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 relationships between North America and the rest of the world at the regional level and identifying possible patterns of regional differentiation in phytogeographical relationships, particularly along latitudinal and longitudinal gradients. 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 divisions north-to-south. Based on the similarity of their world-wide geographical distribution patterns, a total of 1904 indigenous genera of vascular plants found in North America north of Mexico were grouped into ten phytogeographical 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 analyses were used to determine the phytogeographical relationships. Results Along a latitudinal gradient from the Arctic to tropical North America, the proportions of the cosmopolitan and holarctic genera markedly increased, whereas the pantropical, amphi-Atlantic tropical, neotropical and North American endemic genera strikingly decreased. Along a longitudinal gradient from the Atlantic coast to the Pacific coast, the proportions of the pantropical, 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 America could be to a large degree interpreted by the different effects of the direct land connections between North America and Eurasia and South America on intercontinental migration and by the different effects of regional geographical settings within North America on the speciation, dispersal and extinction of plant species.