Inundation frequency determines the post-pioneer successional pathway in a newly created salt-marsh

The effect of inundation frequency on plant community composition, species turnover, total and growth form cover, species richness and abundance of individual common species was investigated. The study area was a newly created salt-marsh located along the Belgian coast with a more or less continuous gradient of inundation frequencies from 0.01% of all high waters for highest elevations to 100% for lowest elevations. Cover of all plant species was estimated in 119 permanent 2m × 2m plots along six randomly chosen transects perpendicular to the main inundation gradient with a 3m distance between the plots in 2003, 2005 and 2007. Detrended correspondence analysis (DCA) scores were used as a proxy for plant composition. Total cover, the cover of annuals and perennials, total species richness, species richness of annuals and perennials and species turnover was calculated for each plot. Repeated measurements and LSD were used to compare all variables in three different years for different inundation frequencies. In addition, TWINSPAN was used to distinguish plant communities in different years.The results showed that plant composition changed differently according to inundation frequency. The cover of annual species increased at a higher pace at higher inundation frequencies. The cover of perennials increased at higher pace at lower inundation frequencies. In total, species richness and the abundance of most species increased in time, indicating absence of a competitive exclusion among species. Nevertheless, the abundance and frequency of Atriplex spp., Chenopodium spp. and Salsola kali strongly decreased in time, indicating a declining natural succession. It seems that perennial species (e.g. Elymus athericus) are spreading by vegetative propagules from upward to downward. Frequent inundations hampered plant species turnover, because of the low number of species that can tolerate that environmental condition. The frequencies of communities dominated by Elymus athericus and Salicornia procumbens strongly increased in time, indicating that these species are getting more and more spatially separated, leading to a stronger separation of plant communities and an appearance of a salt-marsh zonation

The effect of successional stage and salinity on the vertical distribution of seeds in salt-marsh soils

Seed bank density and similarity between seed bank and above-ground vegetation along depth were compared between two salt-marshes different in age. In addition, the effect of salinity on the variation in seed bank density and similarity between seed bank and aboveground vegetation along depth was investigated. The study was conducted in an euhaline saltmarsh that contained both old and newly created habitats. In addition, two other old saltmarshes were selected to study the effect of salinity on the distribution pattern of seed bank and similarity between seed bank and above-ground vegetation at different soil depths. One of them was mesohaline, the other euhaline.Ten plots of 2m × 2m were situated in the new salt-marsh (existing since 2002) and 80 plots in the three old salt-marshes. Soil samples were collected at three different depths (0- 5cm, 5-10cm and 10-15cm) in spring 2006. After washing by fresh water, the soil samples were spread in the greenhouse to allow viable seeds to germinate. Germination experiments lasted 6 months and all seedlings were identified and removed after identification. Aboveground vegetation composition was determined during the growing season in all plots. Viable seed density was calculated for each plot and for the three different depths; the similarity between seed bank and floristic composition of the above-ground vegetation was calculated. A general linear model was used to investigate the effect of soil depth, age and salinity of the salt-marsh on density and similarity between seed bank and above-ground vegetation.The results showed that seed density decreased with depth in all salt-marshes irrespective of their age and soil salinity. Seed density and similarity between seed bank and above-ground vegetation were higher in the new salt-marsh than in the old one in the same study area. This is because in young as well as in old successional stages, the seed bank was mostly composed of new colonizers, while most perennial species were absent from the soil seed bank, although they were dominant in the above-ground vegetation of the old salt-marsh.The characteristics of the seed bank of a mesohaline salt-marsh were found not to be fundamentally different from that of both euhaline salt-marshes

Environmentally determined spatial patterns of annual plants in early salt-marsh succession versus stochastic distribution in old salt-marsh conditions

It is generally accepted that in terrestrial ecosystems the occurrence and abundance of plant species in late succession stages can be well predicted from prevailing soil conditions whereas in early succession their presence is much more influenced by chance events (e.g. propagule availability). In other words late successional vegetation stages would be deterministically structured, while early succession stages would be of a more stochastic nature.To test this hypothesis, we compared the effect of abiotic environmental factors on vegetation composition and probability of occurrence of single species in two adjacent saltmarshes, differing in age (successional stage). A new salt-marsh of about 14ha was created in 2002 by removing a several meters thick layer of sand and slurry which was deposited on the major part of the salt-marsh along the IJzer estuary in the 1960s. From 2002 onwards, primary colonization started on that sterile substrate by hydrochoric seed dispersal, induced by tidal water currents from an adjacent 4ha old salt-marsh and the latter remained untouched as saltmarsh for more than two hundred years. Between mid August and the end of September 2005, three years after the start of the colonization in the new salt-marsh, vegetation and three abiotic environmental factors (soil texture, salinity and elevation) were described in a set of 155 plots (2m × 2m), distributed over the new and the old salt-marsh.In contrast to the general rule for terrestrial ecosystems, the vegetation composition of the early succession stage of the new salt-marsh appears to be at least as much determined by the combined effect of the measured abiotic factors as that of the old salt-marsh. As revealed by logistic regression the presence/absence of perennial species as well as annual species of the young salt-marsh can be well predicted by the measured abiotic environment. For the old salt-marsh, this also holds for the perennials, but not for the annuals. The stochastic appearance of gaps in the perennial vegetation cover appears to be important for the establishment of annuals in the older salt-marsh

Short-term impact of sheep grazing on salt-marsh vegetation succession in a newly created salt-marsh site

In this paper, the effect of three winter seasons of sheep grazing on cover, composition and species richness of vegetation, Elymus athericus expansion and forage quality of saltmarsh species were studied. Four zones were selected: three in real salt-marsh habitat (low, intermediate and high levels) and the fourth in the transition between salt-marsh and sand dune habitat. In each of the three salt-marsh zones, one site was selected and two sites were designated in the transitional zone. Half of each site was excluded from grazing (so-called exclosures) all year round, while the other half was accessible to sheep from mid-August until mid-June (enclosure areas). At each zone, 10 plots (2m × 2m) were established: five within exclosure and five within enclosure sites. In all plots, the cover of all species was estimated in the growing season in 2005 (initial state) and 2007 (state after two years of grazing). In addition, a total of 1516 quadrates (50cm × 50cm) were used to harvest the biomass of species to estimate the forage quality variation during the grazing period inside the plots. The forage quality variables were the percentages of crude protein, acid detergent fibre and neutral detergent fibre. The data of the cover of dominant salt-marsh species, species richness, total cover, plant composition and forage quality parameters are compared between exclosure and enclosure plots and between two sampling years (2005 and 2007) using repeated measurements (general linear model) separately for each zone. The results showed that after three years of plant succession, sheep grazing had a positive influence on plant richness on the high part of the salt-marsh, a negative effect in the transitional zone and no effect in the other zones. Grazing had no effect on cover and plant composition. Forage quality parameters were affected by sheep grazing only in the low salt-marsh zone. Limonium vulgare had the highest forage quality andElymus athericus had the lowest. It would appear that grazing with the current intensity and number of grazers would fail to hamper the expansion of Elymus athericus. A higher intensity of mixed sheep-cattle or cattle grazing would be needed to better control this highly prolific species

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

1. 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.  2. Elevation in the tidal frame and sediment redox potential are major controls on the distribution of saltmarsh plants. We use niche models to characterize ten species’ responses to these, and test whether differences in species occurrence between restored and natural saltmarshes in the UK result from failure to re-create adequate environmental conditions.  3. 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.  4. 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 at higher elevations 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.  5. 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

Spatio-temporal aspects of early vegetation succession in a recently restored salt-marsh ecosystem: a case study of the IJzer estuary (Belgium)

Plant succession is the change in species composition or three-dimensional architecture of the plant cover of a specified place through time or the changes observed in an ecological community, following a perturbation that opens up a relatively large space. Vegetation succession has temporal and spatial aspects and results from many causes and processes, particularly the colonization, growth and mortality of organisms under particular environmental conditions. Successional vegetation processes are an important aspect in ecological restoration, because they determine the final success, and hence, the type and timing of restoration measures. Therefore, vegetation succession and its study should be taken into account in virtually every restoration program. Beside the importance of vegetation succession studies in restoration programs, newly (by man or naturally) created substrates offer us unique opportunities to study plant succession fundamentally, in which salt-marshes are a remarkable habitat. The main objective of this study is to investigate on the characteristics of new colonizers in a newly created salt-marsh and the effect of the most important abiotic (inundation frequency, salinity and texture) and biotic factors (propagule availability and sheep grazing) on vegetation succession in time. The effect of stochastic and deterministic factors on spatial vegetation succession was compared between newly created salt-marsh and old salt-marshes. In addition, seed bank density and similarity between seed bank and above-ground vegetation and their differentiation with depth were compared between salt-marshes in different successional stages (old and newly created). The results showed that viable seed availability might be the most important constraint for plant species to act as early colonizers, and the development of salt-marsh target species could be restricted by limited viable seed production and also unfavourable soil conditions. This study confirms the importance of a nearby salt-marsh to a restoration site and the importance of a continuous water bridge between seed source and sink. The lack of colonization success of some species is most probably caused by the low connectivity between source and sink. Plant composition within the restoration site (newly created salt-marsh) changed over time. In areas of the site with a higher inundation frequency, the rapid expansion of some species (e.g. Suaeda maritime), the appearance of new species (e.g. Salicornia procumbens), and change in the abundance of other species (e.g. Salicornia europaea) resulted in a variation in plant composition. At lower inundation frequencies, the expansion of some species (e.g. Elymus athericus), and turnover of others, resulted in a change in species composition; higher turnover and higher expansion of perennials at lower inundation frequencies. In general, species turnover was lower at higher inundation frequency.The results showed that sheep grazing had some effects on vegetation succession physically and chemically despite the very short time since grazers were introduced. Species richness increased in Elymus athericus- dominated community and decreased in some parts, after sheep were introduced. In addition, sheep grazing changed forage quality in time. Nevertheless, species composition and the cover of dominant species did not change significantly through sheep grazing while natural plant succession (in composition and coverage) was going on.It appears that in our salt-marsh patches, deterministic factors (electric conductivity and texture of soil and elevation) are important in both early and late successional stages. However, annual species in old salt-marsh might germinate in the stochastically appearing vegetation gaps, unrelated to the deterministic factors, where competition with perennials does not prevent the colonization. Seed density was higher in the early successional stage than in the late successional stage. The seed bank composition remains rather constant and is composed of early successional species that produce many seeds that persist during the succession sere. In early successional stages new colonizers become to a large extent incorporated in the seed bank leading to a relatively high similarity between seed bank and above-ground vegetation; in the late successional stages, most dominant species have a transient seed bank or probably no seed input, leading to a relatively low similarity between seed bank and above-ground vegetation. The distribution of seed bank density with depth in old salt-marshes was the same as in newly created salt-marsh with the highest density in upper soil layer

The Effect of the Habitat Type on Soil and Plant Diversity Properties in Natural Ecosystems in the Northern Alborz (Case Study: Vaz Watershed)

This study aimed to compare plant species diversity indices (diversity and richness) and some physico-chemical properties of soil among forest, ecotone and rangeland habitats. Vegetation sampling was done randomly at each habitat. One dominant community was selected in each habitat and one key area was distinguished in each community and 8×1m2 plots were randomly established in each key area. In each plot, the list of existing plants and cover percentage for each species were determined and soil samples were taken from depths of 0-10 cm. The Shannon-Wiener and Simpson diversity indices and Margalef and Menhinic richness indices were estimated using PAST software. Physical and chemical characteristics of the soil were compared at three sites by analysis of variance (One Way ANOVA). The results showed that the lowest and highest values of all species diversity and richness were occurred in forest and ecotone habitats, respectively. Bulk density, sand and pH value of soil were significantly the highest in the rangeland. The percentage of clay and organic carbon in forest habitat were higher than the two other habitats. This study revealed the importance of ecotone in preserving the diversity and species richness