Low Marsh Species Research Articles

Salt marsh vegetation change during a half-century of experimental nutrient addition and climate-driven controls in Great Sippewissett Marsh

Vegetative cover was mapped annually, 1976–2022, in experimental plots in Great Sippewissett Marsh, Cape Cod, USA, chronically fertilized at different doses, and subject to changes in sea level and other climate-related variables. Dominant species within areas of higher elevation in the plots followed different decadal trajectories: rise in sea level diminished cover of Spartina patens; higher N supplies increased cover of Distichlis spicata. The opportunistic growth response of D. spicata to high N supplies unexpectedly fostered increased platform accretion, a feature that persisted for succeeding decades and led to further changes in vegetation: D. spicata functioned as an effective ecosystem engineer with long-term ecological consequences. Shrubs usually found in upper marsh margins expanded into areas where D. spicata had stimulated accretion, then shaded and excluded D. spicata, but subsequently lost cover as sea level rise continued. Increased N supply converted stands of Spartina alterniflora, the dominant low marsh species, from short to taller ecophenotypes; sea level rise had minor effects on S. alterniflora, but during 2019–2022 appeared to reach a tipping point that fostered taller S. alterniflora and bare space even in un-fertilized control plots, and in Great Sippewissett Marsh in general. Model results anticipate that—in spite of potential accretion enhanced by vegetation and ecosystem engineer effects—there will be loss of high marsh, transient increases of low marsh, followed by loss of low marsh, and eventual conversion to shallow open water by the end of the century. Dire local projections match those of the plurality of recent reports from salt marshes around the world. Proposed management strategies may only delay unfortunate outcomes rather than maintain wetlands. Concerted reductions of warming from greenhouse gases, and lower N loads seem necessary to address the coming crises in wetlands—and many other environmental threats.

Metal effects on germination and seedling development in closely-related halophyte species inhabiting different elevations along the intertidal gradient

Seed germination and seedling establishment are very sensitive plant stages to metal pollution. Many halophyte species colonizing salt marshes are able to germinate and establish in highly contaminated habitats and low marsh halophyte species seem to show higher tolerance to metals than high marsh species. We analyzed the effects of copper, zinc and nickel in concentrations up to 2000 μM on seed germination and seedling growth in two closely related species of Sarcocornia, S. perennis, a low marsh species, and S. fruticosa, a high marsh species. Germination of both halophytes was not affected by any metal concentration, and their seedling growth, mainly radicle length, was reduced by increasing metal concentrations. Seedlings of S. perennis showed higher tolerance to the three metals than those of S. fruticosa. Our results are useful for designing ecotoxicological bioassays and planning phytoremediation projects in salt marshes.

Competitive reversal between plant species is driven by species‐specific tolerance to flooding stress and nutrient acquisition during early marsh succession

Abstract Understanding plant species interactions along successional trajectories is critical for managing and restoring ecosystems, as both resource availability and abiotic stresses change over time to affect competitive outcomes and species distributions. Newly created ecosystems experience a succession of plant species and rapid changes in resource availability, which may influence the outcome of biotic interactions. How these biotic interactions vary along abiotic gradients in early successional systems is not well understood. Here, we tested the hypothesis that species‐specific tolerances to flooding would influence their relative ability to capture resources (i.e. nutrients) and affect competition intensity between pioneer and secondary successional species in an early successional created tidal marsh. We transplanted a competitively dominant higher marsh species, saltmeadow cordgrass (Spartina patens), across an elevation gradient within and outside of clones of a pioneer stress‐tolerant low marsh species, smooth cordgrass (S. alterniflora). Within 6 months, S. alterniflora had suppressed the stature and growth of S. patens; however, the magnitude of this competitive effect increased at lower marsh elevations where S. alterniflora was more efficient at capturing available nitrogen (N). In unvegetated areas, where S. patens vigour was high, the amount of available N was approximately 40 times greater than within S. alterniflora clones. Synthesis and applications. Our results demonstrate that competition intensity of the stress‐tolerant species over the competitive species depended on relative resource capture in response to abiotic stress. Managing for specific vegetation communities in marsh restoration, therefore, requires insight into these relationships and interactions. Specifically, marsh restoration in high nutrient environments will limit the succession to high density competitive species due to competition with stress‐tolerant pioneer species, particularly at lower elevations.

Salt marsh ecosystem restructuring enhances elevation resilience and carbon storage during accelerating relative sea-level rise

Salt marshes respond to sea-level rise through a series of complex and dynamic bio-physical feedbacks. In this study, we found that sea-level rise triggered salt marsh habitat restructuring, with the associated vegetation changes enhancing salt marsh elevation resilience. A continuous record of marsh elevation relative to sea level that includes reconstruction of high-resolution, sub-decadal, marsh elevation over the past century, coupled with a lower-resolution 1500-year record, revealed that relative sea-level rose 1.5 ± 0.4 m, following local glacial isostatic adjustment (1.2 mm/yr). As sea-level rise has rapidly accelerated, the high marsh zone dropped 11 cm within the tidal frame since 1932, leading to greater inundation and a shift to flood- and salt-tolerant low marsh species. Once the marsh platform fell to the elevation favored by low-marsh Spartina alterniflora, the elevation stabilized relative to sea level. Currently low marsh accretion keeps pace with sea-level rise, while present day high marsh zones that have not transitioned to low marsh have a vertical accretion deficit. Greater biomass productivity, and an expanding subsurface accommodation space favorable for salt marsh organic matter preservation, provide a positive feed-back between sea-level rise and marsh platform elevation. Carbon storage was 46 ± 28 g C/m2/yr from 550 to 1800 CE, increasing to 129 ± 50 g C/m2/yr in the last decade. Enhanced carbon storage is controlled by vertical accretion rates, rather than soil carbon density, and is a direct response to anthropogenic eustatic sea-level rise, ultimately providing a negative feedback on climate warming.

Is saltmarsh restoration success constrained by matching natural environments or altered succession? A test using niche models.

Restored habitats, such as saltmarsh created through managed realignment, sometimes fail to meet targets for biological equivalence with natural reference sites. Understanding why this happens is important in order to improve restoration outcomes.Elevation in the tidal frame and sediment redox potential are major controls on the distribution of saltmarsh plants. We use niche models to characterize 10 species’ responses to these, and test whether differences in species occurrence between restored and natural saltmarshes in the UK result from failure to recreate adequate environmental conditions.Six species occurred less frequently in recently restored marshes than natural marshes. Failure of restored marshes to achieve the elevation and redox conditions of natural marshes partially explained the underrepresentation of five of these species, but did not explain patterns of occurrence on older (>50 years) restored marshes.For all species, an effect of marsh age remained after controlling for differences in environmental conditions. This could be due to differences in successional mechanism between restored and natural marshes. In recently restored marshes, high‐marsh species occurred lower in the tidal frame and low‐marsh species occurred higher in the tidal frame than in natural marshes. This supports the hypothesis that competition is initially weaker in restored marshes, because of the availability of bare sediment across the whole tidal frame. Species that establish outside their normal realized niche, such as Atriplex portulacoides, may inhibit subsequent colonization of other species that occurred less frequently than expected on older restored marshes. Synthesis and applications. Niche models can be used to test whether abiotic differences between restored sites and their natural counterparts are responsible for discrepancies in species occurrence. In saltmarshes, simply replicating environmental conditions will not result in equivalent species occurrence.

Germination syndromes in response to salinity of Chenopodiaceae halophytes along the intertidal gradient

Germination is a crucial stage in the life cycle of plants and salinity is one of the most important factors conditioning it in salt marshes. This work analyses the effects of salinity on the germination of eight dominant Chenopodiaceae species inhabiting the intertidal gradient in the Gulf of Cádiz. Seeds were collected from Sarcocornia perennis, Atriplex portulacoides, Sarcocornia fruticosa, Salicornia ramosissima, Arthrocnemum macrostachyum, Suaeda vera, Salsola vermiculata and Sarcocornia alpini. They were placed in Petri dishes at 0.0, 0.1, 0.2, 0.3, 0.6 and 0.9M NaCl and germination was recorded during 30days. Ungerminated seeds were transferred to distilled water and germination recovery was recorded. High salinities inhibited germination, although tended to accelerate it in the recovery assays. Beyond these common responses, the studied halophytes showed four germination syndromes: 1) high germination at low-medium salinities and reduced germination after high salinity exposure; 2) no germination at hypersalinity and high recovery germination in fresh water; 3) increased and rapid germination after salinity exposure; and 4) germinating along a wide salinity range. The two species colonising the ecotone between marshes and dunes, Suaeda vera and Salsola vermiculata, shared Syndrome 2. Syndromes 3 and 4 were shared by species colonising different habitats and Syndrome 1 by A. macrostachyum, and fruits of A. portulacoides without bracteoles and S. vermiculata without calyx. The germination of species from high marshes was characterized by marked responses to salinity changes, whereas the germination of low marsh species was based on low sensitivity to salinity fluctuations.

Nutrient enrichment and precipitation changes do not enhance resiliency of salt marshes to sea level rise in the Northeastern U.S.

In the Northeastern U.S., salt marsh area is in decline. Habitat change analysis has revealed fragmentation, displacement of high marsh by low marsh species, and marsh drowning, while development of adjacent uplands limits upslope migration. Measures of marsh vegetation loss for eight sites in Rhode Island and New York between ca.1970 and 2011 indicate that substantial loss has occurred over past decades, with higher loss rates found for lower elevation salt marshes. Using inundation experiments, field surveys, and LiDAR datasets, we developed an elevation-productivity relationship for Spartina alterniflora specific to the U.S. Northeast, and located current salt marsh orthometric heights on this curve. We estimate that 87 % of Northeastern salt marshes are located at elevations where growth is limited by inundation. By manipulating water column nutrients, precipitation, and elevation, we further found that altered precipitation regime was associated with significant reductions in biomass, and that nutrient enrichment adversely impacts organic matter accumulation and peat formation. These results provide evidence that Northeastern U.S. marshes are vulnerable to the effects of accelerated sea level rise, and that neither precipitation changes, nor cultural eutrophication, will contribute positively to long-term salt marsh survival.

The Functions and Values of Fringing Salt Marshes in Northern New England, USA

We compared the functions and values of fringing salt marshes to those of meadow marshes along the southern Maine/New Hampshire coast. Differences included soil organic matter content, plant species richness, and percent cover of high and low-marsh species. More sediment was trapped per unit area in fringing marshes than in meadow marshes, but this difference was not significant. Similarities included aboveground and belowground peak season biomass and the ability to dampen wave energy. Both marsh types reduced the height of waves coming onto the marsh surface by 63% only 7 m into the marsh. Fringing marshes are diverse in terms of their physical characteristics (width, length, slope, elevation, soils). Despite their small size, they are valuable components of estuaries, performing many ecological functions to the same degree as nearby meadow marshes. More effort should be made to include them in regional efforts to conserve and restore coastal habitats.

Seed flotation and germination of salt marsh plants: The effects of stratification, salinity, and/or inundation regime

We examined the effects of cold stratification and salinity on seed flotation of eight salt marsh species. Four of the eight species were tested for germination success under different stratification, salinity, and flooding conditions. Species were separated into two groups, four species received wet stratification and four dry stratification and fresh seeds of all species were tested for flotation and germination. Fresh seeds of seven out of eight species had flotation times independent of salinity, six of which had average flotation times of at least 50 d. Seeds of Spartina alterniflora and Spartina patens had the shortest flotation times, averaging 24 and 26 d, respectively. Following wet stratification, the flotation time of S. alterniflora seeds in higher salinity water (15 and 36 ppt) was reduced by over 75% and germination declined by more than 90%. Wet stratification reduced the flotation time of Distichlis spicata seeds in fresh water but increased seed germination from 2 to 16% in a fluctuating inundation regime. Fresh seeds of Iva frutescens and S. alternflora were capable of germination and therefore are non-dormant during dispersal. Fresh seeds of I. frutescens had similar germination to dry stratified seeds ranging 25–30%. Salinity reduced seed germination for all species except for S. alterniflora. A fluctuating inundation regime was important for seed germination of the low marsh species and for germination following cold stratification. The conditions that resulted in seeds sinking faster were similar to the conditions that resulted in higher germination for two of four species.

Biophysical and morphological leaf adaptations to drought and salinity in salt marsh grasses

Leaf energy budgets were constructed for 13 species of estuarine C 4 grasses (Poaceae) to elucidate the biophysical effects of drought and salinity on the interception and dissipation of solar energy. Spartina alterniflora, S. anglica, S. argentinensis, S. bakeri, S. cynosuroides, S. densiflora, S. foliosa, S. foliosa × S. alterniflora hybrids, S. gracilis, S. patens, S. pectinata, S. spartinae, and Distichlis spicata plants were grown under controlled soil water potential gradients in a greenhouse. Species were grouped into four major ecological functional types, based on elevational zonation ranges: low marsh species, middle marsh species, high marsh species, and freshwater species. Different functional types are adapted to different environmental conditions, and responded differently to reduced water potentials. Latent heat flux decreased similarly across species in response to decreasing water potential. Latent heat loss was found to decrease by as much as 65% under decreasing water potential, leading to an increase in leaf temperature of up to 4 °C. Consequently, radiative and sensible heat losses increased under decreasing water potential. Sensible heat flux increased as much as 336% under decreasing water potential. Latent heat loss appeared to be an important mode of temperature regulation in all species, and sensible heat loss appeared to be more important in high marsh species compared to low marsh species. High marsh species are characterized by narrower leaves than middle and low marsh species, leading to a smaller boundary layer, and providing higher conductance to sensible heat loss. This may be an adaptation for high marsh species to regulate leaf temperature without access to large amounts of water for transpirational cooling. Stomatal conductance decreased with decreasing water potential across species: leaf conductances to water vapor and CO 2 decreased as much as 69% under decreasing water potential. Additionally, oxidative stress appeared to increase in these plants during times of drought or salinity stress. Ascorbate peroxidase activities increased with decreasing soil water potential, indicating increased cellular reactive oxygen species. High marsh species had higher ascorbate peroxidase activities compared to low marsh species, indicating higher tolerance to drought- or salinity-induced stresses. It was concluded that different species of marsh grasses are adapted for growth in different zones of salt marshes. Adaptations include biophysical, biochemical, and morphological traits that optimize heat exchange with the environment.

Root respiration and oxygen flux in salt marsh grasses from different elevational zones

Plants growing in waterlogged environments are subjected to low oxygen levels around submerged tissues. While internal oxygen transport has been postulated as an important factor governing flooding tolerance, respiration rates and abilities to take up oxygen under hypoxic conditions have been largely ignored in plant studies. In this study, physiological characteristics related to internal oxygen transport, respiration, and oxygen affinity were studied in low intertidal marsh species (Spartina alterniflora and S. anglica) and middle to high intertidal species (S. densiflora, S. patens, S. foliosa, a S. alterniflora × S. foliosa hybrid, S. spartinae, and Distichlis spicata). These marsh plants were compared to the inland species S. pectinata and the crop species rice (Oryza sativa), corn (Zea mays), and oat (Avena sativa). Plants were grown in a greenhouse under simulated estuarine conditions. The low marsh species S. anglica was found to transport oxygen internally at rates up to 2.2 μmol O2 g fresh root weight−1 h−1. In contrast, marsh species from higher zones and crop species were found to transport significantly less oxygen internally, although rice plants were able to transport 1.4 μmol g−1 h−1. Under hypoxic conditions, low marsh species were better able to remove dissolved oxygen from the medium compared to higher marsh species and crops. The oxygen concentration at which respiration rates declined due to limited oxygen (Pcrit) was significantly lower in low marsh species compared to inland and crop species; Pcrit ranged from <4 μM O2 in the low marsh species S. anglica up to 20 μM in the inland species corn. Flooding-sensitive crop species had significantly higher aerobic respiration rates compared to flooding-tolerant species in this study. Crop species took up 3.6–6.7 μmol O2 g−1 h−1 while all but one marsh species took up <3.5 μmol O2 g−1 h−1. We conclude that oxygen transport, aerobic demand, and oxygen affinity all play important and interrelated roles in flood tolerance and salt marsh zonation.

Respiratory enzyme activities correlate with anoxia tolerance in salt marsh grasses

Salt marsh communities are known for well-defined species zonation patterns. Lower limits of plant growth are thought to be set by an ability to tolerate anoxic sediments, but the physiological differences between species have not previously been examined. To investigate responses to anoxic sediments, several estuarine species were grown in greenhouse experiments to compare how respiratory processes were affected by flooding. Metabolic characteristics related to respiration and anoxia tolerance were studied in the emergent estuarine species Spartina alterniflora, S. anglica, S. densiflora, S. foliosa, S. alterniflora × S. foliosa hybrids, S. patens, and Distichlis spicata and compared to the inland species maize ( Zea mays). All species showed a strong ability to respire anaerobically, indicating flooding tolerance. High intertidal marsh species had significantly higher root aerobic respiration enzyme activities compared to low intertidal species that may suggest lower aerobic demand in low marsh species. Some higher marsh species showed an apparent high sensitivity to sulfide that may be related to high cytochrome c oxidase activities. In contrast, the low marsh species S. alterniflora and S. anglica had lower aerobic respiration enzyme activities and a lower sensitivity to sulfide. Thus differences in aerobic demand and sulfide sensitivity may influence estuarine species zonation.

Salt marsh vegetation recovery at salt hay farm wetland restoration sites on Delaware Bay

Public Service Enterprise Group's (PSEG) Estuary Enhancement Program (EEP) restored normal daily tidal inundation and drainage to three previously diked farm sites totaling 1780 ha during the period 1996–1998 along the shoreline of the Delaware Estuary in New Jersey, USA. Normal tidal exchange was either prevented or altered by the construction of dikes and water control structures that were used to promote the growth of high marsh “salt hay” species, such as salt meadow cordgrass ( Spartina patens) and salt grass ( Distichlis spicata) at elevations below mean high tide. These elevations would normally support low marsh species, such as smooth cordgrass ( Spartina alterniflora). Prior to restoration these sites were either un-vegetated due to excessive ponding associated with poor drainage or vegetated with “salt hay” species as well as common reed ( Phragmites australis). Extensive S. alterniflora marshes occur in the vicinity of each site at similar elevations and these marshes were relied on as the source of seed for the re-establishment of S. alterniflora on the restoration sites; thus, no other planting or seeding was implemented to initiate re-vegetation. Vegetation coverage at the restoration sites as well as a naturally restored reference marsh was determined during 1996–2003 utilizing geo-referenced remote sensing data and geographic information system (GIS) analyses. Coverage by S. alterniflora or other desirable low marsh vegetation exceeded that required by New Jersey authorities to meet the restoration goal at two sites 8 years ahead of the anticipated 12-year period and re-vegetation of the third site is expected to meet its goal within the established 12-year period. Annual vegetation coverage data have been integral components of an adaptive management program utilized by PSEG to manage the restoration process at each of the sites.

Relative utility of foraminifera, diatoms and macrophytes as high resolution indicators of paleo-sea level in coastal British Columbia, Canada

A multiproxy analysis was carried out on diatom, foraminiferal and macrophyte assemblages across the saltmarsh at Zeballos, Vancouver Island, British Columbia. To determine which group, or combination of groups provided the most accurate elevational zonations, 36 stepwise multiple linear regressions (SMLR) were carried out using a variety of data transformations on an elevational training set. Adjusted R 2 values yielded statistically significant results in all analyses as follows: foraminifera (0.658–0.870); diatoms (0.888–0.974); macrophytes (0.671–0.844); foraminifera/diatoms (0.941–0.981); foraminifera/diatoms/macrophytes (0.958–0.993). The most realistic SMLR results were obtained when data transformations comprised of (ln) normalized fractional abundance data was carried out on species present in statistically significant numbers (NrfaEQ). Of the individual proxies assessed, diatoms yielded the most significant adjusted R 2 results, with the low marsh diatom Achnanthes hauckiana being one of the most important predictor variables (pv's). Amongst the foraminifera, the low marsh species Miliammina fusca and high marsh Balticammina pseudomacrescens were determined to be the most significant pv's. For macrophytes, the low marsh species Carex lyngbyei, the high marsh species Juncus balticus, Shannon–Wiener Diversity Index (SDI) and absence of plant cover on the tidal flat were the most important pv's. As SMLR analysis of all individual groups and combinations of groups yielded statistically significant results, the choice of proxies, or combinations of proxies that are suitable for paleo-sea level research is at the discretion of the researcher.

Do competition and selective herbivory cause replacement of Phragmites australis by tall forbs?

We investigated the role of biotic factors in determining abundance of the low marsh species Phragmites australis and the high marsh species Epilobium hirsutum. In a 2-year field experiment, at a position where Phragmites and Epilobium co-occurred, responses of both species to each other’s removal were measured. In the second year, we also tested if larvae of Archanara geminipuncta, which feed exclusively on Phragmites shoots, affect the competitive ability of Phragmites relative to Epilobium. For both species, removal of aboveground material by clipping did not enhance shoot size or decrease variability in shoot size of the removed species itself. Surprisingly however, shoot numbers of both species increased after removal of the other, which demonstrates that there was a mutual inhibition of each other’s abundance. Comparing the responses of Archanara-infested and non-infested Phragmites shoots revealed no increased competitive suppression by Epilobium due to selective herbivory. Instead, we found that herbivore activity was lower in plots with Epilobium, which demonstrates that Archanara population size is reduced by the presence of non-host plant species. These results contradict the common assumption that biotic factors constrain a species upper limit along flooding gradients. Instead, our result suggest that different biotic interactions may counteract each other and thus slow down replacement by successive species.

COMPETITION AND SALT-MARSH PLANT ZONATION: STRESS TOLERATORS MAY BE DOMINANT COMPETITORS

Although a great deal of research has focused on the effects of nutrient supply on plant competition, few studies have explored how these processes interact with non-resource factors to determine community-level patterns. This study examined how resource competition interacts with physical stress to structure salt-marsh plant communities across a natural gradient in tidal stress. First, nutrient additions at naturally occurring species borders at zonal and patch boundaries in two Rhode Island (USA) marshes revealed that competitive outcomes were typically reversed when nutrients were abundant. These results, which are consistent with earlier findings in a third southern New England marsh, suggest that a nutrient-dependent competitive hierarchy is a general characteristic of salt marshes in this region. To test whether these shifts in competitive outcomes occur only at naturally occurring species borders or can lead to more significant shifts in zonation patterns, lower marsh species were transplanted into the matrix of each zonal species at higher tidal elevations, and the outcomes of plant competition in fertilized plots and unfertilized plots were compared. Results of this experiment indicate that nutrient effects on the competitive relations of marsh plants were independent of where the interactions took place along the tidal gradient. The stress-tolerant species were consistently the best competitors in fertilized treatments, showing that an increase in nutrient availability can lead to drastic shifts in the distributions of plants across marshes. Finally, a third experiment examined the interaction between nutrient supply and the aboveground and belowground components of plant competition using a reciprocal transplant design coupled with nutrient-addition and neighbor-removal treatments. Results suggest that competition is primarily belowground under ambient marsh conditions but is aboveground at high nutrient levels. Thus the mechanism underlying the nutrient-dependent competitive hierarchy may be driven by a trade-off between belowground and aboveground competitive abilities, although the potential interaction between above- and belowground effects was not examined. Together, the results of these experiments suggest that nutrient supplies may significantly affect the competitive dynamics between salt-marsh perennials and their resultant zonation across an environmental gradient in tidal stress. The result that stress tolerators can be dominant competitors is not predicted by any current model of plant competition and must be considered in future empirical and theoretical studies.

Are biotic factors significant in influencing the distribution of halophytes in saline habitats?

The influence of biotic factors on the distribution and establishment of halophytes is being considered in this review. Physicochemical factors, such as salinity and flooding, often are considered to be the determining factors controlling the establishment and zonational patterns of species in salt marsh and salt desert environments. Sharp boundaries commonly are found between halophyte communities even though there is a gradual change in the physicochemical environment, which indicates that biotic interactions may play a significant role in deterining the distribution pattern of species and the composition of zonal communities. Competition is hypothesized to play a key role in determining both the upper and lower limits of species distribution along a salinity gradient. Field and laboratory experiments indicate that the upper limits of distribution of halophytes into less saline or nonsaline habitats is often determined by competition. There appears to be a reciprocal relationship between the level of salt tolerance of species and their ability to compete with glycophytes in less saline habitats. Halophytes are not competitive in nonsaline habitats, but their competitive ability increases sharply in saline habitats. Allelopathic effects have been reported in salt desert habitats, but have not been reported along salinity gradients in salt marshes. Some species of halophytes that are salt accumulators have the ability to change soil chemistry. Chemical inhibition of intolerant species occurs when high concentrations of sodium are concentrated in the surface soils of salt desert plant communities that are dominated by salt-accumulating species. Establishment of less salt-tolerant species is inhibited in the vicinity of these salt-accumulating species. Herbivory is reported to cause both an increase and a decrease in plant diversity in salt marsh habitats. Heavy grazing is reported to eliminate sensitive species and produce a dense cover of graminoids in high marsh coastal habitats. However, in other marshes, grazing produced bare patches that allowed annuals and other low marsh species to invade upper marsh zonal communities. A retrogression in plant succession may occur in salt marshes and salt deserts because of heavy grazing. Intermediate levels of grazing by sheep, cattle, and horses could produce communities with the highest species richness and heterogeneity. Grazing by geese produced bare areas that had soils with higher salinity and lower soil moisture than vegetated areas, allowing only the more salt-tolerant species to persist. Removal of geese from areas by use of inclosures caused an increase in species richness in subarctic salt marshes. Invertebrate herbivores could also inhibit the survival of seeds and the ability of plants to establish in marshes. Parasites could play a significant role in determining the species composition of zonal communities, because uninfected rarer species are able to establish in the gaps produced by the death of parasitized species.

The response of some rain forest insect groups to logging and consersion to plantation: J. D. Holloway, A. H. Kirk-Spriggs & Chey Vun Khen, Philosophical Transactions — Royal Society of London, B, 335(1275), 1992, pp 425–436

Public Service Enterprise Group's (PSEG) Estuary Enhancement Program (EEP) restored normal daily tidal inundation and drainage to three previously diked farm sites totaling 1780 ha during the period 1996–1998 along the shoreline of the Delaware Estuary in New Jersey, USA. Normal tidal exchange was either prevented or altered by the construction of dikes and water control structures that were used to promote the growth of high marsh “salt hay” species, such as salt meadow cordgrass (Spartina patens) and salt grass (Distichlis spicata) at elevations below mean high tide. These elevations would normally support low marsh species, such as smooth cordgrass (Spartina alterniflora). Prior to restoration these sites were either un-vegetated due to excessive ponding associated with poor drainage or vegetated with “salt hay” species as well as common reed (Phragmites australis). Extensive S. alterniflora marshes occur in the vicinity of each site at similar elevations and these marshes were relied on as the source of seed for the re-establishment of S. alterniflora on the restoration sites; thus, no other planting or seeding was implemented to initiate re-vegetation. Vegetation coverage at the restoration sites as well as a naturally restored reference marsh was determined during 1996–2003 utilizing geo-referenced remote sensing data and geographic information system (GIS) analyses. Coverage by S. alterniflora or other desirable low marsh vegetation exceeded that required by New Jersey authorities to meet the restoration goal at two sites 8 years ahead of the anticipated 12-year period and re-vegetation of the third site is expected to meet its goal within the established 12-year period. Annual vegetation coverage data have been integral components of an adaptive management program utilized by PSEG to manage the restoration process at each of the sites.

Water-depth tolerances of the dominant emergent macrophytes of the Delta Marsh, Manitoba

The growth (shoot height, cumulative shoot length, shoot density, above- and below-ground biomass) of seven emergent species growing at five different water depths was measured for 2 years. These species belonged to three different ecological classes: (i) upper marsh species (Carex atherodes, Scolochloa festucacea, and Phragmites australis) that occupy sections of the water-depth gradient that are only seasonally flooded in the Delta Marsh; (ii) lower marsh species (Typha glauca and Scirpus lacutris spp. glaucus) that occupy permanently flooded areas; and (iii) drawdown species (Scirpus lacustris spp. validus and Scirpus maritimus) that become established temporarily during drawdowns. Upper marsh species could not adjust their shoot length if they were growing in water deeper than 20 cm. Lower marsh species were able to do this in water up to 70 cm deep. All three types survived for 1 or 2 years in water too deep for long-term persistence. Scirpus species survived as tubers in areas with water too deep for them to grow. The distributions of the seven species in the experiments overlapped considerably, and all species cooccurred at water depths to 70 cm after 2 years of flooding. The predicted distribution in the four dominant species along a water-depth gradient were consistent with their actual distributions in the Delta Marsh, but their predicted distributions overlapped much more than is the case in the field. Key words: emergent vegetation, experiment, water-depth tolerance, plant growth, distributions.

Community structure of an estuarinemeiobenthic copepod assemblage

The community organization of the meiobenthic copepods of two sub-tidallocations was compared with that of a low marsh, intertidal site in the same South Carolina estuary, over a 12 month period. Twenty-four collections of 4–8 replicate cores were gathered from the soft sediments between Spartina alterniflora plants. Total numbers of post-naupliar copepods ranged from 75 to 620·10 cm −2 . Of the 2 cyclopoid and 19 harpacticoid species, 8 comprised 97% of the fauna. The 4 most abundant species had summer or late fall peaks when copepods were most abundant, while 4 others had their minimal densities at this time. The monthly mean H′, J′ and number of species (1·50, 0·63 and 11·2, respectively) were not statistically different from the diversity values at the two subtidal locations. During the summer and fall months, horizontal, between replicate spatial correlations among the 3 most abundant low marsh species were very strong. Horizontally, Stenhelia (Delavalia) bifidia and Microarthridion littorale were negatively correlated, S. (D.) bifidia and Enhydrosoma propinquum positively correlated, while E. propinquum and M. littorale showed no interaction among the 24 collections. Based on a lack of readily identifiable microhabitat specializations, some combination of predation and physical disturbance seem the most likely regulators of the copepod assemblage at this low marsh site.