Soapweed (Yucca glauca) COSEWIC assessment and status report 2013: chapter 8

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Habitat

Habitat Requirements

In Canada, Soapweed occurs in the Mixed Grass ecoregion (Wilken 1996). The Alberta populations occur in the Dry Mixedgrass Subregion (Natural Regions Committee 2006). This Subregion has very warm summers, high solar gains, long growing seasons and low precipitation with large daily variation and weather extremes. The mean growing season temperature is approximately 16 °C, the regional frost-free season is 100-120 days and the growing season about 180-200 days (Fairbarns 1985). The mean percentage of daylight hours with bright sunshine is the highest in Canada (50%) or 2200 hrs/year (Fairbarns 1985). Total annual precipitation is between 260 to 280 mm, with two thirds of that falling as spring rains (Hurlburt 2001). Due to warm summer temperatures and a high average wind speed, the rate of evaporation is high throughout the summer months (Adams et al. 2005; Knapton et al. 2005). Wind speeds of 70 to 90 km/hr and gusts over 130 km/hr are common.

At the northern limit of its range, Soapweed is found on well-drained coulee slopes that are generally eroded, dry and sparsely vegetated (Figures 5, 6; Milner 1977; Fairbarns 1985; Csotonyi and Hurlburt 2000). They mostly occur on south-facing slopes. At Onefour, the azimuth of these slopes range from 34° (northeast) to 220° (south-southwest), and except for some sites sheltered by adjacent ridges, generally face away from prevailing southwest winds (Csotonyi and Hurlburt 2000). Soils tend to be alkaline and regosolic without shallow hardpan (Milner 1977; Fairbarns 1985).

Figure 5. Soapweed in heavy flower (early June 1999). South-facing coulee slope is representative habitat of northern populations of Soapweed

Photo: D. Hurlburt.

Figure 6. Soapweed habitat is sparsely vegetated and eroded. The Sweetgrass Hills in Montana are visible in the background.

Photo: D. Hurlburt.

Figure 7. Four year old Soapweed seedling at Onefour, AB. Note the grass-like leaves rather than the sharp, erect leaves of an older plant.

Photo: D. Hurlburt.

In Onefour, Soapweed is found on eroding kame slopes dominated by Silver Sagebrush (Artemisia cana). Major grasses include Blue Grama (Bouteloua gracilis) and Plains Muhly (Muhlenbergia cuspidata), while Needle and Thread Grass (Hesperostipa comata), Prairie Junegrass (Koeleria macrantha) and Prairie Sandreed (Calamovilfa longifolia) are locally common. Major forbs include Plains Prickly-pear Cactus (Opuntia polyacantha), Pincushion Cactus (Escobaria vivipara), Smooth Blue Beardtongue (Penstemon nitidus), and Broom Snakeweed (Gutierrezia sarothrae) (Wershler and Wallis 1986).

Habitat Trends

Most of the area occupied by the two previously known populations is designated as Critical Habitat (Environment Canada 2011). Although grazing is the predominant land use in the immediate vicinity of Soapweed, the habitat remains largely undisturbed and has not declined in availability since the area was settled over 100 years ago.

At Onefour, a small proportion of clones grow on the prairie uplands, presumably as a result of a prairie fire in the 1970s. The ability of Soapweed to spread beyond its current range may be dependent on ecological drivers that reduce competitive grasses during years of high fruiting. Historically, heavy grazing and wallowing by bison, and fire may have performed that role (Samson and Knopf 1994; Samson et al. 2004). Fire in the mixed-grass prairie was estimated to occur every three to five years prior to European contact (Samson and Knopf 1994; Samson et al. 2004), but is thought to have occurred only once in Soapweed habitat at Onefour during the last 35 years.