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|Spatio-temporal aspects of early vegetation succession in a recently restored salt-marsh ecosystem: a case study of the IJzer estuary (Belgium)|
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|Erfanzadeh, R. (2009). Spatio-temporal aspects of early vegetation succession in a recently restored salt-marsh ecosystem: a case study of the IJzer estuary (Belgium). PhD Thesis. Ghent University. Faculty of Sciences. Department of Biology. Terrestrial Ecology Unit: Gent. xii, 206 incl. Appendices pp.|
Begrazing; Bodemconditie; Colonisation; Dispersal phenomena; Ecosystemen; Foraging habitats; Gemeenschapssamenstelling; Inundation; Livestock grazing; Milieufactoren; Plantendek; Restauratie; Soortendiversiteit; Succession (ecological); Verticale verdeling; Zaadproductie; Zoutmoerassen; Elymus athericus (Link) Kerguélen [WoRMS]; Salicornia europaea Linnaeus [WoRMS]; Salicornia procumbens Smith [WoRMS]; Suaeda maritima (L.) Dum. [WoRMS]; ANE, België, IJzer R. [gazetteer]; Belgium, Het Zwin natuurreservaat; Marien
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.