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.     

泉州湾蟳埔潮间带大型底栖动物群落的时空分布

为了比较泉州湾蟳埔潮间带沙滩、互花米草滩和牡蛎石泥滩3种生境(3个潮层)的大型底栖动物群落,2011年4月至2012年1月对3种生境的大型底栖动物进行了季度定量取样。在3种生境共获得85种大型底栖动物,其中环节动物39种,软体动物20种,节肢动物21种,刺胞动物、扁形动物、纽虫动物、星虫动物和脊索动物各1种。多维标度排序(MDS)分析表明,春季和冬季泉州湾蟳埔潮间带3种生境的大型底栖动物群落相似性较低;夏季和秋季互花米草滩与牡蛎石泥滩的大型底栖动物群落相似性较高,而与沙滩的大型底栖动物群落相似性较低。沙滩大型底栖动物群落的季节变化较明显,其次是牡蛎石泥滩,而互花米草滩大型底栖动物群落的季节变化较不明显。大型底栖动物栖息密度和生物量随着潮层降低而增加。单变量双因素方差分析(Two-way ANOVA)表明,不同生境之间的大型底栖动物物种数、栖息密度、多样性指数、均匀度指数和丰度指数有显著差异,但生物量无显著差异,这是因为沙滩的物种数较少,栖息密度较低,但优势种弧边招潮蟹(Uca arcuata)个体较大,互花米草滩和牡蛎石泥滩的优势种为加州中蚓虫(Mediomastus californiensis),个体相对弧边招潮蟹小。不同季节之间大型底栖动物物种数、栖息密度、生物量和丰度指数有显著差异,但多样性指数和均匀度指数元显著差异,这是因为沙滩物种数少,但个体分布比较均匀,而互花米草滩和牡蛎石泥滩物种数较多,个体分布较不均匀。以上结果表明,潮汐、沉积物粒径和生境是影响潮间带大型底栖动物群落的主要因素。潮汐导致潮间带的空间异质性,空间异质性导致大型底栖动物群落的差异。     

Invasive alien plants increase CH4 emissions from a subtropical tidal estuarine wetland

Methane (CH₄) is an important greenhouse gas whose emission from the largest source, wetlands is controlled by a number of environmental variables amongst which temperature, water-table, the availability of substrates and the CH₄ transport properties of plants are most prominent and well characterised. Coastal wetland ecosystems are vulnerable to invasion by alien plant species which can make a significant local contribution to altering their species composition. However the effect of these changes in species composition on CH₄ flux is rarely examined and so is poorly understood. Spartina alterniflora, a perennial grass native to North America, has spread rapidly along the south-east coast of China since its introduction in 1979. From 2002, this rapid invasion has extended to the tidal marshes of the Min River estuary, an area that, prior to invasion was dominated by the native plant Cyperus malaccensis. Here, we compare CH₄ flux from the exotic invasive plant S. alterniflora with measurements from the aggressive native species Phragmites australis and the native species C. malaccensis following 3-years of monitoring. CH₄ emissions were measured over entire tidal cycles. Soil CH₄ production potentials were estimated for stands of each of above plants both in situ and in laboratory incubations. Mean annual CH₄ fluxes from S. alterniflora, P. australis and C. malaccensis dominated stands over the 3 years were 95.7 (±18.7), 38.9 (±3.26) and 10.9 (±5.26) g m^?2 year^?1, respectively. Our results demonstrate that recent invasion of the exotic species S. alterniflora and the increasing presence of the native plant P. australis has significantly increased CH₄ emission from marshes that were previously dominated by the native species C. malaccensis. We also conclude that higher above ground biomass, higher CH₄ production and more effective plant CH₄ transport of S. alterniflora collectively contribute to its higher CH₄ emission in the Min River estuary.     

Sulfur storage changed by exotic Spartina alterniflora in coastal saltmarshes of China

For the purpose of ecological engineering, Spartina alterniflora was introduced to China in 1979 and now covers about 112,000 ha of China's coastal lands. It was hypothesized that S. alterniflora could actively change the habitat environment, thus facilitating its competition over native species. In Yancheng Nature Reserve, sulfur storage of sediments and plant tissues was compared among marshes dominated by the exotic S. alterniflora and adjacent native Suaeda salsa and Phragmites australis and bare mudflat. Results showed that the S. alterniflora marsh contained the highest content of water-soluble, adsorbed, carbonate-occluded and total sulfur in the sediment. The sulfur levels were higher in the center than at the edges of the S. alterniflora marsh. Native marshes showed no significant difference in sediment sulfur levels. With greater biomass and higher tissue sulfur concentrations, plant sulfur storage of S. alterniflora vegetation was also larger than those of the native vegetations. Because higher concentrations of sulfur increase the competitive advantage of S. alterniflora over native halophytes, the results of the research showing that S. alterniflora increased marsh sulfur storage may shed light on the mechanism of expansion of monospecific vegetation in coastal China.     

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.     

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.     

Role of biotic interactions and physical factors in determining the distribution of marsh species along an estuarine salinity gradient

Understanding the mechanisms that shape plant distribution patterns is a major goal in ecology. We investigated the role of biotic interactions (competition and facilitation) and abiotic factors in creating horizontal plant zonation along salinity gradients in the Elbe estuary. We conducted reciprocal transplant experiments with four dominant species from salt and tidal freshwater marshes at two tidal elevations. Ten individuals of each species were transplanted as sods to the opposing marsh type and within their native marsh (two sites each). Transplants were placed at the centre of 9‐m² plots along a line parallel to the river bank. In order to disentangle abiotic and biotic influences, we set up plots with and without neighbouring vegetation, resulting in five replicates per site. Freshwater species (Bolboschoenus maritimus and Phragmites australis) transplanted to salt marshes performed poorly regardless of whether neighbouring vegetation was present or not, although 50–70% of the transplants did survive. Growth of Phragmites transplants was impaired also by competition in freshwater marshes. Salt marsh species (Spartina anglica and Puccinellia maritima) had extremely low biomass when transplanted to freshwater marshes and 80–100% died in the presence of neighbours. Without neighbours, biomass of salt marsh species in freshwater marshes was similar to or higher than that in salt marshes. Our results indicate that salt marsh species are precluded from freshwater marshes by competition, whereas freshwater species are excluded from salt marshes by physical stress. Thus, our study provides the first experimental evidence from a European estuary for the general theory that species boundaries along environmental gradients are determined by physical factors towards the harsh end and by competitive ability towards the benign end of the gradient. We generally found no significant impact of competition in salt marshes, indicating a shift in the importance of competition along the estuarine gradient.     

Seasonal variation in CH4 emission and its 13C-isotopic signature from Spartina alterniflora and Scirpus mariqueter soils in an estuarine wetland

Although invasions by non-native species represent a major threat to biodiversity and ecosystem functioning, little attention has been paid to the potential impacts of these invasions on methane (CH₄) emission and its ¹³C-CH₄-isotope signature in salt marshes. An invasive perennial C₄ grass Spartina alterniflora has spread rapidly along the east coast of China since its introduction from North America in 1979. Since its intentional introduction to the Jiuduansha Island in the Yangtze River estuary in 1997, S. alterniflora monocultures have become the dominant component of the Jiuduansha’s vegetation, where monocultures of the native plant Scirpus mariqueter (a C₃ grass) used to dominate the vegetation for more than 30 years. We investigated seasonal variation in soil CH₄ emission and its ¹³C-CH₄-isotope signature from S. alterniflora and S. mariqueter marshes. The results obtained here show that S. alterniflora invasion increased soil CH₄ emissions compared to native S. mariqueter, possibly resulting from great belowground biomass of S. alterniflora, which might have affected soil microenvironments and /or CH₄ production pathways. CH₄ emissions from soils in both marshes followed similar seasonal patterns in CH₄ emissions that increased significantly from April to August and then decreased from August to October. CH₄ emissions were positively correlated with soil temperature, but negatively correlated with soil moisture for both S. alterniflora and S. mariqueter soils (p?<?0.05). The δ¹³C values of CH₄ from S. alterniflora, and S. mariqueter soils ranged from -39.0‰ to -45.0‰, and -37.3‰ to -45.7‰, respectively, with the lowest δ¹³C values occurring in August in both marshes. Although the leaves, roots and soil organic matter of S. alterniflora had significantly higher δ¹³C values than those of S. mariqueter, S. alterniflora invasion did not significantly change the ¹³C- isotopic signature of soil emitted CH₄ (p?>?0.05). Generally, the CH₄ emissions from both invasive S. alterniflora and native S. mariqueter soils in the salt marshes of Jiuduansha Island were very low (0.01–0.26 mg m^-2 h^-1), suggesting that S. alterniflora invasion along the east coast of China may not be a significant potential source of atmospheric CH₄.     

Biomass and aboveground production of four angiosperms in Cantabrian (N. Spain) salt marshes

New data of aboveground biomass and production of four angiosperms over a 12 month period for the Cantabrian Sea salt marshes (Bay of Biscay, N. Spain) are presented. Based on harvest methods, maximum aboveground total biomass values for Spartina maritima (Curtis) Fernald, Spartina alterniflora Loisel, Salicornia ramosissima J. Woods and Halimione portulacoides (L.) Aellen were 628, 1109, 480 and 1267 gm^-2, respectively. We conclude that although a slight latitudinal gradient in biomass is revealed in the data compiled with reference to some of the species studied, more work is neccesary in order to assess the potential productivity of these ecosystems on the coasts of Europe and/or to make comparisons with salt marshes of the American coasts. Annual net aerial primary production estimates using Smalley's method were: 296, 1160, 486 and 952 gm^-2yr^-1, for Spartina maritima, Spartina alterniflora, Salicornia ramosissima and Halimione portulacoides, respectively. These results together with turnover rate estimates point to the lack of vigour of the native S. maritima, while the exotic S. alterniflora, which seems to be spreading along the Cantabrian estuaries, behaves like a veritable pionner throughout the low marshes in this region.     

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.     

Exotic Spartina alterniflora invasion alters ecosystem–atmosphere exchange of CH4 and N2O and carbon sequestration in a coastal salt marsh in China

Coastal salt marshes are sensitive to global climate change and may play an important role in mitigating global warming. To evaluate the impacts of Spartina alterniflora invasion on global warming potential (GWP) in Chinese coastal areas, we measured CH₄ and N₂O fluxes and soil organic carbon sequestration rates along a transect of coastal wetlands in Jiangsu province, China, including open water; bare tidal flat; and invasive S. alterniflora, native Suaeda salsa, and Phragmites australis marshes. Annual CH₄ emissions were estimated as 2.81, 4.16, 4.88, 10.79, and 16.98 kg CH₄ ha^?1 for open water, bare tidal flat, and P. australis, S. salsa, and S. alterniflora marshes, respectively, indicating that S. alterniflora invasion increased CH₄ emissions by 57–505%. In contrast, negative N₂O fluxes were found to be significantly and negatively correlated (P < 0.001) with net ecosystem CO₂ exchange during the growing season in S. alterniflora and P. australis marshes. Annual N₂O emissions were 0.24, 0.38, and 0.56 kg N₂O ha^?1 in open water, bare tidal flat and S. salsa marsh, respectively, compared with ‐0.51 kg N₂O ha^?1 for S. alterniflora marsh and ?0.25 kg N₂O ha^?1 for P. australis marsh. The carbon sequestration rate of S. alterniflora marsh amounted to 3.16 Mg C ha^?1 yr^?1 in the top 100 cm soil profile, a value that was 2.63‐ to 8.78‐fold higher than in native plant marshes. The estimated GWP was 1.78, ?0.60, ?4.09, and ?1.14 Mg CO₂eq ha^?1 yr^?1 in open water, bare tidal flat, P. australis marsh and S. salsa marsh, respectively, but dropped to ?11.30 Mg CO₂eq ha^?1 yr^?1 in S. alterniflora marsh. Our results indicate that although S. alterniflora invasion stimulates CH₄ emissions, it can efficiently mitigate increases in atmospheric CO₂ and N₂O along the coast of China.     

Physical stress, not biotic interactions, preclude an invasive grass from establishing in forb-dominated salt marshes

Background

Biological invasions have become the focus of considerable concern and ecological research, yet the relative importance of abiotic and biotic factors in controlling the invasibility of habitats to exotic species is not well understood. Spartina species are highly invasive plants in coastal wetlands; however, studies on the factors that control the success or failure of Spartina invasions across multiple habitat types are rare and inconclusive.

Methodology and Principal Findings

We examined the roles of physical stress and plant interactions in mediating the establishment of the smooth cordgrass, Spartina alterniflora, in a variety of coastal habitats in northern China. Field transplant experiments showed that cordgrass can invade mudflats and low estuarine marshes with low salinity and frequent flooding, but cannot survive in salt marshes and high estuarine marshes with hypersaline soils and infrequent flooding. The dominant native plant Suaeda salsa had neither competitive nor facilitative effects on cordgrass. A common garden experiment revealed that cordgrass performed significantly better when flooded every other day than when flooded weekly. These results suggest that physical stress rather than plant interactions limits cordgrass invasions in northern China.

Conclusions and Significance

We conclude that Spartina invasions are likely to be constrained to tidal flats and low estuarine marshes in the Yellow River Delta. Due to harsh physical conditions, salt marshes and high estuarine marshes are unlikely to be invaded. These findings have implications for understanding Spartina invasions in northern China and on other coasts with similar biotic and abiotic environments.     

UNDERGROUND BIOMASS PROFILES AND PRODUCTIVITY IN ATLANTIC COASTAL MARSHES

An underground biomass profile and productivity study involved year-long sampling programs in 18 stands of salt marsh plants in Georgia, Delaware, and Maine. As the result of the monthly or bimonthly marsh coring program three types of underground biomass profiles were found. In the first, the concentration of macro-organic matter (MOM) was uniform with depth; the notable example of this type was creekbank Spartina alterniflora in the southern part of the coast. A second type had a high MOM concentration at the surface which decreased with depth. This, the most common type of profile, was exemplified by Spartina patens, S. alterniflora from the high marsh along the southern coast (Georgia), and creekbank S. alterniflora from the northern part of its range (Maine). The third type of profile was seen where a large rhizome mat developed 15–20 cm below the surface. Spartina cynosuroides and Phragmites communis were typical examples of this type of profile, resulting in a low biomass at the surface, a higher biomass somewhat below the surface, and a low concentration at depth. The annual maxima and minima of MOM biomass were used to calculate annual increments, which can be considered minimum annual production values. These productivity values ranged from a low of 80 g C/m² for creekhead S. alterniflora in Maine to a high of 1690 g C/m² for Juncus gerardi in Maine. The mean for all plant stands was 650 g C/m². Since the average carbon content of the MOM was 35.3%, this corresponds to 1850 g dry weight/m² per year. As a measure of the relative activity of the total pool of macro-organic material in the soil, turnover times were calculated by dividing the total macro-organic matter by the annual increments. Within the MOM pool there are several components with turnover times varying from days to years. The turnover time for the entire pool ranged from 18 months in two Georgia salt marsh plant stands to 224 months for one in Maine. In the two instances where values for a species could be compared between Maine and Georgia, the turnover time was shorter at the more southerly site. These results can probably be attributed to slower microbial decay rates in the cooler climate. In Georgia and Maine where the turnover values for a species were determined for two elevations, the time was shorter at the lower elevation.     

The macrobenthos in Spartina alterniflora salt marshes of the Wanggang tidal-flat,Jiangsu coast,China

On the Jiangsu coast, eastern China, Spartina alterniflora, which was introduced artificially into the region, is becoming a dominant plant species in the inter-tidal salt marshes. In order to evaluate the environmental and ecological impact of the colonization of S. alterniflora, we carried out investigations into the benthic macrofauna of the Spartina marshes of the Wanggang area, central Jiangsu coast, in 2006 and 2007. Based on analysis of the data sets obtained, 12 species of macrobenthos have been identified for the Wanggang salt marsh, including S. alterniflora, Cerithidea cingulata, Littorna scabra, Bithynia fuchsiana, Macrophthalmus japonicus, Uca arcuata, Nereis sp., Boleophthalmus petinirostris, Cyclina sinensis, Bullacta exarata, Angustassiminea castanea and Glaucomya chinensis. The results indicate that some of the native species have adapted to the new ecological environment associated with the cordgrass S. alterniflora. The biomass of macrobenthos varies significantly over different parts of the salt marsh. Further, there is a seasonal change in bio-density, with the density in summer (July and August) > autumn (November) > early summer (May). At the landward edge and over the central part of the S. alterniflora marsh, the bio-diversity is higher than the other areas of the marsh.     

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.     

Effects of Spartina alterniflora Invasion on Soil Respiration in the Yangtze River Estuary,China

Many studies have found that plant invasion can enhance soil organic carbon (SOC) pools, by increasing net primary production (NPP) and/or decreased soil respiration. While most studies have focused on C input, little attention has been paid to plant invasion effects on soil respiration, especially in wetland ecosystems. Our study examined the effects of Spartina alterniflora invasion on soil respiration and C dynamics in the Yangtze River estuary. The estuary was originally occupied by two native plant species: Phragmites australis in the high tide zone and Scirpus mariqueter in the low tide zone. Mean soil respiration rates were 185.8 and 142.3 mg CO₂ m⁻² h⁻¹ in S. alterniflora and P. australis stands in the high tide zone, and 159.7 and 112.0 mg CO₂ m⁻² h⁻¹ in S. alterniflora and S. mariqueter stands in the low tide zone, respectively. Aboveground NPP (ANPP), SOC, and microbial biomass were also significantly higher in the S. alterniflora stands than in the two native plant stands. S. alterniflora invasion did not significantly change soil inorganic carbon or pH. Our results indicated that enhanced ANPP by S. alterniflora exceeded invasion-induced C loss through soil respiration. This suggests that S. alterniflora invasion into the Yangtze River estuary could strengthen the net C sink of wetlands in the context of global climate change.     

Determinants of seed bank dynamics of two dominant helophytes in a tidal salt marsh

We investigated dynamics and spatial distribution of Scirpus mariqueter and Spartina alterniflora seed banks at Chongming Dongtan in the Yangtze River estuary, China. Five sites along an elevational gradient were chosen, one in each of the main zones (mudflat, Scirpus monoculture, Scirpus–Spartina mixture, Spartina monoculture and Spartina–Phragmites mixture). Three surveys were performed just after seed rain, before germination and after germination, respectively. During the period of November 2005 to May 2006, soil seed density of Scirpus mariqueter declined by 36%, and that of Spartina alterniflora by 58%. The spatial distributions of their seed banks were also different. Soil seed density of Scirpus mariqueter was not determined directly by seed production, but positively correlated with total aboveground biomass of the whole plant community. On the contrary, soil seed density of Spartina alterniflora just after seed rain (November) was significantly correlated with seed production, but had a poor relationship with the community's aboveground biomass. Our results indicated that other factors such as tidal movement might have had great influence on dispersal of Scirpus mariqueter, which would also affect its population dynamics. The understanding of this process can help us improve the conservation and restoration efforts.     

Porewater oxidation,dissolved phosphate and the iron curtain

The process of dissolved phosphate removal from aqueous solution, which occurs during oxidation of soluble ferrous compounds to insoluble ferric forms, was examined in soils of two tidal freshwater marshes. Sites of amorphous iron deposition and sorption or co-precipitation of phosphate were found to be in surface soils and along creekbanks, where both ion diffusion and porewater advection move dissolved iron and phosphate from reduced to oxidized regions. Profiles of extractable iron and total phosphorus from creekbank and interior soils were consistent with hypothesized differences between a high and a low marsh. Porewater concentrations of dissolved phosphate were higher in creekbank soils of the high marsh, compared with water actually discharging from the creekbank during tidal exposure. We propose that an iron curtain of ferric hydroxides functions as a barrier to diffusive and advective movement of dissolved phosphate along surfaces of tidal freshwater marshes, and has important implications for the distribution and availability of phosphorus in other types of wetlands and aqueous systems.     

Factors influencing aboveground production of Scirpus marshes in the St. Lawrence estuary,Québec,Canada

Aboveground production of macrophytes in tidal marshes was studied for 3 years at Montmagny and Cap St. Ignace on the south shore of the St. Lawrence River in Québec, Canada. Year-to-year variation of production of the dominant species, Scirpus americanus Pers., was observed and related to variation in weather during the growing season. With the exception of Zizania aquatica L., relative production of the macrophytes for different parts of the marshes remained constant in successive years. A multiple regression procedure was used to establish the relationship between production and a series of independent variables including biotic (goose use and plant competition) and abiotic factors (submersion, substrate hardness, soil texture and sedimentation). A lower percentage of organic matter in the soil and a firm substrate were associated with a greater production of S. americanus. Accretion of sediments was positively correlated with the production of S. americanus and Sagittaria spp., whereas the percentage of time of submersion was negatively related with production of Sagittaria spp., Eleocharis spp. and Scirpus torreyi Olney. Competitive interference was apparent between S. americanus and S. torreyi and between Eleocharis spp. and Zizania spp. No measured abiotic characteristic of the marsh explained the variation of Zizania production.     

Growing Season Length and Soil Moisture Interactively Constrain High Elevation Aboveground Net Primary Production

Although high elevation meadows are often considered to be primarily temperature-limited, aboveground net primary production (ANPP) is influenced by both growing season length (GSL) and soil moisture (SM). Progress in understanding these responses comes from studies at individual sites that are often focused on single abiotic drivers, so little is known about how climatic constraints relate to one another and limit ANPP at larger scales. We examined the independent and combined effects of these two key climate drivers (GSL and SM) across a suite of high elevation meadows from two continents. We also sampled gradients of each variable at a single site to explore mechanisms for SM and GSL limitations. Here, we show that high elevation meadows are limited by both GSL and SM levels. GSL constrained maximum ANPP by approximately 4 g m⁻² d⁻¹ whereas average daily ANPP beneath this constraint was significantly influenced by SM. Carbon isotope and plant/soil nitrogen data suggest that SM influenced ANPP through its impact on nitrogen availability. Increases in GSL can increase ANPP unless those increases are accompanied by SM decreases. These interactive effects can produce distinct ecological patterns and must be considered when predicting high elevation meadow responses to future climate changes.