EN
The steeplebush is a shrub growing naturally in North America. In Europe this species has been cultivated as an ornamental plant since the 18th century. It has been observed in spontaneously developing secondary localities in such countries as Belgium, Denmark, Sweden, Norway and Germany. In Poland there are three regions in which Spiraea tomentosa is known to be a permanently established anthropophyte: Puszcza Drawska, Bory Niemodlińskie and Bory Dolnośląskie forests. The greatest number of localities of this shrub can be found in the extensive area of the latter, where its expansion has caused significant changes in the natural environment and problems in woodland management. Therefore a research was carried out to find what conditions made the steeplebush ready for invasion and what types of plant communities were especially susceptible to it. The information about the distribution of S. tomentosa in the Bory Dolnośląskie Forest (SW Poland) was collected during field observations in the years 2011−2014. We also verified and used the results of a survey made in 2011 in three forest districts located in the study area: Ruszów, Węgliniec and Wymiarki. The analysis concerned the frequency of the steeplebush in the forest environment in terms of the forest habitat type, species composition, and stand growth stage as well as in non−forest environments like meadows, peatland, margins of drainage ditches, etc. Phytocoenoses in the habitats of humid mixed coniferous forests and humid coniferous forests with mature tree stands dominated by birches or alders were found to be forest communities the most susceptible to colonisation by S. tomentosa. Especially susceptible to steeplebush invasion were also forest cultures on those habitats. Much fewer steeplebush localities were recorded on fresh and mixed coniferous forests as well as fresh broadleaved forests. In a non−forest environments the steeplebush was mostly noted on margins of drainage ditches. The extensive system of such man−made environmental structures in the Bory Dolnośląskie Forest was found to be the most important factor favouring the expansion of S. tomentosa. Thus, the invasion of the discussed species is primarily determined by a large proportion of humid habitats in a single compact woodland complex connected by a dense network of drainage ditches.