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KASPEREK, G. (1998): Pflanzenökologische Untersuchungen im mittleren Rur-Tal (Nordrhein-Westfalen): Vegetation und Vegetationsdynamik unter besonderer Berücksichtigung von Fluktuationen in Dauerflächen. 344 S. (Archiv Naturwissenschaftlicher Dissertationen, Band 6.) Martina-Galunder-Verlag, Wiehl.
Plant ecological investigation in the valley of river Rur (Northrhine-Westphalia): Vegetation and vegetation dynamics, with special reference to fluctuations in permanent plots
In the Rur valley in the landscape Niederrheinische Bucht (Northrhine-Westphalia), plant ecological investigation have been carried out in order to elaborate an inventory of plant communities and to describe vegetation dynamics in permanent plots; these investigation are part of a long-term biomonitoring programme. The research area as a whole comprises twelve single areas in the floodplain of the middle section of river Rur and one area in the lower Wurm valley, with a total of 1880 acres.
As a result of the phytosociological investigations, 63 plant communities on the level of the association are described in text and vegetation tables. With large shares of the total investigation areas, communities of the alliances Carpinion, Alno-Ulmion and Cynosurion can be found: Stellario-Carpinetum stachyo-typicum is settling on the highest levels; Pruno-Fraxinetum is the characteristical association of the lower reaches in undisturbed parts of the floodplain; and to Lolio-Cynosuretum large areas of intensively grazed grasslands can be ascribed. With stand of smaller size, communities of the alliances Phragmition, Magnocaricion, Agropyro-Rumicion, Aegopodion and Senecion fluviatilis occur regularly in the areas of investigation. Cultivated forests of Poplars can also be found often; they can be subdivided into five phytosociological units.
Of special interest are stands of Alder spring-woods which can be ascribed to Fraxino-Alnetum. They are in contact with Apietum nodosi and Chrysosplenietum oppositifolii. These communities are restricted to few of the areas of investigation; this holds for Carici elongatae-Alnetum, too.
For long-term monitoring of vegetational changes, a total of 100 permanent plots (DF = Dauerflächen) have been established in 1989; they were investigated yearly until 1995, in some cases until 1996, using a conventional cover/abundance-scale with nine classes. Measurements of groundwater level in the permanent plots were carried out monthly.
In the period of investigation, no anthropogenic disturbances of significant importance occured in the major part of the investigation areas. Thus, natural fluctuation of vegetation can be analysed clearly. Especially the community-specific differences in the variation range of ecological parameters in the course of fluctuations can be described. Knowledge of the respective community-specific "normal variation range" is important for early diagnosis of aligned vegetational changes in biomonitoring.
The crucial point in investigating and assessing vegetation dynamics for purposes of biomonitoring is the distinction between fluctuation and succession. To handle this problem, a catalogue of criteria is developed, comprising several geobotanical attributes. With this catalogue, each DF is classified with regard to the type of vegetation dynamics in the period of investigation.
For each DF, vegetation dynamics are quantified by calculating "rates of change". This concept extends earlier approaches in literature of determining rates of succession onto the field of fluctuations. As base parameters, indicator values after Ellenberg, values of species richness, diversity and evenness parameters are calculated; from the spans of their changes over the investigation period, different variants of "rates of change" are derived. Further variants result from calculation of similarity indices after Soerensen and Czekanowski, and from statistics on floristic change.
The group of DF which only showed vegetation dynamics of the type of fluctuations comprises 71 DF; the main focus of the present study is on this group. As an orientation about community-specific extent of fluctuations on the basis of phytosociological alliances, medians of the "rates of change", originally calculated for single DF, are given. Between the communities under study, big differences in the extent of community-specific fluctuations were found. In woodlands, in general only slight changes occur during fluctuations, but there is a pronounced gradient of increasing dynamics parallel to the moisture gradient. Reeds and sedge swamps are characterized by strong fluctuations, and in grazed Flutrasen (communities of Agropyro-Rumicion with winter flooding and summer grazing), extreme values are reached. Based on parameter evenness, as an example, for the alliances Carpinion, Alno-Ulmion and Salicion values of "rates of change" < 20 were found, whereas those vegetation units free of woody plants reached community-specific values of (in most cases to a large extend) > 20.
A comparison of "rates of change" between the two groups "DF with succession" and "DF with fluctuations only" is giving hints on the suitability of the base parameters for the delimitation of fluctuation and succession. Because of the respective community-specific vegetation dynamics, different sets of variants of "rates of change" prove to be expressive for each community. In communities of grasslands and Flutrasen, for instance, similarity indices after Czekanowski are less suitable then those after Soerensen, because the community-specific dynamics are characterized by strong variations of species abundances even in the course of pure fluctuations. Since many of the detected aligned vegetational changes are connected with changes of the habitat factor moisture, those parameters which are derived from Ellenberg's indicator values for moisture are especially suitable; but this in fact is explained by the properties of the used data and study area. Of general importance, going beyond the scope of the present study, is the overall suitablility of similarity indices to indicate succession in the communities under study.
The course of weather has a considerable influence on vegetational fluctuations in DF, but this is often obscured by very complex relations and methodical obstacles. The crucial point is: how can the complex relations between weather and vegetation development be elucidated? In the present study, precipitation data are brought into relationship with indicator values for moisture. To do this, the methodical concept of "periods of influence" is developped. The influence of precipitation anomalies on the vegetational fluctuations in DF is illustrated by the graphical comparison of their courses over time.
The analysis shows that it is not sufficient to use precipitation data on a one year basis or on basis of the vegetation period. Ten different precipitation data sets are calculated for different "periods of influence". Their ecological importance is evaluated by comparison with the course of average indicator values for moisture, and the best fitting periods are called "effective periods of influence". In the study area, precipitation of winter and spring is shown to be of general importance for development of vegetation, but in addition, the influence of one or more preceding years can clearly be recognised in courses of average indicator values for moisture. The current vegetation period is not suitable as a period of influence; precipitation in the current vegetation period has only little influence on the course of average indicator values for moisture. It is stressed that for different phytosociological units, different "effective periods of influence" have to be expected.
In 29 DF, successions were detected. Some possible causes of these successions are discussed. Cyclic variations in precipitation probably play an important role; after the "wet eighties", the period of investigation since 1989 is representing a relatively dry phase, and some successional phenomena can be interpreted as results of these precipitation anomalies. Theoretical aspects of such cyclic variations of weather for the delimitation of fluctuation and succession are discussed.
From the results of the studies on DF, recommendations on methods of phytosociological schemes for preservation of evidence are derived. In the initial phase of long term studies, investigations in yearly intervals are highly recommended. Calculation of similarity indices should be a standard analysis. "Moisture value indices" (after Möller), which are compared to customary average indicator values for moisture (these being not unimpeachable in a mathematical sense), show to be less expressive. Average indicator values for moisture are proven to be suitable to line out the course of precipitation anomalies, even if data from permanent plots are used which were investigated with a relatively rough cover/abundance-scale after Braun-Blanquet.
Copyright: 1998 Martina-Galunder-Verlag, Wiehl, Germany