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: Low 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: Low Terraces Sandy Smooth High Plains Subsection Southern High Plains Section Great Plains-Palouse Dry Steppe Province (Cleland, et al., 2007).
EPA: Low Terraces <26l Upper Canadian Plateau<26 Southwestern Tablelands (Griffith, et al., 2006).
Ecological site concept
The Low Terraces ecological site occurs on stream terraces in river valleys in the CP-LRU. The CP 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 CP LRU ranges from 5,000 to 7,500 feet.
The central concept for the Low Terraces ecological site is a stream valley system that is entrenched into the Cretaceous Canadian Plateau with only subtle relief (Rosgen Valley Type VIII). Since these valleys are not carved down through Dakota Sandstone layers like the riparian sites of the Mesozoic Canyons and Breaks LRU, there are no steeper box canyons. There can be a seasonal water table within rooting depth for woody plants and shrubs as well as enhanced available soil moisture for herbaceous plants due to the run-on contributions of moisture from nearby uplands. During heavy rainfall events from intense summer storms, the Low Terraces ecological site can experience rare flooding for brief periods.
Soil depth for the Low Terraces ecological site is over 78 inches (200 centimeters) to root-restrictive layers. Slope gradient ranges from 0 to 5 percent, causing aspect to have very little effect on site dynamics. Surface texture ranges from sandy loam to silty clay.
This site occurs in soils that have high clay in subsurface horizons. These soils are found on upland positions which may contribute moisture and sediment to riparian systems and their stream terraces.
This site occurs where soils surfaces have strong or violent effervescence and ≥ 5% calcareous rock fragments. These soils are found on upland positions which may contribute moisture and sediment to riparian systems and their stream terraces.
This site occurs on escarpments where soils are ≤ 50 cm to a root-restrictive layer, and on slopes > 10%. These soils are found on upland positions which may contribute moisture and sediment to riparian systems and their stream terraces.
This site occurs on the channels and floodplains of ephemeral streams where salts have not accumulated. Drainageways may connect to low terraces in some places, and contribute moisture to the riparian system.
This site occurs where soils are ≤ 50 cm to lithic contact with sandstone bedrock, and often supports oneseed juniper savannahs. These soils are found on upland positions which may contribute moisture and sediment to riparian systems and their stream terraces.
This site occurs where soils are ≤ 50 cm to lithic contact with limestone bedrock, and often supports oneseed juniper savannahs. These soils are found on upland positions which may contribute moisture and sediment to riparian systems and their stream terraces.
Occurs in perennial stream systems and related floodplains. These sites are situated below Low Terraces and receive run-on moisture from these sites.
This site occurs on old stream terraces. Soils are skeletal, and contain well-developed argillic horizons. Gravelly Terrace sites typically occur right above the Riparian sites and contribute moisture to this site via overland-flow and through-flow.
This site occurs in soils that are > 50 cm to a root restrictive layer and have surface textures of sandy loam or coarser. Such sites typically occur on sand sheets and dunes adjacent to playas.
This site was written for drainageways, alluvial fans, and floodplains, none of which are affected by stream hydrology. The soil systems that are impacted by stream hydrology are significantly different, and yet ecologically important enough from these other types of bottomland soils to differentiate into a separate site. Additionally, most acreage of Low Terraces are not considered significant run-on positions and are not in specifically in the bottom of the landscape.
This site is not specific to Stream terraces and thus does not take into account the soil morphology or hydrology of these specific types of ecology. Thus, while the Loamy Upland site has been applied to areas that currently correlate to Low Terraces, the former is a poor fit for these areas.
Table 1. Dominant plant species
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