Occurrence of alien plant species in all the major habitats in the Czech Republicwas analysed using a data set of 20,468 vegetation plots, classified into 32 habitats according to the EUNIS classification. The plots contain on average 9.0% archaeophytes and 2.3% neophytes; for neophytes, this proportion is much smaller than 26.8% reported for the total flora of the country. Most neophytes are found in a few habitats: only 5.6% of them were recorded in more than ten habitats. By contrast, archaeophytes, and especially native species, tend to occur in a broader range of habitats. Highest numbers of aliens were found on arable land, in annual synantropic vegetation, trampled habitats and anthropogenic tall-forb stands. These habitats contain on average 22–56% archaeophytes and 4.4–9.6% neophytes. Neophytes are also common in artificial broadleaved forestry plantations; they also tend to make up a high percentage of the cover in wet tall-forb stands, but are represented by fewer species there. Entirely or nearly free of aliens are plots located in raised bogs, alpine grasslands, alpine and subalpine scrub and natural coniferous woodlands. Correlations between the number of archaeophytes or neophytes and the number of native species, calculated with habitat mean values, were non-significant, but there was a positive correlation between the numbers of archaeophytes and neophytes. The ratio of archaeophytes to neophytes was high in semi-natural dry and mesic grasslands and low in disturbed habitats with woody vegetation, such as artificial broadleaved forestry plantations, forest clearings and riverine willow stands. When individual plots were compared separately within habitats, the relationships between the number of archaeophytes, neophytes and native species were mostly positive. This result does not support the hypothesis that species-rich communities are less invasible, at least at the scale of vegetation plots, i.e. 10 0–10 2 m2.
Nineteen isolated rocky outcrops of different sizes, tops of which were covered with natural grassland vegetation, were studied in the forested submontane belt (630–1020 m a.s.l.) of the Šumava Mts in the southern part of the Czech Republic, Central Europe. The species of vascular plants present in the treeless sites at each locality were identified. Those species with an Ellenberg indicator value for light equal 6 or higher were considered to be heliophilous. The distance to the nearest secondary treeless area was measured. There were 43 heliophilous species (23%) recorded among the 184 species identified. The number of species and the number of heliophilous species varied independently of the altitude and extent of the treeless area on the rocky outcrops, but were significantly correlated only with the distance to man-made treeless areas. The highest number of heliophilous species was recorded within approximately 400 m from the nearest man-made treeless area. Although the relict occurrence of some of the heliophilous species on the rocky outcrops cannot be completely excluded, obviously most of the species colonized these localities from nearby secondary treeless areas since their creation in the Middle Ages.