XVI International Botanical Congess

Different qualitative aspects of the vascular plant diversity of the U.S.A. are compared mainly on the basis of the PLANTS database (USDA & NRCS 1997). Additionally, we present a map of species numbers of vascular plants in North America based on data from more than 130 different geographical units. While species numbers show a latitudinal gradient and higher values along the coast and the main mountain systems, family numbers as a measure for phylogenetic diversity peak in the south-eastern part of the country. Endemism shows highest values in the western U.S.A., which have a higher diversity of abiotic factors (geodiversity). The floras of different biogeographical units within the U.S.A. are compared in a cluster analysis with different holarctic and neotropical floras to analyse large-scale biogeographical relationships.