Geology topics

Nicole Cormier

Publications and source records attributed to Nicole Cormier.

At least 19 recordsLinked to original sources

USGS2025

Elevated CO2 enables brackish marsh transgression into freshwater forested wetlands while stimulating CH4 emissions

Wetlands are significant carbon (C) sinks and are expected to promote greater C assimilation as atmospheric CO 2 concentrations rise. However, the fate of C with environmental change along fresh-to-oligohaline wetland transitions is not well understood. We established an ex-situ mesocosm experiment to mimic future elevated atmospheric CO 2 concentrations ( e CO 2 , 720 ppm) versus current (380 ppm), and we exposed four co-occurring coastal wetland communities that naturally transgress (i.e., freshwater forest, mixed forest and marsh, marsh, mudflat) to these concentrations for two years. Overall, wetland communities with marsh plants in monoculture and mixed culture maintained high ecosystem C uptake with e CO 2 versus freshwater forested wetlands or mudflats, likely from superior plant species photosynthetic adjustment versus leaf area increases. e CO 2 promoted greater CO 2 uptake by leaves in all communities except mudflats, while promoting CH 4 efflux from whole ecosystems only when marsh plants were present. e CO 2 is projected to stimulate C gain 2.2-fold for forested wetlands and oligohaline marsh and 2.9-fold for forest-marsh mixture through greater CO 2 uptake. However, this comes at a cost of stimulated CH 4 flux by 1.4-to-1.7-fold in mixed and marsh communities versus reduced CH 4 fluxes with e CO 2 by forest and mudflat communities, perhaps through different oxidation pathways. Freshwater forested wetlands limited greenhouse gas emissions compared with transitional habitats, oligohaline marshes, and mudflats as atmospheric CO 2 concentrations increased. Stimulated C uptake in marshes may not offset higher methane emissions from these systems, potentially facilitating greater warming in a future with elevated atmospheric CO 2 .

Estuarine, Coastal and Shelf Science↗

USGS2024

Benthic community metrics track hydrologically stressed mangrove systems

Mangrove restoration efforts have increased in order to help combat their decline globally. While restoration efforts often focus on planting seedlings, underlying chronic issues, including disrupted hydrological regimes, can hinder restoration success. While improving hydrology may be more cost-effective and have higher success rates than planting seedlings alone, hydrological restoration success in this form is poorly understood. Restoration assessments can employ a functional equivalency approach, comparing restoration areas over time with natural, reference forests in order to quantify the relative effectiveness of different restoration approaches. Here, we employ the use of baseline community ecology metrics along with stable isotopes to track changes in the community and trophic structure and enable time estimates for establishing mangrove functional equivalency. We examined a mangrove system impacted by road construction and recently targeted for hydrological restoration within the Rookery Bay National Estuarine Research Reserve, Florida, USA. Samples were collected along a gradient of degradation, from a heavily degraded zone, with mostly dead trees, to a transition zone, with a high number of saplings, to a full canopy zone, with mature trees, and into a reference zone with dense, mature mangrove trees. The transition, full canopy, and reference zones were dominated by annelids, gastropods, isopods, and fiddler crabs. Diversity was lower in the dead zone; these taxa were enriched in 13 C relative to those found in all the other zones, indicating a shift in the dominant carbon source from mangrove detritus (reference zone) to algae (dead zone). Community-wide isotope niche metrics also distinguished zones, likely reflecting dominant primary food resources (baseline organic matter) present. Our results suggest that stable isotope niche metrics provide a useful tool for tracking mangrove degradation gradients. These baseline data provide critical information on the ecosystem functioning in mangrove habitats following hydrological restoration.

Florida↗

USGS2024

Eutrophication saturates surface elevation change potential in tidal mangrove forests

Coastal mangrove forests are at risk of being submerged due to sea-level rise (SLR). However, mangroves have persisted with changing sea levels due to a variety of biotic and physical feedback mechanisms that allow them to gain and maintain relative soil surface elevation. Therefore, mangrove’s resilience to SLR is dependent upon their ability to build soil elevation at a rate that tracks with SLR, or well-enough to migrate inland. Anthropogenic disturbances, such as altered hydrology and eutrophication, can degrade mangrove forest health and compromise this land building process, placing mangroves at greater risk. Much of Florida’s mangroves are adjacent to highly urbanized areas that produce nutrient-loaded runoff. This study assesses how experimental nutrient inputs in the eutrophic Caloosahatchee Estuary influence the soil surface elevation change (SEC) in two distinct mangrove zones. Annual rates of SEC were reduced by phosphorus additions and differed by mangrove zone, ranging from 0.67 ± 0.59 to 2.13 ± 0.61 and 4.21 ± 0.58 to 6.39 ± 0.59 mm year −1 in the fringe and basin zone, respectively. This suggests that eutrophication can reduce the maximum potential SEC response to SLR and that a mangrove forest’s vulnerability to SLR is not uniform throughout forest but can differ by mangrove zone.

Estuaries and Coasts↗

USGS2023

Relative effectiveness of a radionuclide (210Pb), surface elevation table (SET), and LiDAR at monitoring mangrove forest surface elevation change

Sea-level rise (SLR) is one of the greatest future threats to mangrove forests. Mangroves have kept up with or paced past SLR by maintaining their forest floor elevation relative to sea level through root growth, sedimentation, and peat development. Monitoring surface elevation change (SEC) or accretion rates allows us to understand mangrove response to SLR and prioritizes resilient ecosystems for conservation or vulnerable ecosystems for restoration. We compared three methods to measure SEC and accretion in mangrove forests: 210 Pb, surface elevation tables (SETs), and a terrestrial light detection and ranging system (compact biomass LiDAR—CBL). Lead-210 accretion rates were not significantly different than SET SEC rates and differences between the two methods (− 2 to 2 mm/year) were within the error of our measurements. Lead-210 only measures accretion in the upper meter of sediment and cannot capture deeper subsurface processes (e.g., subsidence, compaction) that SETs can. The lack of differences suggests the following: (1) surface processes in the active root zone are influencing forest floor elevation more than subsurface processes, (2) subsurface processes were not large enough to effect elevation, or (3) the SETs were not installed deep enough to capture subsurface processes. CBL SEC rates did not differ significantly from SET SEC rates. The larger spatial scale of the CBL scans resulted in significantly different SEC rates from some of the plots. This was due to the CBL measuring areas missed by the SET. The greater number of points measured by CBL (~ 30,000 vs 36) increased precision and lowered standard error. The traditional SET/rSET method is currently 3–10 × cheaper than the 210 Pb or CBL method, respectively, and can accurately track changes in forest floor elevation. Costs of the use of LiDAR are likely to decrease in the future with the advent of newer and more cost-effective technology.

Estuaries and Coasts↗

USGS2023

Presence of hummock and hollow microtopography reflects shifting balances of shallow subsidence and root zone expansion along forested wetland river gradients

Tidal freshwater forested wetlands (TFFWs) are in an active phase of transition to tidal marsh with sea level rise and salinity incursion along the Atlantic and Gulf Coasts of the United States (U.S.). A prominent feature of TFFWs is hummock/hollow microtopography where hollows represent the flat, base-elevation of the floodplain where inundation occurs relatively frequently, while hummocks provide elevated soil surfaces that often harbor relatively greater numbers and species of trees and shrubs. Hummocks appear at the landscape river boundary just seaward of bottomland hardwoods as tides reach those positions, persist for many years, and disappear as TFFWs eventually transition to marsh. We studied TFFW surface elevation processes along four Atlantic coastal landscape river gradients by using surface elevation tables and marker horizons. Shallow subsidence between trees, equating to as much as 5.5 mm/year, was an important process in hollow maintenance as roots held hummock elevations relatively more stable. However, hummocks were actively subsiding on all sites with little sign of root zone expansion within hummocks, despite hummock elevation gain on some sites. For down-river transitions, hollow infilling through increasing sediment accretion and root zone expansion were predominant processes driving loss of microtopography as marshes replaced TFFWs closer to the estuarine interface; hollows gained elevations to meet hummocks. While these results do not preclude the importance of healthy root zone processes to the maintenance (and formation) of hummocks, our results indicate that reductions in critical sediment supplies to offset natural shallow subsidence explain persistence and eventual loss of hummock and hollow microtopography in TFFWs.

Estuaries and Coasts↗

USGS2023

Mangrove forests and climate change: Impacts and interactions

The position of mangrove forests in the dynamic intertidal zone means that they are expected to be heavily impacted by climate change. Much focus is put on mangroves and their response to sea-level rise, but this ecosystem is exposed to a much broader range of climate change stressors, including increased storminess and waves, more dynamic and unpredictable precipitation patterns, and increases in air and sea surface temperatures, particularly at their latitudinal limits. We show that individual climate change stressors can have (rarely considered) positive, as well as negative impacts on mangroves and associated ecosystem functions such as carbon sequestration. While we generally study climate change stressors individually, they are not expected to act in isolation with other climate change stressors or with anthropogenic stressors. We present a stressor interaction framework previously suggested for coral reefs, and adapt it for use in mangroves, using the recent mangrove dieback in northern Australia as a case study. We show the benefits of moving mangrove and climate change research beyond the study of single stressors and towards identifying key synergistic and antagonistic interactions between climate change stressors.

Book chapter↗

USGS2022

Soil elevation change in mangrove forests and marshes of the greater Everglades: A regional synthesis of surface elevation table-marker horizon (SET-MH) data

Coastal wetlands adapt to rising seas via feedbacks that build soil elevation, which lead to wetland stability. However, accelerated rates of sea-level rise can exceed soil elevation gain, leading to wetland instability and loss. Thus, there is a pressing need to better understand regional and landscape variability in rates of wetland soil elevation change. Here, we conducted a regional synthesis of surface elevation change data from mangrove forests and coastal marshes in the iconic Greater Everglades region of south Florida (USA). We integrated data from 51 sites in which a total of 122 surface elevation table-marker horizon (SET-MH) stations were installed. Several of these sites have been periodically monitored since the 1990s and are among the oldest SET-MH datasets in the world. Rates of surface elevation change ranged from −9.8 to 15.2 mm year −1 , indicating some wetlands are keeping pace with sea-level rise while others are at risk of submergence and conversion to open water. Vertical accretion rates ranged from 0.6 to 12.9 mm year −1 , and subsurface change rates ranged from −13.5 to 8.6 mm year −1 . Rates of surface elevation change were positively related to subsurface change but not vertical accretion. There were no significant relationships between rates of surface elevation change and elevation (NAVD 88) or rates of sea-level rise. Site-specific examples indicate that hurricanes, plant productivity, hydrologic exchange, and proximity to sediment and nutrient inputs are critical but confounding drivers of surface elevation change dynamics in the Greater Everglades region. Collectively, our results reinforce the value of long-term SET-MH data that incorporate spatial variability for advancing understanding of surface elevation change dynamics in coastal wetlands.

Florida↗

USGS2022

Response of soil respiration to changes in soil temperature and water table level in drained and restored peatlands of the southeastern United States

Extensive drainage of peatlands in the southeastern United States coastal plain for the purposes of agriculture and timber harvesting has led to large releases of soil carbon as carbon dioxide (CO 2 ) due to enhanced peat decomposition. Growth in mechanisms that provide financial incentives for reducing emissions from land use and land-use change could increase funding for hydrological restoration that reduces peat CO 2 emissions from these ecosystems. Measuring soil respiration and physical drivers across a range of site characteristics and land use histories is valuable for understanding how CO 2 emissions from peat decomposition may respond to raising water table levels. We combined measurements of total soil respiration, depth to water table from soil surface, and soil temperature from drained and restored peatlands at three locations in eastern North Carolina and one location in southeastern Virginia to investigate relationships among total soil respiration and physical drivers, and to develop models relating total soil respiration to parameters that can be easily measured and monitored in the field.

North Carolina↗

USGS2022

Constraints on the adjustment of tidal marshes to accelerating sea level rise

Much uncertainty exists about the vulnerability of valuable tidal marsh ecosystems to relative sea level rise. Previous assessments of resilience to sea level rise, to which marshes can adjust by sediment accretion and elevation gain, revealed contrasting results, depending on contemporary or Holocene geological data. By analyzing globally distributed contemporary data, we found that marsh sediment accretion increases in parity with sea level rise, seemingly confirming previously claimed marsh resilience. However, subsidence of the substrate shows a nonlinear increase with accretion. As a result, marsh elevation gain is constrained in relation to sea level rise, and deficits emerge that are consistent with Holocene observations of tidal marsh vulnerability.

Science↗

USGS2021

Forest thinning in the seaward fringe speeds up surface elevation increment and carbon accumulation in managed mangrove forests

Mangroves are significant carbon (C) sinks and ecological engineers as they accumulate sediments and increase soil surface elevation. Thus, the forest management practice of thinning may not only alter forest structure, but also facilitate new biogeomorphological processes that affect soil development. Thinning may create additional opportunity for understorey species, such as the light-demanding Acanthus ilicifolius , to become a more prominent vegetation component of mangroves, which may further alter soil surface elevation trajectories. Forest structure and soil surface elevation change (SEC) were monitored along transects from the landward edge (upper intertidal) to seaward fringe before and after thinning of non-native Sonneratia apetala plantations along the seaward edge of a mangrove site in China. Soil C accumulation was also evaluated. Acanthus individuals colonized the forest gaps created by Sonneratia thinning in the seaward forest edge of the intertidal zones. In the absence of other pioneer species or other mangrove propagules/seeds, Acanthus pioneers occupied the areas rapidly (from 33.6 to 72.6 stems/m 2 ), and were successful opportunists. Newly colonizing Acanthus vegetation significantly increased SEC from 25.1 mm/year before Sonneratia was thinned to 46.5 mm/year after thinning-induced Acanthus occupation. Furthermore, Acanthus occupation enhanced soil C accumulation at the seaward edge to 49.9 MgC ha −1 year −1 ; a rate twofold higher than in the Sonneratia plantation before thinning. Synthesis and applications . Prolific Acanthus growth formed small elevation mounds on the seaward forest edge, which was suspected as a positive effect induced by thinning Sonneratia plantations that further facilitated colonization of Acanthus clones. Thinning non-native Sonneratia disturbed the established zonation, altered SEC, and is facilitating further succession of these mangrove forests into un-occupied aquatic areas. Higher elevations created by Acanthus expansion (vertically and horizontally) may further promote a greater percentage of landward edge mangrove species to colonize at these more favourable intertidal elevations yielding at least short-term gains in soil carbon accumulation by altering ecosystem function. Silvicultural activity should consider unintended influences on sedimentation patterns both in situ and in adjacent habitats in tidal forests, where small elevation changes affect mangrove species habitat preferences.

Journal of Applied Ecology↗

USGS2020

Rapid peat development beneath created, maturing mangrove forests: Ecosystem changes across a 25-year chronosequence

Mangrove forests are among the world’s most productive and carbon‐rich ecosystems. Despite growing understanding of factors controlling mangrove forest soil carbon stocks, there is a need to advance understanding of the speed of peat development beneath maturing mangrove forests— especially in created and restored mangrove forests that are intended to compensate for ecosystem functions lost during mangrove forest conversion to other land uses. To better quantify the rate of soil organic matter development beneath created, maturing mangrove forests, we measured ecosystem changes across a 25‐year chronosequence. We compared ecosystem properties in created, maturing mangrove forests to adjacent natural mangrove forests. We also quantified site‐specific changes that occurred between 2010 and 2016. Soil organic matter accumulated rapidly beneath maturing mangrove forests as sandy soils transitioned to organic‐rich soils (peat). Within 25 years, a 20‐cm deep peat layer developed. The time required for created mangrove forests to reach equivalency with natural mangrove forests was estimated as: (1) < 15 years for herbaceous and juvenile vegetation; (2) ~55 years for adult trees; (3) ~25 years for the upper soil layer (0‐10 cm); and (4) ~45‐80 years for the lower soil layer (10‐30 cm). For soil elevation change, the created mangrove forests were equivalent to or surpassed natural mangrove forests within the first five years. A comparison to chronosequence studies from other ecosystems indicates that the rate of soil organic matter accumulation beneath maturing mangrove forests may be among the fastest globally. In most peatland ecosystems, soil organic matter formation occurs slowly (centuries, millennia); however, these results show that mangrove peat formation can occur within decades. Peat development, primarily due to sub‐surface root accumulation, enables mangrove forests to sequester carbon, adjust their elevation relative to sea level, and adapt to changing conditions at the dynamic land‐ocean interface. In the face of climate change and rising sea levels, coastal managers are increasingly concerned with the longevity and functionality of coastal restoration efforts. Our results advance understanding of the pace of ecosystem development in created, maturing mangrove forests, which can improve predictions of mangrove forest responses to global change and ecosystem restoration.

Florida↗

USGS2020

Soil surface elevation dynamics in a mangrove-to-marsh ecotone characterized by vegetation shifts

Mangrove forest encroachment into coastal marsh habitats has been described in subtropical regions worldwide in recent decades. To better understand how soil processes may influence vegetation change, we studied soil surface elevation change, accretion rates, and soil subsurface change across a coastal salinity gradient in Florida, USA, an area with documented mangrove encroachment into saline marshes. Our aim was to identify if variations in the soil variables studied exist and to document any associated vegetation shifts. We used surface elevation tables and marker horizons to document the soil variables over 5 years in a mangrove-to-marsh transition zone or ecotone. Study sites were located in three marsh types (brackish, salt, and transition) and in riverine mangrove forests. Mangrove forest sites had significantly higher accretion rates than marsh sites and were the only locations where elevation gain occurred. Significant loss in surface elevation occurred at transition and salt marsh sites. Transition marshes, which had a significantly higher rate of shallow subsidence compared to other wetland types, appear to be most vulnerable to submergence or to a shift to mangrove forest. Submergence can result in herbaceous vegetation mortality and conversion to open water, with severe implications to the quantity and quality of wetland services provided.

Florida↗

USGS2019

The potential resiliency of a created tidal marsh to sea-level rise

The purpose of this study was to determine the elevation dynamics of a created tidal marsh on the North Carolina coast. Deep rod surface elevation tables (RSET) and feldspar marker horizons (MH) were installed in plots to measure net surface elevation changes and to quantify contributing processes. Twelve total plots were placed on four elevation gradient transects (three transects within the created marsh and one within a reference marsh) with three plots along each transect. Elevation gradient transects included a low marsh plot dominated by Spartina alterniflora, a middle marsh plot dominated by Juncus roemerianus, and a high marsh plot dominated by Spartina patens. RSET and MH measurements were taken in December 2012, January 2014, April 2017, and March 2018. Elevation change ranged from 0.7 to 4.0 mm yr -1 within the created marsh and -0.6 to 2.1 mm yr -1 within the reference marsh. When compared to the long-term linear trend in local relative sea level rise (RSLR) of 3.10 +/- 0.35 mm yr -1 , the middle marsh plots within the created marsh trended toward survival with an observed elevation increase of 3.1 +/- 0.2 mm yr -1 . Alternatively, the low and high marsh plots within the created marsh trended towards submergence with observed elevation increases of 2.1+/- 0.2 and 1.3 +/- 0.2 mm yr -1 , respectively. These results indicate that a created marsh can display elevation dynamics similar to natural marshes. Surface elevation changes were observed over a short time period and in a relatively young marsh, so it is uncertain if these trends will continue or how the longer-term relationship with RSLR will develop. While this study provided the initial data on the ability of created tidal marshes to respond to observed sea-level rise, long-term observations will be continued to evaluate long-term elevation dynamics.

North Carolina↗

USGS2018

The role of the upper tidal estuary in wetland blue carbon storage and flux

Carbon (C) standing stocks, C mass balance, and soil C burial in tidal freshwater forested wetlands (TFFW) and TFFW transitioning to low‐salinity marshes along the upper estuary are not typically included in “blue carbon” accounting, but may represent a significant C sink. Results from two salinity transects along the tidal Waccamaw and Savannah rivers of the US Atlantic Coast show total C standing stocks were 321‐1264 Mg C ha ‐1 among all sites, generally shifting to greater soil storage as salinity increased. Carbon mass balance inputs (litterfall, woody growth, herbaceous growth, root growth, surface accumulation) minus C outputs (surface litter and root decomposition, gaseous C) over a period of up to 11 years were 340‐900 g C m ‐2 yr ‐1 . Soil C burial was variable (7‐337 g C m ‐2 yr ‐1 ), and lateral C export was estimated as C mass balance minus soil C burial as 267‐849 g C m ‐2 yr ‐1 . This represents a large amount of C export to support aquatic biogeochemical transformations. Despite reduced C persistence within emergent vegetation, decomposition of organic matter, and higher lateral C export, total C storage increased as forests converted to marsh with salinization. These tidal river wetlands exhibited high N mineralization in salinity‐stressed forested sites and considerable P mineralization in low salinity marshes. Large C standing stocks and rates of C sequestration suggest that TFFW and oligohaline marshes are considerably important globally to coastal C dynamics and in facilitating energy transformations in areas of the world in which they occur.

Georgia, South Carolina↗

USGS2018

Ghost forests of Marco Island: Mangrove mortality driven by belowground soil structural shifts during tidal hydrologic alteration

Land use changes often create in situ stress and eventual mortality in mangroves as unsuitable hydroperiods are created through tidal flow alterations. Here, we document mangrove forest and soil structural changes within transects established in tidally restricted areas on Marco Island (Collier County, Florida, USA), which has broad swaths of dead-standing or unhealthy mangroves (“ghost forests”). Transects (N = 4) were arranged to include full canopy, transitional, and open canopy (dead) forests, and compared to nearby reference forests. Aboveground and belowground carbon (C) stocks (Total C) ranged from 288 to 304 Mg C ha −1 on full canopy, transitional, and reference sites, which did not differ from each other. However, Total C was lower for dead sites (233 Mg C ha −1 ) dictated entirely by differences in aboveground C (live and dead trees, downed wood); no differences were found among forest condition in belowground C stocks. This belowground C has been persistent in the soil for 85 years since initial tidal restriction. Nevertheless, hydrologic rehabilitation has the potential to increase total C stocks on dead sites by 70–110 Mg C ha −1 . Collapse of the soil surface by 6–8 cm just under the active root zone in chronically stressed mangroves was evident within the bulk density profiles from transitional versus dead sites, suggesting that surface elevation loss as root turnover ceases may work correlatively with chronic stressors (anoxia, P limitation) to affect rapid mortality of forests over short periods of time many years after stress initiation. Hydrologic rehabilitation of stressed or denuded mangroves must also include an understanding of how these soil processes might be re-established.

Florida↗

USGS2017

Assessing coastal wetland vulnerability to sea-level rise along the northern Gulf of Mexico coast: Gaps and opportunities for developing a coordinated regional sampling network

Coastal wetland responses to sea-level rise are greatly influenced by biogeomorphic processes that affect wetland surface elevation. Small changes in elevation relative to sea level can lead to comparatively large changes in ecosystem structure, function, and stability. The surface elevation table-marker horizon (SET-MH) approach is being used globally to quantify the relative contributions of processes affecting wetland elevation change. Historically, SET-MH measurements have been obtained at local scales to address site-specific research questions. However, in the face of accelerated sea-level rise, there is an increasing need for elevation change network data that can be incorporated into regional ecological models and vulnerability assessments. In particular, there is a need for long-term, high-temporal resolution data that are strategically distributed across ecologically-relevant abiotic gradients. Here, we quantify the distribution of SET-MH stations along the northern Gulf of Mexico coast (USA) across political boundaries (states), wetland habitats, and ecologically-relevant abiotic gradients (i.e., gradients in temperature, precipitation, elevation, and relative sea-level rise). Our analyses identify areas with high SET-MH station densities as well as areas with notable gaps. Salt marshes, intermediate elevations, and colder areas with high rainfall have a high number of stations, while salt flat ecosystems, certain elevation zones, the mangrove-marsh ecotone, and hypersaline coastal areas with low rainfall have fewer stations. Due to rapid rates of wetland loss and relative sea-level rise, the state of Louisiana has the most extensive SET-MH station network in the region, and we provide several recent examples where data from Louisiana’s network have been used to assess and compare wetland vulnerability to sea-level rise. Our findings represent the first attempt to examine spatial gaps in SET-MH coverage across abiotic gradients. Our analyses can be used to transform a broadly disseminated and unplanned collection of SET-MH stations into a coordinated and strategic regional network. This regional network would provide data for predicting and preparing for the responses of coastal wetlands to accelerated sea-level rise and other aspects of global change.

Alabama, Florida, Louisiana, Mississippi, Texas↗

USGS2017

Causal mechanisms of soil organic matter decomposition: Deconstructing salinity and flooding impacts in coastal wetlands

Coastal wetlands significantly contribute to global carbon storage potential. Sea-level rise and other climate change-induced disturbances threaten coastal wetland sustainability and carbon storage capacity. It is critical that we understand the mechanisms controlling wetland carbon loss so that we can predict and manage these resources in anticipation of climate change. However, our current understanding of the mechanisms that control soil organic matter decomposition, in particular the impacts of elevated salinity, are limited, and literature reports are contradictory. In an attempt to improve our understanding of these complex processes, we measured root and rhizome decomposition and developed a causal model to identify and quantify the mechanisms that influence soil organic matter decomposition in coastal wetlands that are impacted by sea-level rise. We identified three causal pathways: 1) a direct pathway representing the effects of flooding on soil moisture, 2) a direct pathway representing the effects of salinity on decomposer microbial communities and soil biogeochemistry, and 3) an indirect pathway representing the effects of salinity on litter quality through changes in plant community composition over time. We used this model to test the effects of alternate scenarios on the response of tidal freshwater forested wetlands and oligohaline marshes to short- and long-term climate-induced disturbances of flooding and salinity. In tidal freshwater forested wetlands, the model predicted less decomposition in response to drought, hurricane salinity pulsing, and long-term sea-level rise. In contrast, in the oligohaline marsh, the model predicted no change in response to sea-level rise, and increased decomposition following a drought or a hurricane salinity pulse. Our results show that it is critical to consider the temporal scale of disturbance and the magnitude of exposure when assessing the effects of salinity intrusion on carbon mineralization in coastal wetlands. Here we identify three causal mechanisms that can reconcile disparities between long-term and short-term salinity impacts on organic matter decomposition.

Ecology↗

USGS2017

Created mangrove wetlands store belowground carbon and surface elevation change enables them to adjust to sea-level rise

Mangrove wetlands provide ecosystem services for millions of people, most prominently by providing storm protection, food and fodder. Mangrove wetlands are also valuable ecosystems for promoting carbon (C) sequestration and storage. However, loss of mangrove wetlands and these ecosystem services are a global concern, prompting the restoration and creation of mangrove wetlands as a potential solution. Here, we investigate soil surface elevation change, and its components, in created mangrove wetlands over a 25 year developmental gradient. All created mangrove wetlands were exceeding current relative sea-level rise rates (2.6 mm yr −1 ), with surface elevation change of 4.2–11.0 mm yr −1 compared with 1.5–7.2 mm yr −1 for nearby reference mangroves. While mangrove wetlands store C persistently in roots/soils, storage capacity is most valuable if maintained with future sea-level rise. Through empirical modeling, we discovered that properly designed creation projects may not only yield enhanced C storage, but also can facilitate wetland persistence perennially under current rates of sea-level rise and, for most sites, for over a century with projected medium accelerations in sea-level rise (IPCC RCP 6.0). Only the fastest projected accelerations in sea-level rise (IPCC RCP 8.5) led to widespread submergence and potential loss of stored C for created mangrove wetlands before 2100.

Scientific Reports↗