Spatial Pattern of Vascular Plant Diversity in North America North of Mexico and its Floristic Relationship with Eurasia

Abstract

This paper reports: (1) patterns of taxonomic richness of vascular plants in North America (north of Mexico), an area accounting for 16.6% of the total world land, in relation to latitudinal and longitudinal gradients; (2) floristic relationships between different latitudinal zones, longitudinal zones, and geographic regions of North America; and (3) floristic relationships between North America and Eurasia at various geographic scales. North America was geographically divided into twelve regions, which were latitudinally grouped into four zones, each with three regions, and longitudinally grouped into three zones, each with four regions. The native vascular flora of North America consists of 162 orders, 280 families, 1904 genera and 15352 species. Along the latitudinal gradient, species richness shows a striking increase with decreasing latitude (e.g. the northernmost latitudinal zone has only 11.7% of the number of species in the southernmost latitudinal zone). However, about 63% of the species of the northernmost latitudinal zone are also present in the southernmost latitudinal zone of North America. Among the three longitudinal zones, the zone on the Pacific coast has 1.48 and 1.64-times as many species as the zones in the interior and on the Atlantic coast, respectively. About 36% of the species in the zone of the Atlantic coast also occur in the Pacific coast zone. However, each of over 40% of the species in North America occupies less than 10% of the total land area of North America. Some 48% of the genera and 6.5% of the species of North America are also native to Eurasia. In general, the number of genera common to North America and Eurasia increased from the north to the south and from the west to the east of North America, whereas the number of species common to the two continents decreased along the same two geographic gradients.