Currently there are six subspecies of Artemisia tridentata recognized by the National Plant Data Center: basin (ssp. tridentata), Wyoming (ssp. wyomingensis Beetle & Young), mountain (ssp. vaseyana [Rydb.] Beetle), subalpine (ssp. spiciformis [Ousterhout] Kartesz & Gandhi), xeric (ssp. xericensis Winward ex R. Rosentreter & R. Kelsey) and Parish’s (ssp. parishii [Gray] Hall & Clements). Each is highly variable with multiple ecotypes, but can generally be separated using a number of morphological, geographical and topographical characters.
All chromosome number information was obtained from McArthur and Sanderson (1999) as updated in McArthur (2005). Appendix 1 contains a general summary of characteristics useful to separate subspecies. See McArthur and Stevens (2004) for a detailed review of the characters for the subspecies occurring in the Intermountain West.
Figure 2. Leafy stem of basin big sagebrush. Photo courtesy of the PLANTS database.
Basin big sagebrush usually occurs at the lowest elevational range of the species, being most abundant in the valley bottoms to mountain foothills. Plants typically have a single main trunk and may grow to a height of 4 m (13 ft) under proper conditions, making basin the largest subspecies. Basin big sagebrush plants are generally uneven-topped with loosely branching flowering stems distributed throughout the crown (see figure 1). Floral heads typically contain 3 to 6 small flowers per head. Leaves of the vegetative stems are narrowly cuneate averaging 2 cm (0.8 in) or more and can be as long as 5 cm (2 in) being many times longer than wide (see figure 2). Ultraviolet visible coumarins in leaf extracts are minimal; leaf UV color is none to light blue in water and a rusty red-brown color in alcohol. 2n = 18 or sometimes 36.
Figure 3. Wyoming big sagebrush.
Derek Tilley, USDA NRCS Idaho PMC
Wyoming big sagebrush overlaps in range and elevation with basin big sagebrush. Plants are considerably smaller than those of basin big sagebrush, usually less than 0.9 m (3 ft) tall, and have main stems branching from the ground (see figure 3). Flowering stems are not as widely branching as those of basin, but otherwise closely resemble that subspecies. Leaves are typically shorter, from 1 to 1.5 cm (0.4 to 0.6 in) long, and flabelliform. UV extract color in water is none to light blue and rusty in alcohol. 2n = 36.
The vegetative stems of mountain big sagebrush create a characteristic even topped crown with the panicles rising distinctly and relatively uniformly above the foliage (see figure 3). Plants are normally smaller than those of basin big sagebrush, averaging about 0.9 m (3 ft) tall. Inflorescences are narrow and spicate bearing flower heads containing 4 to 8 flowers per head. Leaves are characteristically wider than those of basin or Wyoming big sagebrush. In extracts, ultraviolet visible coumarins are abundant. Leaf extracts fluoresce blue in water and blue-cream in alcohol. 2n = 18 or sometimes 36.
Figure 4. Even topped mountain big sagebrush.
Derek Tilley, USDA NRCS Idaho PMC
Originally considered a xeric form of mountain big sagebrush, xeric big sagebrush shares similarities with both basin and mountain big sagebrush and may be the result of hybridization between the two subspecies. Xeric big sagebrush plants are large and have an uneven topped crown like those of basin big sagebrush, but in leaf UV color and cytological characters it resembles mountain big sagebrush. Ultraviolet visible coumarins are blue in water, blue-cream in alcohol. 2n = 36.
A new variation of big sagebrush being recognized by some is Bonneville big sagebrush. This as yet undescribed taxon may represent hybridization between Wyoming and mountain big sagebrush. It is reported to have the general growth form of Wyoming plants but bears the leaves and fluorescing characteristics of the mountain subspecies. It has been reported from the bench areas of Lake Bonneville and other ancient lakes of the Intermountain West in Utah and Nevada. Reports of Bonneville big sagebrush have also come from western Wyoming and western Colorado. Of particular importance is this sagebrush’s reported high palatability to wild ungulates and sage grouse.
Subalpine, or spicate big sagebrush, is believed to be a stabilized hybrid between mountain big sagebrush and silver sagebrush (Artemisia cana Pursh ssp. viscidula [Osterhout] Beetle). Plants are similar to those of mountain big sagebrush except that leaves and floral heads are larger, the floral heads having 10 to 18 flowers per head. Ultraviolet visible coumarins in leaf extracts fluoresce blue in water and blue-cream in alcohol. 2n = 18 or 36.
Parish’s big sagebrush is an uncommon taxon restricted to dry, sandy soils in the hills of southern California. It is nearest in appearance and relationship to basin big sagebrush, but differs from basin in having drooping flowering branches and the achenes are hairy. 2n = 36.
One additional taxon that should be mentioned is Lahontan sagebrush (Artemisia arbuscula ssp. longicaulis Winward and McArthur). It is thought to be a stable hybrid between low sagebrush (A. arbuscula) and Wyoming big sagebrush. It bears the flowers of low sagebrush but has the vegetative characteristics of its big sagebrush parent. This subspecies forms dominant communities in northwestern Nevada and adjacent portions of California and Oregon in shallow or clayey soils above and around the shoreline of the Pleistocene Lake Lahontan.
The following key should provide some assistance in separating the subspecies of big sagebrush.
1. plants larger, usually >0.9 m (3 ft) tall, with a
single main trunk; crown uneven with floral stems
2. achenes hairy; floral stems drooping; plants
endemic to sandy soils in southern
2. achenes glabrous; floral stems erect; plants
widely distributed throughout western U.S,
including southern California
3. plants occurring in valley bottoms and low
foothills, occupying deep fertile soils; leaves
narrowly cuneate, 2-5 cm (0.8-2.0 in) long,
UV leaf color in water=none, in alcohol=red
to brown ……...................…...ssp. tridentata
3. UV in water=blue, in alcohol=blue-cream;
plants restricted to well-drained basaltic soils in western Idaho….........ssp. xericensis
1. plants smaller, averaging 0.9 m (3 ft) or less, with
trunks branching at or near ground level; crowns
4. crowns uneven-topped, plants of low
valleys and foothills;
5. UV color in water=none, in alcohol=rust
5. UV color in water=blue, in alcohol=blue-
4. crowns even-topped, floral stems rising
uniformly above the vegetative stems; plants of
6. flowers 4 to 8; leaf tips lobed
6. flowers 10 to 18; leaf tips often pointed
Additional taxonomic information can be found in the Flora of North America, Volume 19 (FNA Editorial Committee 2006) and the Intermountain Flora, Volume 5 (Cronquist et al. 1994).
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