Large-scale biogeographic patterns of vascular plant richness in North America: an analysis at the generic level



The richness pattern of native vascular plants in North America (north of Mexico) was studied at the generic level. North America was divided into thirteen geographical regions, which were latitudinally grouped into four horizontal zones (northern, north-middle, south-middle, and southern zones); and longitudinally grouped into three vertical zones (eastern, central, and western zones). The native vascular flora of North America consisted of 1904 genera in 235 families and eighty-three orders. Along the latitudinal gradient, generic richness (in terms of the number of genera) showed a striking increase with decreasing latitude. The southern zone had more than four times as many genera as did the northern zone (with a difference of 1436 genera). 93.3% of genera in the northern zone also occurred in the southern zone. Along the longitudinal gradient, the central zone had the highest generic richness and the eastern zone had the lowest, but the difference in generic richness between the two zones was only sixty-one genera. The western and eastern zones shared 60% or more of their genera. Generic richness of vascular plants in North America was highly correlated (r=0.965) to available environmental energy (expressed by annual potential evapotranspiration).