Abstract
The riparian tall herb fringe communities of the order <em>Convolvuletalia sepium</em> represent an integral part of the natural vegetation in river valleys. The major objective of this study was to assess the relationships between the diversity and variability of these communities and various environmental factors. The survey was conducted in northwestern Poland, along 101 randomly selected 1–2-km long sections of 24 rivers and the Szczecin Lagoon. Samples were collected in 2008–2013 in all types of tall herb fringe vegetation found in the surveyed river sections. Data collected included hydrogeomorphic variables, soil parameters, potential and actual vegetation, and dominant land use form. A total of 24 vegetation units were documented, based on 300 sample plots (relevés). Tall herb fringe communities occurring in valleys of large rivers (<em>Senecionetum fluviatilis</em>, <em>Fallopio-Cucubaletum bacciferi</em>, <em>Achilleo salicifoliae-Cuscutetum lupuliformis</em>, <em>Convolvulo sepium-Cuscutetum europaeae typicum</em> and <em>chaerophylletosum bulbosi</em> subass. nov., <em>Rubus caesius</em> community, <em>Solidago gigantea</em> community) exhibited floristic and ecological differences in comparison with plant communities from small rivers (<em>Eupatorietum cannabini typicum</em>, <em>aegopodietosum</em> and <em>cardaminetosum amarae</em> subass. nov., <em>Epilobio hirsuti-Convolvuletum sepium</em>, <em>Soncho palustris-Archangelicetum litoralis</em>, <em>Convolvulo sepium-Cuscutetum europaeae aegopodietosum</em>, <em>Urtico-Convolvuletum sepium typicum</em> and <em>aegopodietosum</em>, <em>Urtica dioica</em> community, <em>Galeopsis speciosa</em> community, <em>Rubus idaeus</em> community). This finding fully justified their division into two alliances: the <em>Senecionion fluviatilis</em> and the <em>Archangelicion litoralis</em>, respectively. Significant differences between the tall herb fringe communities associated with large rivers and the plant communities occurring along small rivers included plant species richness, moss layer cover, contribution of river corridor plants, level of invasion, influence of adjacent plant communities on the floristic composition, relative elevation and distance away from the riverbed, degree of shading, proportions of all grain size fractions, soil pH, contents of organic matter, humus, organic carbon, total nitrogen, bioavailable phosphorus, potassium, magnesium, and calcium in the soil. The differences in environmental characteristics of individual plant communities were significant as well; they showed that most vegetation units were well defined. The variables that best discriminated between riparian tall herb fringe communities included the natural potential vegetation unit <em>Salici-Populetum</em>, headwater seeps, soil pH, sample elevation above the river water level, river size, flooding, degree of shading, soil moisture, K<sub>2</sub>O and CaO contents, and C/N ratio. The integration of the main riparian gradients (longitudinal, lateral, vertical) and patch perspective (e.g., natural potential vegetation units, and land use forms) significantly improved the comprehensive riparian vegetation patterns, because these two perspectives underpin different processes shaping the vegetation. This study contributed significantly to the knowledge of riparian tall herb fringe communities. Two subassociations are described here for the first time, whereas six others have not been previously reported from Poland. The data summarized in the synoptic table indicated that the species diagnostic for individual plant communities should be revised at the supra-regional scale. Some syntaxonomic issues were also determined. The inclusion of the order <em>Convolvuletalia sepium</em> to the class <em>Epilobietea angustifolii</em> resolved the problem of classifying the community dominated by <em>Eupatorium cannabinum</em>, a species showing two ecological optima: one in riparian tall herb communities and the other in natural gaps of the tree stands and clearings of fertile alder carrs and riparian woodlands. This also resolved the problem of classifying the communities dominated by <em>Galeopsis speciosa</em> and <em>Rubus idaeus</em>, intermediate between riparian tall herb and clearing communities. The results of this study may serve as a reference for management of the vegetation in river valleys and promote their conservation. They may also be essential for any future syntaxonomic revision of riparian tall herb fringe communities at a larger geographical extent.
Highlights
Riparian ecosystems exhibit high habitat heterogeneity, complex ecological processes, and multidimensional gradients
Subclusters A2 and A3 (11 relevés) had samples dominated by alien plants, being Helianthus tuberosus and Impatiens glandulifera, respectively, which belong to neophytes forming their own plant communities (Helianthus tuberosus community, Impatiens glandulifera community)
Based on the numerical classification of both subclusters and the comparison of the two dendrograms (Fig. S1), four relevés from Subcluster A3 could be assigned to the native plant communities, Eupatorietum cannabini typicum and aegopodietosum, and Epilobio hirsuti-Convolvuletum sepium, because their diagnostic species (Appendix 1) were still present and quite abundant
Summary
Riparian ecosystems exhibit high habitat heterogeneity, complex ecological processes, and multidimensional gradients. Longitudinal transport of organic matter and nutrients, along with lateral mass movement across the floodplain, are essential for ecological processes in river valleys [2]. The vertical dimension in river valleys is important, because it is related to the intensity of flood effects, connectivity of ground- and surface waters, and moisture level [1]. The river continuum concept (RCC) [4] emphasizes the importance of the longitudinal dimension. It focuses on the continuous upstream–downstream gradient of physical conditions and ecological processes, on changes in material supply and circulation, accompanied by alterations in producer and consumer communities along the length of a river. The most severe limitation of the model is the fact that it ignores the effects of floods on the structure and function of riverine landscapes [3]
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