase spatial variability and heterogeneity — indicators that can serve as potential ear...

Rising sea levels lead to the migration of salt marshes into coastal forests, thereby shifting both ecosystem composition and function. In this study, we investigate leaf litter decomposition, a critical component of forest carbon cycling, across the marsh-forest boundary with a focus on the potential influence of environmental gradients (i.e., tem...

Two decades of global satellite observations reveal enhanced greening in mangrove forests relative to adjacent evergreen forests, which highlights important differences in the response of coastal and terrestrial ecosystems to climate change.

Sea level rise is leading to the rapid migration of marshes into coastal forests and other terrestrial ecosystems. Although complex biophysical interactions likely govern these ecosystem transitions, projections of sea level driven land conversion commonly rely on a simplified “threshold elevation” that represents the elevation of the marsh‐upland...

Landward migration of coastal ecosystems in response to sea-level rise is altering coastal carbon dynamics. Although such landscapes rapidly accumulate soil carbon, barrier-island migration jeopardizes long-term storage through burial and exposure of organic-rich backbarrier deposits along the lower beach and shoreface. Here, we quantify the carbon...

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 f...

Transferable and mechanistic understanding of cross-scale interactions is necessary to predict how coastal systems respond to global change. Cohesive datasets across geographically distributed sites can be used to examine how transferable a mechanistic understanding of coastal ecosystem control points is. To address the above research objectives, d...

Coastal landscapes are naturally shifting mosaics of distinct ecosystems that are rapidly migrating with sea-level rise. Previous work illustrates that transitions among individual ecosystems have disproportionate impacts on the global carbon cycle, but this cannot address nonlinear interactions between multiple ecosystems that potentially cascade...

Barrier coastlines and their associated ecosystems are rapidly changing. Barrier islands/spits, marshes, bays, and coastal forests are all thought to be intricately coupled, yet an understanding of how morphologic change in one part of the system affects the system altogether remains limited. Here we explore how sediment exchange controls the migra...

Relative sea level rise (SLR) increasingly impacts coastal ecosystems through the formation of ghost forests. To predict the future of coastal ecosystems under SLR and changing climate, it is important to understand the physiological mechanisms underlying coastal tree mortality and to integrate this knowledge into dynamic vegetation models. We inco...

Marine transgression associated with rising sea levels causes coastal erosion, landscape transitions, and displacement of human populations globally. This process takes two general forms. Along open-ocean coasts, active transgression occurs when sediment-delivery rates are unable to keep pace with accommodation creation, leading to wave-driven eros...

The vulnerability of coastal environments to sea-level rise varies spatially, particularly due to local land subsidence. However, high-resolution observations and models of coastal subsidence are scarce, hindering an accurate vulnerability assessment. We use satellite data from 2007 to 2020 to create high-resolution map of subsidence rate at mm-lev...

Ecosystem connectivity tends to increase the resilience and function of ecosystems responding to stressors. Coastal ecosystems sequester disproportionately large amounts of carbon, but rapid exchange of water, nutrients, and sediment makes them vulnerable to sea level rise and coastal erosion. Individual components of the coastal landscape (i.e., m...

Tidal marshes and mangroves are threatened by relative sea level rise (RSLR) in certain regions on Earth. Elsewhere, these coastal wetlands can adapt through sediment accretion and resulting surface elevation gain. Studies identifying drivers of the global variability in coastal wetland adaptability to RSLR ignored the role of the tidal pattern, va...

Warming temperatures and elevated CO2 are inextricably linked global change phenomena, but they are rarely manipulated together in field experiments. As a result, ecosystem-level responses to these interacting facets of global change remain poorly understood. Here we report on a four-year field manipulation of warming and elevated CO2 in a coastal...

Climate change, land subsidence, and coastal population growth are increasing coastal flood risks and changing land uses. Large-scale levee systems protect many urban areas from flooding, but much less is known about how rural coasts respond to sea level rise and increasing flood risks. This study examined previously unreported, small-scale earthen...

Coastal ecosystems represent a disproportionately large but vulnerable global carbon sink. Sea-level-driven tidal wetland degradation and upland forest mortality threaten coastal carbon pools, but responses of the broader coastal landscape to interacting facets of climate change remain poorly understood. Here, we use 36 years of satellite observati...

The rapid replacement of upland forest by encroaching marshland is a striking manifestation of global sea-level rise (SLR). Timely and high-resolution information on the location and extent of transition forest (the ecotone between upland forest and marsh where tree mortality due to seawater intrusion begins) is fundamental to understanding the pro...

Observations of woody plant mortality in coastal ecosystems are globally widespread, but the overarching processes and underlying mechanisms are poorly understood. This knowledge deficiency, combined with rapidly changing water levels, storm surges, atmospheric CO2, and vapor pressure deficit, creates large predictive uncertainty regarding how coas...

Sea level rise (SLR) and saltwater intrusion are driving inland shifts in coastal ecosystems. Here, we make high‐resolution (1 m) predictions of land conversion under future SLR scenarios in 81 watersheds surrounding Chesapeake Bay, United States, a hotspot for accelerated SLR and saltwater intrusion. We find that 1050–3748 km2 of marsh could be cr...

Coastal marshes are globally important, carbon dense ecosystems simultaneously maintained and threatened by sea‐level rise. Warming temperatures may increase wetland plant productivity and organic matter accumulation, but temperature‐modulated feedbacks between productivity and decomposition make it difficult to assess how wetlands and their thick,...

Spatial self-organization, a common feature of multi-species communities, can provide important insights into ecosystem structure and resilience. As environmental conditions gradually worsen (e.g., resource depletion, erosion intensified by storms, drought), some ecological systems collapse to an irreversible state once a tipping point is reached....

Tidal marsh survival in the face of sea level rise (SLR) and declining sediment supply often depends on the ability of marshes to build soil vertically. However, numerical models typically predict survival under rates of SLR that far exceed field‐based measurements of vertical accretion. Here, we combine novel measurements from seven U.S. Atlantic...

Transgression into adjacent uplands is an important global response of coastal wetlands to accelerated rates of sea level rise. “Ghost forests” mark a signature characteristic of marsh transgression on the landscape, as changes in tidal inundation and salinity cause bordering upland tree mortality, increase light availability, and the emergence of...

The development and expansion of ponds within otherwise vegetated coastal marshes is a primary driver of marsh loss throughout the world. Previous studies propose that large ponds expand through a wind wave-driven positive feedback, where pond edge erosion rates increase with pond size, whereas biochemical processes control the formation and expans...

Coastal tidal wetlands produce and accumulate significant amounts of organic carbon (C) that help to mitigate climate change. However, previous data limitations have prevented a robust evaluation of the global rates and mechanisms driving C accumulation. Here, we go beyond recent soil C stock estimates to reveal global tidal wetland C accumulation...

The impacts of climate change on ecosystems are manifested in how organisms respond to episodic and continuous stressors. The conversion of coastal forests to salt marshes represents a prominent example of ecosystem state change, driven by the continuous stress of sea‐level rise (press), and episodic storms (pulse). Here, we measured the rooting di...

Plain Language Summary Forests along the upland edge of salt marshes are being killed by rising sea levels and replaced with salt marsh in a process called marsh migration. Marsh soils, unlike soils in forest, quickly accumulate carbon in their soils. This indicates that marsh migration could possibly increase carbon storage across the landscape. H...

Sea-level rise is leading to the migration of marshes into coastal forests throughout North America. Marsh migration represents a primary mechanism for marsh survival in the face of sea-level rise and leads to a fundamental reorganization of vegetation communities. Yet, the ecological implications of these changes remain unknown. To evaluate the ef...

Sea-level rise, saltwater intrusion, and wave erosion threaten coastal marshes, but the influence of salinity on marsh erodibility remains poorly understood. We measured the shear strength of marsh soils along a salinity and biodiversity gradient in the York River estuary in Virginia to assess the direct and indirect impacts of salinity on potentia...

Coastal wetland carbon pools are globally important, but their response to interacting facets of global change remain unclear. Numerical models neglect species‐specific vegetation responses to sea level rise (SLR) and elevated CO2 (eCO2) that are observed in field experiments, while field experiments cannot address the long‐term feedbacks between f...

Coastal wetlands accumulate soil carbon more efficiently than terrestrial systems, but sea-level rise potentially threatens the persistence of this prominent carbon sink. Here, we combine a published dataset of 372 soil carbon accumulation rates from across the United States with new analysis of 131 sites in coastal Louisiana and find that the rate...

Salt marshes are valuable but vulnerable coastal ecosystems that adapt to relative sea level rise (RSLR) by accumulating organic matter and inorganic sediment. The natural limit of these processes defines a threshold rate of RSLR beyond which marshes drown, resulting in ponding and conversion to open waters. We develop a simplified formulation for...

Methane (CH4) exchange between trees and the atmosphere has recently emerged as an important, but poorly quantified process regulating global climate. The sources (soil and/or tree) and mechanisms driving the increase of CH4 in trees and degassing to the atmosphere are inadequately understood, particularly for coastal forests facing increased expos...

The presence of bare patches within otherwise vegetated coastal marshes is sometimes considered to be a symptom of marsh dieback and the subsequent loss of important ecosystem services. Here we studied the topographical conditions determining the presence and revegetation of bare patches in three marsh sites with contrasting tidal range, sediment s...

Mangroves are encroaching into salt marshes throughout the world as a result of environmental change. Previous studies suggest mangroves trap sediment more efficiently than adjacent salt marshes, providing mangroves greater capacity to adapt to sea level rise; this may occur by displacing salt marshes. However, sediment transport in adjacent marsh-...

The Defense Coastal/Estuarine Research Program (DCERP) was a 10-year multi-investigator project funded by the Department of Defense to improve understanding of ecosystem processes and their interactions with natural and anthropogenic stressors at the Marine Corps Base Camp Lejeune (MCBCL) located in coastal North Carolina. The project was aimed at...

Salt marshes rely on sufficient sediment inputs and room for lateral migration to maintain vertical and lateral stability under sea-level rise. As the global rate of sea-level rise accelerates, marshes unable to keep pace become vulnerable to drowning. We evaluated the long-term response of a salt marsh in Georgia, USA, to historical (1935–2018) an...

Sea‐level dynamics, sediment availability, and marine energy are critical drivers of coastal wetland formation and persistence, but their roles as continental‐scale drivers remain unknown. We evaluated the timing and spatial variability of wetland formation from new and existing cores collected along the Atlantic and Gulf coasts of the United State...

Plain Language Summary Coastal marshes are highly valued ecosystems, but in some areas with increased sea level rise these vegetated marshes disappear and convert into large ponds. Currently, we do not fully understand how these ponds are formed and why marsh vegetation is not recovering in these areas. In this study we measured the soil elevation...

The lateral extent and vertical stability of salt marshes experiencing rising sea levels depend on interacting drivers and feedbacks with potential for nonlinear behaviors. A two‐dimensional transect model was developed to examine changes in marsh and upland forest lateral extent and to explore controls on marsh inland transgression. Model behavior...

Sediment supply is a primary factor in determining marsh response to sea level rise and is typically approximated through high‐resolution measurements of suspended sediment concentrations (SSCs) from adjacent tidal channels. However, understanding sediment transport across the marsh itself remains limited by discontinuous measurements of SSC over i...

Sea-level rise, saltwater intrusion, and wave erosion threaten coastal marshes, but the influence of salinity on marsh erodibility remains poorly understood. We measured the shear strength of marsh soils along a salinity and biodiversity gradient in the York River estuary in Virginia to assess the direct and indirect impacts of salinity on marsh er...

The presence of bare patches within otherwise vegetated coastal marshes is sometimes considered to be a symptom of marsh die-back and the subsequent loss of important ecosystem services. Here we studied the topographical conditions determining the presence and revegetation of bare patches in three marsh sites with contrasting tidal range, sediment...

Sea level rise (SLR) is threatening coastal marshes, leading to large‐scale marsh loss in several micro‐tidal systems. Early recognition of marsh vulnerability to SLR is critical in these systems to aid managers to take appropriate restoration or mitigation measures. However, it is not clear if current marsh vulnerability indicators correctly asses...

An accelerating global rate of sea level rise (SLR), coupled with direct human impacts to coastal watersheds and shorelines, threatens the continued survival of salt marshes. We developed a new landscape‐scale numerical model of salt marsh evolution and applied it to marshes in the Plum Island Estuary (Massachusetts, U.S.A.), a sediment‐deficient s...

Accelerating sea-level rise and decreasing riverine sediment supply are widely considered to lead to global losses of deltaic marshes and their valuable ecosystem services. However, little is known about the degree to which the related erosion of the seaward delta front can provide sediments to sustain salt marshes. Here, we present data from the m...

We thank Törnqvist et al. for engaging with our modelling study on the future response of global coastal wetlands to sea-level rise (SLR) and their careful and critical discussion of the presented methods and results. However, we disagree with their suggestion that our modelling approach is inadequate, a claim which relies on two arguments: (1) the...

Seagrass provides a wide range of economically and ecologically valuable ecosystem services, with shoreline erosion control often listed as a key service, but can also alter the sediment dynamics and waves within back‐barrier bays. Here we incorporate seagrass dynamics into an existing barrier‐marsh exploratory model, GEOMBEST++, to examine the cou...

Introduced plants provide a unique opportunity to examine how plants respond through plasticity and adaptation to changing climates. We compared plants of Spartina alterniflora from the native (United States, 27–43°N) and introduced (China, 19–40°N) ranges. In the field and greenhouse, aboveground productivity of Chinese plants was greater than tha...

Salt marshes are recognized as valuable resources that are threatened by climate change and human activities. Better management and planning for these ecosystems will depend on understanding which marshes are most vulnerable, what is driving their change, and what their future trajectory is likely to be. Both observations and models have provided i...

Salt marshes are simultaneously among the most valuable and vulnerable ecosystems in the world. We use a simplified formulation for sediment transport across marshes to explain why marshes are most vulnerable to sea level rise (SLR) in microtidal environments. We find inorganic sediment decay length scales with tidal range so that inorganic deposit...

Significance Interactions between nutrient supply and plant demand dictate key terrestrial ecosystem feedbacks to global climate change. We investigated these responses in a marsh ecosystem and found that plants and soils respond to warming at different temperatures. Modest warming caused plant demand for nitrogen (N) to outpace the soil N supply,...

Ghost forests, consisting of dead trees adjacent to marshes, are a striking feature of low-lying coastal and estuarine landscapes, and they represent the migration of coastal ecosystems with relative sea-level rise (RSLR). Although ghost forests have been observed along many coastal margins, rates of ecosystem change and their dependence on RSLR re...

Coastal wetlands provide valuable ecosystem services that are increasingly threatened by anthropogenic activities¹. The atmospheric carbon dioxide (CO2) concentration has increased from 280 ppm to 404 ppm since the Industrial Revolution and is projected to exceed 900 ppm by 2100 (ref. ²). In terrestrial ecosystems, elevated CO2 typically stimulates...

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The sustainability of dynamic natural systems often depends on their capacity to adapt to uncertain climate-related changes, where different management options may be combined to facilitate this adaptation. Salt marshes exemplify such a system. Marsh sustainability under rapid sea level rise requires the preservation of transgression zones - undeve...