Richness of vascular flora in Lublin city, east Poland, under different urban pressure

Cities are commonly described as mosaics of habitats with increasing degrees of human impact along a gradient from the outskirts to the centers, which may lead to both an increase and a decline in species richness and diversity. Data on species richness in the city of Lublin had been collected in a model transect containing 20 1-km(2) study squares. We used ordination methods correspondence analysis (CA) and redundancy analysis (RDA), Spearman's rank correlation, forward selection, and the Monte Carlo permutation test to determine which factors have the greatest effect on species richness in various types of city habitats. There were 795 vascular plant species in squares, with predominance of native (540) rather that alien (255) species. The greatest richness was reported in the city center, where residential areas border railway infrastructure and extensively cultivated agricultural areas. The lowest number of species was found in peripheral areas overgrown by seminatural vegetation. The main factor determining species richness and flora quality in the city is human impact. Species richness of native and alien plants is influenced by the landscape structure created by land-use and land-cover variability, hemeroby level, and the period under urban pressure.