Changes in composition and structure of urban flora over 120 years: a case study of the city of Plzeň

Floristic lists of the industrial city of Plzeň, W Bohemia, Czech Republic, were compiled for the periods 1880-1910, 1960s and 1990s. Data were obtained for the city and its surroundings and abundance of each species was estimated from primary literature and recent field research. Changes in species richness on the time scale of 120 years were described, and dynamics of representation of alien species was analysed. Total number of species recorded decreased from 1173 in 1880-1910 to 988 in 1960s to 1043 in 1990s. These dynamics differed between city and surroundings. In the city, species number was gradually increasing while the trend was opposite in the surroundings. When expressed per log area, initial pattern of higher species richness in the surroundings was reversed in the course of the study period. The floristic similarity between 1880-1910 and 1990s was 0.57 for the surroundings and 0.41 for the city. In the 120 years covered, 805 species remained permanently present, 368 disappeared and 238 immigrated as new. Proportional representation of common species decreased and that of rare species increased. The representation of neophytes in the total flora of the study area increased from 6.2% in 1880-1910 to 13.2% in 1960s to 17.0% in 1990s. The proportion of native species decreased accordingly, and that of archaeophytes was stable over the study period. The representation of woody plants increased in the city, namely among neophytes. Over the study period, there was an increase in the representation of C and CR-strategists, mainly on behalf of CSR and SR strategists. CSR and CS strategies are most typical of native flora elements, C and CR of neophytes, while R and CR are most represented among archaeophytes. No consistent temporal trends in indicator values were recorded but the flora of the city differed from that of the surroundings in higher demands for light, temperature, nitrogen and soil reaction and lower demands for moisture. The present paper demonstrates that historical data on urban floras provide a powerful tool to evaluate the effect of humans on the development of urban landscapes and their plant life.