Jacqueline L. Raw

Mangrove forests raise their soil surface in response to relative sea-level rise (RSLR), a biogenic capacity that underpins their value for coastal protection and blue-carbon storage. Whether this capacity is approaching its upper limit, and whether that limit differs between the far-field Indo-West Pacific and intermediate-field Atlantic-East Pacific settings distinguished by Holocene stratigraphy, has not been tested against contemporary observations at a global scale. Here we synthesise for the first time the global mangrove surface elevation table–marker horizon (SET-MH) record: 601 stations across 141 sites in 21 countries, of which 480 resolve an elevation trend. Mangrove surface elevation gain (3.59 ± 9.51 mm yr⁻¹) lagged behind mean RSLR of 5.21 3.42 mm yr⁻¹. The capacity to build elevation was constrained by a non-linear increase in shallow subsidence with accretion. Structural equation modelling revealed tidal range as the dominant control on this constraint. An elevation deficit becomes very likely (P ≥ 0.90) at 8.1 mm yr⁻¹ of RSLR in the Indo-West Pacific but at 6.3 mm yr⁻¹ in the intermediate-field Atlantic-East Pacific sites sampled, and at only 2.4 mm yr⁻¹ in reef settings, mirroring the relative vulnerability to RSLR inferred from palaeo-stratigraphy. Elevation deficit, rather than RSLR itself, predicts conversion to open water across the network of Surface Elevation Tables. Mangrove resilience to sea-level rise is therefore bounded and regionally unequal, and in intermediate-field Caribbean settings and on reef islands this limit is a present condition rather than a future projection.

Tidal marshes are threatened coastal ecosystems known for their capacity to store large amounts of carbon in their water-logged soils. Accurate quantification and mapping of global tidal marshes soil organic carbon (SOC) stocks is of considerable value to conservation efforts. Here, we used training data from 3710 unique locations, landscape-level environmental drivers and a global tidal marsh extent map to produce a global, spatially explicit map of SOC storage in tidal marshes at 30 m resolution. Here we show the total global SOC stock to 1 m to be 1.44 Pg C, with a third of this value stored in the United States of America. On average, SOC in tidal marshes’ 0–30 and 30–100 cm soil layers are estimated at 83.1 Mg C ha⁻¹ (average predicted error 44.8 Mg C ha⁻¹) and 185.3 Mg C ha⁻¹ (average predicted error 105.7 Mg C ha⁻¹), respectively.

Coastal wetlands are hotspots of carbon sequestration, and their conservation and restoration can help to mitigate climate change. However, there remains uncertainty on when and where coastal wetland restoration can most effectively act as natural climate solutions (NCS). Here, we synthesize current understanding to illustrate the requirements for coastal wetland restoration to benefit climate, and discuss potential paths forward that address key uncertainties impeding implementation. To be effective as NCS, coastal wetland restoration projects will accrue climate cooling benefits that would not occur without management action (additionality), will be implementable (feasibility) and will persist over management-relevant timeframes (permanence). Several issues add uncertainty to understanding if these minimum requirements are met. First, coastal wetlands serve as both a landscape source and sink of carbon for other habitats, increasing uncertainty in additionality. Second, coastal wetlands can potentially migrate outside of project footprints as they respond to sea-level rise, increasing uncertainty in permanence. To address these first two issues, a system-wide approach may be necessary, rather than basing cooling benefits only on changes that occur within project boundaries. Third, the need for NCS to function over management-relevant decadal timescales means methane responses may be necessary to include in coastal wetland restoration planning and monitoring. Finally, there is uncertainty on how much data are required to justify restoration action. We summarize the minimum data required to make a binary decision on whether there is a net cooling benefit from a management action, noting that these data are more readily available than the data required to quantify the magnitude of cooling benefits for carbon crediting purposes. By reducing uncertainty, coastal wetland restoration can be implemented at the scale required to significantly contribute to addressing the current climate crisis.

Tidal marshes are threatened coastal ecosystems known for their capacity to store large amounts of carbon in their water-logged soils. Accurate quantification and mapping of global tidal marshes soil organic carbon (SOC) stocks is of considerable value to conservation efforts. Here, we used training data from 3,710 unique locations, landscape-level environmental drivers and a newly developed global tidal marsh extent map to produce the first global, spatially-explicit map of SOC storage in tidal marshes at 30 m resolution. We estimate the total global SOC stock to 1 m to be 1.44 Pg C, with a third of this value stored in the United States of America. On average, SOC in tidal marshes’ 0-30 and 30-100 cm soil layers are estimated at 83.1 Mg C ha ^-1 (average predicted error 44.8 Mg C ha ^-1 ) and 185.3 Mg C ha ^-1 (average predicted error 105.7 Mg C ha ^-1 ), respectively. Our spatially-explicit model is able to capture 59% of the variability in SOC density, with elevation being the strongest driver aside from soil depth. Our study reveals regions with high prediction uncertainty and therefore highlights the need for more targeted sampling to fully capture SOC spatial variability.

Tidal marshes store large amounts of organic carbon in their soils. Field data quantifying soil organic carbon (SOC) stocks provide an important resource for researchers, natural resource managers, and policy-makers working towards the protection, restoration, and valuation of these ecosystems. We collated a global dataset of tidal marsh soil organic carbon (MarSOC) from 99 studies that includes location, soil depth, site name, dry bulk density, SOC, and/or soil organic matter (SOM). The MarSOC dataset includes 17,454 data points from 2,329 unique locations, and 29 countries. We generated a general transfer function for the conversion of SOM to SOC. Using this data we estimated a median (± median absolute deviation) value of 79.2 ± 38.1 Mg SOC ha⁻¹ in the top 30 cm and 231 ± 134 Mg SOC ha⁻¹ in the top 1 m of tidal marsh soils globally. This data can serve as a basis for future work, and may contribute to incorporation of tidal marsh ecosystems into climate change mitigation and adaptation strategies and policies.

Salt marshes occur globally across climatic and coastal settings, providing key linkages between terrestrial and marine ecosystems. However, salt marsh science lacks a unifying conceptual framework; consequently, historically well‐studied locations have been used as normative benchmarks. To allow for more effective comparisons across the diversity of salt marshes, we developed an integrative salt marsh conceptual framework. We review ecosystem‐relevant drivers from global to local spatial scales, integrate these multi‐scale settings into a framework, and provide guidance on applying the framework using specific variables on 11 global examples. Overall, this framework allows for appropriate comparison of study sites by accounting for global, coastal, inter‐, and intra‐system spatial settings unique to each salt marsh. We anticipate that incorporating this framework into salt marsh science will provide a mechanism to critically evaluate research questions and a foundation for effective quantitative studies that deepen our understanding of salt marsh function across spatial scales.

Latitudinal range limits for mangroves on high‐energy, wave‐dominated coasts are controlled by geomorphological features and estuarine dynamics. Mangroves reach a southern global range limit along the South African coastline, but the distribution is patchy, with stands occurring in only 16% of the estuaries in the region. Yet, the persistence of forests planted >50 years ago beyond the natural distribution limit suggests that additional estuaries could support mangroves. Understanding regional drivers is necessary to inform global‐scale estimates for how this important ecosystem is predicted to respond to climate change. Here, we combine species distribution modelling (MaxEnt), Lagrangian particle tracking using an eddy‐ and tide‐resolving numerical ocean model, and connectivity matrices, to identify suitable mangrove habitats along the South African coastline at present, as well as under the IPCC RCP4.5 and RCP8.5 climate scenarios. Within the current South African distribution range (±900 km), eight more estuaries were identified to be suitable under contemporary conditions. When considering potential range extension (±110 km), an additional 14 suitable estuaries were identified. Connectivity matrices suggest limited long‐distance dispersal, stranding mostly at or near the release location, and a decreased probability of connectivity towards the range limit. Under both future climate scenarios, 30% of estuaries currently supporting mangroves are predicted to become unsuitable, while an additional six estuaries beyond the current distribution are predicted to become suitable. However, there is limited connectivity between these new sites and established forests. Synthesis. This study shows that dispersal substantially limits mangrove distribution at the southern African range limit and highlights the importance of including this process in species distribution models. Ultimately, our results provide new insight into mangrove conservation and management at range limits that are not controlled predominantly by temperature, as it has been assumed that mangroves will largely expand to higher latitudes under climate change.

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.