Natural Resources
Conservation Service
Ecological site FX052X03X160
Thin Breaks (TB) Dry Shrubland
Last updated: 7/08/2019
Accessed: 11/13/2024
General information
Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site.
Figure 1. Mapped extent
Areas shown in blue indicate the maximum mapped extent of this ecological site. Other ecological sites likely occur within the highlighted areas. It is also possible for this ecological site to occur outside of highlighted areas if detailed soil survey has not been completed or recently updated.
MLRA notes
Major Land Resource Area (MLRA): 052X–Brown Glaciated Plains
The Brown Glaciated Plains, MLRA 52, is an expansive, agriculturally and ecologically significant area. It consists of approximately 14.5 million acres and stretches across 350 miles from east to west, encompassing portions of 15 counties in north-central Montana. This region represents the southwestern limit of the Laurentide Ice Sheet and is considered to be the driest and westernmost area within the vast network of glacially derived prairie pothole landforms of the northern Great Plains. Elevation ranges from 2,000 feet (610 meters) to 4,600 feet (1,400 meters).
Soils are primarily Mollisols, but Entisols, Inceptisols, Alfisols, and Vertisols are also common. Till from continental glaciation is the predominant parent material, but alluvium and bedrock are also common. Till deposits are typically less than 50 feet thick, and in some areas glacially deformed bedrock occurs at or near the soil surface (Soller, 2001). Underlying the till is sedimentary bedrock largely consisting of Cretaceous shale, sandstone, and mudstone (Vuke et al., 2007). The bedrock is commonly exposed on hillslopes, particularly along drainageways. Significant alluvial deposits occur along glacial outwash channels and major drainages, including portions of the Missouri, Teton, Marias, Milk, and Frenchman Rivers. Large glacial lakes, particularly in the western half of the MLRA, deposited clayey and silty lacustrine sediments (Fullerton et al., 2013).
Much of the western portion of this MLRA was glaciated towards the end of the Wisconsin age, with the maximum glacial extent occurring approximately 20,000 years ago (Fullerton et al., 2004). The result is a geologically young landscape that is predominantly a level till plain interspersed with lake plains and dominated by soils in the Mollisol and Vertisol orders. These soils are very productive and generally are well suited to dryland farming. Much of this area is aridic-ustic. Crop-fallow dryland wheat farming is the predominant land use. Areas of rangeland typically are on steep hillslopes along drainages.
The rangeland, much of which is native mixedgrass prairie, increases in abundance in the eastern half of the MLRA. The Wisconsin-age till in the north-central part of this area typically formed large disintegration moraines with steep slopes and numerous poorly drained potholes. A large portion of Wisconsin-age till occurring on the type of the level terrain that would typically be optimal for farming has large amounts of less-suitable sodium-affected Natrustalfs. Significant portions of Blaine, Phillips, and Valley Counties were glaciated approximately 150,000 years ago during the Illinoisan age. Due to erosion and dissection of the landscape, many of these areas have steeper slopes and more exposed bedrock than areas glaciated during the Wisconsin age (Fullerton and Colton, 1986).
While much of the rangeland in the aridic-ustic portion of MLRA 52 is classified as belonging to the “dry grassland” climatic zone, sites in portions of southern MLRA 52 may belong to the “dry shrubland” climatic zone. The Dry Shrubland climatic zone represents the northernmost extent of the big sagebrush (Artemisia tridentata) steppe on the Great Plains. Because similar soils occur in both southern and northern portions of the MLRA, it is currently hypothesized that climate is the primary driving factor affecting big sagebrush distribution in this area. However, the precise factors are not yet fully understood.
Sizeable tracts of largely unbroken rangeland in the eastern half of the MLRA and adjacent southern Saskatchewan are home to the northern Montana population of greater sage-grouse (Centrocercus urophasianus), and large portions of this area are considered to be a Priority Area for Conservation (PAC) by the U.S. Fish and Wildlife Service (U.S. Fish and Wildlife Service, 2013). This population is unique among sage grouse populations because many individuals overwinter in the big sagebrush steppe (dry shrubland) in the southern portion of the MLRA and then migrate to the northern portion of the MLRA, which lacks big sagebrush (dry grassland), to live the rest of the year (Smith, 2013).
Areas of the till plain near the Bearpaw and Highwood Mountains as well as the Sweetgrass Hills and Rocky Mountain foothills are at higher elevations, receive higher amounts of precipitation, and have a typic-ustic moisture regime. These areas have significantly more rangeland production than the drier aridic-ustic portions of the MLRA and have enough moisture to produce crops annually rather than just bi-annually, as in the drier areas. Ecological sites in this higher precipitation area are classified as the Moist Grassland climatic zone.
Classification relationships
NRCS Soil Geography Hierarchy
• Land Resource Region: Northern Great Plains
• Major Land Resource Area (MLRA): 052 Brown Glaciated Plains
• Climate Zone: Dry Shrubland
National Hierarchical Framework of Ecological Units (Cleland et al., 1997; McNab et al., 2007)
• Domain: Dry
• Division: Temperate Steppe
• Province: Great Plains-Palouse Dry Steppe Province 331
• Section: Northwestern Glaciated Plains 331D
• Subsection: Montana Glaciated Plains 331Dh
• Landtype Association/Landtype Phase: N/A
National Vegetation Classification Standard (Federal Geographic Data Committee, 2008)
• Class: Xeromorphic Woodland, Scrub and Herb Vegetation Class (3)
• Subclass: Cool Semi-Desert Scrub and Grassland Subclass (3.B)
• Formation: Cool Semi-Desert Scrub and Grassland Formation (3.B.1)
• Division: Cool Semi-Desert Scrub and Grassland Division (3.B.1.Ne)
• Macrogroup: Artemisia tridentata - Artemisia tripartita ssp. tripartita - Purshia tridentata Steppe and Shrubland Macrogroup (3.B.1.Ne.3)
• Group: Artemisia tridentata ssp. wyomingensis - Artemisia tridentata ssp. tridentata Steppe and Shrubland Group (3.B.1.Ne.3.a)
• Alliance: Artemisia tridentata ssp. wyomingensis Dry Steppe and Shrubland Alliance
• Association: No existing correlation
EPA Ecoregions
• Level 1: Great Plains (9)
• Level 2: West-Central Semi-Arid Prairies (9.3)
• Level 3: Northwestern Glaciated Plains (42)
• Level 4: North-Central Brown Glaciated Plains (42o) and Glaciated Northern Grasslands (42j)
Ecological site concept
This provisional ecological site occurs in the Dry Shrubland climatic zone of MLRA 52. Figure 1 illustrates the distribution of this ecological site based on current data. This map is approximate, is not intended to be definitive, and may be subject to change. Thin Breaks Dry Shrubland is a somewhat extensive ecological site occurring on steep or dissected landscapes in MLRA 52. This ecological site occurs on hillslopes, bluffs, and escarpments. This site is typically found on side slopes, nose slopes, and head slopes in a badlands landscape. Due to the complex nature of the landscape, soil texture, soil depth, and slope on this ecological site can be highly variable.
The distinguishing characteristics of this site are lithic or paralithic bedrock within 40 inches of the soil surface and a relatively young, undeveloped soil profile, which lacks features such as a mollic epipedon and an argillic horizon. Soils range from very shallow (less than 12 inches to bedrock) to moderately deep (between 20 to 40 inches to bedrock) and are typically derived from residuum. Soil surface textures vary from sandy loam to clay loam and clay content is less than 35 percent. Slopes vary, but are typically greater than 15 percent. Characteristic vegetation is needle and thread (Hesperostipa comata), western wheatgrass (Pascopyrum smithii), and threadleaf sedge (Carex filifolia). Little bluestem (Schizachyrium scoparium) becomes common in the eastern portion of this ecological site. The principle shrub on this site is Wyoming big sagebrush (Artemisia tridentata wyomingensis).
Associated sites
FX052X03X131 |
Shallow Clay (Swc) Dry Shrubland This site is adjacent to the Thin Breaks Dry Shrubland ecological site where soils contain greater than 35 percent clay. It typically occupies a backslope position similar to the Thin Breaks Dry Shrubland ecological site. |
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FX052X03X007 |
Coarse Clay (Coc) Dry Shrubland This site occurs on moderate to steeply sloping hillslopes adjacent to the Thin Breaks Dry Shrubland ecological site where soils contain greater than 35 percent clay and have a strong granular structure. It is commonly in the same landscape positions as Thin Breaks Dry Shrubland ecological site. |
FX052X03X029 |
Limy-Steep (Lystp) Dry Shrubland This site is generally upslope from the Thin Breaks Dry Shrubland ecological site. It is most commonly on shoulder positions above Thin Breaks Dry Shrubland ecological site. |
Similar sites
FX052X01X160 |
Thin Breaks (TB) Dry Grassland This site differs from the Thin Breaks Dry Shrubland ecological site in that it has slightly cooler annual temperatures and supports silver sagebrush rather than big sagebrush. |
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FX052X03X131 |
Shallow Clay (Swc) Dry Shrubland This site differs from the Thin Breaks Dry Shrubland ecological site in that the clay content is greater than 35 percent and depth to bedrock is less than 20 inches. |
FX052X03X029 |
Limy-Steep (Lystp) Dry Shrubland This site differs from the Thin Breaks Dry Shrubland ecological site in that the soil depth is greater than 40 inches and soils are typically derived from glacial till rather than residuum. Herbaceus production, particularly of mid-statured bunchgrasses, is significantly higher. |
FX052X03X040 |
Loamy-Steep (Lostp) Dry Shrubland This site differs from the Thin Breaks Dry Shrubland ecological site in that the soil is typically greater than 40 inches deep and derived from glacial till. When moderately deep (20 to 40 inches) soils are well developed (evidenced by a mollic epipedon and/or argillic horizon). |
Table 1. Dominant plant species
Tree |
Not specified |
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Shrub |
Not specified |
Herbaceous |
Not specified |
Legacy ID
R052XY726MT
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