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.
Major Land Resource Area (MLRA): 070A–High Plateaus of the Southwestern Great Plains
This site is only applicable to the Canadian Plateaus LRU of MLRA 70A (LRU 70A.1).
This site is only applicable to the Canadian Plateaus LRU of MLRA 70A (LRU 70A.1). Please refer to the following key:
Land Resource Unit (LRU) Key for MLRA 70A
– High Plateaus of the Southwestern Great Plains
1a. The site exists on a landform of volcanic origin, such as a basalt plateau, or is part of an escarpment system that rises directly to a volcanic structure. These escarpments are included if they have volcanic alluvium or colluvium (i.e. basalt, rhyolite, tuff, cinders) overlying non-volcanic residuum or bedrock (i.e. sandstone, shale). → VOLCANIC PLATEAUS LRU (VP)
User tip: Other alluvial or colluvial landform features extending below the escarpments are not included unless they have a predominance of volcanic fragments at the surface. Also, note that playas atop volcanic plateaus are included within the VP-LRU.
1b. All other sites. → 2
2a. The site exists in the annulus or floor of a playa. → CANADIAN PLATEAUS LRU (CP)
User tip: Small islands of playas occur within large areas of HP-LRU. These sites may be far from the nearest CP landform but will still key-out to the CP-LRU. The playa rim components, however, may key out to either LRU, so it is important to properly identify their soil properties.
2b All other sites. → 3
3a. The site is part of an escarpment landscape complex (defined below) or is within a canyon, valley, or small basin confined by such escarpments. At the upper boundary of the LRU, the soil surface meets at least 4 of the following 5 criteria:
I. Shallow or very shallow soils are present in at least 50% of the landform area;
II. Soils are underlain by sandstone bedrock of the Cretaceous Dakota Formation or older;
III. Presence or historical evidence of a conifer stand (≥ 2% canopy cover);
IV. The ground surface has a slope of at least 10%;
V. The landforms drain towards steep-walled escarpments or canyons below the Dakota sandstone (older Jurassic and Triassic Formations underlie this sandstone mesa cap).
→ MESOZOIC CANYONS AND BREAKS LRU (MCB)
User tip: The MCB sites also occur on any colluvial or alluvial bottomlands confined within escarpments or canyons. Some valleys transition from CP to MCB, or back to CP, and the turning point can be difficult to determine. Generally, the landforms are part of the MCB when confined between Dakota sandstone breaks or escarpments on both sides. Much of the acreage in the MCB is aproned by colluvial debris fans—composed of sandy materials with large sandstone fragments visible on the soil surface, including large stones or boulders. The soils in the bottoms of these confined valleys will also be in the MCB. When the valley opens, or there is only a single escarpment opening to the plains, the landforms below the steeper, rockier escarpments will be members of the CP-LRU.
3b. Fewer than 4 of the above criteria are met. → 4
4a. The soil is on a plateau summit position (tread) and is within 50 cm to contact with either plateau bedrock (non-soil bedrock of cemented sandstone, limestone, or shale) or strath terrace cobbles, but not a petrocalcic contact (caprock or caliche of cemented calcium carbonate). → CANADIAN PLATEAUS LRU (CP)
4b. No plateau bedrock or strath terrace cobbles within 50 cm. → 5
5a. Fragments (>2 mm) are visible within the soil profile and/or on the surface. If fragments cannot be found in the profile, it is acceptable to look nearby on ant mounds or around burrows. If site is in a drainageway, one can look for fragments on landforms immediately upslope.→ 6
5b. Fragments are entirely absent. → 7
6a. Fragments are mostly petronodes or High Plains gravels. → HIGH PLAINS LRU (HP)
6b. Fragments are mostly plateau bedrock fragments. → CANADIAN PLATEAUS LRU
7a. All horizons in the upper 100 cm of soil have textures of sandy clay loam or sandier.
→ CANADIAN PLATEAUS LRU (CP)
7b. At least one horizon in the upper 100 cm of soil has a texture that is less sandy than sandy clay loam. → HIGH PLAINS LRU (HP)
NRCS and BLM: Gravelly Terraces Canadian Plateaus LRU Major Land Resource Area 70A, High Plateaus of the Southwestern Great Plains Land Resource Region G, Western Great Plains Range and Irrigated Region (United States Department of Agriculture, Natural Resources Conservation Service, 2006).
USFS: Gravelly Terraces Sandy Smooth High Plains Subsection Southern High Plains Section Great Plains-Palouse Dry Steppe Province (Cleland, et al., 2007).
EPA: Gravelly Terraces <26l Upper Canadian Plateau<26 Southwestern Tablelands (Griffith, et al., 2006).
Ecological site concept
The Gravelly Terraces ecological site occurs on treads of elevated stream terraces in the Canadian Plateaus LRU. This LRU occupies the western portion of MLRA 70A and extends from Las Vegas, NM at the southern end to beyond Raton, NM at its northern end. Elevation for the LRU ranges from 5,000 to 7,500 feet.
The concept for this ecological site is a soil with a depth over 20 inches (50 centimeters) to root-restrictive layers. Surface textures are variable, but are typically high in both sand and coarse fragments and moderate in clay. There are at least 35 percent coarse fragments in a layer greater than or equal to 20 inches thick within the upper 100 40 inches of soil. An increase in clay with depth is common, causing a subsurface horizon to behave as an aquitard (a layer that impedes the percolation of water). This concentration of clay in the profile affects the rooting zone of plants by creating a zone by which water is temporarily “perched” and, thus, more accessible to shallow roots. Such soil properties have only been observed on older, elevated stream terraces formed by high-velocity waters than are typically observed today, carrying sediment from the Sangre De Cristo Mountains into the Great Plains.
This site occurs on alluvial flats with slopes ≤ 1% and soils a that contain ≥ 35% in a layer beginning within 15 cm of the surface.
This site occurs in soils that contain layers in the upper 50 cm with ≥ 35% clay.
This site occurs on the channels and floodplains of ephemeral streams where salts have not accumulated. Adjacent Gravelly Terraces sites may contribute water to this site via run-on and through-flow.
This site occurs on terraces above perennial streams where the flooding frequency interval is ≥ 10 years. This site is often used for hay and small grain production. Adjacent Gravelly Terraces sites may contribute water to this site via run-on and through-flow.
This site occurs on slopes ≥ 10%. Its soils lack a layer within the upper 100 cm that is ≥ 50 cm thick and contains ≥ 35% rounded rock fragments. This is the most common site to directly subtend Gravelly Terraces.
This site occurs in soils that lack layers in the upper 50 cm with ≥ 35% clay.
This site occurs on slopes < 10% where soils surfaces have strong or violent effervescence and ≥ 5% calcareous rock fragments.
Occurs in perennial stream systems and related floodplains. These sites are situated below Gravelly Terraces and receive run-on moisture from these sites in some cases.
Table 1. Dominant plant species
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