Floristic analysis of vascular plant genera of North America north of Mexico: spatial patterning of phytogeography

Aim This paper aims at determining and quantifying phytogeographical relati onships between North America and the rest of the world at the regional lev el and identifying possible patterns of regional differentiation in phytoge ographical relationships, particularly along latitudinal and longitudinal g radients. Location North America north of Mexico. Methods North America north of Mexico was divided into twelve geographical regions arrayed in a grid with three divisions east-to-west by four divisio ns north-to-south. Based on the similarity of their world-wide geographical distribution patterns, a total of 1904 indigenous genera of vascular plant s found in North America north of Mexico were grouped into ten phytogeograp hical elements. Phytogeographical relationships of the twelve geographical regions of North America north of Mexico to the world flora were examined, based on the composition of phytogeographical elements. Multivariate analys es were used to determine the phytogeographical relationships. Results Along a latitudinal gradient from the Arctic to tropical North Amer ica, the proportions of the cosmopolitan and holarctic genera markedly incr eased, whereas the pantropical, amphi-Atlantic tropical, neotropical and No rth American endemic genera strikingly decreased. Along a longitudinal grad ient from the Atlantic coast to the Pacific coast, the proportions of the p antropical, amphi-Pacific tropical, neotropical and North American-eastern Asian genera tended to decrease, whereas the North American-South American and North American endemic genera tended to increase. Main conclusions The spatial patterns of the phytogeography of North Americ a could be to a large degree interpreted by the different effects of the di rect land connections between North America and Eurasia and South America o n intercontinental migration and by the different effects of regional geogr aphical settings within North America on the speciation, dispersal and exti nction of plant species.