Effects of abiotic factors on the life span of the invasive cordgrass <Emphasis Type="Italic">Spartina densiflora</Emphasis> and the native <Emphasis Type="Italic">Spartina maritima</Emphasis> at low salt marshes

We analyzed variations in the life span of the invasive cordgrass Spartina densiflora at low marshes of SW Iberian Peninsula, and identified the abiotic factors limiting the plant in the absence of competition. With these objectives, clump survivorship, flowering, and growth of S. densiflora were studied in two natural populations at different low marsh elevations during more than three years, and at a transplant experiment in comparison with the native Spartina maritima. The life spans of both cordgrasses changed depending on small variations of a few centimeters in elevation. S. maritima, which tolerates better than S. densiflora the stressful abiotic environment of lower marshes, showed a significant lower distribution limit for its perennial habit, with survivorship longer than three years (from 1997 to 2000), than the neophyte (+1.57 m SHZ vs. +2.00 m SHZ). S. densiflora clumps flowered before dying at mostly all elevations, showing low relative growth rates. In contrast, clumps of S. maritima, with non-viable seeds, only flowered when they were three years old at higher elevations in the low marsh. Our results have applications for salt marshes bioengineering projects and to prevent S. densiflora from invading European marshes since our data improve the knowledge of its colonization mechanisms through salt marsh zonation and so identify those portions of restored and native marshes most susceptible to invasion due to the establishment of perennial populations.     

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.     

Plant zonation at salt marshes of the endangered cordgrass <Emphasis Type="Italic">Spartina maritima</Emphasis> invaded by <Emphasis Type="Italic">Spartina densiflora</Emphasis>

The South American cordgrass Spartina densiflora is invading European salt marshes getting into contact with the indigenous and endangered low-marsh dominant, Spartina maritima. This work describes the evolution of the plant zonation during 7 years in a marsh of S. maritima invaded by S. densiflora. S. maritima appeared throughout the whole intertidal gradient from 1.72 to 3.33 m over Spanish Hydrographic Zero (SHZ), showing its higher biomasses and shoot densities at low elevations. In contrast, S. densiflora only invaded upper areas (>+2.59 m SHZ) at the centre of circular tussocks of S. maritima. Above-ground biomass of S. maritima dropped drastically at maximum occupation of space by the alien, and its shoot density and above-ground biomass decreased at S. densiflora zone during the study. The competitive potential of S. densiflora was reflected in high above- and below-ground biomass and shoot densities, accompanied by elevated wrack accumulation and the absence of other marsh plants presented together with S. maritima from areas dominated by S. densiflora. S. densiflora altered the native vegetational zonation pattern through the invasion of the centre of S. maritima tussocks; however, the alien invasion may be limited by the presence of the autochthonous cordgrass at lower elevations. Handling editor: Luis Mauricio Bini     

Interactive effects of elevation and burial with wrack on plant community structure in some Rhode Island salt marshes

1 The effects of disturbances and elevation on marsh plant communities were examined using experimental disturbances along an elevation gradient in marshes with different disturbance histories. In addition, differences in species composition among five marshes were determined at elevations at which the greatest concentration of burial by wrack occurred.
2 Experimental wrack burial generally caused significant mortality of the high-marsh competitive dominants, Juncus gerardi and Spartina patens , and strongly increased the abundance of the fugitive perennial, Distichlis spicata .
3 The effects of experimental wrack burial interacted strongly with abiotic factors associated with elevation to influence the distributions of both competitive dominants and annual fugitive plants.
4 Frequent wrack burial in a marsh appears to lead to a persistent assemblage of plants dominated by competitively subordinate fugitives. This assemblage of fugitives tends to occur at intermediate elevations within the marsh, where wrack gets stranded for long periods of time and where the resistance of Juncus gerardi to wrack burial is lowest.
5 We suggest that wrack-burial disturbances interact strongly with marsh elevation to influence the zonation of plants in New England salt marshes, and discuss some implications of our results.     

Vegetation and ecological characteristics of the northernmost salt marshes of the European continent

We surveyed 17 locations of salt marshes along the Barents Sea coast in northern Norway (Finnmark, Nordkinnhalvøya and Varangerhalvøya), where 86 phytosociological relevés were recorded and analyzed. Two main vegetation groups were identified: Caricion glareosae and Puccinellion phryganodis, both alliances belonging to the class Juncetea maritimi, order Puccinellietalia phryganodis. The first, Caricion glareosae (upper marshes) is subject to shorter‐term flooding and is saturated by brackish water in river estuaries on sandy or sandy gravel soils. Four units were distinguished: Festuceto‐Caricetum glareosae, Caricetum mackenzie, Caricetum salinae and the Plantago maritima × borealis community. The second group (lower marshes on fine muddy surface, higher salinity, inundated for a longer period directly by the sea) belongs to the alliance Puccinellion phryganodis which encompasses two units: the associations Caricetum subspathaceae and Puccinellietum phryganodis. The floristic composition and habitat preferences of each vegetation cluster are described and our study revealed that there are no significant differences in the vegetation composition of salt marshes beyond the Arctic line and the southern subarctic sites. The observed variations are related to the geomorphology of the sites rather than to their latitude. The comparison of historical records with our recent data demonstrates the great stability of habitat conditions and species composition. The salt marshes studied meet all criteria of good quality and we found no evidence for any threatening trends like those seen in most other habitats in Europe.     

Impact of different sheep grazing intensities on salt marsh vegetation in northern Germany

Abstract. Mainland salt marshes in Schleswig-Holstein (northern Germany) have been grazed intensively by sheep for several decades. In 1988 experimental sites were established in the lower and middle salt marsh of Sönke-Nissen-Koog and subjected to different grazing intensities. From 1989 to 1993 the impact of sheep grazing on the composition and structure of the salt marsh vegetation was studied through the yearly analysis of permanent plots, vegetation mapping and measurements of the vegetation height. The intensively grazed site (10 sheep/ha) is covered by a short monotonous Puccinellia maritima sward with Salicornia europaea and Suaeda maritima. Halimione portulacoides and Aster tripolium, especially flowering plants, are rare. On the sites with 1.5 and 3 sheep/ha Puccinellia maritima remained dominant. The population density of Salicornia europaea decreases after reduction of the grazing intensity, whereas Suaeda maritima finds optimal growing conditions. Stands of Halimione portulacoides and flowering Aster tripolium plants are rare near the sea dike but their cover and size increases further away from the dike. In the plot with 1.5 sheep/ha the height of the vegetation increases along the gradient from the dike towards the tidal flats, due to local differences in actual grazing intensity. When grazing is stopped, Puccinellia maritima is successively replaced by Festuca rubra, Halimione portulacoides and Aster tripolium. High variability of vegetation height indicates structural diversity. Patches of higher and lower vegetation correspond with the distribution pattern of different plant species. In terms of nature conservation cessation of grazing is recommended.     

Variation with depth and season in metal sulfides in salt marsh soils

Seasonal monitoring of metal sulfides was carried out in four soils ofthe Ría de Ortigueira salt marshes. Soils from the high salt marsh (withsuboxic redox conditions at the surface), had low concentrations of ironsulfides (AVS and pyrite fraction) and thus a low degree of trace metalpyritization (DTMP) in surface layers (0–10 cm), butconcentrations of metals associated with the pyrite fraction increasedconsiderably at depth (27.5 cm). In the low salt marsh soils (withanoxic conditions at the surface) maximum concentrations of metal sulfides werefound in the surface layers of soils colonized by Spartina maritima. These results are explained by the double effectexerted by roots in strongly reduced soils. On the one hand, they stimulate theactivity of sulfate-reducing bacteria and on the other, they favour the partialoxidation of the soil, thus generating polysulfides with which Fe²⁺immediately precipitates as pyrite, whereas in the deepest, permanently anoxiclayers, pyrite must be formed in a reaction in which FeS is an intermediate, asfollows: FeS + H₂S FeS₂ + H₂.Concentrations of metal sulfides also varied greatly with the season, with twopatterns being distinguished. In soils colonized by S. maritima in both high and low salt marshes, the lowestconcentrations were found in summer. At this time of the year there is a netloss of metal sulfides throughout the profile, presumably due to physiologicalactivity of plants (evapotranspiration and release of oxygen from roots). Incontrast, maximum concentrations of AVS and pyritic metals were found in thesummer in the low salt marsh soils not colonized by vascular plants (creekbottom). In this case, the higher temperatures increased the activity ofsulfur-reducing bacteria leading to synthesis and accumulation of metalsulfidesin the soil.     

Constraints on Salt Marsh Development Following Managed Coastal Realignment: Dispersal Limitation or Environmental Tolerance?

Salt marshes restored through managed coastal realignment (MR) often develop slowly and show persistent differences in vegetation from natural marshes. Development might be constrained by the availability of propagules or poor suitability of the abiotic environment for their establishment. To distinguish between these factors, we compared vegetation colonization and environmental conditions at a salt marsh created by MR at Brancaster, Norfolk, UK, with five reference marshes, varying in age from 30 to circa 6,000 years. After 5 years, plant communities of the MR site remained different from those in mature reference marshes. In contrast, the communities of the youngest reference marsh were not significantly different from mature reference marshes. At the MR site, abundance of perennial and later‐successional species was low and large areas remained unvegetated. These differences are unlikely to be due to dispersal limitation, because 76% of the species from the local species pool colonized the site within 5 years. Although the annuals Salicornia europaea and Suaeda maritima were abundant by year 2, they were not ubiquitous until the end of the study. Tidal elevations of the MR site were suitable for vegetation development, but soil redox potentials were lower than that at the reference sites. Reducing conditions in the MR site appear to be the major cause of vegetation differences from the reference marshes, as they are associated with an abundance of bare ground and a small range of vegetation clusters. Measures to avoid low sediment redox potentials may have a great benefit in some salt marsh restoration projects.     

Rafting on wrack as a mode of dispersal for plants in coastal marshes

Here, I examine the potential for dead plant material, or wrack, to act as a vector of dispersal for plants among coastal marshes along the Palmer River in New England, USA. Three hundred mats of wrack floating along a 2.5 km section of river spanning tidal fresh, brackish and salt marshes were sampled for propagules of plants from May 1997 to October 1998. Thirty species of plants and 2 species of algae were associated with the wrack, and 24 of the plant species were viable under greenhouse conditions. Common and abundant species of tidal fresh, brackish and salt marshes were found in the wrack, as well as introduced and invasive species. Wrack contained plants suited to different modes of dispersal, including water, wind and adherence to animals. Measurements of the movement of individual mats of wrack along a 200 m section of the river indicated that wrack travelled at speeds ranging from 1.1 to 2.6 km h⁻¹, which translates to potential dispersal distances ranging from 6.5 to 15.9 km for one ebb or flood tide of 6 h duration. Tagged mats of wrack tracked for one ebb or flood tide travelled at least 2.5 km along the river. These results suggest that wrack may be an important agent of plant dispersal among coastal marshes in estuaries.     

The performance of the leaf mining microlepidopteran Bucculatrix maritima (Stt.) on the salt marsh halophyte,Aster tripolium (L.), exposed to different salinity conditions

Summary The performance of phytophagous insects is influenced by the nutritional quality of the food plant, which may vary with environmental conditions. Hardly any information exists on food-plant mediated effects of variable soil salinity on the performance of phytophagous insects. Conspicuous differences in salinity levels, however, are found in soils of intertidal wetlands such as salt marshes and mangroves. The growth of larvae of Bucculatrix maritima, a leaf miner of the salt marsh halophyte Aster tripolium, was studied on the host plant along the salinity gradient of the Westerschelde estuary (S.W. Netherlands). In addition, its performance on A. tripolium grown on low or high salinity culture medium was investigated experimentally. Although salinity conditions significantly influenced the chemistry of the host plants, insect performance seemed almost unaffected, although near the mouth of the estuary high environmental salinities may have caused some inhibition of larval growth. The results contrast with our previous studies on the stem-borer Agapanthia villosoviridescens, which showed that growth and development was conspicuously influenced by the changing characteristics of Aster tripolium along the estuarine salinity gradient. The location-dependent qualities of halophytes in an estuary thus appear to have species-specific effects on insect performance. We hypothesize that this phenomenon contributes to the existence of non-identical distribution patterns of phytophagous insects associated with the same halophyte in an estuary.     

Utilisation of Wadden Sea salt marshes by geese in relation to livestock grazing

To arctic breeding geese, the salt marshes of the International Wadden Sea are important spring staging areas. Many of these marshes have always been grazed with livestock (mainly cattle and sheep). To evaluate the influence of livestock grazing on composition and structure of salt-marsh communities and its consequences for habitat use by geese, a total of 17 pairs of grazed and ungrazed marshes were visited both in April and May 1999, and the accumulated grazing pressure by geese was estimated using dropping counts. Observed grazing pressure was related to management status and to relevant vegetation parameters.The intensity of livestock grazing influences the vegetation on the marsh. Salt marshes that are not grazed by livestock are characterised by stands with a taller canopy, a lower cover of grasses preferred by geese, and a higher cover of plants that are not preferred.Overall goose-dropping densities are significantly lower in ungrazed marshes compared to marshes grazed by livestock. Some ungrazed marshes had comparatively high goose grazing pressure, and these were all natural marshes on a sandy soil, or artificial mainland marshes with a recent history of intensive livestock grazing. Goose grazing is associated with a short canopy. The plant communities with short canopy, dominated by Agrostis stolonifera, Festuca rubra and Puccinellia maritima, together account for 85% of all goose droppings in our data.The sites that were not visited by geese differed very little from those that were visited, in the parameters we measured. This might indicate that there was no shortage of available habitat for spring staging geese in the Wadden Sea, in the study period.     

Salt-marsh vegetation in the Shetland Islands

Details are given of a preliminary study of salt marshes near Sullom Voe, Mainland, Shetland, using 50×50 cm quadrats placed systematically along transects. Computer-generated clusters are shown to match well against generally accepted syntaxa, whilst high-similarity clusters in certain alliances possess internal structure related to dominance and to effects of freshwater irrigation. The syntaxa provisionally identified are the Eleocharion uniglumis, Armerion maritimae, Puccinellion maritimae, and grazed cliff top grasslands showing affinity to the Puccinellio-Spergularion salinae. Computer clusters with Puccinellia maritima and much Fucus muscoides are associated with active pioneer grass growth and with slumping and erosion on wet marshes, whilst more species-rich clusters reflect a loss of vigour in Puccinellia maritima at higher elevations. Drier stonier marshes also display depositional and erosional features, the former being associated with active Puccinellia maritima growth at lower levels on the marsh. It is suggested that most of these processes involve the recycling of local marsh sediment material.Nomenclature follows Clapham et al. (1981) for the angiosperms, Parke & Dixon (1976) for the algae, and Adam (1981) for plant communities.My thanks are due to the Sheland Oil Terminal Environmental Advisory Group for allowing me to make use of data obtained during contract work for them at Sullom Voe, and to the Nature Conservancy Council and Institute of Terrestrial Ecology for permission to refer to their report on Shetland cliffs and salt marshes. Thanks are due to the Central Research Fund of the University of London and to the Department of Pure and Applied Biology, Imperial College for financial assistance. Computing was done at the Imperial College forputer Centre. I would also like to thank R. G. Davies for lending me some of his FORTRAN IV programs, and my wife for assistance with the field work.     

Mobility of metals in salt marsh sediments colonised by <Emphasis Type="Italic">Spartina maritima</Emphasis> (Tagus estuary,Portugal)

Chemical associations of Zn, Pb, Cu, Co and Cd were determined using a sequential extraction procedure in sediments colonised by S. maritima in three salt marshes within the Tagus estuary: Rosário, Corroios and Pancas. Concentrations of these metals were also analysed in above- and belowground parts of Spartina maritima, as well as in sediments colonised by the plant. The highest metal concentrations in sediments were found in the marshes near the industrial and urban areas, whereas metal concentrations in plants were not significantly different among sites. This was thought to be a consequence of differences observed in metal bioavailability: Metals in Pancas, the least polluted location, were largely associated to easily accessible fractions for plant uptake, probably as a result of low organic matter content and high sandy fraction in sediments. S. maritima was able to induce the concentration of metals between its roots in the three salt marshes. The results obtained in this study indicate that S. maritima could be useful to induce phytostabilisation of metals in sediments, although the effectiveness to modify chemical associations is highly dependent on existing sediment parameters, and thus different results could be obtained depending on site characteristics. Guest editors: J. Davenport, G. Burnell, T. Cross, M. Emmerson, R. McAllen, R. Ramsay & E. Rogan Challenges to Marine Ecosystems     

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.     

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.     

Long‐term vegetation changes in experimentally grazed and ungrazed back‐barrier marshes in the Wadden Sea

Abstract. Vegetation succession in three back‐barrier salt marshes in the Wadden Sea was studied using a data set comprising 25 years of vegetation development recorded at permanent quadrats. The effect of livestock grazing on succession was assessed by comparing quadrats where grazing was experimentally prevented or imposed. We studied changes at the species level as well as at the level of the plant community. Special attention is given to effects on plant species richness and community characteristics that are relevant for lagomorphs (hares and rabbits) and geese. Inundation frequency and grazing were most important in explaining the variation in species abundance data. The three marshes studied overlap in the occurrence of different plant communities and the observed patterns were consistent between them. Clear differences in frequency and abundance of plant species were observed related to grazing. Most plant species had a greater incidence in grazed treatments. Species richness increased with elevation, and was 1.5 to 2 × higher in the grazed salt marsh. Grazing negatively influenced Atriplex portulacoides and Elymus athericus, whereas Puccinellia maritima and Festuca rubra showed a positive response. The communities dominated by Elymus athericus, Artemisia maritima and Atriplex portulacoides were restricted to the ungrazed marsh. Communities dominated by Puccinellia maritima, Juncus gerardi and Festuca rubra predominantly occurred at grazed sites. As small vertebrate herbivores prefer these plants and communities for foraging, livestock grazing thus facilitates for them.     

Flood tolerance and the distribution of Iva frutescens across New England salt marshes

Summary Tidal flooding is widely believed to be an important determinant of marsh plant distributions but has rarely been tested in the field. In New England the marsh elder Iva frutescens often dominates the terrestrial border of salt marshes and we examined its flood tolerance and distribution patterns. Marsh elders only occur at elevations where their roots are not subject to prolonged water table flooding. Consequently they are found on the terrestrial border of marshes and at lower elevations associated with drainage ditches and locally elevated surfaces. Marsh elders transplanted to elevations lower than they normally occur died within a year with or without neighbors and greenhouse tests revealed that I. frutescens is much less tolerant of flooded soil conditions than plants found at lower marsh elevations. We also manipulated the water table level of field plots and found that increasing or decreasing water table drainage led to enhanced and diminished I. frutescens performance, respectively. Our results demonstrate the importance of water table dynamics in generating spatial patterns in marsh plant communities and provide further evidence that supports the hypothesis that the seaward distributional limits of marsh plant populations are generally dictated by physical processes.     

Niche expansion, body size, and survival in Galápagos marine iguanas

Foraging theory predicts that dietary niche breadth should expand as resource availability decreases. However, Galápagos marine iguanas often die during algae shortages (El Niños) although land plants abound where they rest and reproduce. On Seymour Norte island, a subpopulation of iguanas exhibited unique foraging behavior: they consistently included the succulent beach plant B. maritima in their diet. We investigated the consequences of land-plant feeding for body size and survival. Batis-eaters supplemented their algae diet both before and after intertidal zone foraging, and more Batis was eaten during tides unfavorable for intertidal zone foraging (dawn and dusk). Larger, energy-constrained iguanas fed more on land than did smaller animals. Compared to intertidal zone algae, Batis was 39% lower in caloric content (1.6 vs. 2.6 kcal g^–1 dry mass), 56% lower in protein (8.3 vs. 18.9% dry mass) and 57% lower in nitrogen (1.3 vs. 3.0% dry mass). In spite of its lower nutrient value, iguanas that supplemented their diet with this plant were able to attain nearly twice the body size of other iguanas on the island. Age estimates indicate that many Batis-eaters survived repeated El Niño episodes during which animals of their relative size-class experienced high mortality on other islands. The larger animals were, however, completely dependent upon this supplementary source of food to maintain condition, and all perished in the 1997–1998 El Niño when high tides inundated and killed Batis on Seymour Norte Island. We hypothesize that Batis feeding developed as a local foraging tradition, and that dietary conservatism and strong foraging site fidelity explain why the inclusion of land plants in the diet has been observed in only a single population. Ultimately, a unique algae-adapted hindgut morphology and physiology may limit a switch from marine to terrestrial diet.     

Changes in the species richness,spatial pattern and species frequency associated with the decline of oak forest

Nomenclature: follows Flora Europea (Tutin et al. 1964–1980). Since the extension of the irrigation system, the water regime of most of the permanent marshes of the Camargue (southern France) have been intensively controlled. Considerable quantities of nutrient rich Rhone water are pumped into these marshes, leading to lower salinities and a higher biomass production and consequently an increasing organic matter concentration of the sediments. Myriophyllum spicatum has become abundant in these permanent marshes since large quantities of freshwater entered these systems. It has displaced Potamogeton pectinatus in several of these marshes. The different factors likely to influence the distribution of P. pectinatus and M. spicatum were investigated experimentally. The impact of Cl^- concentrations between 0 and 6 g l^-1 on the biomass production of both species was tested. P. pectinatus appears to be more salt tolerant than M. spicatum. The influence of sediment quality on the biomass production of both species was investigated using six sediments differing in organic matter concentration. Compared to P. pectinatus, M. spicatum had a lower total biomass production when grown on sediments with low organic matter concentration (2–4% organic matter) and a higher biomass production on sediments with relatively high organic matter concentration (9–13% organic matter).Nitrogen addition to the sediments yielded an increased biomass production of P. pectinatus and M. spicatum. On some sediments M. spicatum needed higher concentrations of nitrogen than P. pectinatus to increase its biomass production.The creation of freshwater marshes by the introduction of irrigation water, resulting in lower salinities and an increase in sediment organic matter concentration, stimulates the biomass production of M. spicatum.As M. spicatum grows less well on poor sediments and at higher salinities it seems to be unable to displace P. pectinatus in more natural systems in the Camargue.     

Germination and early establishment of lower salt-marsh species in grazed and mown salt marsh

Abstract. Lower salt-marsh species occur more in the grazed than in the mown sites of a salt marsh in Schiermonnikoog in the Netherlands. This was demonstrated by a sowing experiment which determined which characteristic of the stand structure, height of the canopy or percentage of bare soil, is responsible for this difference. The total number of seedlings which emerged was negatively related to the height of the canopy and positively to the percentage of bare soil. The survival of seedlings of Suaeda maritima and Plantago maritima could be explained by the height of the canopy and for the latter species also by the percentage of bare soil. The survival of Aster tripolium and Plantago maritima was higher in grazed than in mown sites. Since the amount of bare soil was higher than seemed necessary for germination and survival, it was concluded that the taller canopy was responsible for the absence of lower salt-marsh species in the mown sites.