increased - its site dominance (Mack 1981). There is 

 low potential for the return of natural vegetation on 

 Prairie Dog Island. A large sward of Canada thistle 

 (Cirsium arvense) adjoins the southeast end where bird 

 use seemed heaviest. The plains cottonwood (Populus 

 dekoides) and tamarisk (Tamarisk chmensis) that had 

 established along the shoreline were innundated and 

 killed in the high water conditions of 1997-98. 



Spring Creek Research Natural 

 Area 



ENVIRONMENT: 



Spring Creek RNA is a 160 acre tract that encompasses 

 an incised ravine system, adjoining tableland, and 

 valley slopes along the Dry Arm of Fort Peck Reservoir. 

 It is part of the Missouri River Breaks (Subsection of 

 331D (Nesser et al. 1997). The 190 ft. of vertical relief 

 dissects both sandstone and underlying siltstone 

 bedrock. It is situated north of Spring Creek and Spring 

 Creek Bay, with an ephemeral streamcourse that 

 empties westward into the Reservoir. Upland soils, 

 upper ravine slopes, and ravine bottom soils are fine 

 sandy loams developed from the underlying bedrock. 

 Lower ravine slopes, benches and valley slopes are silt 

 loams (shale derived) . The climate is essentially 

 Continental (refer to the characterization of Fort Peck 

 monitoring data, presented for Prairie Dog Island 

 RNA.) 



VEGETATION: 



Well-developed expressions of tableland and ravine 

 slope plant communities are found, as well as a stringer 

 of woody draw vegetation that has a prevalent green 

 ash (Fraxinus pensylvardca) component in the canopy 

 mixture. The original designation of this RNA to 

 preserve an unusual stand of aspen (Populus 

 iremuloides) may have been based on the interpretation 

 that this species, being the tallest in the ravine, is the 

 stand dominant. The complement of plant associatioiis 

 does not have one that is clearly more extensive than 

 any other. Perhaps the most extensive, though 

 variable, cover type is comprised of the Wyoming big 

 sagebrush shrubland (Artemisia trideruata ssp. 

 wyomingensis Shrubland) on benches at the mouth of 

 the ravine and valley slopes above the reservoir. 



Artemisia tridentata ssp. wycmingensis I Pascopyrum 



smithii Shrubland 



[ARTTSW / PASSMI] 



Wyoming big sagebrush / western wheatgrass shrubland 



This association is a major vegetation type both in the 

 study area and across the Northern Great Plains from 

 Colorado north to Saskatchewan, on both glaciated 

 and non-glaciated surfaces. The majority of the type 

 occurs in Wyoming and Montana. It generally occurs, 

 as on this RNA, over large areas, except at the 

 periphery of its range, where it may occur in small 

 patch fragments. There are several recognized names 

 and variations for the Wyoming big sagebrush/ western 

 wheatgrass shrubland (including Artemisia trideruata I 

 Pascopyrum smithii, Artemisia tridentata ssp. tridentata I 

 Pascopyrum smithii, Artemisia tridentata I Elymus 

 larv:eolatus, A. tridentata I Pascopyrum smitHii - (Elymus 

 larKeolatus) [Schneider et al. 1997, Vanderhorst et al. 

 1998].) Some of these differ only in that the subspecies 

 of big sagebrush was not stipulated or known at the 

 time the investigations were conducted. Only the 

 Artemisia tridentata ssp. tridentata association 

 (ARTTST) represents a distinct habitat, one more 

 associated with swales and drainages and found to the 

 west of the ARTTSW / PASSMI type. The ARTTSW 

 / PASSI type is generally associated with low relief 

 uplands, benches, plateaus, or rolling terrain but within 

 most of the RNA these flatter surfaces are sandstone 

 capped and favor needle-and-thread (Stipa comata}- 

 dominated grasslands, whereas the Artemisia tridentata 

 communities are found downslope on gentle inclines 

 with heavier-textured soils (silty clay loams). This 

 association, including the RNA representation, has 

 considerable exposed surfaces (soils mainly) , generally 

 in excess of 50%. 



The shrub canopy of Artemisia tridentata ssp. 

 wyomingensis is variable in cover, but generally in the 

 vicinity of 20-30%, straddling the shrubland to herb- 

 dominated structural break according to NVCS. 

 Winter fat (Krascheninrukovia lanata) and Fringed sage 

 (Artemisia frigida) are present in trace amounts, though 

 areas of disturbance support more Artemisia frigida. 

 Western wheatgrass (Pacopyrum smithii) and 

 threadleaved sedge (Carex filifolia) are the dominant 

 graminoids, their combined cover usually not exceeding 

 50%. Green needlegrass (Naselia viridula) is present, 

 mosdy under the protective canopy of A. tridentata, 

 raising some speculation, at least for the footslope 

 positions, that ARTTSW / PASSMI - Naselia viridula 

 may be the potential community type. CertaiiJy in the 

 past, livestock grazing pressure might have been 

 extreme and led to sigruficantly reduced coverage for 

 the highly palatable Naselia iiiridula. The forb 

 component is both sparse and species poor. In the plot, 

 two of the four species, including plains prickly-pear 

 (Opuntia polyacarxtha) and brittle prickly-pear (Opuntia 

 fragilis) are associated with overgrazing, though their 



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