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Riparian zones in two rivers in southern Poland were studied in terms of species composition and soil parameters in patches dominated by three knotweed taxa (Fallopia japonica, F. sachalinensis and the hybrid F. ×bohemica). The main purpose was to detect any differences in species diversity, environmental conditions and in the impact of the three Fallopia spp. on resident species. Fieldwork was conducted in spring and summer in 30 invaded plots (in total 90 subplots). It was demonstrated that vegetation dominated by particular knotweed taxa differed in response to soil pH and ammonium, nitrate, and magnesium content. Fallopia spp. (living plants and necromass) had a stronger negative impact on the cover and species diversity of the resident species in summer in comparison with spring. Vegetation patches differed significantly in species composition in relation to the knotweed taxa present. These differences may be the consequence of the differentiated biotopic requirements of Fallopia taxa and the coexisting plants, or to the different impact of the knotweed taxa on the resident species.
Northern red oak (Quercus rubra) is the most common alien tree species in Polish forests that is considered as an invasive one. It is native to North America. Negative impact of Q. rubra on natural habitats in Europe is well−known, but there is still a lack of information about correlations between red oak and individual species or groups of plants. Especially knowledge about impact of Q. rubra on fern communities is still unsatisfactory. The goal of research was to check if ferns could be neutral to the negative oak impact. In order to verify, populations of ferns occurring in patches of forests dominated by northern red oak and population from natural forests with the same abiotic conditions were compared. Moreover, quantitative relations between ferns and other herb species were analyzed. The research was carried out in southern Poland. Field surveys were conducted in 100 m² plots with domination of Q. rubra. Plots were settled in 3 different types of plant communities: mixed broadleaf forest, acidophilous oak forest and acidophilous beech forest. In the same types of plant communities, but without northern red oak, control plots were located. Control patch was settled in place with similar inclination, exposition, altitude and water regimes. For each surface list of all herb species with their percentage coverage and biometric measures of fern fronds were made. At maximum 50 specimens of each fern species were taken into account. Plots were located in areas of 3 geographical regions: the Śląskie Foothills, the Oświęcimska Basin and Śląska Upland. Results show that ferns in monocultures of the alien oak are bigger, more lush and their coverage is higher than in the control plots. What is more, coverage of other herb species and species diversity are significantly higher in control patches. It suggest that ferns are not sensitive to Q. rubra influence, so it can be concluded that they are competitive to other species which are sensitive to northern red oak impact.
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