Symptoms of anthropogenic changes in the flora include processes of extinction in some species and spread in others. These tendencies have increased in magnitude in recent centuries, adversely affecting natural biodiversity on a regional, national and continental scale. The main idea behind the project presented here was to investigate the diversity of the alien vascular flora at the regional scale and to update the list of invasive alien species for the Silesian Upland. The aim of this study is also to provide a synthesis of the knowledge accumulated to date on the conditions for their occurrence, their distribution patterns and the possible threat they might pose. The distribution of particular species at the regional scale was investigated by mapping the species on a 2 km x 2 km grid. The study showed that the list of alien plant species for the Silesian Upland encompasses 338 species, including 125 archaeophytes, 195 kenophytes (=neophytes) and 18 species of an uncertain status in the Polish flora. A threat evaluation was performed for particular species on the basis of the number of localities, the diversity of preferred habitats recorded, and their current (over the last decade) tendency to colonise new localities. As a result, a final list of 101 invasive species has been selected. Among the alien plant species considered to be invasive in the area of the Silesian Upland, 20 species are identified as Çweedsí (13 archaeophytes and 7 kenophytes), another 25 (exclusively kenophytes), penetrating into natural habitats, are defined as Çtransformersí.

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Differentiation of arable field weed communities in the northern part of the Silesian Upland (S Poland) The purpose of this paper is to present the role of the selected species (i.e. associations character species, endangered as well as the most expansive weeds) in arable field communities in the area of the Tarnowskie Góry Ridge and the Chełm, two mesoregions of the Silesian Upland (southern Poland). On the basis of 750 phytosociological relevés taken using the method of Braun-Blanquet (Braun-Blanquet 1964) between 1995 and 2009, eight weed associations as well as numerous rump communities were distinguished. Phytocoenoses without a significant contribution of the association character species comprised about 68% of the patches analysed in the cereal crop and about 12% of the communities established in the root plant crop. Very often the character species of cereal weed associations occurred sporadically or in small numbers. Some expansive weeds (e.g. Apera spica-venti (L.) P. Beauv., Avena fatua L., A. xvilis Wallr., Chenopodium album L., Echinochloa crus-galli (L.) P. Beauv., Elymus repens L., Galinsoga ciliata (Raf.) S. F. Blake, G. parviflora Cav.) played an important role in the communities analysed. The vanishing of some weed species and the spread of others has resulted in the transformation of the composition and structure of the segetal communities, a vast majority of rare and endangered weed species in the studied area are considered to be very sensitive to modern agriculture methods, e.g. chemicalization, introduction of new crop cultivars, effective cleaning of seed materials, drainage etc. (Siciński 1998; Warcholińska 1998).