These rare plant communities in the Pryor Mountain Desert 

 are threatened by (1) encroachment of exotic species, (2) 

 livestock and feral horse grazing, (3) off-road vehicle (ORV) 

 use, and (4) oil and gas development (Lesica and Achuff 1992). 



Exotic Species 



Haloqeton qlomeratus is an Asian species that was introduced 

 into arid v/estern North America in 1934. It has since spread 

 throughout much of the arid and semi-arid lands in the western 

 U.S. It is poisonous to livestock, and it outcompetes many 

 native species by increasing the salinity of the soil (Sauer 

 1988) . This plant is present in the Bighorn Basin (Fisser and 

 Joyce 1984) and has become locally common in the Pryor Mountain 

 Desert region and may be able to displace native plants, 

 especially annuals. Haloqeton qlomeratus was found in three of 

 the rare community types, Artemisia tridentata / Atriplex 

 nuttallii , Atriplex nuttallii / Monolepis nuttalliana and 

 Chrysothamnus nauseosus / Erioqonum brevicaule , and it is expected 

 to be able to invade the Artemisia pedatifida - Atriplex nuttallii 

 community type. Haloqeton qlomeratus is a threat to individual 

 rare plant species (Lesica and Achuff 1992) as well as the 

 integrity of rare communities. 



Grazing 



Recent livestock grazing appeared to be light to moderate 

 throughout most of the lov/er portions of the study area. 

 However, severe effects of overgrazing were apparent in areas 

 near water, such as Gyp Springs and Bear Canyon. Overgrazing in 

 the Gyp Springs area is a particularly serious problem because 

 this area supports the only stands of the rare Atriplex 

 nuttallii - Artemisia spinescens community type. Four or five 

 species of grass occur in this type, but total grass cover was 

 always less than 5%. Low grass cover may be due to the harsh 

 edaphic environment, but grazing is likely an exacerbating 

 factor. Most stands of Artemisia pedatif ida - Atriplex nuttallii , 

 another rare community type, had very low grass cover, but one or 

 two stands had much more grass. Stands on steep slopes often had 

 more grass cover than stands on adjacent terraces. These 

 differences in grass cover are likely caused by livestock 

 grazing. ATRNUT-ARTSPI and ARTPED-ATRNUT are particularly 

 vulnerable to livestock grazing because they occur on nearly 

 level terrain v/here use is heaviest. 



Damage due to feral horses was observed in the desert west 

 of the mouth of Big Coulee. Steep slopes and banks had been 

 severely terraced and destabilized. Fragile vegetation in this 

 arid environment is slov; to recover, particularly v;hen the 

 effects of trampling and terracing are compounded by erosion. 

 Subalpine grasslands appear to have been degraded by feral horse 

 grazing. We sampled inside a horse exclosure (T3S R28E S18 



38 



