Abstract. Artemisia subgenus Tridentatae (Asteraceae–Asteroideae–Anthemideae–Artemisiinae) comprises 13 species, including 12 subspecies, of shrubs endemic to western North America, including the coastal areas of Baja California, the grasslands of the Great Plains, the basalt scablands of the Columbia Plateau, the western shrub lands of Canada, and the warm deserts of the Colorado Plateaus. The Tridentatae lineage underwent a period of rapid diversification and expansion, especially since the last glacial period. The greatest abundance of shrubs occurs within the arid Great Basin, a cold desert that was occupied by Pleistocene lakes. Taxa apparently representing ancestral lineages (A. rigida and A. tripartita) occur outside the margins of this inland desert. In spite of the extraordinary ecological specializations among the taxa, there is relatively little genetic differentiation, and morphological differences are often subtle. Differences in soil type, temperature, and moisture regimes distinguish the habitats of species as well as subspecies. Hybridization between species is rare, although hybridization among subspecies is common where populations are sympatric or habitats have been disturbed. Morphological differences among taxa primarily include discontinuities in growth form, the shape of the crown, the structure of the inflorescence, and habit (evergreen or deciduous, “root-sprouting” or not). Differences in leaf anatomy are significant and physiologically correlated, helping to define species boundaries but of no utility in field identification. Pollen varies notably in shape and size, and may prove to be useful in distinguishing species in stratigraphic profiles. Floral morphology varies little (florets and cypselae are nearly identical), but sexual arrangement within floral heads (capitula) defines sections: sect. Tridentatae is homogamous (all florets are perfect and fertile), and sect. Nebulosae is heterogamous (central florets are perfect or sterile, marginal florets are pistillate). An expanded circumscription and the geographic range of subg. Tridentatae is proposed. In order to accommodate morphological differences while keeping alliances indicated by molecular studies, subg. Tridentatae is divided into two new sections: sect. Tridentatae (10 species) and sect. Nebulosae; the latter includes A. californica and A. nesiotica (formerly placed in subg. Artemisia), and A. filifolia (formerly placed in subg. Dracunculus). Morphology and anatomy, ploidy levels, phytogeography, and phylogeny are discussed. Full synonymies and descriptions are provided for all taxa, as well as a key, specimen citations, illustrations, and maps.
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