Salt Marsh Species Research Articles

Tolerance of the Australian halophyte, beaded samphire, Sarcocornia quinqueflora, to Pb and Zn under glasshouse conditions: Evaluating metal uptake and partitioning, photosynthetic performance, biomass, and growth

Saltmarsh sediments are susceptible to accumulation of excessive concentrations of anthropogenically elevated metals such as lead (Pb) and zinc (Zn). The resident salt tolerant plants of saltmarsh ecosystems form the basal underpinning of these ecosystems. As such, metal-associated adverse impacts on their physiology can have detrimental flow-on effects at individual, population, and community levels. The present study assessed the accumulation and partitioning of ecologically relevant concentrations of Pb, Zn, and their combination in a dominant Australian saltmarsh species, Sarcocornia quinqueflora. Plants were hydroponically maintained under glasshouse conditions for 16 weeks exposure to either Pb (20 µg l–1), Zn (100 µg l–1), or their mixture. We evaluated the chronic toxicological effects of single and mixed metal treatments with reference to metal uptake and partitioning, photosynthetic performance, photosynthetic pigment concentration, biomass and growth. Lead was more toxic than Zn, and Zn appeared to have an antagonistic effect on the toxicological effects of Pb in S.quinqueflora in terms of metal uptake, photosynthetic performance, photosynthetic pigment concentrations, and growth. Indeed, the tolerance index was 55 % in plants treated with Pb compared to 77 % in Zn treated plants and 73 % in Pb+Zn treated plants. Finally, Sarcocornia quinqueflora primarily accumulated both Pb and Zn in roots at concentrations exceeding unity whilst translocation of these metals to above ground tissues was restricted regardless of treatment. This suggests that S. quinqueflora may be suitable for phytostabilisation of Zn, and of Pb particularly in the presence of Zn.

Contribution of soil seed banks to vegetation resilience in coastal freshwater wetlands of subtropical Australia

AbstractQuestionsWhat role do soil seed banks play in the resilience of coastal freshwater wetland vegetation communities? How might soil seed bank composition and similarity to standing vegetation drive changes in vegetation expression, particularly given projected changes in climate?LocationSixty wooded coastal freshwater wetlands in southeast Queensland, Australia.MethodsWe surveyed standing vegetation and investigated soil seed bank composition through an 8‐month‐long emergence experiment.ResultsSoil seed bank assemblages were dominated by forb and sedge species (23% exotic), but composition varied throughout the study region. Spatial (north–south) and land‐use (urban–rural) gradients explained some variation in soil seed bank composition. Soil moisture and groundwater dependence also influenced species distributions, particularly for freshwater wetland species. The similarity of soil seed banks to standing vegetation was low. Species present in both extant and soil seed bank assemblages were commonly native wetland taxa, including one salt marsh species (Juncus kraussii).ConclusionsProjected climatic changes will likely drive changes in coastal freshwater wetland vegetation communities through increases in the frequency and intensity of disturbances (e.g., storm surge). Our results suggest that regeneration from soil seed banks could promote four potential scenarios: (1) expansion of weed communities, (2) expansion of salt marsh communities, (3) maintenance and expansion of wetland/terrestrial species, and (4) transformation to an unvegetated open water zone because of reduced regeneration success under changing conditions. These diverse vegetation futures highlight the vulnerability of wooded coastal freshwater wetlands and the need for research and management interventions to maintain their biodiversity and ecosystem services.

Retreating coastal forest supports saltmarsh invertebrates

AbstractAs sea‐level rise converts coastal forest to salt marsh, marsh arthropods may migrate inland; however, the resulting changes in arthropod communities, including the stage of forest retreat that first supports saltmarsh species, remain unknown. Furthermore, the ghost forest that forms in the wake of rapid forest retreat offers an unknown quality of habitat to marsh arthropods. In a migrating marsh in Virginia, USA, ground‐dwelling arthropod communities were assessed across the forest‐to‐marsh gradient, and functional use of ghost forest and high marsh habitats was evaluated to determine whether marsh arthropods utilized expanded marsh in the same way as existing marsh. Diet and body condition were compared for two marsh species found in both high marsh and ghost forest (the detritivore amphipod, Orchestia grillus, and the hunting spider, Pardosa littoralis). Community composition differed among zones along the gradient, driven largely by retreating forest taxa (e.g., Collembola), marsh taxa migrating into the forest (e.g., O. grillus), and unique taxa (e.g., Hydrophilinae beetles) at the ecotone. The low forest was the most inland zone to accommodate the saltmarsh species O. grillus, suggesting that inland migration of certain saltmarsh arthropods may co‐occur with early saltmarsh plant migration and precede complete tree canopy die‐off. Functionally, O. grillus occupied a larger trophic niche in the ghost forest than the high marsh, likely by consuming both marsh and terrestrial material. Despite this, both observed marsh species primarily consumed from the marsh grass food web in both habitats, and no lasting differences in body condition were observed. For the species and functional traits assessed, the ghost forest and high marsh did not show major differences at this site. Forest retreat and marsh migration may thus provide an important opportunity for generalist saltmarsh arthropods to maintain their habitat extent in the face of marsh loss due to sea‐level rise.

Suaeda salsa spectral index for Suaeda salsa mapping and fractional cover estimation in intertidal wetlands

Suaeda Salsa (S. salsa), with short and red-purplish plants, is a typical pioneer saltmarsh species in the intertidal wetlands of temperate East Asia. It has important ecological, economic, and recreational values. In the past few decades, S. salsa has severely degraded in coastal China, which has brought widespread attention from regional and local governments. As a result, extensive S. salsa restoration projects have been initiated in recent years. It is urgently needed to develop satellite-based methods for both S. salsa mapping and fractional cover (FC) estimation because degradation and recovery of S. salsa are manifested by changes in both area and plant cover. However, accurate mapping and FC estimation of S. salsa are challenging because (1) S. salsa in intertidal areas have low FC and (2) heterogeneous soil backgrounds in wetlands greatly impact the spectral reflectance observed by satellites. To address these issues, this study proposed a new Suaeda Salsa Spectral Index (SSSI) to support accurate detection and FC estimation of S. salsa. The SSSI was designed based on the laboratory spectral measurements by considering variations in wetland soil moisture and by taking advantage of the reddish color of S. salsa. It consists of two components, one of which utilized blue, green and red bands to separate S. salsa from green vegetation, and the other component utilized a modification of the Soil Adjusted Vegetation Index (SAVI) to reduce the impact of soil background and maintain a linear relationship with S. salsa FC. SSSI was then applied on Sentinel-2/GF-1 images over the Yellow River Delta (YRD) and Liao River Delta (LRD), China. Based on SSSI, a simple thresholding approach was used to identify S. salsa, and a linear regression model was used to estimate FC. With reference datasets provided from field investigations, Unmanned Aerial Vehicle multispectral images and high-spatial resolution satellite images, our results show that the SSSI was able to detect low-coverage S. salsa (FC > 20 % in YRD and FC > 10 % in LRD), and the S. salsa maps had an overall accuracy over 94%. The SSSI-FC models achieved good estimation accuracies (R2 = 0.77 ∼ 0.86, RMSE = 7.55 % ∼ 9.79 %). Compared to the Normalized Difference Vegetation Index (NDVI) and SAVI, SSSI alleviated the impacts from soil backgrounds and provided better S. salsa FC estimations, particularly for low-coverage S. salsa. SSSI has great potential in supporting continuous monitoring of S. salsa dynamics and evaluating the effectiveness of S. salsa restoration projects.

Influence of Seasonal Abiotic Factors and Co-Existing Salt Marsh Plants on the Growth and Reproduction of Zostera japonica in Fluctuating Estuarine Environments

In the biodiverse Nakdong River estuary, the predominant seagrass and salt marsh species, Zostera japonica and Bolboschoenus planiculmis, are declining due to human and natural pressures. Our study investigated how environmental factors and co-existing salt marsh vegetation impact the growth and reproduction of Z. japonica. Understanding the reproductive dynamics of Z. japonica in this estuary is crucial, as sexual reproduction ensures the resilience and stability of seagrass populations in challenging environments. This study revealed that approximately 49% of Z. japonica shoots flowered, yet none persisted to the subsequent growth season, indicating a reliance on sexual reproduction for population resilience. The presence of competing B. planiculmis shoots and Ulva pertusa indirectly suppressed the growth and reproduction of Z. japonica by reducing light availability. Additionally, environmental stresses that occurred during summer, such as elevated temperatures, reduced salinity, and sediment transport, likely affected the vegetative and reproductive performance of Z. japonica in this estuary. Consequently, Z. japonica in this estuary has adopted a mixed annual life history strategy in response to these environmental oscillations. Our findings highlight the vulnerability of the Z. japonica population to seasonal environmental shifts and interspecies competition in this estuary, offering essential considerations for its conservation and effective management.

A multi-sensor approach to monitor the ongoing restoration of edaphic conditions for salt marsh species facing sea level rise: An adaptive management case study in Camargue, France

The Camargue or Rhône delta is a coastal wetland in southern France of which parts formerly devoted to salt production are undergoing a renaturation process. This study assessed a multisensor approach to investigate the link between sediment size distribution, habitat development mapped with Worldview 2, flooding durations estimated with time series of SENTINEL 2 images and elevation modelled with a LIDAR point cloud in former saltworks. A Random Forest classification algorithm was used to map the vegetation distributions of Sarcocornia fruticosa and Arthrocnemum macrostachyum, main representatives of the NATURA 2000 “Mediterranean and thermo-Atlantic halophilous scrubs (Sarcocornetea fruticosi)” habitat on the site. The best habitat map was obtained when considering the species separately. The random forest Out-of-bag errors were 1.43 % for S. fruticosa and 2.18 % for A. macrostachyum. Both species were generally distributed on different elevation and flooding duration zones considering mean values. Flooding duration was estimated with the Water In Wetland index (WIW) based on 15 Sentinel-2 scenes. Two models related to sediment grain size distribution were developed: one predicting the flooding duration and one predicting the halophilous scrub distribution. Maps of flooding duration, sediment grain size distribution and elevation highlighted two main zones in the study area: a western section with coarser sediments, shorter flooding durations and higher elevations under sea influence; an eastern section with finer sediments, longer hydroperiods and lower elevations under a historic river influence. This multidisciplinary approach offers perspectives for using space-based data over large scales to monitor changes in edaphic conditions of coastal areas facing natural and anthropogenic forcings. The results call for further investigations to predict the dynamic distribution of other coastal habitats following climate change impacts, such as sea level rise.

Projecting future wave attenuation by vegetation from native and invasive saltmarsh species in the United States

Saltmarshes are naturally found along gently sloped coastlines of temperate zones including low-energy intertidal estuaries, shallow bays, and on the landward side of barrier islands. Often referred to as one of the most productive, dynamic, and valuable ecosystems on Earth, saltmarshes have been increasingly used as natural and nature-based features (NNBFs) for coastal defense. In preserved conditions, low-lying, regularly flooded marshes in the United States are naturally dominated by a single native species, Spartina alterniflora. However, over the last century the invasive Common Reed (i.e., Phragmites australis), which is directly associated with decreases in plant biodiversity and disruption of natural biochemical cycles, has become the dominant marsh species in North America. Our study quantified the influence of Phragmites australis invasion on future wave attenuation by vegetation when compared to native Spartina alterniflora marshes. We combined the coupled ADCIRC+SWAN with the point-based landscape model SLAMM and field-work-based explicit representation of vegetation to develop a series of modeling simulations based on a combination of low and high-intensity storms, SLR projections, and marsh migration scenarios. Wave attenuation by different marsh species was investigated using average vegetation characteristics (stem height, density, and diameter) for both Phragmites australis-dominated and Spartina alterniflora-dominated bay-wide modeling scenarios. Results are presented as spatially distributed differences in wave attenuation and average wave attenuation curves. Our results show that under current SLR conditions Phragmites australis-dominated marshes can provide significantly more wave attenuation than native Spartina alterniflora marshes during extreme hurricane events. On the contrary, under high-frequency and low-intensity storm events, Spartina alterniflora-dominated marshes are slightly more efficient than invasive Phragmites australis marshes. With the addition of SLR and the increase in the incoming wave heights Phragmites australis marshes can support more wave attenuation than Spartina alterniflora.

Assessing Spectral Band, Elevation, and Collection Date Combinations for Classifying Salt Marsh Vegetation with Unoccupied Aerial Vehicle (UAV)-Acquired Imagery

New England salt marshes provide many services to humans and the environment, but these landscapes are threatened by drivers such as sea level rise. Mapping the distribution of salt marsh plant species can help resource managers better monitor these ecosystems. Because salt marsh species often have spatial distributions that change over horizontal distances of less than a meter, accurately mapping this type of vegetation requires the use of high-spatial-resolution data. Previous work has proven that unoccupied aerial vehicle (UAV)-acquired imagery can provide this level of spatial resolution. However, despite many advances in remote sensing mapping methods over the last few decades, limited research focuses on which spectral band, elevation layer, and acquisition date combinations produce the most accurate species classification mappings from UAV imagery within salt marsh landscapes. Thus, our work classified and assessed various combinations of these characteristics of UAV imagery for mapping the distribution of plant species within these ecosystems. The results revealed that red, green, and near-infrared camera image band composites produced more accurate image classifications than true-color camera-band composites. The addition of an elevation layer within image composites further improved classification accuracies, particularly between species with similar spectral characteristics, such as two forms of dominant salt marsh cord grasses (Spartina alterniflora) that live at different elevations from each other. Finer assessments of misclassifications between other plant species pairs provided us with additional insights into the dynamics of why classification total accuracies differed between assessed image composites. The results also suggest that seasonality can significantly affect classification accuracies. The methods and findings utilized in this study may provide resource managers with increased precision in detecting otherwise subtle changes in vegetation patterns over time that can inform future management strategies.

Seed settling and trapping during submerged secondary dispersal: Implications for saltmarsh recruitment and restoration

Given the decline of global salt marshes, there is a pressing need to pinpoint the key processes that limit and facilitate seed-based pioneer recruitment. Secondary seed dispersal, in the form of short-distance submerged movement, is a prerequisite for initiating pioneer establishment in adjacent tidal flats but has not been fully appreciated and understood. In this study, using a settling tube and race-track flume, seeds of four global occurring saltmarsh species were studied in terms of their settlement speed and trapping opportunity to understand how seed traits and physical settings affect submerged dispersal behavior and thus seed-based saltmarsh recruitment. Present study led to the following novel insights: 1) Seeds have density-dependent settling speeds, which are comparable to that of fine sand, but much faster than that of very fine sand and silt. Since the latter is the type of sediment commonly found in many estuaries worldwide (such as the Scheldt), seeds will typically settle faster than local sediments. A sufficiently long hydrodynamic-calm period allows slowly settling sediment to bury settled seeds, otherwise, seeds will remain uncovered if the period is short. 2) Seed trapping ratio increased linearly with surface roughness (a proxy for local topographic complexity), but this effect becomes smaller with increasing hydrodynamic intensity. Seed drag coefficient was identified as the key biotic factor contributing to interspecies variability in trapping ratio. Overall, present results suggest that submerged seed dispersal may form a primary bottleneck for salt marsh recruitment by limiting seed availability via two mechanisms: i) reduced chance of seed burial through asynchronous settling of seeds and sediment particles; ii) reduced probability of seed trapping due to encountering smooth tidal flat surfaces. This study provide mechanistic and data basis for the targeted application of biophysical models in predicting outcomes of saltmarsh recruitment and long-term maintenance, thereby informing seed-based conservation and restoration.

Nutrient enrichment undermines invasion resistance to Spartina alterniflora in a saltmarsh: Insights from modern coexistence theory

Abstract The ability of native communities to maintain invasion resistance in the face of environmental changes is crucial for biodiversity conservation. However, previous studies have primarily focused on the inhibitive effects experienced by invaders, providing limited insights into the ultimate outcome of resistance. In this study, we integrated modern coexistence theory into biotic resistance research to investigate the impact of nutrient enrichment on the resistance of Phragmites australis marshes to Spartina alterniflora invasion in the Yangtze estuary saltmarshes of China. Our results demonstrated that under non‐enriched conditions, successful invasion resistance was facilitated by stable coexistence between native and invasive species in the field. This prediction of invasion resistance aligned with the distribution dynamics of the two species in the Yangtze estuary saltmarshes over the past two decades. However, nutrient enrichment was likely to lead to a fundamental shift in their coexistence and ultimately, the failure of resistance. Synthesis and applications. Integrating modern coexistence theory into biotic resistance studies advances the assessment of invasion resistance by shifting from quantifying relative strength to predicting explicit resistance outcomes. Ecosystem managers can draw explicit conclusions about the potential establishment and impact of invaders by analysing observational or experimental data within the framework of modern coexistence theory. This information aids in identifying the most efficient strategies for addressing invasive species.

Field practice of Scirpus mariqueter restoration in the bird habitats of Chongming Dongtan Wetland, China

The coastal saltmarshes in China have been seriously degraded and artificial restoration strategies have become one of the primary measures to protect the coastal habitats. In this study, we investigated efficient restoration techniques for native saltmarsh plants in vacant habitats within the project area of the Ecological Control of Spartina alterniflora and Improvement of Birds Habitats in the Shanghai Chongming Dongtan Bird National Nature Reserve in the Yangtze Estuary. Through field and pilot experiments, we analyzed the effects of propagule types and transplanting methods on the restoration efficiency of Scirpus mariqueter, a dominant saltmarsh species in the Yangtze Estuary. We further evaluated the cost and efficiency of various restoration strategies. Our results indicated that: 1) The corm seedlings of S. mariqueter had higher flood tolerance, and were suitable for use in the habitats with higher environmental heterogeneity. 2) Across the four treatments for S. mariqueter restoration, the most economically efficient treatment was to transplant low-density corm seedlings without sediment, with an investment cost of approximately ￥10100 per hectare. The costs were ￥41100, ￥30000, and ￥120100 per hectare for high-density without sediment, low-density with sediment, and high-density with sediment restoration, respectively. 3) After nearly 5-yr efforts, the S. mariqueter community of Plot C3 in project area had achieved a coverage of over 60%, marking a relatively successful large-scale field restoration. This study could provide the foundation and support for large-scale coastal saltmarsh restoration projects.

WAVE ATTENUATION OF SALTMARSH VEGETATION UNDER STORM CONDITIONS

Nature-based solutions (NbS) for coastal protection has recently gained increased attention worldwide as a sustainable, economical and eco-friendly alternative to conventional grey structures, particularly under the threat of climate change (Temmerman et al. 2013). Wave energy dissipation by vegetation can be parameterized by the total horizontal force acting on the plant; expressed using a Morison-type equation considering only the form drag component (Dalrymple et al. 1984). Modelling wave-vegetation interaction is challenging in a laboratory environment (Lara et al. 2016) and it is difficult to accomplish a realistic representation of a plant’s biomechanical behavior and geometry using plant mimics or surrogates. Few studies have modelled real saltmarsh vegetation in large scale laboratory facilities (Moller et al. 2014; Maza et al. 2015) and quantified wave attenuation, particularly for engineered living shorelines (Maryland DoE, 2013). Further research is needed, particularly in the Canadian context, to investigate the capacity of different saltmarsh species to effectively attenuate waves and wave runup under storm conditions, to examine the plant’s drag coefficient and to bridge the gap to develop technical design specifications for the detailed design of living shorelines.

Response of native and exotic saltmarsh species to sediment deposition addition

The native saltmarsh species Scirpus mariqueter (hereafter S. mariqueter) and the exotic species saltmarsh cordgrass (Spartina alterniflora Loisel., hereafter S. alterniflora), have been found commonly in regional saltmarsh ecosystems which received a large amount of sediment inputs from Yangtze River, eastern coasts of China. For the purpose of saltmarsh restoration and invasive species management, it is important to understand the response of vegetation species to various sediment inputs. This study investigated and compared the effects of sediment addition on S. mariqueter and S. alterniflora through laboratory experiment using vegetation samples collected from a natural saltmarsh with a high sedimentation rate (12 cm a−1). Plant growth parameters over their growth period, including survival rate, height and biomass were measured against sediment addition gradient (0 cm, 3 cm, 6 cm, 9 cm, and 12 cm in thickness). The results showed that sediment addition significantly affected the growth of vegetation but this effect varied between two species. Compared with the control group, the growth of S. mariqueter was promoted with sediment addition of 3–6 cm, but it turned to inhibition when the sediment thickness exceeded 6 cm. The growth of S. alterniflora was increased with increasing sediment addition till 9–12 cm, but the survival rate of each group kept stable. Overall, against a gradient sediment addition, S. mariqueter was found to benefit from low to moderate sediment addition (3–6 cm) but higher addition showed inhabitation effects. S. alterniflora benefited from increasing sediment addition to a point. When facing high sediment inputs, S. alterniflora was found to be more adaptable than S. mariqueter. These results have important implications for further studies on saltmarsh restoration and interspecific competition against a high sediment input background.

The Role of Estuarine Wetlands (Saltmarshes) in Sediment Microplastics Retention

Concerns regarding plastic pollution, especially microplastics, have increased, as they can be present in different environmental compartments, including estuarine areas and saltmarshes. Although saltmarshes are highly vulnerable to different human activities and pressures, they have the ability to trap/retain contaminants in their vegetated sediments. However, there is still little information regarding the role of saltmarshes in microplastic retention. Thus, the present study aims to investigate the capability of an estuarine saltmarsh to trap microplastics by comparing microplastic concentrations in vegetated (saltmarsh) and non-vegetated sediments. Microplastic content from sediment (vegetated and non-vegetated) samples collected at different sampling sites in Lima River estuary was estimated using previously optimised extraction protocols, and the observed particles were then characterised accordingly to their size, colour, shape, and polymer (by FTIR). Water samples were also collected and analysed for their microplastics content to complement MPs characterisation within the estuarine area. Microplastics were detected in all sediment samples, with fibres being the most common type of microplastic found, followed by fragments/particles. Overall, vegetated sediments, especially those of saltmarsh species Juncus maritimus, presented a higher number of plastic items. These results indicated that microplastics tend to be trapped in vegetated sediments, supporting the fact that saltmarshes have a significant influence on the transport, distribution, and accumulation of MPs in estuarine areas.

Deformation of Spartina patens and Spartina alterniflora stems under irregular wave action

ABSTRACT Physical modeling studies have endeavored to quantify the influence of plant biophysical parameters and hydrodynamics on wave-vegetation interactions and coastal protection. The accuracy with which such studies have characterized stem motion is limited by the predominant use of plant surrogates, and the few saltmarsh species considered in live vegetation studies. To address this, prototype-scale experiments were conducted in the outdoor Large Wave Canal of the Institut National de la Recherche Scientifique, Québec, in collaboration with the University of Ottawa and the National Research Council Canada, allowing novel characterization of live vegetation deformation under wave action. Two saltmarsh species were investigated (Spartina alterniflora, Spartina patens) under various irregular wave conditions (0.03 m< H s <0.28 m, T s 2.5, 10 s). Stem deformation was characterized using submerged cameras and bending angle tracking, coupled with wave height and velocity measurements. Significant differences in stem flexibility were observed between species, with S. alterniflora exhibiting more rigid stems (EI alterniflora =0.051 Nm2) than S. patens (EI patens =0.0015 Nm2; t-test; p<0.05). The two species consequexhibited different bending angles under similar hydrodynamic conditions, expected to influence their relative coastal protection capacity. These findings provide critical insight into the design of marsh construction or restoration for coastal protection.

Factors driving the halophyte rhizosphere bacterial communities in coastal salt marshes.

Root-associated microorganisms promote plant growth and provide protection from stresses. Halophytes are the fundamental components maintaining ecosystem functions of coastal salt marshes; however, it is not clear how their microbiome are structured across large spatial scales. Here, we investigated the rhizosphere bacterial communities of typical coastal halophyte species (Phragmites australis and Suaeda salsa) in temperate and subtropical salt marshes across 1,100 km in eastern China. The sampling sites were located from 30.33 to 40.90°N and 119.24 to 121.79°E across east China. A total of 36 plots were investigated in the Liaohe River Estuary, the Yellow River Estuary, Yancheng, and Hangzhou Bay in August 2020. We collected shoot, root, and rhizosphere soil samples. the number of pakchoi leaves, total fresh and dry weight of the seedlings was counted. The soil properties, plant functional traits, the genome sequencing, and metabolomics assay were detected. The results showed that soil nutrients (total organic carbon, dissolved organic carbon, total nitrogen, soluble sugars, and organic acids) are high in the temperate marsh, while root exudates (measured by metabolite expressions) are significantly higher in the subtropical marsh. We observed higher bacterial alpha diversity, more complex network structure, and more negative connections in the temperate salt marsh, which suggested intense competition among bacterial groups. Variation partitioning analysis showed that climatic, edaphic, and root exudates had the greatest effects on the bacteria in the salt marsh, especially for abundant and moderate subcommunities. Random forest modeling further confirmed this but showed that plant species had a limited effect. Taken together, the results of this study revealed soil properties (chemical properties) and root exudates (metabolites) had the greatest influence on the bacterial community of salt marsh, especially for abundant and moderate taxa. Our results provided novel insights into the biogeography of halophyte microbiome in coastal wetlands and can be beneficial for policymakers in decision-making on the management of coastal wetlands.

Tidal channel meanders serve as stepping-stones to facilitate cordgrass landward spread by creating invasion windows.

Understanding the mechanisms by which the geomorphic structures affect habitat invasibility by mediating various abiotic and biotic factors is essential for predicting whether these geomorphic structures may provide spatial windows of opportunity to facilitate range-expansion of invasive species in salt marshes. Many studies have linked geomorphic landscape features such as tidal channels to invasion by exotic plants, but the role of tidal channel meanders (i.e., convex and concave sides) in regulating the Spartina alterniflora invasion remains unclear. Here, we examined the combined effects of tidal channel meander-mediated hydrodynamic variables, soil abiotic stresses, and propagule pressure on the colonization of Spartina in the Yellow River Delta, China, by conducting field observations and experiments. The results showed that lower hydrodynamic disturbance, bed shear stress, and higher propagule pressure triggered by eddies due to the convex structure of channel meanders facilitated Spartina seedling establishment and growth, whereas the concave side considerably inhibited the Spartina invasion. Lower soil abiotic stresses also significantly promoted the invasibility of the channel meanders by Spartina. Based on these findings, we propose a conceptual framework to illustrate the effects of the meandering geomorphology of tidal channels on the mechanisms that might allow the landward spread of Spartina and related processes. Our results demonstrate that the meandering geomorphic structures of tidal channels could act as stepping-stones to significantly facilitate the landward invasion of Spartina along tidal channels. This implies that geomorphic characteristics of tidal channels should be integrated into invasive species control and salt marsh management strategies.

A new perspective on the impacts of Spartina alterniflora invasion on Chinese wetlands in the context of climate change: A case study of the Jiuduansha Shoals, Yangtze Estuary

Spartina alterniflora, an invasive plant, was introduced to the Chinese coastal zone in the early 90s. As an eco-engineering species, S. alterniflora not only alters saltmarsh species distributions, previously described as habitat degradation, but it also plays a vital role in coastal protection, especially for the development of recently emerged intertidal shoals. To provide a reference for coastal management under global change, we quantified the impact of the invasion process on provided ecological and coastal protection functions, exemplified at the emerging Jiuduansha Shoals (JDS) in the Yangtze Estuary. Results obtained by high-precision satellite monitoring and numerical modelling showed that the establishment and growth of S. alterniflora can exert considerable changes on local environment. The invasion of S. alterniflora to JDS wetland can be divided into three distinct phases, (1) establishment 1998–2003, (2) expansion 2003–2009, and (3) dominant 2009–2018 stages according to the changes in saltmarsh composition. Spatially, S. alterniflora continuously replaced Scirpus mariqueter, forcing S. mariqueter and Phragmites australis slowly to the lower and higher intertidal habitats, respectively. Notably, S. alterniflora expansion was the main driver that contributed to over 70 % of recent JDS wetland expansion even under sediment deficit conditions. Established S. alterniflora marsh (directly) dampens more waves because of aboveground stems, but it also causes more accretion and indirectly leads to higher “morphological” wave dampening. Thus, it increases coastal defense provided by the saltmarsh in the context of sea-level rise and strengthening storms. In conclusion, the role of S. alterniflora invasion to the local environment under global changes is controversial. For sustainable coastal management, we need context-dependent S. alterniflora management to maximize the benefit of coastal protection and minimize the impact on local ecology, especially in sediment-starving estuaries with expected coastline retreat.

Species-based Mapping of Carbon Stocks in Salt Marsh: Tianjin Coastal Zone as a Case Study

Because of geographical position and high carbon storage potential, coastal salt marshes are recognized as an essential component of blue carbon and play an indispensable role in regulating climate and reaching carbon neutrality targets. Nonetheless, accurately mapping salt marsh carbon stock on a regional scale remains challenging. The framework of mapping salt marsh carbon stock was developed by using machine learning (temporal–phenological–spatial) models, vegetation index aboveground biomass inversion models, and above/belowground biomass allometric models. Here, we employed Sentinel-2 time series images based on Google Earth Engine in combination with field survey data to produce a 10-m map of salt marsh carbon stocks in the Tianjin coastal zone (TCZ). The total and average carbon stocks of TCZ salt marsh vegetation in 2020 were approximately 6.24 × 10 3 Mg C and 45.02 Mg C/ha, respectively. In terms of vegetative species, the carbon stock was ranked by Spartina alterniflora (2.89 × 10 3 Mg C) &gt; Phragmites australis (1.74 × 10 3 Mg C) &gt; Suaeda salsa (1.61 × 10 3 Mg C). The carbon density of 3 representative salt marsh species sampled in Tianjin were calculated: S. alterniflora (18.63 Mg/ha) &gt; P. australis (6.49 Mg/ha) &gt; S. salsa (1.40 Mg/ha). The random forest algorithm shows the best performance in classifying, with an overall accuracy of 87.21%. This work created the replicable and generic technical framework for mapping carbon stocks in salt marshes, which supports blue carbon accounting and provides case support for “nature-based solutions.”

Estimating fractional cover of saltmarsh vegetation species in coastal wetlands in the Yellow River Delta, China using ensemble learning model

Saltmarshes in coastal wetlands provide important ecosystem services. Satellite remote sensing has been widely used for mapping and classification of saltmarsh vegetation, however, medium-spatial-resolution satellite datasets such as Landsat-series imagery may induce mixed pixel problems over saltmarsh landscapes which are spatially heterogeneous. Sub-pixel fractional cover estimation of saltmarsh vegetation at species level are required to better understand the distribution and canopy structure of saltmarsh vegetation. In this study, we presented an approach framework for estimating and mapping the fractional cover of major saltmarsh species in the Yellow River Delta, China based on time series Landsat 8 Operational Land Imager data. To solve the problem that the coastal area is frequently covered by clouds, we adopted the recently developed virtual image-based cloud removal (VICR) algorithm to reconstruct missing image values under the cloud/cloud shadows over the time series Landsat imagery. Then, we developed an ensemble learning model (ELM), which incorporates Random Forest Regression (RFR), K-Nearest Neighbor Regression (KNNR) and Gradient Boosted Regression Tree (GBRT) based on temporal-spectral features derived from the time-series cloudless images to estimate the fractional cover of major vegetation types, i.e., Phragmites australis, Suaeda salsa and the invasive species, Spartina alterniflora. High spatial resolution imagery acquired by the Unmanned Aerial Vehicle and Gaofen-6 satellites were used for reference sample collections. The results showed that our approach successfully estimated the fractional cover of each saltmarsh species (average of R-square:0.891, RMSE: 7.48%). Through four scenarios of experiments, we found that the ELM is advantageous over each individual model. When the images during key months were absent, cloud removal for the Landsat images considerably improved the estimation accuracies. In the study area, Spartina alterniflora covers the largest area (5753.97 ha), followed by Phragmites australis with spatial extent area of 4208.4 ha and Suaeda salsa of 1984.41 ha. The average fractional cover of S. alterniflora was 58.45%, that of P. australis was 51.64% and that of S.salsa was 51.64%.