Drivers of floristic change in large cities - A case study of Frankfurt/Main (Germany)

The main aim of the study is to reconstruct diversity of established vascular plants between 1800 and 2000 in today's area of Frankfurt/Main (Germany, Hesse) - using historical literature, herbarium vouchers, and a recent landscape survey - and to interpret changes (extinctions and gains) in the light of land use changes (drivers). Species numbers declined from 1800 (1232 species; 926 indigenous-228 archaeophytes-78 neophytes) to 1900 (1200 species; 871-219-110), and 2000 (1107 species; 698-166-252). The most important drivers for the loss of species (319 spp.) are use of fertilizers and herbicides, abandonment of wood pastures, abandonment and melioration of wet grassland. Most species gained (193 spp.) were intentionally introduced as ornamentals. Among the neophytes naturalized after 1800, short-living plants adapted to dry open habitats dominate. Plants using the C4 carbon fixation pathway (C4-plants) are predominantly neophytes. Drivers for the loss of species are general changes in land use. Nature conservation slackened the loss of species in the last decades; today about 3% of the vascular plants are conservation dependent. Comparatively high rates of short-living plants adapted to dry open habitats and C4-plants among neophytes indicate a warming of the climate. We expect a rising number of garden escapes. Furthermore, the flora will become more dependent on conservation measures. Remnants of now obsolete forms of land use like low nutrient grassland shall be integrated into parks and urban open spaces.