Hybridization . --XanthocephaZiArn sarothrae has a basic chromosome number of x = 4. 

 It is a polyploid species consisting most commonly of diploid (2n = 8) and occasionally 

 of tetraploid (2n = 16) populations. Both the diploids and the tetraploids show con- 

 siderable variation and grow intermixed through the range of species (Harrington 1954; 

 Solbrig 1971; Pinkava and Keil 1977). 



Distribution and habitat . --Broom snakeweed is the most abundant snakeweed in west- 

 ern North America. Many ecotypes have been found. It occurs in plains, valleys, 

 foothills, and mountainsides from Saskatchewan and Alberta to southeastern Washington, 

 southward to Texas and Baja California. It grows in a wide range of soil types at ele- 

 vations between 900 and 3,000 meters (2,950 and 9,850 feet) and may be associated with 

 such plants as creosote bush, mesquite {Prosopis spp.), soapweed (Yuaoa spp.), rabbit- 

 brush, big sagebrush, black sagebrush, shadscale, pinyon- juniper , oak {Queraus spp.), 

 Indian ricegrass {Oryzopsis hymenoides) , grama grass (Bouteloua spp.), buffalo grass 

 [Buohloe daotyZoides) , and cheatgrass {Brorms tectorum) (USDA Forest Service 1937) . 



The type locality of X. sarothrae is listed as the "plains of Missouri River" 

 (Abrams and Ferris 1960). 



i/se.--This shrub is a very aggressive invader into areas where the climax vegeta- 

 tion has been depleted. It is considered an indicator of range deterioration because 

 it is one of the first plants to invade when the range has been overgrazed and often 

 becomes the principal plant cover. As such, it is important in protecting the soil 

 against erosion. Broom snakeweed has usefulness as a pioneer shrub on other types of 

 disturbed areas as well. It is not, however, competitive with other perennial cover. 



Throughout its range, J. sarothrae is generally considered as poor forage and is 

 grazed mainly because of lack of anything better. It is a secondary or facultative 

 selenium absorber (Kingsbury 1964), and heavy use by livestock may cause sickness and 

 death where it takes up this chemical. On many areas it is substantially browsed by 

 livestock and game without any obvious ill effects. On the winter ranges of western 

 Utah and eastern Nevada, this shrub is rated as fair forage of sheep which browse it 

 most heavily after growth begins in the spring (USDA Forest Service 1937) . 



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