Vegetation successions of coastal wetlands in southern Laizhou Bay, Bohai Sea, northern China, influenced by the changes in relative surface elevation and soil salinity

Organic matter composition and stability in estuarine wetlands depending on soil salinity

Coastal wetlands are key players in mitigating global climate change by sequestering soil organic matter. Soil organic matter consists of less stable particulate organic matter (POM) and more stable mineral-associated organic matter (MAOM). The distribution and drivers of MAOM and POM in coastal wetlands have received little attention, despite the processes and mechanisms differ from that in the upland soils. We explored the distribution of POM and MAOM, their contributions to SOM, and the controlling factors along a salinity gradient in an estuarine wetland. In the estuarine wetland, POM C and N were influenced by soil depth and vegetation type, whereas MAOM C and N were influenced only by vegetation type. In the estuarine wetland, SOM was predominantly in the form of MAOM (> 70 %) and increased with salinity (70 %-76 %), leading to long-term C sequestration. Both POM and MAOM increased with SOM, and the increase rate of POM was higher than that of MAOM. Aboveground plant biomass decreased with increasing salinity, resulted in a decrease in POM C (46 %-81 %) and N (52 %-82 %) pools. As the mineral amount and activity, and microbial biomass decreased, the MAOM C (2.5 %-64 %) and N pool (8.6 %-59 %) decreased with salinity. When evaluating POM, the most influential factors were microbial biomass carbon (MBC) and dissolved organic carbon (DOC). Key parameters, including MBC, DOC, soil salinity, soil water content, aboveground plant biomass, mineral content and activity, and bulk density, were identified as influencing factors for both MAOM abundance. Soil water content not only directly controlled MAOM, but together with salinity also indirectly regulated POM and MAOM by controlling microbial biomass and aboveground plant biomass. Our findings have important implications for improving the accumulation and increased stability of soil organic matter in coastal wetlands, considering the global sea level rise and increased frequency of inundation.

Changes of Tamarix austromongolica forests with embankment dams along the Laizhou bay

Background

Embankment dams were built south of the Laizhou bay in China for controlling storm surge disasters, but they are not enough to replace coastal forests in protecting the land. This study was designed to evaluate the effects of embankment dams on natural forests dominated by Tamarix austromongolica and test whether the dam-shrub system is a preferable updated defense.

Methods

Coastal forests on two typical flats, one before and one behind embankment dams, were investigated using quadrats and lines. Land bareness, vegetation composition and species co-occurrence were assessed; structures of T. austromongolica populations were evaluated; and spatial patterns of the populations were analyzed using Ripley's K and K1,2 functions.

Results

In the area before embankment dams, 84.8% of T. austromongolica were juveniles (basal diameter ≤ 3 cm), and 15.2% were adults (basal diameter > 3 cm); behind the dams, 52.9% were juveniles, and 47.1 were adults. In the area before the dams, the land bareness was 13.7%, four species occurred, and they all were ready to co-occur with T. austromongolica; behind the dams, the land bareness was 0%, and 16 species occurred whereas they somewhat resisted co-occurrence with T. austromongolica. In the area before the dams, the T. austromongolica population was aggregated in heterogeneous patches, and the juveniles tended to co-occur with the adults; behind the dams, they were over-dispersed as nearly uniform distributions, while the juveniles could recruit and were primarily independent of the adults. These results indicate that the T. austromongolica species did not suffer from the unnatural dams, but benefited somehow in population expansion and development. Overall, the T. austromongolica species can adapt to artificial embankment dams to create a synthetic defense against storm surges.

Mechanisms for carbon stock driving and scenario modeling in typical mountainous watersheds of northeastern China

Watershed ecosystems play a pivotal role in maintaining the global carbon cycle and reducing global warming by serving as vital carbon reservoirs for sustainable ecosystem management. In this study, we based on the "quantity-mechanism-scenario" frameworks, integrate the MCE-CA-Markov and InVEST models to evaluate the spatiotemporal variations of carbon stocks in mid- to high-latitude alpine watersheds in China under historical and future climate scenarios. Additionally, the study employs the Geographic Detector model to explore the driving mechanisms influencing the carbon storage capacity of watershed ecosystems. The results showed that the carbon stock of the watershed increased by about 15.9 Tg from 1980 to 2020. Fractional Vegetation Cover (FVC), Digital Elevation Model (DEM), and Mean Annual Temperature (MAT) had the strongest explanatory power for carbon stocks. Under different climate scenarios, it was found that the SSP2-4.5 scenario had a significant rise in carbon stock from 2020 to 2050, roughly 24.1 Tg. This increase was primarily observed in the southeastern region of the watersheds, with forest and grassland effectively protected. Conversely, according to the SSP5-8.5 scenario, the carbon stock would decrease by about 50.53 Tg with the expansion of cultivated and construction land in the watershed's southwest part. Therefore, given the vulnerability of mid- to high-latitude mountain watersheds, global warming trends continue to pose a greater threat to carbon sequestration in watersheds. Our findings carry important implications for tackling potential ecological threats in mid- to high-latitude watersheds in the Northern Hemisphere and assisting policymakers in creating carbon sequestration plans, as well as for reducing climate change.

Exploring main soil drivers of vegetation succession in abandoned croplands of Minqin Oasis, China

Background

The Minqin Oasis, which is located in Wuwei City, Gansu Province, China, faces a very serious land desertification problem, with about 94.5% of its total area desertified. Accordingly, it is crucial to implement ecological restoration policies such as cropland abandonment in this region. In abandoned croplands, abiotic factors such as soil properties may become more important than biotic factors in driving vegetation succession. However, the connections between soil properties and vegetation succession remain unclear. To fill this knowledge gap, this study investigated these connections to explore major factors that affected vegetation succession, which is meaningful to designing management measures to restore these degraded ecosystems.

Methods

This study investigated seven 1-29-year-old abandoned croplands using the "space for time" method in Minqin Oasis. Vegetation succession was classified into different stages using a canonical correlation analysis (CCA) and two-way indicator species analysis (Twinspan). The link between soil properties and vegetation succession was analyzed using CCA. The primary factors shaping community patterns of vegetation succession were chosen by the "Forward selection" in CCA. The responses of dominant species to soil properties were analyzed using generalized additive models (GAMs).

Results

Dominant species turnover occurred obviously after cropland abandonment. Vegetation succession can be classified into three stages (i.e., early, intermediate, and late successional stages) with markedly different community composition and diversity. The main drivers of vegetation succession among soil properties were soil salinity and saturated soil water content and they had led to different responses of the dominant species in early and late successional stages. During the development of vegetation succession, community composition became simpler, and species diversity decreased significantly, which was a type of regressive succession. Therefore, measures should be adopted to manage these degraded, abandoned croplands.

Decomposition of exotic versus native aquatic plant litter in a lake littoral zone: Stoichiometry and life form analyses

The decomposition rates and stoichiometric characteristics of many aquatic plants remain unclear, and our understanding of material flow and nutrient cycles within freshwater ecosystems is limited. In this study, an in-situ experiment involving 23 aquatic plants (16 native and 7 exotic species) was carried out via the litter bag method for 63 days, during which time the mass loss and nutrient content (carbon (C), nitrogen (N), and phosphorus (P)) of plants were measured. Floating-leaved plants exhibited the highest decomposition rate (0.038 ± 0.002 day-1), followed by submerged plants and free-floating plants (0.029 ± 0.002 day-1), and emergent plants had the lowest decomposition rate (0.019 ± 0.001 day-1). Mass loss by aquatic plants correlated with stoichiometric characteristics; the decomposition rate increased with an increasing P content and with a decreasing C content, C:N ratio, and C:P ratio. Notably, the decomposition rate of submerged exotic plants (0.044 ± 0.002 day-1) significantly exceeded that of native plants (0.026 ± 0.004 day-1), while the decomposition rate of emergent exotic plants was 55 ± 4 % higher than that of native plants. The decomposition rates of floating-leaved and free-floating plants did not significantly differ between the native and exotic species. During decomposition, emergent plants displayed an increase in C content and a decrease in N content, contrary to patterns observed in other life forms. The P content decreased for submerged (128 ± 7 %), emergent (90 ± 5 %), floating-leaved (104 ± 6 %), and free-floating plants (32 ± 6 %). Exotic plants released more C and P but accumulated more N than did native plants. In conclusion, the decomposition of aquatic plants is closely linked to litter quality and influences nutrient cycling in freshwater ecosystems. Given these findings, the invasion of the littoral zone by submerged and emergent exotic plants deserves further attention.

A study of typical plant growth changes in response to drainage water and salt in ditch wetland in arid area

Agricultural drainage significantly affected the changes of soil moisture and salinity in ditch wetlands. These changes can profoundly impact the spatial distribution and evolution of ditch wetland vegetation, thereby affecting the ecological environmental effects of these wetlands. Consequently, it is imperative to investigate the response of typical plant growth to drainage and soil salt in ditch wetlands in arid regions. Based on the classical metapopulation conceptual framework model (Levins model), this study established a new model of plant growth change in ditch wetlands, incorporating the key variables (water level and soil salinity) of arid area ditch wetlands. The application of the Gaussian model facilitates the resolution of species growth rates and mortality rates within this model. The study focused on the main drainage ditch (ditch M) and the drainage bucket ditch (ditch N) in the Lubotan saline-alkali land in Fuping, Shaanxi Province. The results revealed the following key findings: 1) the model effectively simulates the response of plant growth changes to water level and soil salinity in ditch wetlands in arid regions, particularly plants in the reed area and transition area disturbed by single factors such as water level and soil salinity; 2) the germination period of Phragmites australis in the reed area thrives in a shallow moisture environment, and adjusting the water level of the drainage ditch can maintain optimal growth conditions for Phragmites australis; 3) during the germination period of Suaeda salsa in the transition area, soil salinity should not be excessively high, though a moderate increase in soil salinity can promote the germination and growth of Suaeda salsa; and 4) Suaeda salsa in the symbiotic area has a higher adaptability to the soil salinity, with change in biomass consistent with plants in the transition area. The model provides an explanation and prediction for the growth changes of plant communities in ditch wetlands under drainage conditions. By integrating this model with the impact of farmland drainage on water level and soil salinity in drainage ditches, effective drainage management measures can be formulated, offering scientific support for the construction of ecological irrigation areas.

Estimating Soil Salinity with Different Levels of Vegetation Cover by Using Hyperspectral and Non-Negative Matrix Factorization Algorithm

Hyperspectral technology has proven to be an effective method for monitoring soil salt content (SSC). However, hyperspectral estimation capabilities are limited when the soil surface is partially vegetated. This work aimed to (1) quantify the influences of different fraction vegetation coverage (FVC) on SSC estimation by hyperspectra and (2) explore the potential for a non-negative matrix factorization algorithm (NMF) to reduce the influence of various FVCs. Nine levels of mixed hyperspectra were measured from simulated mixed scenes, which were performed by strictly controlling SSC and FVC in the laboratory. NMF was implemented to extract soil spectral signals from mixed hyperspectra. The NMF-extracted soil spectra were used to estimate SSC using partial least squares regression. Results indicate that SSC could be estimated based on the original mixed spectra within a 25.76% FVC (R2cv = 0.68, RMSEcv = 5.18 g·kg-1, RPD = 1.43). Compared with the mixed spectra, NMF extraction of soil spectrum improved the estimation accuracy. The NMF-extracted soil spectra from FVC below 63.55% of the mixed spectra provided acceptable estimation accuracies for SSC with the lowest results of determination of the estimation R2cv = 0.69, RMSEcv = 4.15 g·kg-1, and RPD = 1.8. Additionally, we proposed a strategy for the model performance investigation that combines spearman correlation analysis and model variable importance projection analysis. The NMF-extracted soil spectra retained the sensitive wavelengths that were significantly correlated with SSC and participated in the operation as important variables of the model.

Study on the rare waterbird habitat networks of a new UNESCO World Natural Heritage site based on scenario simulation

As a newly established World Natural Heritage site, the conservation of rare waterbird habitats in the Yancheng coastal wetlands has attracted wide international attention. In view of the importance of this area in international biodiversity conservation and waterbird habitat conservation, the study of the current situation of rare waterbird habitat networks with spatial isolation features of great demonstration significance to improve the habitat conservation quality of the heritage site. Based on the data obtained from habitat suitability assessments of rare waterbirds, this paper used complex network theory, ecological stepping-stone theory and circuit models to analyze the current status and robustness of rare waterbird habitat networks in the Yancheng coastal wetlands. The results showed that the stepping stones of the red-crowned cranes and the Oriental storks were mainly distributed around important habitat areas, including aquaculture ponds and reed marshes, with areas of 1275.68 hm² and 1247.74 hm², respectively, while the stepping stones of Saunders's gulls were mainly distributed within the Tiaozini habitat site, with an area of 1180.76 hm². The stability and connectivity of the habitat networks of red-crowned cranes and Oriental storks in the northern habitat area were better. At the Tiaozini habitat site, there was spatial isolation and low connectivity among the habitats and stepping stones of Saunders's gulls. In the optimal protection scenario, the stability and connectivity of the habitat network structure of red-crowned cranes and Oriental storks were the best, and the networks tended to exhibit assortativity. In the random destruction scenario, the connectivity of the habitat networks of Saunders's gulls was the lowest, and the network structure was the most fragile. Finally, the optimization and restoration patterns of habitat network based on improving the support capacity of important habitat nodes and the enhancement of the stability and connectivity of the stepping-stone networks were proposed.

The inhibitory effects of Ulva prolifera extracts on early growth of Spartina alterniflora and the underlying mechanisms

Spartina alterniflora, a highly invasive plant, has caused a serious threat to ecosystem biodiversity and economic development in coastal areas of many countries. In this study, the allelopathic effect of Ulva prolifera extracts on seed germination and seedling growth of S. alterniflora was studied. The results showed that three different treatments (water, methanol and ethyl acetate extract) could inhibit the seed germination and seedling growth of S. alterniflora by reducing the germination proportion and germination index of seeds, decreasing the seedling length and root length of seedlings, and affecting the lipid peroxidation and enzyme activity. The higher the concentration of the extracts, the higher the inhibition effect. When the aqueous extract concentration reached 0.20 g/mL, the germination proportion of S. alterniflora decreased to 49.53% of the control. RNA-seq analysis showed that the expression of genes related to amino acid metabolism and photosynthesis were both upregulated, and genes related to energy generation and metabolism were both downregulated after adding the extracts. GC-MS analysis indicated that the U. prolifera extract was rich in organic acids, alcohols and esters, among which butanoic acid, butyl ester, Valine and Hexanedioic acid, bis (2-ethylhexyl) ester might be the dominant allelochemicals. In order to facilitate field dosing, prolong action time and control release effect, PVA/SA hydrogel embedded U. prolifera extract was used to obtain a sustained-release agent. In addition, the survival rate of S. alterniflora was significantly reduced, which was only 21.67% at the salinity of 30 ppt. The results of this study provide a feasible method for controlling the invasion of S. alterniflora and achieving the waste utilization of U. prolifera.

Distribution Characteristics and Restoration Application of Vegetation in Chengcun Bay Surrounding Areas of Yangjiang City

In recent years, global warming and sea level rise have further aggravated the risk of coastal erosion. Coastal vegetation plays an important role in resisting storm surges and alleviating coastal erosion. Therefore, screening plant species for the purpose of constructing ecological seawalls to protect or repair damaged coastal zones has become a hot issue. In this paper, a field survey was conducted to investigate the vegetation in Chengcun Bay surrounding areas of Yangjiang City by combining a line survey and sample plot survey. By understanding the vegetation types, distribution and community structure in the bay's surrounding areas and analyzing the restricting environmental factors of those plants, we put forward some countermeasures for coastal vegetation restoration in difficult site conditions from the aspects of plant species selection, vegetation configuration and restoration technology, so as to provide reference for ecological vegetation restoration in similar locations.

Assessing Coastal Reclamation Success in the East China Coast by Using Plant Species Composition

Quantitative analysis of the species composition and succession law of a plant community in a coastal reclamation area is of great significance for revealing the community construction and species coexistence mechanisms, and provides a basis for the rational use and conservation in coastal reclamation areas. Through the investigation of natural plant communities in Dongtai reclamation area and the adjacent national nature reserves in Jiangsu Province, eastern China, the composition and succession of plant communities were studied. A quantitative method was explored to analyze the process of plant succession and its representative species. The results showed that (1) A total of 65 species were found in the vegetation survey. These belonged to 26 families and 61 genera, and Poaceae is the most common plant species. The plant communities in the unreclaimed areas were mainly composed of Poaceae and Cyperaceae. The plant species increased after reclamation, which were mainly composed of Poaceae and Asteraceae; (2) The plant coverage greatly reduced after three years of reclamation, from 80% of the tidal flat to 37.34%, then gradually increased, and remained generally between 50% and 70%; (3) The above-ground biomass of the plant community was sharply reduced after reclamation, from 1.823 kg/m2 in the tidal flat to 0.321 kg/m2 in three years of reclamation, and then maintained at 0.11~0.27 kg/m2; (4)The species succession process of the plant community in the coastal wetland ecosystem that was affected by the reclamation activities transformed from a halophyte community that was dominated by a salt marsh plant community (Suaeda salsa, Spartina alterniflora, Scirpus mariqueter, and Phragmites australis) to a mesophyte plant community that was constructed with pioneer species such as Setaria viridis, Eleusine indica, etc., and eventually succeeded to a xerophyte plant community that was dominated by Humulus scandens and Cyperus difformis, etc. Reclamation activities have a profound impact on the characteristics and succession rules of natural vegetation communities along coastal wetland ecosystems. The period of seven years is presumed to be the tipping point in the succession of the plant community in coastal reclamation areas. The results of this study can provide a basis and reference for ecological protection and restoration in coastal reclamation areas.

Evolutions of 30-Year Spatio-Temporal Distribution and Influencing Factors of Suaeda salsa in Bohai Bay, China

Suaeda salsa (L.) Pall. (S. salsa) acts as a pioneer species in coastal wetlands due to its high salt tolerance. It has significant biodiversity maintenance, socioeconomic values (e.g., tourism) due to its vibrant color, and carbon sequestration (blue carbon). Bohai Bay region, the mainly distributed area of S. salsa, is an economic intensive region with the largest economic aggregate and population in northern China. The coastal wetland is one of the most vulnerable ecosystems with the urbanization and economic developments. S. salsa in Bohai Bay has been changed significantly due to several threats to its habitat in past decades. In this paper, we analyzed all available archived Landsat TM/ETM+/OLI images of the Bohai Bay region by using a decision tree algorithm method based on the Google Earth Engine (GEE) platform to generate annual maps of S. salsa from 1990 to 2020 at a 30-m spatial resolution. The temporal-spatial dynamic changes in S. salsa were studied by landscape metric analysis. The influencing factors of S. salsa changes were analyzed based on principal component analysis (PCA) and a logistic regression model (LRM). The results showed that S. salsa was mainly distributed in three regions: the Liao River Delta (Liaoning Province), Yellow River Delta (Shandong Province), and Hai River Estuary (Hebei Province, Tianjin). During the past 31 years, the total area of S. salsa has dramatically decreased from 692.93 km2 to 51.04 km2, which means that 92.63% of the area of S. salsa in the Bohai Bay region was lost. In the 641.89 km2 area of S. salsa that was lost, 348.80 km2 of this area was converted to other anthropic land use categories, while 293.09 km2 was degraded to bare land. The landscape fragmentation of S. salsa has gradually intensified since 1990. National Nature Reserves have played an important role in the restoration of suitable S. salsa habitats. The analysis results for the natural influencing factors indicated that precipitation, temperature, elevation, and distance to the coastline were considered to be the major influencing factors for S. salsa changes. The results are valuable for monitoring the dynamic changes of S. salsa and can be used as effective factors for the restoration of S. salsa in coastal wetlands.