Spartina alternifloramarshes on dredged material: a critical review of the ongoing debate over success

After three decades ofexperience, environmental managers continue toquestion the use of dredged material for creation andrestoration of Spartina alternifloramarshes. Different uses of the term `success' have led to someconfusion, and poor recognition of the limitations ofresearch design have contributed to confusion instudies comparing dredged material and naturalmarshes. This paper uses data from a number ofsources to compare dredged material and naturalmarshes. Quantitative analyses based on dataavailable in the scientific literature suggest thatmean values for aboveground and belowground biomass,organic carbon in sediments, polychaete densities, andcrustacean densities may differ in dredged materialand natural marshes. There is not sufficient evidenceto conclude that differences are present for othervariables, such as densities of stems, oligochaetes,or fish. Cumulative quantitative data do not supportthe contention that dredged material sites becomeincreasingly similar to nearby natural marshes overtime as a general rule, although some attributes ofindividual dredged material marshes may becomeincreasingly similar to some natural marshes. Although it is clear that dredged material marshesprovide habitat for birds, limited data suggest thatdredged material marshes may provide habitat for adifferent community of birds than natural marshes. Similarly, limited data suggest that geomorphologicalfeatures found in natural marshes are not duplicatedin dredged material marshes. In short, datasummarized from the literature suggest that dredgedmaterial marshes provide some of the functions ofnatural marshes but probably do not replace all of thefunctions of lost natural marshes.     

Comparison of physical characteristics between created and natural estuarine marshes in Galveston Bay,Texas

Five natural and ten created Spartinaalternifloramarshes in the Lower Galveston BaySystem were compared to determine if there weresignificantly different physical characteristicsassociated with each type of marsh. The saltmarsheswere compared on the basis of microhabitats,length-width ratio, area-perimeter ratio, marsh-wateredge ratio, total size of S. alternifloraplantcommunities, fetch distances, angle of exposure,orientation, and elevation. All physicalmeasurements, except for elevation, were obtained fromphotography analyzed with the use of a GeographicInformation System with digital image processingcapabilities. Differences existed between natural andcreated marshes. The natural marsh sites in this studywere characterized by highly undulant marsh-wateredges, island-like S. alternifloraplant stands,concave shorelines, and low elevations. Createdmarshes were characterized by relatively smoothmarsh-water edges, an unbroken shoreline morphology,convex to straight shoreline configurations, andelevations on the edge and inner portions of the marshhigher than those of natural marshes. The lowelevations of the natural marsh appear to be due tocoastal subsidence in the Galveston Bay area alongwith rising sea level. Reticulated marshes andundulant shorelines appear to be caused by consequentdrowning of the natural marshes. High elevations insome of the created marshes are related to erosion ofthe low elevation marsh or deposition of coarsesediments at the marsh-water edge.     

Utilization of a citizen monitoring protocol to assess the structure and function of natural and stabilized fringing salt marshes in North Carolina

Narrow fringing salt marshes dominated by Spartina alterniflora occur naturally along estuarine shorelines and provide many of the same ecological functions as more extensive marshes. These fringing salt marshes are sometimes incorporated into shoreline stabilization efforts. We obtained data on elevation, salinity, sediment characteristics, vegetation and fish utilization at three study sites containing both natural fringing marshes and nearby restored marshes located landward of a stone sill constructed for shoreline stabilization. During the study, sediment accretion rates in the restored marshes were approximately 1.5- to 2-fold greater than those recorded in the natural marshes. Natural fringing marsh sediments were predominantly sandy with a mean organic matter content ranging between 1.5 and 6.0%. Average S. alterniflora stem density in natural marshes ranged between 130 and 222 stems m⁻², while mean maximum stem height exceeded 64 cm. After 3 years, one of the three restored marshes (NCMM) achieved S. alterniflora stem densities equivalent to that of the natural fringing marshes, while percentage cover and maximum stem heights were significantly greater in the natural than in the restored marshes at all sites. There was no significant difference in the mean number of fish, crabs or shrimp captured with fyke nets between the natural and restored marshes, and only the abundance of Palaemonetes vulgaris (grass shrimp) was significantly greater in the natural marshes than in the restored ones. Mean numbers of fish caught per 5 m of marsh front were similar to those reported in the literature from marshes adjacent to tidal creeks and channels, and ranged between 509 and 634 fish net⁻¹. Most of the field data and some of the sample analyses were obtained by volunteers as they contributed 223 h of the total 300 h spent collecting data from three sites in one season. The use of fyke nets required twice as many man-hours as any other single task. Vegetation and sediment parameters were sensitive indicators of marsh restoration success, and volunteers were capable of contributing a significant portion of the labor needed to collect these parameters. The U.S. Government's right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Salt marsh stabilization affects algal primary producers at the marsh edge

As sea level rise and human activities erode coastal wetlands, managers rebuild or preserve wetlands that can perform the ecosystem services of a natural system. One increasingly common mitigation activity is the construction of rock sills in the low marsh zone to stabilize marsh elevation. Sills dramatically alter the physical structure of marshes by changing elevation, adding hard substrate and potentially altering the spatial structure of benthic algal communities in and adjacent to the low marsh. We documented differences in benthic algal abundance at the seaward marsh edge in silled and unsilled marshes in North Carolina. We found that sills were associated with reduced standing stocks of benthic algal primary production and reduced macroalgal taxonomic richness, and this difference was driven primarily by differences in macroalgal abundance. We experimentally tested the effect of macroalgal abundance on cordgrass (Spartina alterniflora) growth in the low zone of an unmanipulated marsh, and found that macroalgal removal had no effect on final cordgrass abundance. Our study suggests that salt marsh management through the construction of sills in low marsh zones impacts benthic primary production in the low marsh zone, but that benthic algal production does not affect cordgrass growth over a growing season.     

SEASONAL PATTERNS IN THE BELOWGROUND BIOMASS OF SPARTINA ALTERNIFLORA (GRAMINEAE) ACROSS A TIDAL GRADIENT

Belowground biomass of two ht forms of the salt marsh cordgrass, Spartina alterniflora, in a New England salt marsh exhibited a seasonal cycle. Biomass was highest in midsummer with no secondary peak in biomass corresponding with the autumn dieback of aboveground parts. Total annual biomass production and the maximum depth that living tissue penetrated into the substrate decreased with increasing tidal ht. Substrate characteristics (soil aeration, pH, nutrient levels) known to affect aboveground biomass of S. alterniflora also decreased with increasing tidal ht and may similarly affect belowground biomass across the same tidal gradient.     

Fifteen Years of Vegetation and Soil Development after Brackish-Water Marsh Creation

Aboveground biomass, macro‐organic matter (MOM), and wetland soil characteristics were measured periodically between 1983 and 1998 in a created brackish‐water marsh and a nearby natural marsh along the Pamlico River estuary, North Carolina to evaluate the development of wetland vegetation and soil dependent functions after marsh creation. Development of aboveground biomass and MOM was dependent on elevation and frequency of tidal inundation. Aboveground biomass of Spartina alterniflora, which occupied low elevations along tidal creeks and was inundated frequently, developed to levels similar to the natural marsh (750 to 1,300 g/m²) within three years after creation. Spartina cynosuroides, which dominated interior areas of the marsh and was flooded less frequently, required 9 years to consistently achieve aboveground biomass equivalent to the natural marsh (600 to 1,560 g/m²). Aboveground biomass of Spartina patens, which was planted at the highest elevations along the terrestrial margin and seldom flooded, never consistently developed aboveground biomass comparable with the natural marsh during the 15 years after marsh creation. MOM (0 to 10 cm) generally developed at the same rate as aboveground biomass. Between 1988 and 1998, soil bulk density decreased and porosity and organic C and N pools increased in the created marsh. Like vegetation, wetland soil development proceeded faster in response to increased inundation, especially in the streamside zone dominated by S. alterniflora. We estimated that in the streamside and interior zones, an additional 30 years (nitrogen) to 90 years (organic C, porosity) are needed for the upper 30 cm of created marsh soil to become equivalent to the natural marsh. Wetland soil characteristics of the S. patens community along upland fringe will take longer to develop, more than 200 years. Development of the benthic invertebrate‐based food web, which depends on organic matter enrichment of the upper 5 to 10 cm of soil, is expected to take less time. Wetland soil characteristics and functions of created irregularly flooded brackish marshes require longer to develop compared with regularly flooded salt marshes because reduced tidal inundation slows wetland vegetation and soil development. The hydrologic regime (regularly vs. irregularly flooded) of the “target” wetland should be considered when setting realistic expectations for success criteria of created and restored wetlands.     

The interactive effects of created salt marsh substrate type,hydrology, and nutrient regime on <Emphasis Type="Italic">Spartina alterniflora</Emphasis> and <Emphasis Type="Italic">Avicennia germinans</Emphasis> productivity and soil development

Recent salt marsh and barrier island restoration efforts in the northern Gulf of Mexico have focused on optimizing self-sustaining attributes of restored marshes to provide maximum habitat value and storm protection to vulnerable coastal communities. Salt marshes in this region are dominated by Spartina alterniflora and Avicennia germinans, two species that are valued for their ability to stabilize soils in intertidal salt marshes. We conducted a controlled greenhouse study to investigate the influences of substrate type, nutrient level, and marsh elevation on the growth and biomass allocation of S. alterniflora and A. germinans, and the consequent effects on soil development and stability. S. alterniflora exhibited optimal growth and survival at the lowest elevation (? 15 cm below the water surface) and was sensitive to high soil salinities at higher elevations (+ 15 cm above the water surface). A. germinans performed best at intermediate elevations but was negatively affected by prolonged inundation at lower elevations. We found that although there was not a strong effect of substrate type on plant growth, the development of stressful conditions due to the use of suboptimal materials would likely be exacerbated by placing the soil at extreme elevations. Soil shear strength was significantly higher in experimental units containing either S. alterniflora or A. germinans compared to unvegetated soils, suggesting that plants effectively contribute to soil strength in newly placed soils of restored marshes. As marsh vegetation plays a critical role in stabilizing shorelines, salt marsh restoration efforts in the northern Gulf of Mexico and other storm impacted coasts should be designed at optimal elevations to facilitate the establishment and growth of key marsh species.     

Seasonal variation of fungal biomass in the sediment of a salt marsh in New Brunswick

In a marsh in New Brunswick, Canada, belowground biomass of Spartina alterniflora consistently exceeded aboveground biomass by a factor of approximately 9. Both values peaked in July. Redox potential of the sediment was negative at all levels tested (2, 6, and 11 cm below surface), and was negatively correlated with depth. Concentrations of ergosterol, a sterol typical of higher fungi, were negatively correlated with redox potential and were highest in roots and rhizomes in July and August, 1–3 cm below the surface. These maxima corresponded to a fungal content of approximately 0.6% per ash-free dry mass of Spartina material. Balsa wood panels buried in anaerobic salt marsh sediment were colonized by fungi within 12 weeks. Eight fungal species isolated from S. alterniflora roots did not grow in the absence of oxygen, but were able to grow downward into an anaerobic medium. Offprint requests to: F. Bärlocher.     

闽江河口春季互花米草入侵过程对短叶茳芏沼泽土壤碳氮分布特征的影响

2014年4月,选择闽江口鳝鱼滩湿地中未被入侵的短叶茳芏群落(A)、互花米草入侵斑块边缘(B)以及互花米草入侵斑块中央(C)为研究对象,基于时空互代研究方法,探讨了互花米草入侵序列下湿地土壤碳氮空间分布特征的差异。结果表明,互花米草入侵显著降低了土壤的NO_3~--N含量(P0.05),但整体增加了NH_4~+-N含量,这与其入侵后导致湿地土壤颗粒组成发生显著变化(砂砾含量增加33.81%),进而促进了土壤的矿化作用和硝化作用,并有助于硝态氮的垂直淋失有关。互花米草入侵整体增加了土壤的碳氮含量和C/N比,与入侵进程和入侵前相比,互花米草入侵后湿地土壤的碳储量分别增加了8.73%和24.37%,氮储量则分别增加了10.22%和17.87%,这主要与其对闽江口湿地植物群落格局、养分生物循环以及强促淤作用引起的土壤颗粒组成等显著改变有关。研究发现,闽江口互花米草入侵对短叶茳芏湿地土壤碳氮含量的影响相对于江苏盐城、长江口以及杭州湾湿地的影响可能更为显著,其互花米草入侵较大改变了土壤中陆源和海源有机质的来源比例,使得入侵后湿地土壤养分的自源性增强。     

Temporal development of salt marsh value for nekton and epifauna: utilization of dredged material marshes in Galveston Bay,Texas, USA

Densities of nekton and other fauna were measured inthree created salt marshes to examine habitatdevelopment rate. All three marshes were located onPelican Spit in Galveston Bay, Texas, USA and werecreated on dredged material from the Gulf IntracoastalWaterway. The youngest marsh was planted on 1-mcenters in July of 1992. At the time sampling wasinitiated in fall 1992, the marshes were 9, 5, andless than 1 year in age; sampling continued in thefall and spring through spring 1994. Animaldensities were measured within the vegetation at twoelevations using an enclosure sampler. In the fall of1992, 4 months following the planting of the 92Marsh,densities of most marsh organisms were lower in thismarsh compared with the older two marshes. Significantly lower densities were observed fordominant crustaceans (including three species of grassshrimps, two species of commercially-important penaeidshrimps, thinstripe hermit crabs Clibanarius vittatus,and juvenile blue crabs Callinectes sapidus), adominant fish (Gobionellus boleosoma), and thedominant mollusc (Littoraria irrorata). By the fallof 1993, however, densities of most nekton specieswere similar among the three created salt marshes. Incontrast, reduced densities of less mobile epifauna(C. vittatusand L. irrorata) persisted in the 92Marshthroughout the 2 years of sampling. The patterns ofnekton utilization exhibited in these marshes suggestthat the 92Marsh reached its maximum habitat supportfunction for these animals in less than 1 year. Comparisons of the older marshes with natural marshesin the bay system, however, suggest that all three ofthese created marshes are functioning at lower levelsthan natural marshes in terms of supporting productionof commercially important fishery species such aspenaeid shrimps and C. sapidus.     

Differential population response of allocation,phenology, and tissue chemistry in <Emphasis Type="Italic">Spartina alterniflora</Emphasis>

Phenotypic variation within species is widespread among salt marsh plants. For Spartina alterniflora, the dominant species of low intertidal wetlands across the Altantic and Gulf coasts of the US, distinct phenological and morphological differences among populations from different latitudes have been found. To determine whether S. alterniflora plants from lower latitudes and those regenerated from Delaware tissue cultures would maintain differences from that of native plants, we conducted a field study in a natural salt marsh in Delaware, US. After two growing seasons, plant height, stem density, above- and belowground biomass, elemental composition, and nutrient resorption were measured. Natural variation in porewater salinity influenced physiological traits of Na⁺/K⁺ ratio regulation and nitrogen resorption efficiency similarly across populations. While plant height exhibited plasticity where populations tended to converge to a similar height, several other traits remained distinct. Delaware plants had a greater rate of rhizome growth than Georgia and Louisiana plants, which correlated with a greater magnitude of fall senescence. If traits such as seasonal translocation are plastic and can change with the length of the growing season, climate warming may alter belowground biomass production of S. alterniflora in wetlands of the mid-Atlantic.     

闽江河口湿地不同植被带土壤全硅的含量及分布特征

硅是湿地系统元素循环所关注的重要内容之一,土壤全硅的分布特征与湿地土壤-植物系统活跃程度密切相关。于2016年1-12月,以闽江河口鳝鱼滩湿地为研究对象,通过野外原位采样及室内实验分析,对短叶茳芏、短叶茳芏与互花米草交错带、互花米草3种植物类型湿地的土壤全硅含量和储量的变化特征进行观测。结果表明：短叶茳芏、短叶茳芏与互花米草交错带、互花米草湿地土壤全硅的年平均含量依次为197.67、201.21、210.33 mg/g,表现为由陆向海方向逐渐增加的趋势;季节上均呈现秋冬高于春夏的趋势;土壤全硅含量和储量均表现为上部土层（0-30cm）高于下部土层（30-60cm）。经统计分析发现,湿地土壤全硅含量与有机质显著负相关（P < 0.05）,与含水率和pH有极显著正相关关系（P < 0.01）。除短叶茳芏湿地外,其余2种湿地土壤全硅含量与有效硅含量显著负相关（P < 0.05）。研究闽江河口湿地不同植被带土壤全硅含量和储量及其分布特征,旨在揭示湿地土壤全硅水平在不同类型湿地植被生长影响下的变化过程,为本研究区的硅素研究补充关键数据。     

Drainage and Elevation as Factors in the Restoration of Salt Marsh in Britain

Intertidal restoration through realignment of flood defenses has become an important component of the U.K. coastal and estuarine management strategy. Although experimentation with recent deliberate breaches is in progress, the long‐term prognosis for salt marsh restoration can be investigated at a number of sites around Essex, southeast England where salt marshes have been reactivated (unmanaged restoration) by storm events over past centuries. These historically reactivated marshes possess higher creek densities than their natural marsh counterparts. Both geomorphology and sedimentology determine the hydrology of natural and restored salt marshes. Elevation relative to the tidal frame is known to be the primary determinant of vegetation colonization and succession. Yet vegetation surveys and geotechnical analysis at a natural marsh, where areas with good drainage exist in close proximity to areas of locally hindered drainage at the same elevation, revealed a significant inverse relationship between water saturation in the root zone and the abundance of Atriplex portulacoides, normally the physiognomic dominant on upper salt marsh in the region. Elsewhere in Essex natural and restored marshes are typified by very high sediment water contents, and this is reflected in low abundance of A. portulacoides. After a century of reestablishment no significant difference could be discerned between the vegetation composition of the storm‐reactivated marshes and their natural marsh counterparts. We conclude that vegetation composition may be restored within a century of dike breaching, but this vegetation does not provide a reliable indicator of ecological functions related to creek structure.     

Seasonal patterns of daily net photosynthesis,transpiration and net primary productivity of Juncus roemerianus and Spartina alterniflora in a Georgia salt marsh

Summary Studies of the seasonal CO₂ and water vapor exchange patterns of Juncus roemerianus and Spartina alterniflora were conducted in an undisturbed marsh community on Sapelo Island, Georgia. Daily patterns of net photosynthesis, transpiration, leaf diffusive conductance and water-use efficiency in response to ambient conditions were monitored on intact, in situ plants. Net primary productivity was calculated from the daytime CO₂ fixation totals, nighttime CO₂ loss, leaf standing stock and aboveground to belowground biomass ratios for each plant type.The tall form of S. alterniflora had higher rates of photosynthesis and higher water-use efficiency values which, in conjunction with low respiratory losses and large leaf standing crop, results in high values of net primary productivity. The environmental factors in the marsh which permit these physiological responses occur in less than 10% of the marsh. Overall, the physiological capabilities of the short form of S. alterniflora were reduced in comparison to the tall form, but the combination of environmental factors which determine the physiological responses of this form occur in a much greater portion of the marsh, and the short form of S. alterniflora dominates the Sapelo Island marshes.The response patterns of the C₃ species, J. roemerianus, differed somewhat from the height forms of S. alterniflora. A large, seasonally constant leaf standing crop coupled with moderate rates of photosynthesis resulted in a net primary productivity value which was between the tall and short height forms of S. alterniflora. However, as with the tall S. alterniflora, the environmental conditions under which this high productivity and high water loss rate can be sustained are restricted to specific regions of the environmental gradient in the marsh.Contribution No. 435 from the University of Georgia Marine Institute     

Effects of regular salt marsh haying on marsh plants, algae, invertebrates and birds at Plum Island Sound, Massachusetts

The haying of salt marshes, a traditional activity since colonial times in New England, still occurs in about 400 ha of marsh in the Plum Island Sound estuary in northeastern Massachusetts. We took advantage of this haying activity to investigate how the periodic large-scale removal of aboveground biomass affects a number of marsh processes. Hayed marshes were no different from adjacent reference marshes in plant species density (species per area) and end-of-year aboveground biomass, but did differ in vegetation composition. Spartina patens was more abundant in hayed marshes than S. alterniflora, and the reverse was true in reference marshes. The differences in relative covers of these plant species were not associated with any differences between hayed and reference marshes in the elevations of the marsh platform. Instead it suggested that S. patens was more tolerant of haying than S. alterniflora. Spartina patens had higher stem densities in hayed marshes than it did in reference marshes, suggesting that periodic cutting stimulated tillering of this species. Although we predicted that haying would stimulate benthic chlorophyll production by opening up the canopy, we found differences to be inconsistent, possibly due to the relatively rapid regrowth of S. patens and to grazing by invertebrates on the algae. The pulmonate snail, Melampus bidendatus was depleted in its δ¹³C content in the hayed marsh compared to the reference, suggesting a diet shift to benthic algae in hayed marshes. The stable isotope ratios of a number of other consumer species were not affected by haying activity. Migratory shorebirds cue in to recently hayed marshes and may contribute to short term declines in some invertebrate species, however, the number of taxa per unit area of marsh surface invertebrates and their overall abundances were unaffected by haying over the long term. Haying had no impact on nutrient concentrations in creeks just downstream from hayed plots, but the sediments of hayed marshes were lower in total N and P compared to references. In sum, haying appeared to affect plant species composition but had only short-term affects on consumer organisms. This contrasts with many grassland ecosystems, where an intermediate level of disturbance, such as by grazing, increases species diversity and may stimulate productivity. From a management perspective, periodic mowing could be a way to maintain S. patens habitats and the suite of species with which they are associated.     

Population structure of the ribbed mussel <Emphasis Type="Italic">Geukensia demissa</Emphasis> in salt marshes in the southern Gulf of St. Lawrence,Canada

The ribbed mussel, Geukensia demissa, is highly dependent on the cordgrass Spartina alterniflora for amelioration from environmental stress and substrate stabilization. Spartina alterniflora is a foundation species in marshes, and G. demissa is typically associated with cordgrass beds. Marshes in the southern Gulf of St. Lawrence are experiencing erosion and degradation, presumably as a result of increases in sea level, which increases salinity exposure and negatively impacts S. alterniflora. The population structure of the ribbed mussel, Geukensia demissa, was studied at nine sites in six estuaries in the southern Gulf of St. Lawrence in Nova Scotia, Canada, where marsh degradation is occurring. Mussel length was used as a proxy for age of G. demissa in three salt marsh zones characterized by density and elevation of Spartina alterniflora: (1) a lower zone in which the S. alterniflora was dead, but where the basal mat was coherent, (2) a zone of living, but low density S. alterniflora at the margin of the living marsh, and (3) a zone of dense S. alterniflora one to three meters back from the edge. Mussel length was significantly different across the three zones in seven of the nine sites. Mean length decreased as elevation increased, and small mussels (i.e., 1–3 cm) were absent at seven sites. The smallest mussels occurred in the dense S. alterniflora zone, higher in the marsh. Mussel length in the two western sites did not differ between zones, and small mussels (i.e., 1–3 cm) were present, but rare. The absence of small mussels in seven of the nine sites, and the size frequency distribution at remaining sites, suggests a lack of recent recruitment and a long-term threat to the survival of G. demissa. Salt marsh degradation and the death of S. alterniflora have negatively impacted G. demissa recruitment, and population decline is evident.     

Nitrogen sequestration capacity of two salt marshes from the Tagus estuary

The role of salt marshes as nitrogen sink is examined taking into consideration the seasonal variation of above and belowground biomass of Spartina martima and Halimione portulacoides in two marshes from Tagus estuary, Pancas and Corroios, and the degradation rates of belowground litter. Total nitrogen was determined in plant components, decomposing litter and sediment. Biomass was higher in Corroios, the saltier marsh, with 7190 g m⁻² y⁻¹ dw of S. maritima and 6593 g m⁻² y⁻¹ dw of H. portulacoides and the belowground component contributed to 96% and 90% of total biomass, respectively. In the other marsh, Pancas, belowground biomass contributed to 56% and 76% of total biomass for S. maritima and H. portulacoides, respectively. Litterbag experiment showed that between 25% and 50% of nitrogen is lost within the first month and remained relatively constant in the next four months. Slower decomposition is observed in sediments with higher nitrogen concentration (max. 0.7% N in the saltier marsh). Higher concentrations of N were found in the sediment upper layers. Considering the sediment-root system, most of the nitrogen is stored in the sediment compartment and only about 1–4% of the total N was found in the roots. Considering these results, Tagus salt marshes act as a sink for nitrogen.     

Where temperate meets tropical: multi-factorial effects of elevated CO2, nitrogen enrichment, and competition on a mangrove-salt marsh community

Our understanding of how elevated CO₂ and interactions with other factors will affect coastal plant communities is limited. Such information is particularly needed for transitional communities where major vegetation types converge. Tropical mangroves (Avicennia germinans) intergrade with temperate salt marshes (Spartina alterniflora) in the northern Gulf of Mexico, and this transitional community represents an important experimental system to test hypotheses about global change impacts on critical ecosystems. We examined the responses of A. germinans (C₃) and S. alterniflora (C₄), grown in monoculture and mixture in mesocosms for 18 months, to interactive effects of atmospheric CO₂ and pore water nitrogen (N) concentrations typical of these marshes. A. germinans, grown without competition from S. alterniflora, increased final biomass (35%) under elevated CO₂ treatment and higher N availability. Growth of A. germinans was severely curtailed, however, when grown in mixture with S. alterniflora, and enrichment with CO₂ and N could not reverse this growth suppression. A field experiment using mangrove seedlings produced by CO₂‐ and N‐enriched trees confirmed that competition from S. alterniflora suppressed growth under natural conditions and further showed that herbivory greatly reduced survival of all seedlings. Thus, mangroves will not supplant marsh vegetation due to elevated CO₂ alone, but instead will require changes in climate, environmental stress, or disturbance to alter the competitive balance between these species. However, where competition and herbivory are low, elevated CO₂ may accelerate mangrove transition from the seedling to sapling stage and also increase above‐ and belowground production of existing mangrove stands, particularly in combination with higher soil N.     

Phenotypic plasticity of <Emphasis Type="Italic">Spartina alterniflora</Emphasis> and <Emphasis Type="Italic">Phragmites australis</Emphasis> in response to nitrogen addition and intraspecific competition

Phenotypic plasticity of the two salt marsh grasses Spartina alterniflora and Phragmites australis in salt marshes is crucial to their invasive ability, but the importance of phenotypic plasticity, nitrogen levels, and intraspecific competition to the success of the two species is unclear at present. Spartina alterniflora Loisel. is an extensively invasive species that has increased dramatically in distribution and abundance on the Chinese and European coasts, and has had considerable ecological impacts in the regions where it has established. Meanwhile, Phragmites australis Cav., a native salt marsh species on the east coast of China, has replaced the native S. alterniflora in many marshes along the Atlantic Coast of the US. This study determined the effects of nitrogen availability and culm density on the morphology, growth, and biomass allocation traits of Spartina alterniflora and Phragmites australis. A large number of morphological, growth, and biomass parameters were measured, and various derived values (culm: root ratio, specific leaf area, etc.) were calculated, along with an index of phenotypic plasticity. Nitrogen addition significantly affected growth performance and biomass allocation traits of Spartina alterniflora, and culm density significantly affected morphological characteristics in a negative way, especially for Spartina alterniflora. However, there were no significant interactions between nitrogen levels and culm density on the morphological parameters, growth performances parameters, and biomass allocation parameters of the two species. Spartina alterniflora appears to respond more strongly to nitrogen than to culm density and this pattern of phenotypic plasticity appears to offer an expedition for successful invasion and displacement of Phramites australias in China. The implication of this study is that, in response to the environmental changes that are increasing nitrogen levels, the range of Spartina alterniflora is expected to continue to expand on the east coast of China.     

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.