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Significant efforts have been invested to restore mangrove forests worldwide through reforestation and afforestation. However, blue carbon benefit has not been compared between these two silvicultural pathways at the global scale. Here, we integrated results from direct field measurements of over 370 restoration sites around the world to show that mangrove reforestation (reestablishing mangroves where they previously colonized) had a greater carbon storage potential per hectare than afforestation (establishing mangroves where not previously mangrove). Greater carbon accumulation was mainly attributed to favorable intertidal positioning, higher nitrogen availability, and lower salinity at most reforestation sites. Reforestation of all physically feasible areas in the deforested mangrove regions of the world could promote the uptake of 671.5–688.8 Tg CO 2 -eq globally over a 40-year period, 60% more than afforesting the same global area on tidal flats (more marginal sites). Along with avoiding conflicts of habitat conversion, mangrove reforestation should be given priority when designing nature-based solutions for mitigating global climate change. Blue carbon benefit has not been compared among mangrove reforestation and afforestation pathways at the global scale. This study shows that mangrove reforestation could perform a greater carbon storage potential per hectare than afforestation as its higher nitrogen availability and lower salinity.

Coastal wetlands (mangrove, tidal marsh and seagrass) sustain the highest rates of carbon sequestration per unit area of all natural systems 1 , 2 , primarily because of their comparatively high productivity and preservation of organic carbon within sedimentary substrates 3 . Climate change and associated relative sea-level rise (RSLR) have been proposed to increase the rate of organic-carbon burial in coastal wetlands in the first half of the twenty-first century 4 , but these carbon–climate feedback effects have been modelled to diminish over time as wetlands are increasingly submerged and carbon stores become compromised by erosion 4 , 5 . Here we show that tidal marshes on coastlines that experienced rapid RSLR over the past few millennia (in the late Holocene, from about 4,200 years ago to the present) have on average 1.7 to 3.7 times higher soil carbon concentrations within 20 centimetres of the surface than those subject to a long period of sea-level stability. This disparity increases with depth, with soil carbon concentrations reduced by a factor of 4.9 to 9.1 at depths of 50 to 100 centimetres. We analyse the response of a wetland exposed to recent rapid RSLR following subsidence associated with pillar collapse in an underlying mine and demonstrate that the gain in carbon accumulation and elevation is proportional to the accommodation space (that is, the space available for mineral and organic material accumulation) created by RSLR. Our results suggest that coastal wetlands characteristic of tectonically stable coastlines have lower carbon storage owing to a lack of accommodation space and that carbon sequestration increases according to the vertical and lateral accommodation space 6 created by RSLR. Such wetlands will provide long-term mitigating feedback effects that are relevant to global climate–carbon modelling. Wetlands exposed to rapid sea-level rise over the late Holocene contain more soil carbon than those that experienced a long period of sea-level stability.

Mangrove ecosystems store large quantities of organic carbon for long periods of time. This study explores organic carbon stock change through the first comparative study of radiometric analysis and repeat field measures over a multi-decadal period in a mangrove system. Examining one tall gallery forest of Avicennia marina, and an adjacent interior scrub mangrove of mixed Avicennia marina and Aegiceras corniculatum, radiometric analysis estimated a soil organic carbon accumulation rate of 4.3 ± 0.6 Mg C ha⁻¹ y⁻¹ in the tall gallery forest and 2.2 ± 0.5 Mg C ha⁻¹ y⁻¹ in a stunted mangrove. Repeat measures of root carbon separated by 30 years estimated an increase of 5.06 Mg C ha⁻¹ y⁻¹ in the tall forest and 6.63 Mg C ha⁻¹ y⁻¹ in the stunted forest—suggesting an underestimate of carbon accumulation by radiometric dating of 15% and 67% in the tall and stunted forest, respectively. A higher carbon stock in the interior forest was attributed to root mass increase, associated with landward mangrove encroachment. Extrapolated to the entire region of NSW we estimate that mangrove encroachment has contributed at least about 1.8 Tg C sequestration over the 70 years for which this has been observed in New South Wales, Australia.

Mangroves can retain both autochthonous and allochthonous marine and/or terrestrial organic carbon (OC) in sediments. Accurate quantification of these OC sources is essential for the proper allocation of blue C credits. Here, we conduct a global-scale analysis of sediments autochthonous and allochthonous OC contributions in estuarine and marine mangroves using stable isotopes. Globally, mangrove-derived autochthonous OC was the main contributor to estuarine and marine mangrove top-meter soil organic carbon (SOC) (49% and 62%, respectively). Less marine allochthonous OC (21%) was deposited than terrestrial allochthonous OC (30%) in estuarine mangrove sediments. Estuarine mangroves accumulated more SOC in sediments than marine mangroves (282 ± 8.1 Mg C ha −1 and 250 ± 5.0 Mg C ha −1 , respectively), primarily due to the additional terrestrial OC inputs. Globally, marine mangroves held 67% of the total mangrove SOC, reaching 3025 ± 345 Tg C, while 1502 ± 154 Tg C was stored in estuarine mangrove sediments. The findings emphasize the substantial influence of coastal environmental settings on OC contributions, underlining the necessity of accurate OC source quantification for the effective allocation of blue carbon credits. Mangrove-derived organic carbon (OC) contributes 49% and 62% to estuarine and marine mangrove soil OC (SOC). Globally, 1502 Tg and 3025 Tg SOC were stored in estuarine and marine mangroves.

Assessment of mangrove forest surface elevation changes across the Indo-Pacific coastal region finds that almost 70 per cent of the sites studied do not have enough sediment availability to offset predicted sea-level rise; modelling indicates that such sites could be submerged as early as 2070. Mangrove forests on the verge Intertidal mangrove forests provide a wide range of ecosystem services, including coastal protection and carbon storage. Their survival can be threatened by sea-level rise, but the forests can avoid inundation if there is sufficient sediment supply to allow them to maintain soil elevations suitable for plant growth. This study analyses recent trends in mangrove surface elevation changes across the Indo-Pacific region and finds that sediment availability is important to maintaining rates of soil-surface elevation gain that matched or exceeded that of sea-level rise, but that 69% of the forest study sites had rates of soil-surface elevation gain less than that of sea-level rise. Numerical modelling based on the field data suggests that mangrove forests at sites with low tidal range and low sediment supply could be submerged as early as 2070. Sea-level rise can threaten the long-term sustainability of coastal communities and valuable ecosystems such as coral reefs, salt marshes and mangroves 1 , 2 . Mangrove forests have the capacity to keep pace with sea-level rise and to avoid inundation through vertical accretion of sediments, which allows them to maintain wetland soil elevations suitable for plant growth 3 . The Indo-Pacific region holds most of the world’s mangrove forests 4 , but sediment delivery in this region is declining, owing to anthropogenic activities such as damming of rivers 5 . This decline is of particular concern because the Indo-Pacific region is expected to have variable, but high, rates of future sea-level rise 6 , 7 . Here we analyse recent trends in mangrove surface elevation changes across the Indo-Pacific region using data from a network of surface elevation table instruments 8 , 9 , 10 . We find that sediment availability can enable mangrove forests to maintain rates of soil-surface elevation gain that match or exceed that of sea-level rise, but for 69 per cent of our study sites the current rate of sea-level rise exceeded the soil surface elevation gain. We also present a model based on our field data, which suggests that mangrove forests at sites with low tidal range and low sediment supply could be submerged as early as 2070.

19 I. 19 II. 20 III. 20 IV. 21 V. 26 VI. 29 VII. 30 31 References 31 SUMMARY: Mangroves are among the most well described and widely studied wetland communities in the world. The greatest threats to mangrove persistence are deforestation and other anthropogenic disturbances that can compromise habitat stability and resilience to sea‐level rise. To persist, mangrove ecosystems must adjust to rising sea level by building vertically or become submerged. Mangroves may directly or indirectly influence soil accretion processes through the production and accumulation of organic matter, as well as the trapping and retention of mineral sediment. In this review, we provide a general overview of research on mangrove elevation dynamics, emphasizing the role of the vegetation in maintaining soil surface elevations (i.e. position of the soil surface in the vertical plane). We summarize the primary ways in which mangroves may influence sediment accretion and vertical land development, for example, through root contributions to soil volume and upward expansion of the soil surface. We also examine how hydrological, geomorphological and climatic processes may interact with plant processes to influence mangrove capacity to keep pace with rising sea level. We draw on a variety of studies to describe the important, and often under‐appreciated, role that plants play in shaping the trajectory of an ecosystem undergoing change.

The future of coastal wetlands and their ecological value depend on their capacity to adapt to the interacting effects of human impacts and sea-level rise. Even though extensive wetland loss due to submergence is a possible scenario, its magnitude is highly uncertain due to limited understanding of hydrodynamic and bio-geomorphic interactions over time. In particular, the effect of man-made drainage modifications on hydrodynamic attenuation and consequent wetland evolution is poorly understood. Predictions are further complicated by the presence of a number of vegetation types that change over time and also contribute to flow attenuation. Here, we show that flow attenuation affects wetland vegetation by modifying its wetting-drying regime and inundation depth, increasing its vulnerability to sea-level rise. Our simulations for an Australian subtropical wetland predict much faster wetland loss than commonly used models that do not consider flow attenuation. Predictions of coastal wetland response to sea-level rise often neglect attenuation effects due to vegetation and infrastructure. Here, the authors show that including attenuation effects improves prediction of wetland evolution and suggests increases in wetland vulnerability to sea-level rise.

Dryland wetlands are resilient ecosystems that can adapt to extreme periodic drought–flood episodes. Climate change projections show increased drought severity in drylands that could compromise wetland resilience and reduce important habitat services. These recognized risks have been difficult to evaluate due to our limited capacity to establish comprehensive relationships between flood–drought episodes and vegetation responses at the relevant spatiotemporal scales. We address this issue by integrating detailed spatiotemporal flood–drought simulations with remotely sensed vegetation responses to water regimes in a dryland wetland known for its highly variable inundation. We show that a combination of drought tolerance and dormancy strategies allow wetland vegetation to recover after droughts and recolonize areas invaded by terrestrial species. However, climate change scenarios show widespread degradation during drought and limited recovery after floods. Importantly, the combination of degradation extent and increase in drought duration is critical for the habitat services wetland systems provide for waterbirds and fish.

The Australian ‘New South Wales Estuary health assessment and biodiversity monitoring program’ has set state-wide targets for estuary health. A selection of water bodies is being monitored by in situ chlorophyll a concentration and turbidity measurements, indicators for water quality. We investigate whether the current monitoring program can benefit from the use of remote sensing derived data, analyzing chlorophyll a and water clarity estimates by the C2RCC and ACOLITE products based on Sentinel-2 MSI imagery for three lakes along the New South Wales coast. The C2RCC and ACOLITE products were partly successful in predicting chlorophyll a concentration and water clarity. Estimates based on Sentinel-2 MSI imagery were in the range of in situ measurements. However, results varied across years and lakes, and a significant correlation could not be found in every case. It is likely that the physical differences between the systems, such as nutrient input, tannins, and suspended algae/sediment matrix, influence the output of the algorithm. This may preclude the application of a ‘one size fits all’ monitoring approach, given the importance of local ecological phenomena in both influencing remote sensing observations and the nature of appropriate targets. However, the design of a monitoring program that incorporates remote sensing provides a way forward.

The fate of coastal wetlands and their ecosystem services is dependent upon maintaining substrate elevations within a tidal frame that is influenced by sea-level rise. Development and application of morphodynamic models has been limited as few empirical studies have measured the contribution of key processes to surface elevation change, including mineral and organic matter addition, autocompaction of accumulating sediments and deep subsidence. Accordingly, many models presume that substrates are in equilibrium with relative sea-level rise (RSLR) and the composition of substrates are relatively homogenous. A 20-year record of surface elevation change and vertical accretion from a large tidal embayment in Australia coupled with analyses of inundation frequency and the character of sediments that have accumulated above mean sea level was analyzed to investigate processes influencing surface elevation adjustment. This study confirms the varying contribution of addition, decomposition and compression of organic material, and mineral sediment consolidation. Autocompaction of substrates was proportional to the overburden of accumulating sediments. These processes operate concurrently and are influenced by sediment supply and deposition. Vertical accretion was linearly related to accommodation space. Surface elevation change was related to vertical accretion and substrate organic matter, indicated by carbon storage above mean sea level. Surface elevation change also conformed to models that initially increase and then decrease as accommodation space diminishes. Rates of surface elevation change were largely found to be in equilibrium with sea-level rise measured at the nearest tide gauge, which was estimated at 3.5 mm y –1 over the period of measurements. As creation of new accommodation space with sea-level rise is contrary to the longer-term history of relative sea-level stability in Australia since the mid-Holocene, striking stratigraphic variation arises with deeper sediments dominated by mineral sands and surficial sediments increasingly fine grained and having higher carbon storage. As the sediment character of substrates was found to influence rates of surface elevation gain, we caution against the unqualified use of models derived from the northern hemisphere where substrates have continuously adjusted to sea-level rise and sediment character is likely to be more homogenous.