Abstract
Invasive alien plant species represent an important threat to various protected areas of the world, and this threat expected to be further enhanced due to climate change. This is also the case for the most important network of protected areas in Europe, the Natura 2000 network. In the current study we evaluated the distribution pattern of alien plant taxa across selected continental and insular Natura 2000 sites in Greece and their potential spread 15 years since first being recorded in the field. A total of seventy-three naturalized plant taxa were recorded in the 159 sites under study. At the site level and regardless of the habitat group, the ratio of invaded areas increased between the two monitoring campaigns. An increase in the ratio of invaded plots was also detected for all habitat groups, except for grassland and riparian-wetland habitats. Precipitation during the dry quarter of the year was the factor that mainly controlled the occurrence and spread of alien plant taxa regardless of the site and habitat group. It is reasonable to say that the characterization of an area as protected may not be sufficient without having implemented the proper practices for halting biological invasions.