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 M exico) was studied at the generic level. North America was divided into thi rteen geographical regions, which were latitudinally grouped into four hori zontal zones (northern, north-middle, south-middle, and southern zones); an d longitudinally grouped into three vertical zones (eastern, central, and w estern zones). The native vascular flora of North America consisted of 1904 genera in 235 families and eighty-three orders. Along the latitudinal grad ient, generic richness tin terms of the number of general showed a striking increase with decreasing latitude. The southern zone bad more than four ti mes as many genera as did the northern zone (with a difference of 1436 gene ra), 93.3% of genera in the northern zone also occurred in the southern zon e. Along the longitudinal gradient, the central zone had the highest generi c richness and the eastern zone had the lowest, but the difference in gener ic richness between the two zones was only sixty-one genera. The western an d eastern zones shared 60% or more of their genera. Generic richness of vas cular plants in North America was highly correlated (r = 0.965) to availabl e environmental energy (expressed by annual potential evapotranspiration).