Chongming Dongtan Nature Reserve Research Articles

UAV Photogrammetry in Intertidal Mudflats: Accuracy, Efficiency, and Potential for Integration with Satellite Imagery

The rapid, up-to-date, cost-effective acquisition and tracking of intertidal topography are the fundamental basis for timely, high-priority protection and restoration of the intertidal zone. The low cost, ease of use, and flexible UAV-based photogrammetry have revolutionized the monitoring of intertidal zones. However, the capability of the RTK-assisted UAV photogrammetry without ground control points, the impact of flight configuration difference, the presence of surface water in low-lying intertidal areas on the photogrammetric accuracy, and the potential of UAV/satellite Synergy remain unknown. In this paper, we used an RTK-assisted UAV to assess the impact of the above-mentioned considerations quantitatively on photogrammetric results in the context of annual monitoring of the Chongming Dongtan Nature Reserve, China based on an optimal flight combination. The results suggested that (1) RTK-assisted UAVs can obtain high-accuracy topographic data with a vertical RMSE of 3.1 cm, without the need for ground control points. (2) The effect of flight altitude on topographic accuracy was most significant and also nonlinear. (3) The elevation obtained by UAV photogrammetry was overestimated by approximately 2.4 cm in the low-lying water-bearing regions. (4) The integration of UAV and satellite observations can increase the accuracy of satellite-based waterline methods by 51%. These quantitative results not only provide scientific insights and guidelines for the balance between accuracy and efficiency in utilizing UAV-based intertidal monitoring, but also demonstrate the great potential of combined UAV and satellite observations in identifying coastal erosion hotspots. This establishes high-priority protection mechanisms and promotes coastal restoration.

A dynamic biomass model of emergent aquatic vegetation under different water levels and salinity

Emergent aquatic vegetation (EAV) is an important part of wetland ecosystems that provide multiple ecological services. However, human activities and natural changes often influence wetland hydrological regimes such as water levels, salinity, and other factors, which greatly influence the survival and growth of wetland plants. Based on field measurements and control experiments, we developed an EAV model to simulate biomass dynamics under changing conditions of water levels and salinity. This model successfully reproduced the seasonal biomass variation of three typical emergent plants, Phragmites australis, Typha angustifolia and Scirpus mariqueter, and simulated the response of EVA biomass under multiple scenarios of water levels and salinity in the Chongming Dongtan Nature Reserve (CDNR), Shanghai, China. Results suggest that there is a negative correlation between salinity and biomass. An optimal range of water levels are suitable for EAV, and biomass will decrease when the water levels are below or above their optimal range. Applying this dynamic EAV model is a cost-effective approach to find a sustainable and nature-based solution to managing and predicting wetland vegetation changes. The model and approach used in this study may provide a sustainable and nature-based solution for management and protection of wetland ecosystems, and may be transferrable to other wetland systems as well.

Assessing the ecological health of the Chongming Dongtan Nature Reserve, China, using different benthic biotic indices

To assess the ecological health of the Chongming Dongtan Nature Reserve (CDNR), univariate and multimetric indices—AZTI's Marine Biotic Index (AMBI), multivariate AMBI (M-AMBI), abundance-biomass comparison curves, taxonomic diversity, Shannon-Wiener index (H′) (log2), and Benthic Opportunistic Polychaetes Amphipods index (BOPA) — were used to translate the macrobenthic community into classifications of ecological health in 2016 and 2017. Based on the results of the various indicators, the overall habitat status of the CDNR ranged from good to high status. Sites 4 and 5, which fell in the remaining tidal flats following reclamation activities, were classified as disturbed, whereas the degree of disturbance at site 3 was low. The results indicate that the CDNR is under some form of human disturbance, which includes reclamation and animal (mainly cattle) grazing. H′ (log2) and M-AMBI index may be more suitable for assessing ecological quality in intertidal zones, including the CDNR.

Alternative stable states in coastal intertidal wetland ecosystems of Yangtze estuary, China

Alternative stable states phenomenon widely exists in a variety of ecosystems and is closely related to ecosystem health and sustainable development. Although alternative stable states research has become the focus and hotspot of the ecology researches, only a few empirical evidences supported its behavior and mechanisms in coastal wetland ecosystems up to now. In our study, ta-king the intertidal wetland ecosystem in Chongming Dongtan Nature Reserve as study area, we aimed to: 1) test the existence of alternative stable states based on judgment conditions (bimodal characteristic and threshold effect) and determine the relative stable state types; 2) explore the formation mechanisms of alternative stable states by monitoring hydrological conditions, sediment accretion dynamics as well as vegetation growth parameters and analyzing the positive feedbacks between saltmarsh vegetation and sedimentary geomorphology. Our results showed that: 1) Normalized difference vegetation index (NDVI) frequentness distribution revealed obvious bimodality at saltmarsh pioneer zone. Propagule biomass threshold limited the establishment of plant patches representing the "saltmarsh" state. The presence of bimodality and biomass threshold demonstrated there are "mudflat" stable state and "saltmarsh" stable state with distinct structure and function in intertidal wetland ecosystem. 2) Current velocities, turbidities and direction perpendicular to the vegetation zone were the most important factors responsible for the sediments rapid accretion at saltmarsh pioneer zone in spring and summer. Sediments accretion significantly promoted the growth of saltmarsh plant. The positive feedbacks between plant growth and sediments accretion resulted in the formation of alternative stable states. 3) The expansion pattern of saltmarshes in the Chongming Dongtan intertidal wetland ecosystem also suggested that increases of sediments accretion could trigger the formation of "mudflat" stable state and "saltmarsh" stable state on landscape scale. The results from this study could enrich regime shift mechanisms researches and provide the scientific supports for coastal zone protection, restoration and comprehensive management, which could have important theoretical and practical meaning.

The spatial relationship between salt marsh vegetation patterns, soil elevation and tidal channels using remote sensing at Chongming Dongtan Nature Reserve, China

The analysis of vegetation-environment relationships has always been a study hotspot in ecology. A number of biotic, hydrologic and edaphic factors have great influence on the distribution of macrophytes within salt marsh. Since the exotic species Spartina alterniflora (S. alterniflora) was introduced in 1995, a rapid expansion has occurred at Chongming Dongtan Nature Reserve (CDNR) in the Changjiang (Yangtze) River Estuary, China. Several important vegetation-environment factors including soil elevation, tidal channels density (TCD), vegetation classification and fractional vegetation cover (FVC) were extracted by remote sensing method combined with field measurement. To ignore the details in interaction between biological and physical process, the relationship between them was discussed at a large scale of the whole saltmarsh. The results showed that Scirpus mariqueter (S. mariqueter) can endure the greatest elevation variance with 0.33 m throughout the marsh in CDNR. But it is dominant in the area less than 2.5 m with the occurrence frequency reaching 98%. S. alterniflora has usually been found on the most elevated soils higher than 3.5 m but has a narrow spatial distribution. The rapid decrease of S. mariqueter can be explained by stronger competitive capacity of S. alterniflora on the high tidal flat. FVC increases with elevation which shows significant correlation with elevation (r=0.30, p<0.001). But the frequency distribution of FVC indicates that vegetation is not well developed on both elevated banks near tidal channels from the whole scale mainly due to tidal channel lateral swing and human activities. The significant negative correlation (r=–0.20, p<0.001) was found between FVC and TCD, which shows vegetation is restricted to grow in higher TCD area corresponding to lower elevation mainly occupied by S. mariqueter communities. The maximum occurrence frequency of this species reaches to 97% at the salt marsh with TCD more than 8 m/m2.

Evaluation of the combined threat from sea-level rise and sedimentation reduction to the coastal wetlands in the Yangtze Estuary, China

In recent decades, the prospect of climate change, in particular sea level rise (SLR) and its impacts on low-lying coastal areas has generated worldwide attention. The coastal wetlands located in the Yangtze Estuary, with their low elevation and decreasing sediment loading, are susceptible to both SLR and anthropogenic activities. Taking the Chongming Dongtan wetlands as a study area, two scenarios of SLR were evaluated: the present trend (1980–2010) of SLR 2.6mm/yr (PSLR) and the IPCC A1F1 scenario of 5.9mm/yr (HSLR). In addition, two scenarios of sedimentation rate were adopted for the Yangtze Estuary: the current sedimentation rate (CSR) and a reduced sedimentation rate of half of the present rate (1/2CSR). The combined effects of SLR and sedimentation reduction were evaluated by projection of the combinations of these phenomena for 2025, 2050 and 2100, using a Sea Level Affecting Marshes Model (SLAMM). The results showed that the combined effects of sea-level rise, reduced sedimentation and land subsidence could result in a considerable decrease in or even complete loss of the coastal wetland habitat in the Chongming Dongtan nature reserve, particularly under the medium-term (2050) and long-term (2100) scenarios. Without proper mitigation measures, the potential decrease in and loss of habitats and ecosystem services is inevitable. Such mitigation measures should be considered in the future for securing the coastal wetland ecosystems, which include the management of sedimentation, the rehabilitation and re-creation of wetland habitat and the control and limitation of reclamation.

Evaluation of ecosystem health for the coastal wetlands at the Yangtze Estuary, Shanghai

Despite the growing awareness of the important ecological functions and values provided by coastal and estuarine wetlands, wetland degradation continues worldwide due to increasing anthropogenic disturbances. Chongming Dongtan wetlands, adjacent to Shanghai, the largest city and industrial and trading port in China in rapid urban expansion and socioeconomic development are currently threatened with biodiversity reduction, wetland loss, contamination, and invasion of exotic plant. Sustainable protection and management of Dongtan Nature Reserve necessitate research to develop diagnostic tools and indicators for a comprehensive and objective assessment of wetland ecosystem health condition. Based on the pressure-state-response framework and ecological and environmental surveys at the Dongtan wetlands, an indicator system was established for evaluating the coastal wetlands ecosystem health, using indicators detected from satellite imagery and current field surveys. Through the establishment of health assessment units and spatial quantification of the indicators, the spatial clustering analysis, integrated with remote sensing and geographic information system technique was applied to make an accurate diagnosis of ecosystem health for Chongming Dongtan wetlands and highlight the areas in subhealthy and unhealthy condition and urgent need of conservation and management. The results from this research indicated that the ecosystem health condition at the Dongtan wetlands showed spatial variation, to a certain extent, corresponding to the distributions of elevation and land cover types. More than 75 % of the total study area was at a relatively healthy level, with 34.19 km2 for the very healthy zone and 41.08 km2 for the healthy zone, while the subhealthy and unhealthy zones covered 18.23 and 4.76 km2, respectively. This study demonstrated the potential for this integrated approach to give objective and effective evaluation of ecosystem health for the dynamic coastal and estuarine wetlands and provide up-to-date information to assist with early warning for ecological security and management decisions for Chongming Dongtan Nature Reserve.

Effect of Spartina alterniflora invasion and its controlling technologies on soil microbial respiration of a tidal wetland in Chongming Dongtan, China

The exotic species Spartina alterniflora has recently rapidly expanded in the Chongming Dongtan Nature Reserve. As a result, many technologies to control this organism have been developed. To clarify the effects of S. alterniflora invasion as well as the controlling technologies on the organic carbon reservation capability of the Chongming Dongtan Wetland, variations in soil microbial respiration (SMR) after S. alterniflora invasion and the implementation of different controlling technologies in Chongming Dongtan were analyzed, and their influencing mechanisms were studied by analyzing the soil and plant litter characteristics. The results showed that the SMR of S. alterniflora rhizosphere was 1.87 times (P=0.000<0.05) higher than that of the Phragmites communis rhizosphere, but that their annual organic matter input was almost identical. These findings implied that the organic carbon reservation capability of the S. alterniflora wetland was lower than that of the native P. communis wetland. S. alterniflora stalks more easily decayed into soil than P. communis stalks due to their lower lignin content and C/N ratio. This resulted in increased soil organic carbon (SOC) and TN content in soil of the S. alterniflora rhizosphere, which likely promoted microbial activities, thereby accelerating SMR and organic carbon output. In addition, three artificial measures for controlling the spread of S. alterniflora were found to generally improve the soil microbial activities, thereby increasing organic carbon output when compared to the control. However, when compared to other measures, the effects of mowing plus tillage and waterlogging (MTW) on soil microbial activities and SMR were relatively low owing to restriction by waterlogging. Overall, MTW was a relatively low carbon method for control of the spread of S. alterniflora.

Reproductive success of the threatened Reed ParrotbillParadoxornis heudeiin non-harvested and harvested reedbeds in the Yangtze River estuary, China

SummaryUnderstanding the factors that affect the nesting success of threatened birds is essential in designing effective conservation strategies. Here we compare nesting success of the Reed ParrotbillParadoxornis heudeiin annually harvested and non-harvested reedbed habitats in the Chongming Dongtan Nature Reserve in China, by recording the number of nestlings produced per nest and the causes of nest failure. We modelled daily nest survival rate (DSR) by considering the effects of harvesting, vegetation characteristics, year, date, advancement within the breeding season, nest age and nearest-nest distance, using the program MARK. Nest densities, but not the number of fledglings per nest, were significantly lower in harvested than non-harvested reedbed habitats. The best-fit DSR model estimated constant survival; none of the tested co-variables had significant effects. Moreover, harvesting did not affect the date of breeding initiation, likelihood of nest failure, or causes of nest failure, as vegetation cover was not significantly different between harvested and non-harvested reedbeds during the whole breeding season. Nest failure following adverse weather conditions was unusually common in harvested and non-harvested reedbeds, accounting for as many nest failures as depredation. However, comparisons with other studies suggest that deriving a conclusion on the impact of harvesting on nesting success is not straightforward and is probably linked to environmental characteristics affecting reed growth.

The application of cutting plus waterlogging to control Spartina alterniflora on saltmarshes in the Yangtze Estuary, China

Control and eradication of the exotic and invasive plant Spartina alterniflora within the Chongming Dongtan nature reserve, Shanghai, China, is vital for the management and conservation of the saltmarshes. A demonstration project was established using waterlogging and cutting to control this invasive species. Results from 2007 to 2008 showed that, although the managed waterlogging significantly reduced biomass and seed production of S. alterniflora at an early stage, the species subsequently showed rapid adaptation to the long-term waterlogging stress. Thus, managed waterlogging alone was insufficient for the effective eradication of S. alterniflora. However, managed waterlogging for around 3 months, combined with cutting the above-ground part of S. alterniflora at a key stage (flowering period in July), controlled and eradicated the plant successfully. Both the above-ground and below-ground parts of S. alterniflora were killed and the plants began to decompose after 3 months. Furthermore, there was no re-growth of the emergent part of S. alterniflora in the following years. However, once the impounded water was released restoring the natural hydrodynamic regime of the saltmarshes, the seeds and seedlings of S. alterniflora reinvaded the controlled site from the neighboring areas and the S. alterniflora community was re-established. Thus, after eradication of S. alterniflora, control measures should be maintained to prevent the re-establishment of S. alterniflora. The results of this demonstration project indicate a potentially useful and effective approach for the control and management of large-scale invasion by S. alterniflora on saltmarshes in the Yangtze Estuary, China.

Forecasting the effects of sea-level rise at Chongming Dongtan Nature Reserve in the Yangtze Delta, Shanghai, China

Located at the mouth of the Yangtze Estuary, the Chongming Dongtan Nature Reserve is extremely vulnerable to climate change and especially to accelerated sea-level rise. We use a variety of data from remote sensing, an in situ global positioning system (GPS), tidal gauges, nautical charts, geographic spatial analysis modeling and IPCC sea-level rise scenarios to forecast the potential impacts of increased sea level on the coastal wetland habitat at Chongming Dongtan Nature Reserve. The results indicate that around 40% of the terrestrial area of the Dongtan Reserve will be inundated by the year 2100 due to an estimated 0.88 m increase in sea level. In particular, the Scirpus mariqueter communities and bare tidal flats are more vulnerable to sea-level rise. The limitations of this approach and the implication of the results for wetland and ecosystem conservation as well as management are discussed.

The Classification of Salt Marsh Vegetation for Chongming Dongtan Nature Reserve Based on Knowledge Engineer

This paper used Chongming Dongtan Nature Reserve as the research object for salt marsh vegetation classification based on Landsat TM image.According to such image preprocessing measures as image geometric correction and subset image and on the basis of analyses of Landsat TM remotely sensed images integrated with field survey and other studies of spatio-temporal dynamics of Chongming Dongtan Nature Reserve,this paper confirmed the species of the vegetation in this area.The authors used knowledge engineer to classify the vegetation,built knowledge base on the basis of vegetation spectral information and presented a vegetation classification method based on the spectral information.The overall precision of the vegetation classification method based on knowledge engineer is 92.35%,and the kappa coefficient is 0.907 2.The precision is higher than the overall precision of the vegetation classification based on unsupervised classification and supervised classification(maximum likelihood): the overall precisions of unsupervised classification and supervised classification are respectively 86.92% and 90.10%.The result shows that the vegetation classification method can classify and discriminate vegetation effectively and the precision is higher than that of other methods.The vegetation classification method provides a theoretical foundation and effective method for automatic extraction of vegetation.

Impacts of Spartina alterniflora invasion on the benthic communities of salt marshes in the Yangtze Estuary, China

As a species for ecological engineering, Spartina alterniflora was introduced to Chongming Dongtan in 1995, and over the last 10 years, this species has rapidly invaded large areas of the Chongming Dongtan nature reserve. In this study, use of a normalized biomass size-spectra (NBSS) approach was explored to evaluate the possible impacts of S. alterniflora invasion on the benthic communities along gradients of intertidal zones and the invasion history of S. alterniflora within the nature reserve. The results showed that the characteristics of macrobenthic communities and the variation in macrobenthic communities described by the first two CCA axes revealed clearly the gradients of elevation and invasion history of S. alterniflora. The differences in the macrobenthic assemblages between the Spartina alterniflara marshes and the native Phragmites australis marshes decreased with increasing of invasion history of S. alterniflara. The macrobenthic biomass showed a decreasing trend, while the meiobenthic biomass showed a reverse trend along the elevation gradient. The macrobenthic biomass of S. alterniflora marshes with longer invasion history was higher than that at recently invaded S. alterniflora marshes, while the meiobenthic biomass was lower. The slopes of NBSS for the sampling sites showed a trend of steeper slopes with decreasing of elevation and at the recently invaded S. alterniflora marshes than that at marshes with longer invasion history, while the differences between the native P. australis marshes and the S. alterniflora marshes with long invasion history tended to be diminished. The NBSS approach could thus be used more widely to detect possible impacts of S. alterniflara invasion on benthic assemblages. This study also indicated the potential for this approach to provide valuable insights into the ecosystem ecology of invasive species, which could be very important for wetland biodiversity conservation and resource management in the Yangtze River Estuary and other such impacted areas.

The range expansion patterns of Spartina alterniflora on salt marshes in the Yangtze Estuary, China

The range expansion patterns of Spartina alterniflora and the roles which sexual reproduction and asexual propagation play in range expansion were investigated at the Chongming Dongtan nature reserve in the Yangtze Estuary, China. Two range expansion patterns of S. alterniflora at its advancing fronts could be found (1) S. alterniflora–mudflat front (S–M) and (2) S. alterniflora– Scirpus mariqueter–mudflat front (S–S–M). One feature revealed by this study was that a flush of seedling recruitment and establishment in spring was a crucial way for S. alterniflora to colonize new habitats and achieve a fast rate of range expansion. The mean number of seedlings recruited at the S–M front was much higher than that at the S–S–M front. Once established, the survivorship of seedlings was high, both at the S–M and S–S–M fronts. The established seedlings formed new tussocks quickly by vegetative tillering and growth of rhizomes and these finally merged into dense meadows. The mean distance of range expansion of S. alterniflora, after one growing season at the S–M front, was 25.4 ± 3.1 m yr −1 and 2.7 ± 0.5 m yr −1 at the S–S–M front. Sexual reproduction by seedlings and asexual propagation by tillering and growth of rhizomes were the two main means by which S. alterniflora could maintain a fast rate of range expansion on the salt marshes of the Yangtze Estuary. The colonization behaviors of S. alterniflora on advancing fronts differed as a reaction to various external and internal factors. The impact of abiotic and biotic factors governing the range expansion of S. alterniflora and its implications for the spatial structure of tidal wetlands are discussed.

A study on seed characteristics and seed bank of Spartina alterniflora at saltmarshes in the Yangtze Estuary, China

Since Spartina alterniflora was introduced into the Chongming Dongtan Nature Reserve in 1995, there has been a rapid expansion of this species, seriously threatening the overall biodiversity. In this study, the seed production, germination characteristics and soil seed bank of S. aterniflora were studied at the nature reserve along an intertidal gradient where this invasive species distributed and spread. The results showed that the middle intertidal zone (MIT) had the largest seed production and higher viability than those of lower (LIT) and higher (HIT) intertidal zones. The differences in seed production among these sites were largely dependent on the higher percentage of fruiting culms, longer spike and higher seed number per spike. The differences in seed viability among these sampling sites seemed largely dependent on the higher seed weight at the site MIT. The chilling treatment (at low temperature and in moist conditions) could significantly enhance the germinability of S. alterniflora seeds and shorten the time of onset seed germination. The seeds from the site MIT had much higher germinability than the sites LIT and HIT. The highest density of soil seed bank was recorded at the site of MIT, where had the highest seed production. By July, before there was any replenishment with fresh seeds from the current year, the soil seed bank was completely exhausted and the persistent time of soil seed bank for S. alterniflora was less than 9 months, which is in agreement with that of the transient seed bank. The results from this study indicated that the seed propagation could have an important attribution to the spreading of this invasive plant and the implications in terms of controlling and managing the invasion of S. alterniflora at the nature reserve were discussed.

Analyzing the habitat suitability for migratory birds at the Chongming Dongtan Nature Reserve in Shanghai, China

The Dongtan Nature Reserve, one of the largest nature reserves in East Asia, is located in the mouth of the Yangtze Estuary and is an important habitat for migratory birds. The Dongtan wetlands were listed in the Chinese Protected Wetlands in 1992, and were designated as internationally important under the Ramsar Wetlands Convention in 2001 and as a national nature reserve in 2005. By their very nature and location, the estuarine and coastal habitats are dynamic and their biodiversity conservation and management relies on up-to-date spatial information. By establishing qualitative and quantitative relationships between bird populations and key habitat factors such as elevation, land cover type, tidal creek density and macrophytobenthos biomass, we developed an object-oriented image approach, in conjunction with Geographical Information Systems and the data from current field surveys, to analyze and assess the habitat suitability for the main types of birds, namely Anatidae, Charadriidae, Ardeidae and Laridae, at the reserve. The results from this study showed that about 40% of the total area of the Dongtan Reserve contained suitable habitats for these four bird families. The Scirpus mariqueter zone, mudflat zone, and tidal creeks were the most important habitats for these birds. This study indicated the potential of this approach for objective and effective evaluation of the species habitat suitability in a dynamic estuarine and coastal area. The implications of the results as a tool for biodiversity conservation, wetland conservation, and ecosystem management are discussed.

An experimental study on physical controls of an exotic plant Spartina alterniflora in Shanghai, China

Since Spartina alterniflora was introduced into the Chongming Dongtan Nature Reserve in 1995, there has been rapid expansion of this species, seriously threatening the overall biodiversity. During 2005 and 2006, a field experiment to examine physical controls on S. alterniflora, including digging and tillage, breaking of rhizomes, mowing and biological substitution with Phragmites australis, was conducted to find a means of controlling this invasive plant. The growth parameters of plant density, coverage and above-ground biomass were used to evaluate the efficiency of different treatments. The results showed that for all treatments, the plant density, coverage and above-ground biomass were significantly lower than those of the control in the first growing season. However, in the second season, the differences between the treatment and the control were not significant and there were no significant differences by the end of the second growing season. The breaking of rhizomes treatment inhibited the growth of S. alterniflora significantly in the first growing season and inhibition increased with the depth of the treatment. However, the inhibition of growth disappeared after two growing seasons and there were no significant differences among the treatments and the control. The mowing treatment significantly inhibited the growth of S. alterniflora in the first growing season. By the end of the second growing season, the growth of S. alterniflora had recovered to some extent, and only the treatments of JUN, JUL, AUG and SEP significantly inhibited its growth. The treatment of AUG might be the most suitable time for controlling via mowing. In the biological substitution treatment, the transplanted P. australis survived quite well over both growing seasons and both the plant height and fruiting percent increased considerably in the second growing season. A realistic strategy for controlling and managing the invasion of S. alterniflora in the nature reserve should involve integrating all four control measures on the basis of their intensity, frequency, timing and area. Further work on longer-term field experiments is required in order to test these conclusions further and provide useful information for the wetland management of the nature reserve.

Multi-seasonal spectral characteristics analysis of coastal salt marsh vegetation in Shanghai, China

Remote sensing technology has become the primary tool for ecological research on a large scale. The spectral characteristics of the salt marsh vegetation canopy, including four main communities, Phragmites australis community, Spartina alterniflora community, Scirpus mariqueter community and Carex scabrifolia community, were measured in the seasons of spring, summer and autumn by a ground FieldSpec™ Pro JR spectroradiometer, at the Chongming Dongtan Nature Reserve in Shanghai. The spectral data were converted to the reflectance curves, and their first derivative curves between 350 nm and 1000 nm were calculated. The differences in the reflectance and first derivative curves were then analyzed, with particular attention paid to the characteristics in the bands of visible, “green peak”, “red edge” and near infrared. The results from this study showed that the different salt marsh communities had different and rather unique spectral characteristics during the three seasons studied. The near-ground spectral reflectance varied with the growing season, community type and its phenology. The discrimination ability, in terms of variations in the spectral reflectance, among the four major salt marsh communities was highest in autumn. It is proposed that a realistic strategy for classifying the salt marsh vegetation could be adopted by integrating and analyzing their remotely sensed images of multi-seasons. The implications of the results from this study in terms of serving as a tool for monitoring and mapping the distribution pattern and dynamics of salt marsh vegetation on the eastern coast of China are discussed.

IDENTIFICATION OF THE SPECTRAL CHARACTERISTICS OF SALTMARSH VEGETATION USING INDIRECT ORDINATION: A CASE STUDY FROM CHONGMING DONGTAN, SHANGHAI

Remote sensing is a major source of spatial information of the attributes of the earth's surface, and remote sensing technology has become a primary tool for vegetation classification at large scales. The relationship between vegetation and their spectral characteristics is key for interpreting remote sensing images. This study related characteristics of saltmarsh vegetation at the intertidal zone of Chongming Dongtan Nature Reserve, Shanghai, to patterns of their spectral reflectance. Paired measurements of saltmarsh community and spectral characteristics were carried out along three transects covering the major variations in vegetation and environment within the study area. The spectral characteristics were measured by a ground FieldSpec Pro JR spectroradiometer and the spectral data were converted to simulate the 10 bands of Compact Airborne Spectrographic Imager (CASI) bandset. The spectral data sets were then ordinated using Principal Component Analysis (PCA), an indirect ordination technique. The eigenvalue of the first PCA axis was 19.9, which represented 98.5% of the variation in the spectral data. A sequence of bare mudflat, Scirpus mariqueter community, Spartina alterniflora community and Phragmites australis community changed along the first ordination axis showed a strong correspondence with variation in the bands six to ten, i.e. the simulated CASI wavebands covering the 736 to 870 nm wavelengths. A significant relationship between the first axis PCA scores for the spectral data of quadrats and their percentage community cover and height also was identified. The second axis accounted for only 1.4% of the variation in the spectral data and it proved impossible to demonstrate any close link between any specific plant community type and a distinct set of spectral characteristics because of its low representation of the variation in the spectral data. Our study has demonstrated that the variation in the spectral data of saltmarsh vegetation at Chongming Dongtan Nature Reserve can be identified using the indirect ordination technique of PCA and then applying correlation and regression analyses to explain the relationships between the variation in the spectral data with the vegetation and ecological data. Our results indicate that PCA is of value for identifying relationships between community types and the spectral data of CASI bandsets for saltmarsh vegetation. This could provide an effective and practical approach for classification of saltmarsh vegetation at large scales and for monitoring spatio_temporally dynamic patterns of saltmarshes at Chongming Dongtan Nature Reserve. Further work is required in order to test these conclusions. The method has been evaluated for a single season with a limited sample of saltmarsh plant communities, and the application of PCA and spectral data of CASI bandsets to other saltmarsh vegetation as well as multi_seasonal spectral data are necessary to determine whether similar patterns emerge and whether the approach has more general applicability in terms of characterizing remote sensing/spectral imagery and community types in saltmarsh vegetation.