ARTEMISIA subg. TRIDENTATAE in Flora of North America @ efloras.org

119b. ARTEMISIA Linnaeus subg. TRIDENTATAE (Rydberg) McArthur, Amer. J. Bot. 68: 590. 1981.

Artemisia [unranked] Tridentatae Rydberg in N. L. Britton et al., N. Amer. Fl. 34: 282. 1916

Shrubs; fibrous rooted, caudices woody, rhizomes absent. Stems not wandlike (relatively numerous; new stems may sprout from caudices). Leaves (pungently aromatic) deciduous or persistent, cauline (in lateral fascicles on vegetative shoots). Heads discoid (except A. bigelovii with, rarely, 1–2 raylike florets). Receptacles epaleate, glabrous. Pappi 0. Florets: 3–20, bisexual, fertile; corollas (pale yellow) funnelform.

Species 10 (10 in the flora): North America, nw Mexico.

Difficulty in classification of Artemisia subg. Tridentatae has been complicated by transfer of North American species to Seriphidium (Y. R. Ling 1995b; W. A. Weber 1984b), a disposition not followed here. Species circumscription varies among authors, but most modern treatments recognize the species as defined here. The most useful field characteristics in sagebrush taxonomy are size of the plant, shape and lobing of the vegetative leaves, and size and shape of the flowering heads (A. A. Beetle 1960; A. H. Winward 1970). Differences in chromosome number are more useful in defining subspecies than species (E. D. McArthur et al. 1981; G. H. Ward 1953), and introgression among subspecies is common (McArthur et al. 1988; McArthur and S. C. Sanderson 1999). The following key relies on vegetative characteristics, and unless noted, descriptions of leaf size and lobing refer to the leaves found in the vegetative shoots proximal to arrays of heads. These ‘vegetative leaves’ occur in bundles, or fascicles that are part of the lateral shoots. They are subtended by an elongate leaf (termed ‘ephemeral’), which is attached to the primary stem and falls off early in the season. With the exception of Artemisia spiciformis, which retains its ephemeral leaves through most of the growing season, ephemeral leaves normally drop from the plant before the onset of flowering.

SELECTED REFERENCES

Beetle, A. A. 1960. A study of sagebrush. The section Tridentatae of Artemisia. Wyoming Agric. Exp. Sta. Bull. 368. Ling, Y. R. 1995b. The New World Seriphidium (Besser) Fourr. In: D. J. N. Hind et al., eds. 1995. Advances in Compositae Systematics. Royal Botanic Gardens, Kew. Pp. 283–291. McArthur, E. D., C. L. Pope, and D. C. Freeman. 1981. Chromosomal studies of subgenus Tridentatae of Artemisia: Evidence for autopolyploidy. Amer. J. Bot. 68: 589–605. McArthur, E. D. et al. 1998. Randomly amplified polymorphic DNA analysis (RAPD) of Artemisia subgenus Tridentatae species and hybrids. Great Basin Naturalist 58: 12–27. McArthur, E. D. and S. C. Sanderson. 1999. Cytogeography and chromosome evolution of subgenus Tridentatae of Artemisia. Amer. J. Bot. 86: 1754–1775. Shultz, L. M. 1983. Systematics and Anatomical Studies of Artemisia subgenus Tridentatae. Ph.D. dissertation. Claremont Graduate School. Shultz, L. M. 1986. Taxonomic and geographic limits of Artemisia subgenus Tridentatae (Beetle) McArthur. In: E. D. McArthur and B. L. Welch, eds. 1986. Proceedings, Symposium on the Biology of Artemisia and Chrysothamnus, Provo, Utah, July 9–13, 1984. Ogden. Pp. 20–28. Shultz, L. M. 1986b. Comparative leaf anatomy of sagebrush. In: E. D. McArthur and B. L. Welch, eds. 1986. Proceedings, Symposium on the Biology of Artemisia and Chrysothamnus, Provo, Utah, July 9–13, 1984. Ogden. Pp. 253–264. Ward, G. H. 1953. Artemisia section Seriphidium in North America, a cytotaxonomical study. Contr. Dudley Herb. 4: 155–206. Winward, A. H. 1970. Taxonomic and Ecological Relationships of the Big Sagebrush Complex in Idaho. Ph.D. dissertation. University of Idaho.

1 Leaves deciduous, blades usually entire, sometimes irregularly lobed; moist habitats 11 Artemisia cana

+ Leaves deciduous or persistent, blades usually lobed, sometimes entire; dry habitats (2)

2 (1) Leaves bright green, pinnately lobed, lobes 3–7 (gypsum or shale) 13 Artemisia pygmaea

+ Leaves gray-green, usually palmately lobed, lobes 0 or 3 or 3–6 (3)

3 (2) Leaf lobe lengths ¹/3+ blade lengths, widths 1–1.5 mm (4)

+ Leaf lobe lengths to ¹/3 blade lengths, widths (1–)1.5–5 mm (5)

4 (3) Leaves rigid (lava scablands, Oregon and Washington) 14 Artemisia rigida

+ Leaves not rigid 18 Artemisia tripartita

5 (3) Shrubs, 50–200(–250) cm (6)

+ Shrubs, 10–50 cm (8)

6 (5) Leaves mostly deciduous (variable in size and shape, entire or irregularly 3–6-lobed, lobes rounded or acute); involucres broadly campanulate. 16 Artemisia spiciformis

+ Leaves persistent (lobes 3, uniform, lengths to ¹/3 blade lengths); involucres lanceoloid or ovoid (7)

7 (6) Leaves light or dark gray-green, sticky-resinous; involucres ovoid, 3–5 × 4–6 mm; florets 12–20 (California) 15 Artemisia rothrockii (in part)

+ Leaves gray-green, not sticky-resinous (widespread, including California); involucres lanceoloid, (1–)1.5–4 × 1–3 mm; florets 3–8. 17 Artemisia tridentata

8 (5) Leaves silver-green, blades narrowly cuneate, lobes acute; heads mostly nodding; involucres globose 10 Artemisia bigelovii

+ Leaves dark green to gray-green, blades broadly cuneate, lobes obtuse or rounded; heads mostly erect; involucres campanulate, globose-ovoid, or turbinate. (9)

9 (8) Leaves on flowering stems entire (heads mostly pedunculate); involucres narrowly turbinate; phyllaries sparsely hairy or glabrous 12 Artemisia nova

+ Leaves on flowering stems entire or lobed (heads mostly sessile); involucres campanulate or globose-ovoid; phyllaries densely pubescent or tomentose (10)

10 (9) Leaves on flowering stems 3-lobed, not sticky-resinous 9 Artemisia arbuscula

+ Leaves on flowering stems entire, sticky-resinous or densely hairy and not sticky 15 Artemisia rothrockii (in part)

List of lower taxa