Causes and directions of changes of meadow-pasture vegetations of the Wielki Leg Obrzanski (WLO)

The paper presents results of the studies conducted to determine causes and directions of transformations in vegetation of meadow-pasture communities in the area of the Wielki Łeg Obrzanski (Great Obra Floodmeadows). Multifaceted natural and habitat analyses were conducted on over 1800 releves prepared in the vegetation seasons of 2005-2009 according to Braun-Blanquet. On their ba- sis the phytosociological and floristic structure was analysed and habitat conditions were determined by phytoindication. Moreover, laboratory methods were applied in soil analyses to determine con- tents of basic macro- and microelements and groundwater table levels were recorded. Results referred to the documentation and archive literature made it possible to determine causes of transformations in plant communities in that area and outline probable directions in which succession was progressing. A lack of regular land use patterns is seen as the primary cause for the transformation in the flora of the Wielki Łeg Obrzanski, particularly in the last 20 years. A considerable limitation of land use in certain sec- tions of this area has led to the initiation of dominance of nitrophilic species such as Urtica dioica, Cirsium arvense, Potentilla anserina or Galium mollugo, while in the thinned sward it led to the development of grass species with very limited economic importance. Another equally important factor affecting the structure of vegetation cover is connected with air and water relations in soils, determined by the drainage system operating there for the last 200 years and by precipitation.