The present study deals with vegetation analyses of swamp forests, moorland, forests as well as bog-myrtle scrubs of the natural regions 'Niederrheinische Sandplatten' and 'Bergische Heideterrassen' and their syntaxonomical divisions. The study is focused on the impact of disturbances in syntaxa Sphagno palustre- and Carici elongatae-Alnetum glutinosae, Vaccinio uliginosi-Betuletum pubescentis and Myricetum gale. Different degrees of impairment are pointed out by comparison of moorlands near the conurbation metropolitan regions in the western part of Ruhr area in the centre of both natural regions to the northern and southern adjacent rural regions. Basis for the phytosociological classification and the elaboration of interference patterns are 1766 relevés (179 by the author, 213 previous covering the study area and 1374 supra-regional from literature). The succession processes, caused or modified by negative impact is the study focus. Based on current and previous relevés of the studied area from 1953 until 1985, the present status and the former successive processes are presented. General changes in plant communities have been traced back to the beginnings of the phytosociological research by comparison of relevés from the studied area with available previous relevés from central Europe in the 1920th until the 1940thdecades. Besides the chronological comparison the current relevés were also compared with those from other natural regions of Germany and thus spatially placed in a larger context. Based on the assumption that nutrient and water balance conditions are the key factors shaping the plant communities of the examined azonal vegetation types, the consideration of the succession processes focused on these ecologically relevant species. These include all species that characterize the nutrient and water balance of the site in a special way. As bryophyte species react often much more sensitive to environmental changes, their suitability is used as an indicator to explain the complex cause-effect structure, emerging in the vegetation pattern. Until now, however, the value for nitrogen (nitrogen number) has been missing for such environmental analysis in the list of the values for moss species (DÜLL 2001). According to the specifications of ELLENBERG et al. (2001) a nitrogen indicator value number was developed and applied in the present study. After evaluation of all proposed concepts in literature, alder carrs and swamp forests (Alnion glutinosae) are subdivided primarily by criteria of nutrient balance. A classification on association level based mainly by criteria of water balance and chorology is rejected. Both, the Sphagno palustris-Alnetum glutinosae (peatmoss alder carrs) and the Carici elongatae-Alnetum glutinosae (sedge alder carrs) are found in the investigated natural regions. The widespread nutrient-poor sands and gravels as the source material of the soil development in the investigation area assume that the Sphagno palustris-Alnetum glutinosae, with few exceptions, would be the natural alder carr community. According to the characterizing features of the nutrient and water balance for the differential species
this community can be divided in 5 different near-natural or degraded associations. The sedge alder forest of the study area is a heterogenic complex and can be divided in 4 variants and a monostructural stadium of Carex acutiformis. Compared with previous investigations it is shown that wetness indicator species disappear especially from nutrient-poor sites, that nitrogen indicator species increase, and that alien plant species such as Impatiens glandulifera immigrate. In addition, some of the sedge alder carrs characterized forests show a development from former peatmoss alder carrs. Formerly sharp boundaries between the two communities gradually disappear and also the synsystematic dependency of species is more difficult to detect. Examples of such leveling are Polytrichum commune, which show a close synsystematical linking to the peatmoss alder carrs in the relevés of the 1920th to 1940th decades, and Carex elongata, which was only found in sedge alder carrs. Nowadays the peat moss Sphagnum squarrosum is much less frequently found in alder carrs than in former times. However, it can be assumed that a number of peat moss species in the past has been wrongly determinated as Sphagnum squarrosum. Not only Sphagnum palustre (as previously assumed) can form ecadic modifications with right angles recurved branch leaves (S. palustre mod. squarrosulum), that are confused with the similar Sphagnum squarrosum. Compared to recent conditions S. squarrosum was possibly also rare in alder carrs and swamp forests previously. Using the principle of the formation of species-specific identification method, both the young and older successional stages dominated by Betula pubescens are treated as downy birch swamp forests (Vaccinio-Betuletum pubescentis) in the present paper. The autonomy of swamp forests due to characteristic species adapted to oligotrophic sites and a preliminary summary of all swamp forest types in a separate class (WAGNER & WAGNER 2000) are recognized. Also the downy birch swamp forest implies structurally and floristically diversity and is divided into 7 variants that are perceived as young and older successional stages and more or less degraded vegetation, according to an ecological gradient, which also reflects the level of impairment. The discernible stronger degradation of the vegetation nowadays can be recognized, besides the increase in nitrogen indicator species of drier sites, in particular by the higher abundance of Molinia caerulea and Dryopteris carthusiana. Concerning mosses Sphagnum denticulatumoccurs as an indicator species of interference, which in the present study is considered as a characteristic species of the Sphagnum-alder carrs. Among the invasive alien plant species growing in downy birch swamp forests in particular Carex brizoides and Prunus serotina are mentioned, which can obtain high abundances. Regional floristic differences in the species composition of the Vaccinio-Betuletum pubescentis are particularly evident in the differential characteristic groups of species on nutrient-poor sites. Many of these species, adapted to extremely low nutrient sites, are found only in the relevés outside the agglomeration area and illustrate the significantly lower impact by eutrophication in these rural areas.
The Myricetum gale, which in the study area can only be found in the southern heathland, is structurally and floristically heterogeneous and can be divided into 4 variants. The temporal comparison shows for the Myricetum gale in the study area in particular the displacement of eutrophication moos species by vascular plants. In the analysis of the site conditions through indicator species, the Myricetum gale is compared to the Vaccinio-Betuletum pubescentis a society of nutritious poorer locations. Older investigations found as nutrient indicators only the bryophytes Brachythecium rutabulum, Eurhynchium praelongum and Lophocolea bidentata, but today there are also vascular plants like Dryopteris dilatata and Phragmites australis. In Myricetum gale the high frequency of degraded stocks also outside the study area is noticeable, which highlights the particular sensitivity of this community to perturbances. Disproportionately often the subassociation 'peucedanetosum', named by WEBER (1998), occurs. The numerous outlines of site impairment concerning this subtype as mentioned in the subject literature contribute to the suggestion that it seems to be an anthropogenic community. According to investigation results of the present study a differentiation of subassociations is not followed, but a subdivision into different variants in concordance with the nutrient and maturity gradient. Regarding the associations Vaccinio uliginosi-Betuletum and Myricetum gale not only a deprivation of species, but also an increase of species from wet and low-nutrient habitats is recognized. This positive tendency of stock development is an obvious result of maintenance measures in the investigation area. In supra-regional comparison of the investigated syntaxa to recent relevés from all over Germany it is pointed out that the phenomenon of advanced succession of stocks is not locally occurring, but also present in other regions. Apart from the natural connection of wet and swamp forests, anthropogenic impairment as drainage and eutrophication lead to a decrease or extinction of syntaxonomic relevant species – or occurrences of not typical species which cause problems in recognition of units. The evaluation of floristic data comprises 218 vascular plant species and 99 bryophytic species with present occurrences in swamp forest, moorland forest and wet heath vegetation, additionally 35 vascular plants and 17 bryophytes that are disappeared by now. The species inventory in the investigation area was formerly documented by old floras, dating back to the end of the 18th century. Therefore the floristic changes in moorland, swamp forest and wet heath vegetation could be reconstructed. Negative influences are not a recent phenomenon, they already lead to a decrease of species 170 years ago (OLIGSCHLÄGER 1837). In the framework of the present investigations not only the decrease of typical moorland species, especially among the bryophytes, was detected. A number of typical moorland bryophytes were discovered or rediscovered: Mylaanomala (Gartroper Busch), Sphagnummagellanicum (Hemmings-Schlinke, Gartroper Busch), Cephalozia macrostachya (Gartroper Busch), Sphagnum warnstorfii und Sphagnum molle (Ohligser Heide, FUCHS & SONNENBURG 2009), Odontoschisma sphagni (Hildener Heide) etc. The highest species numbers are situated in the Duisburg-Mülheimer Wald area in the centre of the investigation area. But high species diversity is only assessed positively concerning the typical swamp and moorland forest species. With increasing proximity to agglomeration, the number of not biotope typical and neo- and industriophytic species increases. Compared to the neophytic proportion of the total flora, the part of non-native species in moorland and swamp forest and wet heath vegetation is relatively low. This illustrates that the investigated vegetation under wet and nutrient-low conditions forms a high barrier for immigration of neophytes. The most frequent perturbation indicator vascular plant species are Athyrium filix-femina, Dryopteris dilatata, Oxalis acetosella, Pteridium aquilinum and Rubus idaeus. Important bryophytic perturbation indicators are Plagiomnium affine, Calliergonella cuspidata, Sphagnum denticulatum, Brachythecium rutabulum and Eurhynchium praelongum. The neo- and industriophytic species inventory of the vegetation in the investigation area includes 15 tree and shrub species and 8 herbs. Unlike with tree and shrub species, which mainly immigrated from neighbouring forests in the moorland and swamp forest and wet heath areas, the non-native herbs are predominantly escaped from cultivation or garden waste. The most remarkable recent occurrences from garden escapes include the North American species Lysichiton americanus and Onoclea sensibilis. Their establishment in a swamp forest relict and a spring mire indicates an agriophytic persistence in near-natural habitats (FUCHS et al. 2003, FUCHS & KEIL 2004). Campylopus flexuosus is the only non-native bryophyte in the moorland and swamp forests. Disturbances in the bryophytic flora are therefore not caused by non-native species, but by increase of number and abundance of native but not biotope typical species. This process is enhanced by dry hammock sites. Increased dehydration leads to nitrification and subsequently to an enrichment of vascular plants, while bryophytes decrease. Overall it appears that in the investigation area the moorland and swamp forest and the wet heath vegetation were destroyed except of a few relicts by different causes in a period of less than 200 years. In a first stage the moor areas were destructed by land use intensification from agriculture, settlement building and industry, but the causes of present impairment are more complex, as there are influences of substance discharges by immissions, small-sized drainages within forest and water management measures and leisure activity. Conurbation sources of impairment are caused by urban settlement, industry and in particular by mining, while in the rural regions the impairment sources are mainly forestry, agriculture and gravel quarrying. Impairment sources from water management, traffic and recreation show no significant spatial differences. In sum of reasons the moorland and swamp forests near conurbation areas are more affected than those in rural regions. Nearly all recognized perturbance based processes in the studied communities directly or indirectly depend on interventions in nutrient and water balance. Processes in intra- and interspecific competition, soil alterations and the resulting transformations of communities can be distinguished. Regional dissimilarities are caused by different level and different duration of interventions. In rural regions first stages of drainage and eutrophication of an oligotrophic moor can be observed, while those processes are completed in the conurbation area. Other processes, triggered after long periods of impairment of nutrient and water balance, are noticed only or predominantly in the Duisburg-Mülheimer Wald area. For all investigated vegetation units the patchwork of habitats consisting of hummocks and hollows is common. Therefore disturbances as eutrophication and drainage within a single vegetation unit vary over time before being noticed. While drainage affects the hummock sites faster, the influences of eutrophication affect first the hollows. The total of all influences of perturbances leads to partly irreversible transformation processes in plant communities.