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Floristic elements and floristic areas for North America were circumscribed using principle components analysis (PCA) of a sample of 245 local floras from Canada, the United States and Mexico. Three analyses were conducted: (1) a PCA on a matrix of Otsuka similarity indices based on shared species, which identified 27 floristic subprovinces; (2) a PCA on a matrix of Pearson correlations on the log number of species per genus in each flora, which identified 12 floristic provinces; and (3) a PCA on a matrix of Pearson correlations on the log number of species per family in each flora, which identified 4 floristic regions.
Seventy-eight percent of the 245 floras formed nested hierarchical groups across all three analyses; 98% formed nested groups over two levels of the hierarchy. When compared with earlier biogeographic treatments of North America by Dice, Udvardy, and Cronquist, my results supported different aspects of each one but also showed that none completely captured the major floristic patterns on the continent.