RESUMEN
Ailanthus altissima is an invasive species for the native flora of Greece and it could pose a serious threat to the biodiversity and the functioning of ecosystems. The purpose of this study was to investigate the spreading of Ailanthus altissima in urban and non urban areas of North and Central Greece and also to evaluate the effects of its spreading on species composition and floristic diversity in natural ecosystems. The spreading of Ailanthus altissima in urban areas is very intense, mainly in abandoned places (35.29%). It is commonly found in non urban areas of Greece, especially in hedgerows of arable lands (36%) and adjacent wetlands (17%). It is less common in forests (4%), shrublands (11%) and grasslands (9%). The spread of Ailanthus altissima in urban and natural ecosystems is relatively recent. Although it has been recorded at altitudes of up to 640 m, it usually appears at low altitudes of up to 200 m. Floristic diversity was found to be higher in the stands that it dominated (H´=1.574, H´=1.890) in comparison to stands that were dominated by Quercus pubescens (H´=1.468) or Q. coccifera (H´=1.716). This may be contributed to the fact that in those stands synanthropic species, which are usually found in regions of intense human activity, were present together with typical forest vegetation species.