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ABSTRACT.-This study assesses the floristic diversity and affinities of pinyon-juniper (PJ) woodlands in northern Arizona at 2 different scales: regionally within the context of North American floristic patterns and locally within the 2500-m elevation gradient of the San Francisco Volcanic Field (SFVF). An analysis based upon 245 North American floras indicates that the PJ woodlands of the SFVF share strong affinities with the adjacent Colorado Plateau and Apachian floristic elements but also show high floristic similarity to the Great Plains. Data suggest that mid-elevation woodlands of the Colorado Plateau share floristic affinities with the Great Plains that are as strong as or stronger than those shared with the Great Basin. A geostatisical analysis provides a spatially explicit depiction of these findings. A comparison of species occurrences between 6 adjacent biotic communities in northern Arizona reveals that the PJ woodlands host the most distinctive flora among local life zones. Despite what their simple woodland structure may suggest, PJ woodlands of the SFVF host a moderately species-rich flora. This study suggests that the floras of PJ woodlands vary in significant and important ways across the range of PJ woodlands in western North America.
Key words: biogeography, phytogeography, floristics, pinyon-juniper woodlands, San Francisco Volcanic Field, Colorado Plateau.
Regional Floristic Affinities
Phytogeography, or the biogeography of plants, examines the spatial distributions and relationships of plant species (Good 1974). Individual plants inhabit specific geographic areas based upon a myriad of factors including climate, topography, physiology, and evolutionary and migrational history (Westoby and Wright 2006). Plant geographers have long struggled to classify regions of the world by cohesive and characteristic floristic elements (Takhtajan 1986). While it is often impossible to elucidate how or why the synergy of climate, topography, history, and geography favors a specific assemblage of plants in a specific region, biogeographers nevertheless strive to identify these unified regions. Plant geographers commonly delimit floristic elements based on geographically restricted and endemic taxa (Takhtajan 1986) or based on shared taxa (Stott 1981). McLaughlin (1986, 1989, 1994) has used overlap in the ranges of many species to extensively define areas of floristic similarity in the southwestern United States.
The San Francisco Volcanic Field (SFVF) of northern Arizona lies near the junction of the Colorado Plateau and Apachian floristic zones (Fig. 1)...