Artemisia tridentata ssp. wyomingensis / Opuntia polyacantha Community Type 



ARTTSW / OPUPOL; Wyoming big sagebrush / prickly-pear cactus 

 MTNHP rank: to be determined 



Environment: This highly localized community type occurs on nearly level valley floors and 

 benches on erodable alluvial outwash materials weathered from shale. It was sampled by two plots 

 and observed elsewhere numerous times in the course of reconnaissance, and has not been described. 

 It occurs as small patches (« 1 acre) in matrices of sagebrush steppe (ARTTSW / PASSMI) with 

 higher grass cover, and in grasslands {Pascopyrum smithii or Elymus /awceo/afw^-dominated) which 

 occupy less eroded substrates. Additional examination of the soil profile is needed to determine 

 whether these settings represent pan spot range sites, often referred to as "scablands." 



Vegetation: The overall sparse canopy cover is dominated by Artemisia tridentata ssp. 

 wyomingensis (15-25 %) with a stature considerably shorter (0.15 — 0.25 m.) than specimens of the 

 surrounding matrix communities (mostly 0.3-0.6 m). There are only eight other species in the two 

 plots, of these, the grass Elymus lanceolatus, the forb Polygonum ramosissimum, and the cactus 

 Opuntia polyacantha are constant. The relative paucity of £. lanceolatus in these sites (not 

 exceeding trace amounts) compared to its canopy cover in the surrounding matrix is significant. 

 Opuntia polyacantha has 10% cover in one plot, but much higher cover values were noted during 

 reconnaissance. These cursory observations, though not supported by quantitative data, do express a 

 consistency of structure with widely-spaced, short stature A. tridentata ssp. wyomingensis and the 

 high constancy, if not forb-layer dominance, ofO. polyacantha; in many instances Poa secunda 

 and/or P. cusickii are noted to be the herb-layer dominants. 



Soils: The depauperate vegetation is reflected in the lack of litter and more than 80% exposed soil 

 surface. Parent materials are alluvial outwash derived from shales. We have inadequate data to 

 compare this community type to its closest analogue, ARTTSW / ELYLAN-BOUGRA but a cursory 

 examination shows a complete overlap in ranges for all variables. Branson et al. (1970) note a ^4. 

 tridentata / O. polyacantha community type in northeastern Montana developed on Bearpaw Shale. 

 It differs fi-om a commimity type they call simply A. tridentata by having higher pH values (all over 

 7 versus all less than 5) and higher soluble sodium values (> 50% versus < 50% soluble sodium). 



Comments: Only Branson et al. (1970) have described (or at least named) an A. tridentata / O. 

 polyacantha community type, which occurs on Bearpaw Shale in northeastern Montana. The 

 Artemisia subspecies they encountered is undoubtedly wyomingensis as it is the only subspecies 

 distributed in this comer of Montana (Shultz 1984). They list Elymus lanceolatus as having as high 

 a coverage in ARTTSW / OPUPOL as in the ARTTSW community type. Not knowing the grazing 

 history of their site it is difficult to assess the ecological meaning of the closely comparable 



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