No abstract

The complexity of invasion process of alien species is very high and requires better understanding. Nowadays, the successful range expansion of Erechtites hieraciifolia is being observed in Poland. In the study, the distribution of the species in Poland was summarized, based on the available literature and new field data. The fieldworks were conducted in the part of the species expansion zone located in central and north-eastern Poland. Two case studies on species encroachment into post-fire and transitional bog communities were performed and a case inventory of the species presence on the border of its main secondary range was conducted. The measurements of height, counting of the number of inflorescences per individual and the number of achenes per flower head were conducted in selected stands. There was performed a checking of local conditions of growth using ecological indicator values for vascular plants. The analyzed specimens of the species had significantly less inflorescences and achenes than in the other parts of the species expansion zone. They grew also in worse light conditions and in less fertile sites. The species colonized most disturbed patches at first. It spread to the less disturbed ones and omitted natural sites. The massive colonization of the transitional bog could be caused by groundwater level lowering. In the area of the edge of the species main range it was found in many diffuse stands within disturbed patches of the landscape. Based on the obtained results the species is considered to create now still only a moderate threat in the analyzed part of the expansion zone. The possibility of its expansion to wetland sites and sudden appearance in Kampinos National Park needs much more attention. Key words: alien invasive species, climate change, American burnweed, expansion, disturbance