Northern Great Plains Mixed Grass Prairie

This system extends from northern Nebraska into southern Canada and westward through the Dakotas to the Rocky Mountain Front in Montana and eastern Wyoming, on both glaciated and non-glaciated substrates. Soil texture (which ultimately effects water available to plants) is the defining environmental descriptor; soils are primarily fine and medium-textured and do not include sand, loamy sand, or sandy loam soils. This system occurs on a wide variety of landforms (e.g., rolling uplands stream terraces, ridgetops) and in proximity to a diversity of other systems. Most usually it is found in association with Western Great Plains Sand Prairie (CES303.670) which occupies the coarser-textured substrates. Northwestern Great Plains Shrubland (CES303.662) is intermixed on the landscape in draws and ravines which receive more precipitation runoff and are somewhat protected from fires. In various locales generally north and east of the Missouri River, the topography where this system occurs is broken by many glacial pothole lakes, and this system may be proximate to Great Plains Prairie Pothole (CES303.661). On the eastern Montana and western Dakota plains, mixedgrass prairie is by far the predominant system. Here it occurred continuously for hundreds of square kilometers, interrupted only by riparian areas or sand prairies, which were associated with gentle rises, eroded ridges, or mesas derived from sandstone. The growing season and rainfall are intermediate to drier units to the southwest and mesic tallgrass regions to the east. Graminoids typically comprising the greatest canopy cover include Pascopyrum smithii, Nassella viridula, and Festuca spp. In Montana these include Festuca campestris and Festuca idahoensis. Other commonly dominant species in Montana are Bouteloua gracilis, Hesperostipa comata, and Carex filifolia, while Festuca campestris and Festuca idahoensis may be more abundant in the north and foothill/montane grassland transition areas. Bouteloua curtipendula, Elymus lanceolatus, Muhlenbergia cuspidata, and Pseudoroegneria spicata are common, and sometimes abundant, components of this system. Remnants of Hesperostipa curtiseta-dominated vegetation are found in northernmost Montana and North Dakota associated with the most productive sites (largely plowed to cereal grains); this species, usually in association with Pascopyrum smithii, is much more abundant in Canada. Sites with a strong component of Nassella viridula indicate a more favorable moisture balance and perhaps a favorable grazing regime as well because this is one of the most palatable of the mid-grasses. Hesperostipa comata is also an important component and becomes increasingly so as improper grazing practices favor it at the expense of (usually) Pascopyrum smithii; progressively more destructive grazing can result in the loss of Pascopyrum smithii from the system followed by drastic reduction in Hesperostipa comata and ultimately the dominance of Bouteloua gracilis (or Poa secunda and other short graminoids) and/or a lawn of Selaginella densa. Koeleria macrantha, at least in Montana and southern Canada, is the most pervasive grass; if it has high cover, past intensive grazing is the presumed reason. In the eastern portion of this system's range, tallgrass species, especially Andropogon gerardii, Panicum virgatum, and Sorghastrum nutans, are often present to common on more mesic sites. Shrub species such as Symphoricarpos spp., Artemisia frigida, and Artemisia cana occur in the western and central portions while Symphoricarpos spp. and Prunus spp. can be found in the eastern portion. Sites with slightly to moderately saline soils have small to moderate amounts of salt-tolerant species such as Distichlis spicata and Sporobolus airoides. Fire, grazing and climate constitute the primary dynamics affecting this system. Drought can also impact this system, in general favoring the shortgrass component at the expense of the mid-grasses. With intensive grazing, cool-season exotics such as Poa pratensis, Bromus inermis, and Bromus tectorum can increase in dominance; both of the rhizomatous grasses have been shown to markedly depress species diversity. Shrub species such as Juniperus virginiana can also increase in dominance with fire suppression. This system is one of the most disturbed grassland systems in Nebraska, North and South Dakota, and Canada.

This system contains greater than 50% cover of natural, cool-season grasses such as Festuca spp., Pascopyrum smithii, Elymus lanceolatus, Hesperostipa comata, Hesperostipa curtiseta, and Nassella viridula. Hesperostipa comata becomes increasingly important where improper grazing regimes have favored it at the expense of (usually) Pascopyrum smithii; progressively more destructive grazing can result in the loss of Pascopyrum smithii from the system followed by drastic reduction in Hesperostipa comata and ultimately the dominance of Bouteloua gracilis (or Poa secunda and other short graminoids) and/or a lawn of Selaginella densa. Koeleria macrantha, at least in Montana and southern Canada, is the most pervasive grass; if it has high cover, past intensive grazing is the presumed reason. Shrub species such as Symphoricarpos spp. and Artemisia frigida also occur. Cover of native, nongrazing-induced shrubs typically does not exceed 25% in conjunction with topographic relief (breaks); otherwise the stand would be considered part of Northwestern Great Plains Shrubland (CES303.662). Cool-season exotics such as Poa pratensis, Bromus inermis, and the annual Bromus tectorum can increase in dominance with overgrazing; both of the above-named rhizomatous grasses are sufficiently aggressive to outcompete natives regardless of disturbance regime. Likewise, shrub species such as Juniperus virginiana can also increase in dominance with fire suppression.

This system extends from northern Nebraska into southern Canada and westward through the Dakotas to the Rocky Mountain Front in Montana and eastern Wyoming, on both glaciated and non-glaciated substrates. It occurs on a wide variety of landforms (e.g., rolling uplands, mesatops, stream terraces) and in proximity to a diversity of other systems. Elevations range typically from 430-1220 m, and up to 1980 m in the northwestern extent (LANDFIRE 2007a).

Climate: The climate is cool, continental, ranging from hot summers (mean daily temperature in July of 15°C in the northwest to 25°C in the southeast) to cold winters (mean daily temperature of -16°C in the northeast to -5°C in the southwest). Precipitation increases from west (25 cm) to east (55 cm) with most falling as rain or snow from April through June (LANDFIRE 2007a). Climate and growing season length for the region this system occurs are intermediate to the shortgrass regions to the west and southwest and the tallgrass regions to the east with a shorter growing season and less humid climate compared to the range of Central Mixedgrass Prairie (CES303.659). Moisture conditions are generally semi-arid.

Physiography/landform: Given the system's rather extensive geographic range, it is not surprising to find it occurring on a wide variety of landforms (e.g., rolling uplands, mesatops, stream terraces) and in proximity to a diversity of other systems.

Soil/substrate/hydrology: Soils are variable as it occurs on both glaciated and non-glaciated substrates generally with Entisols in the west and Mollisols in the east (LANDFIRE 2007a). Soil texture (which ultimately effects water available to plants) is the defining environmental descriptor; soils are primarily fine- and medium-textured, ranging from silt and clay loams, silty clay loams, silt loams to gravelly loam and do not include sands, sandy soils, or coarse sandy loams (Rolfsmeier and Steinauer 2010). In unglaciated areas, soils are derived primarily from fine-textured sedimentary rocks and deposits, primarily Cretaceous Pierre Shales, and to a lesser extent in Tertiary siltstones and chalky shales (Rolfsmeier and Steinauer 2010). Other rock types are included so long as their weathering products are not coarse-textured, namely not sandy soils. In glaciated areas, this system is found over glacial till and sometimes glacial lakeplains. It is found primarily on planar to gently rolling topography but is found on broken topography hillslopes as well. Some examples may include an impermeable or slowly permeable subsoil claypan layer. Other northern soils may be solonetzic and characterized by a subsoil hardpan layer with an excess of sodium (Adams et al. 2013).

This grassland system evolved with fire, grazing, and drought, which constitute the primary dynamics affecting this system. The diversity in this mixedgrass system likely reflects both the short- and long-term responses of the vegetation to these often concurrent disturbance regimes (Collins and Barber 1985). Drought, rather than fire, is the primary driver maintaining the dry mixed grassland because it occurs more frequently than fire, inhibits expansion of woody shrubs and reduces the abundance of tallgrasses and mesophytic forbs, and prevents an accumulation of fuel that would maintain a frequent fire regime (Sala et al. 1996). Although variable in area, severe drought years in the Great Plains tend to occur in clusters periodically (1890s, 1930s, mid-1950s, late 1970s, late 1980s to early 1990s, and early 2000s) and have major ecological impacts.

Historic fire-return intervals have been estimated at 8-12 years (LANDFIRE 2007a), but fires burn patchily across the landscape, consuming vegetation in some areas and missing others because of natural firebreaks such as badlands, break in topography/ridge, and rivers. Fire-return intervals were likely longer in the drier, less vegetated central and western portions of this system's range and shorter in the east, near the transition to tallgrass prairie-dominated landscapes. Grazing and prairie dog towns also reduced fuel loads and fire frequency, size and intensity, with the most substantial impacts in valley bottom shrublands and grasslands, and upland grasslands near water (LANDFIRE 2007a). Historically, the majority of human-caused ignitions were concentrated in spring and fall seasons, while the more common lightning-caused fires were concentrated in late summer (Higgins 1984, 1986, LANDFIRE 2007a). This combined with the differential responses of species to burning results in greater diversity across the landscape (Wright and Bailey 1980).

Grazing by native ungulates, primarily bison (Bos bison) and small mammals, principally prairie dogs (Cynomys ludovicianus) added a further degree of patchy disturbance to the mixedgrass prairie (Whicker and Detling 1988). Available soil moisture drives species composition in this grassland, with a higher percentage of tall grasses on relatively moist, and cooler north-facing slopes, and mid and short grasses on drier steep and warmer southerly exposures (Rolfsmeier and Steinauer 2010). Long-term precipitation variance affects diversity of the central mixedgrass prairie, creating conditions more favorable to shortgrass species during droughts while allowing mixedgrass species to spread during wetter years (Sims et al. 1978, Singh et al. 1983). Extended drought in similar mixedgrass prairie in central Kansas caused loss of most forbs and cool-season grasses, and severe reductions of warm-season grasses (70-80%) (Albertson 1937) and likely has the same effects on mixedgrass prairie further north.

The absence of grazing and replacement fire for many years (e.g., 50 years) would lead to an increased shrub component (often Symphoricarpos spp. and Fraxinus pennsylvanica, but also possibly Prunus spp., Amelanchier alnifolia, Elaeagnus commutata, Dasiphora fruticosa ssp. floribunda, and Juniperus horizontalis) in precipitation zones greater than 35 cm, and a buildup of dead grass (LANDFIRE 2007a). Within the semi-arid (25-35 cm) precipitation zones, Artemisia tridentata ssp. wyomingensis and Artemisia cana may also increase. Productivity of the grasses is decreased, resulting in greater mortality from smoldering fire (LANDFIRE 2007a). Mormon crickets (Anabrus simplex), grasshoppers (Orthoptera) and extinct Great Plains locust (Melanoplus spretus) probably had more of an impact in this system than currently defined, but the historical impact and frequency are unknown.

LANDFIRE developed a VDDT model for this system which has two classes (LANDFIRE 2007a, BpS 2911410).

A) Early Development 1 All Structures (25% of type in this stage): Herbaceous cover is 0-40%. Class A is the post-fire early-seral stage, combined with the very short-statured vegetation resulting from prairie dog disturbance or repeated high-intensity herbivory or trampling (e.g., watering points or buffalo wallows). This class may also be a short-term response to severe drought, combined with other impacts. This class lasts approximately three years. If in a prairie dog state, then the class would last longer in order to transition out of it; however, this is accounted for by having a prairie dog disturbance in the model, resetting succession and keeping it in this class. The 3-year interval attempts to capture what would happen post-fire or post-drought. Also post-heavy-grazing in current conditions would take longer to transition out of this class. Drought can occur every 30 years, not causing a transition. Replacement fire occurs but not as frequently, due to lack of fuel, every 20 years.

B) Mid Development 1 Closed (75% of type in this stage): Herbaceous cover (41-90%). Class B represents the intact historic plant community functioning under grazing and/or fire, dominated by taller, cool and warm-season rhizomatous perennial grasses, as well as bunchgrasses. This is the all-encompassing mid-to late-development, functioning final stage. Little below-ground mortality occurs after replacement fire, and resprouting of perennial grasses and forbs often occurs within days or weeks, depending on season. Grasses show greater vigor; some forb establishment may occur as a result of exposure of mineral soil. Canopy cover recovers quickly after resprouting. Shrub species could be present at 0-10% cover. Silver sagebrush and winterfat (on deeper soils) are the most common shrub, and would start resprouting. Wyoming big sagebrush can also be a component (on shallower soils) of this BpS, although a small component. Clubmoss might be present in Glaciated Plains at 0-5% cover, but not on shallow clay sites or dense clay sites, sands, saline upland, saline lowland, subirrigated or wet meadow. Replacement fire occurs every 5-15 years. Drought occurs every 30 years and maintains this stage. Native grazing by large ungulates could have occurred, including bison grazing. It is likely heavy locally due to increased succulence of young grasses. It might occur with a probability once every five years or 20% of this class each year. Native grazing by prairie dogs could also occur on a small portion of the landscape, bringing this state to A. Insect/disease occurs very infrequently. Grasshoppers and Mormon crickets might have a larger impact historically; however, there is uncertainty of impact and frequency. With a lack of fire, this class might shift to having more shrubs and tree invasion.

In the LANDFIRE BpS 2011410 model (3 Classes), drought was also thought to occur once every 30 years on average (LANDFIRE 2007a). It was also acknowledged that this system occurs within the very same biotope as Inter-Mountain Basins Big Sagebrush Steppe (CES304.778) or Inter-Mountain Basins Big Sagebrush Shrubland (CES304.777), the only difference being that fire has not been present where the sagebrush systems occur, a purely stochastic outcome (LANDFIRE 2007a).

Historically, this system covered approximately 61.4 million ha (614,000 square km) in Nebraska, North and South Dakota, and Canada; now it covers approximately 29.9 million ha (299,000 square km) in this region, a 51% reduction in extent. Major threats to this system are loss through direct conversion to crop fields and heavily grazed pastures. Farmland development has fragmented the natural landscape and has eliminated the large-scale processes of fire and grazing by native ungulates and small mammals that were necessary to maintain this system. Lack of fire, grazing, or mowing result in a decrease in productivity as sites accumulate more litter. Lack of fire allows tree cover to increase rapidly, especially on lower, more mesic slopes and in the eastern, more mesic edges of the system's range. Encroachment by Juniperus virginiana as a result of fire suppression is problematic in some portions of the system's distribution (Rolfsmeier and Steinauer 2010). This system is well-adapted to moderate grazing over time or heavy grazing for short periods but when used as long-term pasture and with high stocking rates many of the dominant native grasses are reduced or eliminated (Branson and Weaver 1953). Heavy haying or grazing done for extended periods results in a selective reduction in more palatable mid- and tallgrass species. This results in a relative increase in short graminoids, such as Bouteloua dactyloides, Bouteloua gracilis, Carex spp., and Poa secunda and, where there are nearby seed sources, shrubs such as Artemisia tridentata, Artemisia cana, and Symphoricarpos spp. or if those are done consistently during the mid-summer months negatively affects the taller warm-season grasses (if present) by removing their biomass before they have flowered. Cool-season grasses and forbs which set seed earlier are favored by these activities as are short and/or less palatable species. Native and non-native forbs, woody species, and C3 grasses increase in the absence of fire, especially when combined with grazing by livestock. Drier sites on hilltops or rocky soils persist longer but mesic sites on lower slopes can be invaded by trees and shrubs after just several years without fire.

Invasion by non-native species degrade the biotic integrity of many stands of this grassland system reducing the abundance of native species (Ogle et al. 2003, Pritekel et al. 2006, Mack et al. 2007, Davies 2011, Fink and Wilson 2011). Exotic grasses (Agropyron cristatum, Bromus inermis, Poa compressa, and Poa pratensis) have been planted for forage and erosion control on many sites. Invasive upland forb species such as Acroptilon repens, Centaurea stoebe ssp. micranthos, Linaria spp., Melilotus officinalis, and mesic site species Cirsium sp. and Euphorbia esula have become naturalized in many areas (LANDFIRE 2007a). Invasion by annual bromes, especially Bromus arvensis and Bromus tectorum, has impacted many mixedgrass prairie sites, especially those dominated by cool-season grasses Pascopyrum smithii and Nassella viridula (Ogle et al. 2003).

The natural grazing regime has been replaced with domestic livestock grazing that is targeted toward "moderate" grazing intensity. This is often characterized by grazing each year with removal of herbage over an extended period of the growing season without adequate rest and recovery from grazing. This is contrasted with the expected historic shorter, episodic grazing patterns. One result is more structural homogeneity. Under the current livestock-grazing regime, taller, palatable grasses such as Nassella viridula and Pseudoroegneria spicata decrease and shorter grasses (Bouteloua gracilis, Hesperostipa comata, Pascopyrum smithii, and Poa secunda) increase. Only under season-long grazing will warm-season grasses such as Schizachyrium scoparium decrease. Season of use and/or twice-over grazing will impact the prevalence of Schizachyrium scoparium and other C4 plants. Heavy grazing causes cool-season exotics such as Poa pratensis and Bromus inermis to increase in dominance.

Shrubs (Artemisia cana, Artemisia frigida, Artemisia tridentata ssp. wyomingensis, Ericameria spp., Symphoricarpos occidentalis, and Symphoricarpos oreophilus) increase greatly over the historic plant community. Compare the ecological site description to avoid using a shrub model for historic plant community when considering a grass site that has changed as a result of uncharacteristic grazing or unnaturally long fire-return intervals. Unnaturally long intervals without fire may contribute to an increased shrub component. Xeric sites will experience an increase in sagebrush (Artemisia spp.), whereas western snowberry (Symphoricarpos spp.) will increase in mesic areas.

Long-term high-intensity grazing by domestic livestock without periods of rest and recovery can result in a conversion in the vegetation states from a midgrass-dominated community to shortgrass-dominated communities (Bouteloua gracilis, Bouteloua dactyloides (in southern portions), Carex spp., Koeleria macrantha, and Poa secunda). This should be distinguished from the class (class B) that's influenced more by presence of prairie dog towns - which have a higher forb component with less of a midgrass component than the other classes. In species composition, communities grazed by prairie dog versus domestic livestock are very different.

In current conditions, there has also been an increase in the amount of woody vegetation on the plains, particularly increases in Symphoricarpos spp. on mesic sites and expansion of Pinus ponderosa into grasslands and shrublands which were probably maintained in a grassland state under historic fire frequencies. The lack of fire has shifted grassland systems to shrublands or woodlands.

Conversion to agriculture also impacts this system; however, the degree of agricultural alteration of this system is highly variable by geographic region with Montana and Wyoming having experienced much less impact than the estimated 75% percent of the Nebraska-Dakota-south-central Canada region, where this system has been heavily altered. In Montana, this system is the major sustainer of livestock grazing with overall far less than half of it having been lost to agriculture; several Montana counties have more than 90% of this system remaining intact, though impacted by grazing to varying degrees. The shortgrass Bouteloua gracilis is frequently abundant and conspicuous on mowed and heavily grazed sites, but on lightly grazed or spring-burned sites the tall grasses are frequently most conspicuous, creating the appearance of tallgrass prairie.

This system is found in the western Great Plains north of the shortgrass prairie and west of the northern tallgrass prairie and extends from northern and western Nebraska into southern Canada, and west to central Montana and eastern Wyoming. The U.S. range corresponds to Bailey et al. (1994) sections 331D, 331E, 331F (mostly), 331G, 332A, 332B, 332D, and perhaps minor extensions into 251B, and in Canada to the Moist Mixed Grassland and Fescue Grassland.