Lake Mason National Wildlife Refuge 



Lake Mason Research Natural 

 Area 



environment: 



Lake Mason RNA is in a broad, open natural basin, 

 comprised of two parcels totaling 1,420 acres, lying on 

 either side of Lake Mason at the lake perimeter. The 

 lake outlet is South Willow Creek, and it has a spillgate 

 to artificially maintain lake levels. Lacustrine deposits 

 and alluvium derived from shale and sandstone are the 

 primary parent materials in the basin, though residuum 

 derived soils occur in the western half of Section 22. 

 All soils appeared to have a heavy texture, with silty 

 clays and silty clay loams predominant. The semi-arid 

 continental climate has peak precipitation in June 

 followed by May, and a mean annual precipitation of 

 12.4 inches (Climate data from Roundup, Western 

 Regional Climate Center, 1914-1997). This RNA 

 occurs within the Montana Sedimentary Plains 

 Subsection of the Powder River Basin Section where 

 annual precipitation ranges from 11 to 14 inches 

 annually, about a third of which is snow. 



VEGETATION: 



The three main vegetation types are distributed 

 primarily according to soil moisture regimes, which vary 

 with distance from Lake Mason and South Willow 

 Creek with the exception of the uplands in Section 22. 

 All but the standing water (herbaceous emergent) plant 

 communities were sampled. 



Pascopyrum smtkii Herbaceous Vegetation 



IPASSMI] 



western wheatgrass wet meadow 



The exterwive alluvial flats are dominated by western 

 wheatgrass (Pascopyrum smithii). There are at least six 

 plant associations named across the Northern Great 

 Plains that have Pascopyrum smithii as the first-named 

 indicator species; only the type named here is defined 

 by the virtual monospecific dominance of P. smithii and 

 is rated G3G5 by TNC. This type is typically strongly 

 associated with subirrigated alluvial flats and most of 

 this plant association in the RNA meets definitions of 

 jurisdictional wedand (Hansen et al. 1995). Consistent 

 with this characterization, both sample plots displayed 

 gleyed and mottled soils. It is dominated by a sward of 



P. smithii ranging in cover from 50 to in excess of 80 

 percent, making up a relatively homogeneous expanse 

 on the flats (and beyond) around the perimeter of the 

 lake. At least in the sampled locations, weedy or 

 increaser with disturbance species, e.g. Japanese brome 

 (Bromusjaponicus), povertyweed (Iva axillaris), wild 

 lettuce (Lactuca canadensis), flixweed tansymustard 

 (Descurairua sophia) , and common dandelion 

 (Taraxacum officinale) dominate the herbaceous layer to 

 the near exclusion of native species except grasses. This 

 may reflect a history of heavy grazing by livestock. 

 [Plots NHMTECLM97SC0001, NHMTECLM970006] 



Pascopynmi smithii - Nasella viridula 



Herbaceous Vegetation 



[PASSMI-NASVIR] 



western wheatgrass - green needlegrass grassland 



This association is the prevailing type on non-wetland 

 alluvial flats and on gentle upland slopes; it constituted 

 a major plant association prior to agricultural 

 development. It is ranked as G4 and occurs in North 

 Dakota, South Dakota, Wyoming, and Saskatchewan. 

 Stands on the flats grade to the western wheatgrass wet 

 meadow (Pascopyrum smithii association) and those of 

 the uplands grade to western wheatgras- needle-and- 

 thread grassland (Pascopyrum smithii / Stipa comata) of 

 drier sites. In the vicinity of South Willow Creek this 

 type appeared to be in good to excellent condition with 

 Nassella viridula (green needlegrass) canopy cover 

 ranging from 10 to 40 percent, exceeded only by that of 

 Pascopyrum smithii. The uplands in northwest comer of 

 Section 22 also support fair to good quality occurrences 

 of this type. Other graminoids represented with more 

 than 5 percent cover include needle-and-thread (Stipa 

 comata) and prairie junegrass (Koeleria macramha). 

 Exotic graminoids, including Japanese brome (Bromus 

 japonicus), crested wheatgrass (Agropyron cristatum), 

 and Kentucky bluegrass (Poa pratensis), are much less 

 prevalent than on the association described above. In 

 the curtailment of grazing, populations of Brumus 

 japonicus and Agropyron cristatum often decline, but Poa 

 prater\sis has shown a propensity on similar mesic sites 

 to increase without disturbance. Povertyweed (Iva 

 axillaris) and scarlet globemallow (Sphaeralcea coccinea) 

 are the only forbs occurring with more than trace 

 coverages; other high constancy herbs include prickly 

 pear (Opunda polyacantha) , American vetch (Vicia 



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