Nicole Cormier

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 ¹³C 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.

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.

This article is a Commentary on Arnaud et al. (2021), 232: 1591–1602.

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/m²), 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.

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‐yr 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 yr, 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 yr for herbaceous and juvenile vegetation, (2) ~55 yr for adult trees, (3) ~25 yr for the upper soil layer (0–10 cm), and (4) ~45–80 yr 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 5 yr. 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 (over centuries, millennia); however, these results show that mangrove peat formation can occur within decades. Peat development, primarily due to subsurface 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.

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 were measured in December 2012, January 2014, April 2017, and March 2018. Elevation change ranged from 1.0 to 4.0 mm year-1 within the created marsh and from-0.4 to 2.0 mm year-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 year-1 , the middle marsh plots within the created marsh trended toward survival, with an observed elevation increase of 3.1 0.2 mm year-1. Alternatively, the low and high marsh plots within the created marsh trended toward submergence, with observed elevation increases of 2.1 0.2 and 1.3 0.2 mm year-1 , respectively. These results indicate that a created marsh can display elevation dynamics similar to a natural marsh and can be resilient to current rates of RSLR if constructed with a high elevation capital. 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 long-term relation with RSLR will develop. While this study provided initial data on the ability of created tidal marshes to respond to observed sea level rise, subsequent observations are needed to evaluate the long-term elevation dynamics.

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 U.S. Atlantic Coast show that total C standing stocks were 322–1,264 Mg C/ha among all sites, generally shifting to greater soil storage as salinity increased. Carbon mass balance inputs (litterfall, woody growth, herbaceous growth, root growth, and 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 · year⁻¹. Soil C burial was variable (7–337 g C · m⁻² · year⁻¹), and lateral C export was estimated as C mass balance minus soil C burial as 267–849 g C · m⁻² · year⁻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.