Great Plains Sand Prairie

Sand prairies are often considered part of the tallgrass or mixedgrass regions in the western Great Plains but can contain elements from Western Great Plains Shortgrass Prairie (CES303.672), Central Mixedgrass Prairie (CES303.659), and Northwestern Great Plains Mixedgrass Prairie (CES303.674). The largest expanse of sand prairies (approximately 5 million ha) can be found in the Sandhills of north-central Nebraska and southwestern South Dakota. These areas are relatively intact. The primary use of this system has been grazing (not cultivation), and areas such as the Nebraska Sandhills can experience less degeneration than other prairie systems. Although greater than 90% of the Sandhills region is privately owned, the known fragility of the soils and the cautions used by ranchers to avoid poor grazing practices have allowed for fewer significant changes in the vegetation of the Sandhills compared to other grassland systems. Nonetheless, the sustained annual grazing within pastures by cattle has altered the mix of vegetation. The unifying and controlling feature for this system is coarse-textured soils, and the dominant grasses are well-adapted to this condition. Soils in the sand prairies can be relatively undeveloped and are highly permeable. Soil texture and drainage along with a species' rooting morphology, photosynthetic physiology, and mechanisms to avoid transpiration loss are highly important in determining the composition of the sand prairies. In the northwestern portion of its range, stand size corresponds to the area of exposed caprock sandstone, and small patches predominate, but large patches are also found embedded in the encompassing Northwestern Great Plains Mixedgrass Prairie (CES303.674). Another important feature is their susceptibility to wind erosion. Blowouts and sand draws are some of the unique wind-driven disturbances in the sand prairies, particularly where there are fine sands, such as in the Nebraska Sandhills (where the rare Penstemon haydenii occurs). In most of eastern Montana, substrates supporting this system have weathered in place from sandstone caprock; thus the solum is relatively thin, and the wind-sculpted features present further east, particularly in Nebraska, do not develop. Graminoid species dominate the sand prairies, although relative dominance can change due to impacts of wind disturbance. Andropogon hallii and Calamovilfa longifolia are the most common species, but other grass and forb species such as Hesperostipa comata, Schizachyrium scoparium, Carex inops ssp. heliophila, and Panicum virgatum are often present. Apparently only Calamovilfa longifolia functions as a dominant throughout the range of the system. In the western extent, Hesperostipa comata becomes more dominant, and Andropogon hallii is less abundant but still present. Communities of Artemisia cana ssp. cana are included here in central and eastern Montana. Patches of Quercus havardii can also occur within this system in the southern Great Plains. Fire and grazing constitute the other major dynamic processes that can influence this system. In the Western Great Plains in Texas, prairies on deep sands and sandhills which currently represent far southern outliers of this system, are dominated by species such as Andropogon gerardii, Andropogon hallii, Calamovilfa gigantea, Cenchrus spinifex, Hesperostipa comata, Paspalum setaceum, Schizachyrium scoparium, Sporobolus cryptandrus, and Sporobolus giganteus. Some woody species may be present, including Artemisia filifolia and Quercus havardii. Shrub species such as Artemisia filifolia, Prunus angustifolia, Rhus trilobata, and Quercus havardii may be present but constitute relatively little cover.

This system is distinguished by the dominance of graminoids such as Andropogon hallii and Calamovilfa longifolia. Other graminoids such as Hesperostipa comata, Carex inops ssp. heliophila, and Panicum virgatum may be present. Characteristic forbs differ by region, but species of Psoralidium, Pediomelum, and Eriogonum are a common feature, along with sand-loving annuals such as Helianthus petiolaris and Oenothera rhombipetala. Penstemon haydenii is endemic to the sand prairie system and of special conservation concern because of its probable decline due to grazing and fire suppression. Very diffuse patches of Rhus trilobata are found on shallow sandy soils, often associated with breaklands; other shrubs occasionally occurring include Artemisia cana ssp. cana, Betula occidentalis, Juniperus horizontalis, Prunus pumila var. besseyi (= Prunus besseyi), Prunus angustifolia, and Yucca glauca. Many of the warm-season graminoids extend at least to the Rocky Mountain Front as dominant components on appropriate sites or as a response to disturbance. All the characteristic species mentioned for Nebraska and South Dakota are also found in Montana stands (and possibly Wyoming and perhaps the rest of the states cited). Some of the communities cited as part of the concept in Nebraska and South Dakota are only marginally present in Montana, but others are found throughout Montana's Great Plains region. In the southern range of this system, patches of Quercus havardii can also occur.

The distribution, species richness and productivity of plant species within the sand prairie ecological system are controlled primarily by environmental conditions, in particular, the temporal and spatial distribution of soil moisture and topography. Soils in the sand prairies can be relatively undeveloped and are highly permeable. Soil texture and drainage along with a species' rooting morphology, photosynthetic physiology, and mechanisms to avoid transpiration loss are highly important in determining the composition and distribution of communities/associations within the sand prairies. Another important aspect of soils in the sand prairies is their susceptibility to wind erosion. Blowouts and sand draws are some of the unique wind-driven disturbances in the sand prairies, particularly the Nebraska Sandhills, which can profoundly impact vegetation composition and succession within this system. This tallgrass prairie is found primarily on sandy and sandy loam soils that can be relatively undeveloped and highly permeable as compared to Western Great Plains Tallgrass Prairie (CES303.673), which occurs on deeper loams. This system is usually found in areas with a rolling topography and can occur on ridges, midslopes and/or lowland areas within a region. It often occurs on moving sand dunes, especially within the Sandhills region of Nebraska and South Dakota. In Montana, occurrences are intimately associated with Northwestern Great Plains Mixedgrass Prairie (CES303.674), usually occupying higher positions in local landscapes where sandy members of some geologic formations (that are predominantly marine shales) constitute the highest (and most weathering-resistant) points in the landscape. In Texas, this system occurs on rolling to level, eolian or alluvial, deep sand deposits classed as Deep Sand or Sandhill Ecological Sites.

The distribution, species richness and productivity of plant species within the sand prairie ecological system are controlled primarily by environmental conditions, and in particular, the temporal and spatial distribution of soil moisture and topography. Another important aspect of this system is its susceptibility to wind erosion. Blowouts and sand draws are some of the unique wind-driven disturbances in the sand prairies, particularly the Nebraska Sandhills, which can profoundly impact vegetation composition and succession within this system.

Fire and grazing constitute the other major disturbances that can influence this system. The most extensive fires are likely to have occurred in years with wet springs followed by hot, dry summers when grazing pressure was low. Wet springs would have resulted in more productive and more continuous plant cover (i.e., fuel) that would have supported and expanded fires ignited under dry conditions occurring later in the season. In addition, litter accumulation over several fire-free years would also have supported widespread fire, in any conditions. The litter component, a determining factor in fire size and frequency, is correlated with seral stage. Several fire-free years produce enough litter to carry another fire (LANDFIRE 2007a).

Drought has additional impact in these very sandy soils and the high water table of the sandhills also affects the vegetation and encourages invasive trees (K. Kindscher pers. comm.). Extended periods of severe drought are likely to have affected both species composition and the stability of the sandhill soil, particularly when compounded by temperature, wind and heavy grazing. These conditions may have led to the development of blowouts making it difficult for vegetation to re-establish quickly. The occurrence of blowout penstemon (Penstemon haydenii) suggests long periods when blowouts were common across the landscape although causes resulting in this feature have not been determined (LANDFIRE 2007a).

Overgrazing, fire and trampling that leads to the removal of vegetation within those areas susceptible to blowouts can either trigger a blowout or perpetuate one already occurring. Overgrazing can also lead to significant erosion. The major large grazer, bison (Bos bison), occurring in large numbers in this system has largely been replaced by cattle. Both species impact the range by grazing and trampling; however, bison also significantly impacted local areas by wallowing. Unlike elsewhere in the Great Plains mixed and shortgrass prairie dog towns were a minor component of the Sandhills landscape and limited to where soils were finer-textured and in flat uplands and in valleys and the eastern Sandhills where the water table was not high (LANDFIRE 2007a).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has two classes in total (LANDFIRE 2007a, BpS 3111480). These are summarized as:

A) Early Development 1 Open (25% of type in this stage): Herbaceous cover is 0-20%. Class A represents immediate to three-year post-disturbance conditions. Vegetation consists of resprouting and seedling grasses and forbs. Total bare soil is greater than before the disturbance particularly on less productive sites. The vigor of new growth and the specific species affected depend on the season of the disturbance and on pre- and post-disturbance environmental conditions (e.g., available soil moisture). Litter is low initially but increases until, by year three, there is enough to support fire under average burning conditions. Fire was therefore modeled as occurring somewhat less frequently than in class B. In uplands, where soil type is dominated by coarse-grained sands with low water-holding capacity, post-disturbance primary production initially decreases, thus fire may only carry under ideal conditions. Under these conditions, grazing is likely to be light. In lowlands, with finer-textured soils, primary production is determined largely by moisture availability. Artemisia cana can resprout immediately after fire, so it could be present in this stage as well. It could, however, be killed following intensive fires. But since there is not much litter in these sites, possibility of intense fire is reduced. Repeated grazing of these areas will prevent succession to class B. Grazing occurs with a probability of 0.05. Prairie dog grazing was modeled as optional 1, with a very unlikely probability of 0.0007. Both will set succession back to the beginning.

B) Mid Development 1 Closed (80% of type in this stage): Herbaceous cover is 21-80%. Class B is sandhill grassland, the dominant historical condition. This class has a moderately dense herbaceous layer (20-80% cover) up to 1 m tall. Fire (every 10 years) would return this class to A, while lack of fire (after 40 years) would move it toward class C. Shrubs may make up to 25% of the cover but is more commonly 0-10%. Native grazing maintains this class. Severe, multiple-year drought (every 100 years) moves this to class C by reducing grass cover and fuel loads and giving a competitive advantage to the usually spare shrub cover.

C) Late Development 1 All Structures (shrub-dominated - 10% of type in this stage): Shrub cover is 21-100%. Class C is the shrub-dominated sandhill grassland and differs from the sandhill shrubland (BpS 1094) which is modeled separately based on edaphic differences. Fire returns this to class A (MFRI = 0.10). Dominate shrubs include sand sagebrush, shinnery oak and sand cherry.

Conversion to agriculture can impact this system, and its range has decreased from human activities. Impacts from energy extraction in oil and gas fields in have recently fragmented larges areas with road networks to well pads and pipelines. Overgrazing by livestock grazing and fires can remove vegetation cover and promote blowouts.

The dominant species are adapted to frequent fires, sprouting from rhizomes post-fire. Fire suppression and moderate grazing have caused unevenness in structure and favored invasion of introduced grasses Poa pratensis and Bromus inermis across the sandhills (Sims 1988, Hauser 2005). A variety of seral stages are desirable to provide habitat for all phases of the lesser or greater prairie-chicken life cycle. The vegetation ideally exhibits a diversity of native short to tall grasses and native forbs interspersed with sparse to somewhat dense low-growing shrubby cover which includes sufficient cover for nesting and brood-rearing, as well as open areas suitable for leks.

This system is found throughout the Western Great Plains Division. The largest and most intact example of this system is found within the Sandhills region of Nebraska and South Dakota. However, it is also common (though occurring in predominantly small patches) farther west into central and eastern Montana. Its western extent in Wyoming is still to be determined, but it does occur in mapzone 29 on weathered-in-place sandy soils, where Calamovilfa longifolia is found, along with Artemisia cana. In addition, outliers have been described from the Western Great Plains in Texas (Monahans Sandhills State Park).