The vegetation dynamics in six permanent plots in two lowland rivulets of the Federal Republic of Germany are analyzed. The year-to-year change in species cover is displayed by means of tables. In each site there are core species (both hydrophytes and helophytes) which have been able to successfully reproduce within the sampling plots over the total observation period. There are also transient hydrophytes which regularly become washed in from the upper course, and transient helophytes growing permanently into the river from the banks.A numerical analysis of the performance of the 12 most frequent and abundant hydrophytes in relation to various independent variables was carried out using canonical correspondence analysis. There is no directional temporal variation within the vegetation data set. The hydrochemical variables were almost constant within the observation period. Rainfall in summer has some influence via discharge and turbidity. Most of the variance in the data set is explained by the position of the sites along the rivers. Most of the residual variance can be explained by a binary disturbance variable. The processes observed can mostly be explained from life history characteristics of the dominant species, particulary Sparganium emersum, Ranunculus peltatus and Potamogeton natans. The spatial scale of the study site was relatively adequate. A smaller size would have produced noisy data (suggesting erratic change), while a greater size would have produced no change at all. The adequate temporal scale for observation is the comparison of the yearly maxima because of the seasonality of most of the species. An exact prediction of dominance and species composition of the following year is impossible.
This resampling study in 338 semi-permanent plots analyses changes in river macrophyte diversity in 70 water courses (small streams to medium-sized rivers) from four regions of the northwest German lowlands during the last six decades. The total macrophyte species pool decreased between the 1950s and 2010/2011 by 28% (from 51 to 37 species), mean plot-level species richness by 19% (from 4.7 to 3.8 species per relevé) and the number of red-listed species by 40% (from 30 to 18 species). Species loss was associated with marked change in species traits: species with presumably higher mechanical stress tolerance (indicated by low specific leaf area and short leaf longevity) are more abundant today. Nearly, half of the species present in the 1950s had either disappeared or been replaced by other species in the recent relevés. The dramatic impoverishment is likely a consequence of continued nutrient input that drove oligo-and mesotraphent species to extinction, and of restructuring and maintenance works in the water courses that reduced stagnant and undisturbed river habitats, where stress-intolerant species can persist. Efficient measures to reduce the nutrient load and to re-naturalise stream and river beds are urgently needed to halt and reverse the loss of macrophyte diversity.
SUMMARY1. Continuous multiyear data sets can help to detect long-term trends and allow predictions with respect to environmental and biological variables. While the use of taxon-based approaches for biomonitoring of watercourses is well established, little information exists concerning long-term variability of dominance and growth form composition of aquatic macrophyte communities. We analysed patterns in dominance and growth forms of macrophytes over a period of 21 years in relation to changes in water chemistry, disturbance regime and extreme weather conditions in two reference streams in the north-west German lowlands. 2. Rank-abundance curves of annual observations indicated five core species. The pattern of abundance resembled a broken-stick rather than a log-normal distribution. The core species, which reached both the highest absolute and average cover values, were Sparganium emersum, Elodea canadensis, Callitriche hamulata, Potamogeton natans and Ranunculus peltatus.3. An analysis of change in dominance showed that in 67.5% of all observed cases, the same species became dominant in the subsequent year. Sparganium emersum (80.6%) and E. canadensis (68.8%) were the most successful species in retaining their relative dominance. Calculation of transition probabilities showed a strong attractor function of states dominated by Sparganium and Elodea. However, dominance patterns broke down over time and after 20 years, only one of the sites still showed the same dominance type. On average, dominance was preserved for around 4 years. 4. Reaches were mostly dominated by six growth forms, namely vallisnerids, potamids, peplids, nymphaeids, myriophyllids and graminids. In each reach, two, three or five growth forms shared the highest cover percentages. An analysis of traits of successfully established and dominant species identified three strategies: a facultative wintergreen growth habit in combination with effective means of vegetative reproduction; strong maintenance traits (in particular rhizomes); short-life span in combination with seed and turion production. 5. Strong oscillations of growth form types were observed. Six growth form clusters were defined, up to four of which may occur in one reach. Most frequent clusters were one characterised by strong vallisnerid dominance and another characterised by dominant potamids with peplids and myriophyllids. Both clusters showed the highest frequencies, durations and transition probabilities from other clusters. On average, growth form clusters persisted for 4.5 years.6. An analysis of the influence of environmental factors on growth form dynamics showed that chemical and physical parameters distinguish the sample sites but cannot explain the observed changes. 7. We conclude that dynamics of species dominance as well as the occurrence of growth form assemblages are not controlled by physicochemical environmental factors such as nutrient composition at the scale of observation in this type of river. Instead, stochastic interactions between species characteristics and...
The changes in habitat quality of lowland rivers in Lower Saxony (Germany) during the past 40 years are outlined. Almost all chemical, physical, and morphological parameters have changed, resulting in most cases in an enhanced potential productivity, accompanied by a complex disturbance regime. Historical reconstruction of the change in river vegetation is presented to compare the frequency of macrophyte species 40 years ago with the situation of today. For a total of 289 sampling sites, the floristic change was exactly reconstructed. Nearly all species show a decline in frequency. This trend is also recognizable in Potamogeton, with the exception of two narrow-leaved species.An attempt is made to explain both decline and maintenance in terms of life history characters (vital attributes) allowing the species to react to the changing habitat conditions. The successful species are characterized by certain life history characteristics which enable them to survive under the current disturbance regime. Most important aspects of life history are the ability to reproduce by means ofturions and other fragments, a long-lived, deep-rooting rhizome system, phenotypic plasticity of above-ground parts, synchronous shoot polymorphism, and the potential to regenerate quickly from remaining buds after disturbance. The decline of formerly frequent species can be attributed mainly to the lack of certain key characters; however, physiological characters also may be important. The extirpation of some rare species could also be caused by random fluctuations in small populations. The general importance of population ecological research, particularly demography, life history theory, and the modelling of clonal populations in conservation ecology is stressed.
a b s t r a c tIn the present study, we present a synopsis of two macrophyte surveys of physiographic units in northwest Germany carried out over one decade. Data were used to test a set of hypotheses on macrophyte distribution at the regional level. Rank-frequency curves resembled the broken stick model. Twentyone species of the 59 most frequent species occurred at high frequencies above 15%. Helophytes made up a high percentage (12 of 21) of the frequent species. Phalaris arundinacea was the most frequent species in both sampling periods. Most species showed no considerable change in frequency over time, among them the core hydrophytes. Spatial variation of species frequencies among physiographical units showed a unimodal distribution in relation to frequency. Spatial variation of frequencies of functional groups was significantly lower. Most uneven distribution among physiographical units was found in cryptogams. DCA ordinations of physiographical units showed a spatial gradient from alluvial plains to higher grounds units, which remained constant over time. CCA ordination of physiographical units in relation to environmental parameters identified two main axes, an altitudinal gradient and an alkalinity gradient. Species composition of units corresponded to the main landscape pattern of alluvial plains, glacial lowlands, and higher grounds on Mesozoic rock. Species diversity showed a complex behavior. Diverse units were found both in alluvial plains and glacial lowlands of intermediate elevation. The study may help defining regionally differentiated reference states for stream management, benchmarking indicator scores of species and avoiding application of assessment methods outside their range of applicability.
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