Forming a transitional zone and shifting mosaic with the tallgrass prairie to the east and shortgrass prairie to the west, the mixed-grass prairie is the central third of the North American Great Plains. It arches below the boreal forests of Alberta, Saskatchewan, and Manitoba, passing through the east-central Dakotas and central Nebraska to western Oklahoma. It also stretches west of the tallgrass prairie through the western Dakotas, northeast Wyoming, eastern Montana, and southern portions of Canada.
Overall, grasslands are the largest vegetation formation in North America with the mixed-grass prairie comprising about 22%. Historically, the mixed-grass prairie ecosystem covered around 26,000,000 ha (260,000 km2, 156,000 sq miles) a value which varies based on yearly precipitation. Its current distribution has been reduced to less than 30% of the historical range or approximately 7,800,000 ha (78,000 km2or 46,800 sq miles). The ecosystem can be split into three types based on plant community: northern mixed prairie, sandhills or central mixed prairie, and southern mixed prairie.
The mixed-grass prairie occurs in a semiarid climatic zone. The mean temperature lies between 11º C and 14º C (52º F and 57º F). It is typified by low and irregular precipitation with distinct dry and wet seasons and temperature and precipitation extremes. While average precipitation is 50 cm (20″) and ranges from 30 to 70 cm (12-28″), there is an east-west precipitation gradient resulting in a range from 30 cm to 60 cm (12-24″) in the north and from 40 cm to 80 cm (16-32″) in the south. A deficiency of precipitation late in the growing season favors grassland ecosystems over forested. Two thirds of the precipitation occurs during the growing season, often in the form of thunderstorms. In the region, high wind velocities contribute to rates of evaporation that exceed precipitation. The area has anywhere from 168 to 226 growing degree days with 180 days in the south and 140 in the north.
Topography and Soils
The mixed-grass prairie occurs on relatively level and rolling topography cut by steeply-sloped drainages. From east to west, the elevation rises from 400 m to around 1,130 m (1300-3700′). The region is underlain by the Ogallala aquifer.
Large rivers in the mixed-grass prairie tend to have sandy bottoms and broad river valley floodplains. They have seasonal high flows in the spring and greatly reduced flows in the summer. The seasonal flow differences create ephemeral habitats such as sandbars. Five major rivers in the region are the Arkansas, Cimarron, North Canadian, Canadian, and Red; other rivers include the South Fork of the Arkansas, Chickasha, Washita, South and North Forks of the Red, Republican, Loup, Big Blue, and Little Blue.
The soils of the mixed-grass prairie are mostly Mollisols, but Entisols, Inceptisols, and Aridisols are also present. Soil texture ranges from fine or loamy sands to loams to clays, and the soils are deep and fertile loess deposits. In the northern areas, the soil is formed on glacial till. The soils have limited profile development and are more basic compared to forest soils. The coloring ranges from black in more mesic areas to browner in semiarid areas.
Current Plant Communities
Large areas of the mixed-grass prairie have been converted to cropland, pasture, and other uses. Most of the southern portion is in cultivation but the north is not entirely suitable for crop production. Relict prairies can be found along railroad corridors and right-of-ways; abandoned railroad tracks are often used as hiking trails.
The majority of the remnants of mixed-grass prairie exist in a deteriorated state. For example, large areas are suffering from shrub encroachment by species such as eastern redcedar. This has been accelerated by the common practice of planting woodlands and shelterbelts. Also, severe anthropogenic disturbances switched the community from perennials to mostly annuals. Degradation and alterations are also a result of fire suppression, heavy grazing, and invasive and non-native species. Plowing, erosion, and development have decreased the amount of prairie wetlands. Additionally, reservoirs, impoundments, channelization, and water withdrawal alters the flow of waterways, threatens ephemeral habitats, and incises stream channels.
Historic Plant Communities
The shortgrass prairie is a relatively young ecosystem, having formed between the middle Miocene and the early Holocene, It resulted from a gradual shift from semi-open forest with occasional grassy areas to open grasslands with few trees due to factors such as increasing aridity, drought, and natural and anthropogenic fire. Due to the young age, the ecosystem lacks many endemic species. In fact, the only endemic plant species is blowout penstemon (Penstemon haydenii).
Due to its position as a transition zone between the tallgrass and shortgrass prairie, the vegetative community is a blend of warm- and cool-season vegetation from both. This intermix results in a greater number of plant species than any other prairie type. Differing grass heights create distinctive grass layers that add to spatial diversity. The mixture of warm and cool season species results in varying or bimodal peaks of live biomass. Most biomass is located belowground.
There is a gradient of species from more mesic tallgrasses in the east to drier habitats with shortgrasses in the west. Moving north, cool season grasses increase in dominance. Common grasses in this region include: little bluestem (Schizachyrium scoparium), blue grama (Bouteloua gracilis), big bluestem (Andropogon gerardii), western wheatgrass (Western wheatgrass), tall dropseed (Sporobolus compositus), buffalograss (Buchloe dactyloides), switchgrass (Panicum virgatum), indiangrass (Sorghastrum nutans), and sand lovegrass (Eragrostis trichodes).
Although perennial grasses dominate production, forbs supply the majority of the plant diversity. There are generally 3 to 4 times as many forb species as grass species. Forb productivity ranges from 0-40% of the net primary production. Common forbs and shrubs of the mixed-grass prairie include: purple coneflower (Echinacea atrorubens), leadplant (Amorpha canescens), dotted gayfeather (Liatris punctate), heath aster (Aster ericoides), hairy sunflower (Helianthus hirsutus), roundleaf bladderpod (Lesquerella ovalifolia), western ragweed (Ambrosia psilostachya), lamb’s quarter (Chenopodium belandieri), and Patagonian plantain (Plantago patagonica).
Shrubs, succulents, and trees are minor components of the mixed-grass prairie vegetative community. Historically, woody species, such as eastern redcedar and hackberry, were restricted to ravines, canyons, and riparian areas and covered a mere 1% of the region. Common woody species of the mixed-grass prairie include: eastern redcedar (Juniperus virginiana), southern hackberry (Celtis laevigata),redberry juniper (Juniperus pinchotii), honey mesquite (Prosopis glandulosa), ashe juniper (Juniperus ashei), western snowberry (Symphoricarpos occidentalis), eastern cottonwood (Populus deltoids), green ash (Fraxinus pennsylvanica), and sand lovegrass (Eragrostis trichodes).
The shortgrass prairie also includes wetland communities that are crucial components of the Great Plains Flyway. These commonly ephemeral prairie wetlands go by many terms such as playas, prairie potholes, sloughs, bottoms, river flats, sandhill lakes, desert sinks, thermal seeps, rainwater basins, and gilgais. There are mainly herbaceous wetlands in small, seasonally flooded depressions in between dunes and hills or waterway floodplains.
Large herbivores include pronghorn antelope (Antilocapra Americana), elk (Cervus Canadensis) which is locally extirpated, white-tailed deer (Odocoileus virginianus),and American bison (Bos bison) also locally extirpated. Other mammals include the black-tailed prairie dog (Cynomys ludovicianus), mountain lion (Puma concolor), Great Plains wolf (Canis lupus nubilus) which has been locally extirpated, the endemic Texas kangaroo rat (Dipodomys elator), Western big-eared bat (Plecotustownsendii), and eastern spotted skunk (Spilogale putorius).
Birds of the mixed-grass prairie include the greater prairie chicken (Tympanuchus cupido), northern bobwhite quail (Colinus virginianus), bald eagle (Haliaeetus leucocephalus) and burrowing owl (Athene cunicularia). Several birds are endangered, including Baird’s sparrow (Ammodramus bairdii), loggerhead shrike (Lanius ludovicanus), ferruginous hawk (Buteo regalis), and black-capped vireo (Vireo atricapilla).
Insects and reptiles of the mixed-grass prairie include the prairie mole cricket (Gryllotalpa major), Texas horned lizard (Phrynosoma cornutum), diamond-backed rattlesnake (Crotalus atrox), alligator snapping turtle (Macrochelys temminckii), and massasauga (Sistrurus catenatus).
The formation of Great Plains grasslands began 5 to 7 million years ago in the Miocene-Pliocene transition with periods of forest, woodland, tundra and ice dominance, depending upon climatic cycles and glacial/interglacial periods. The previous vegetation consisted of temperate and tropical forests, but that declined as the climate became drier and colder. The northern Great Plains were glaciated many times by continental ice sheets with the current landform resulting from retreat of the last Wisconsin glaciers approximately 10,000 years ago. The current grassland biome formed during the Altithermal period, 3000 to 4000 years ago. In the last 1500 years, the climate has been cooler and wetter to produce the current soils and vegetation. More recently farms, once cultivated, were then abandoned following the Dust Bowl. They were left to revegetate to rangeland in the 1940s and the 1950s but plowed again in the 1980s.
Yearly and Seasonal Variation
Vegetation in the mixed-grass prairie fluctuates with climate, fire, and the degree and frequency of grazing. The major vegetation changes occur due to drought and grazing pressure; fire is of secondary importance. Long-term negative effects on the prairie promote increased plant species, less palatable plant species, and invasive species.
The historical fire frequency in level prairies was 5-10 years and 10 to 20 years in prairies cut by breaks and streams. Fire once burned across vast distances, but grassland fires are now limited by cultivated lands, roads, and fire suppression.
The majority of grasses, except for cool-season bluegrasses, are relatively tolerant to fire. Fire has a greater effect on the forb community than on grasses, with forbs peaking three to four years following a burn and then declining. Fire reduces standing crop in dry years and maintains or decreases biomass in wet years. Vegetative responses to fire depend on season. For example, winter burning will favor cool season species over warm season species and early spring burns enhance native grasses, whereas later growing season fires reduce both warm and cool season grasses. Fire recovery is strongly affected by periodic droughts. The benefits of fire include increased forage production and palatability, decreased litter, and control of undesirable annual grasses, forbs, and woody plants.
Historically, the tallgrass prairie was grazed by large herds of bison, antelope, deer, and elk along with colonies of prairie dogs. Except for prairie dogs, these species were migratory, continuously searching for green forage and responding to environmental variables such as precipitation, drought, and fire. This resulted in rotational grazing that allowed vegetation to recover in the absence of the herbivores. It also caused repetitive seasonal grazing pressures to which the vegetation adapted. Due to anthropogenic changes, prairie dogs now can cause overgrazing.
Palatable plants are selected first by grazers and are continually pressured. Grazing has a greater effect on the forb community than on grasses and favors rhizomatous species over bunchgrasses. Moderate grazing increases decomposition and affects the chemical properties of the soil. It can also provide bio-control of invasive species such hedge bindweed (Convolvulus sepium). Short-term light grazing increases and long-term heavy grazing decreases plant productivity. A lack of grazing results in litter accumulations causing degeneration and reduced productivity. Because blue grama and buffalograss retain nutritional value when “cured on the vine,” they support winter grazing even when dormant.
There are many grazing strategies that are used today: some have altered the plant species composition, while others have promoted heterogeneity of the landscape. Improper grazing management techniques detrimentally alter the plant communities and historical fire patterns. Improper management can be especially harmful to riparian areas.
Drought is a major limiting factor and risk to ranching in the mixed-grass prairie. During droughts, grass cover declines but tall grasses expand following wet years.
Thunderstorms in the mixed-grass prairie can produce hail and tornadoes. The area is prone to an especially high frequency of large hailstones. The effects of storms cause crop damage and endanger livestock and humans.
Dust storms are a natural occurrence and occur frequently during dry years. The Dust Bowl of the 1930s occurred across the Great Plains during a 22 year drought cycle. It contained the most severe dust storms on record and blame was placed on sod busters who plowed up the prairie. It resulted in many lands being placed under the Conservation Reserve Program.
Buffalo wallows are small, concave depressions with hardpan bottom formed by bison rolling in exposed soil. They contain water in rainy periods. These wetter conditions result in different vegetation composition.
Prairie Dog Mounds
Higher plant diversity exists within mounds. The mounds subject surrounding vegetation to continuous disturbance. There are also disturbances from mounds of other species, such as badgers.
Fire in the mixed-grass prairie now occurs less frequently than historically, resulting in dramatic ecosystem changes. These changes include decreased overall grassland vigor, woody encroachment, and mulch accumulations that favor invasion of exotics such as Kentucky bluegrass. These changes degrade wildlife habitat.
Prescribed fire is a useful management tool for implementing a fire regime on a landscape that may have a lower cost than other options. However, it comes with management concerns such as the public’s fear of fire, the challenges of smoke management, and complex liability issues. Benefits of appropriately and wisely used prescribed fires include enhanced biodiversity, suppressed woody encroachment, protection against catastrophic wildfires, improved forage for livestock, increased soil fertility, and improved wildlife habitat. The eventual alternative to prescribed fire is wildfires that can be catastrophic. Fire suppression activities contribute to wildfires by allowing fuel to build up, which increases the intensity of wildfires.
Woody Plant Encroachment
Woody species encroachment in the mixed-grass prairie results from a combination of factors including fires, prairie dogs, large grazers, overgrazing, farming, oil and gas development, road building, and climatic change. It degrades the land, causing the loss of tallgrasses and all midgrasses since they are increasers and/or invaders. Species that encroach include eastern redcedar (Juniperus virginiana), redberry juniper (Juniperus pinchotii), honey mesquite (Prosopis glandulosa), ashe juniper (Juniperus ashei), western snowberry (Symphoricarpos occidentalis), and fringed sagebrush (Artemisia frigid).
Nurse plants, such as honey mesquite, allow development of multi-storied, mixed-species brush thickets that out-compete grass species for both light and water. To control these thickets, more costly control measures are required that have a lower success rate than single-species control measures.
Exotic species that invade the mixed-grass prairie include cheatgrass (Bromus tectorum), fireweed (Kochia scoparia), saltcedar (Tamarix spp.), Sericea lespedeza (Lespedeza cuneata), brown-headed cowbirds (Molothrus ater), Kentucky bluegrass (Poa pratensis), smooth brome (Bromus inermis), yellow sweetclover (Melilotus officinalis), leafy spurge (Euphorbia esula) and Canada thistle (Cirsium arvense).
Erosion is a common occurrence in the shortgrass prairie with large-scale wind erosion resulting in dust storms. The historic Dust Bowl of the 1930s illustrated the need for protective measures from wind erosion. These measures include tree wind breaks, plowing transverse to the dominant wind direction, protective stubble mulch left on soil surface, and old tires placed around sacrifice areas.
The mixed-grass prairie has been fragmented by several land management practices. For example, cultivation only uses 42% of the Great Plains but fragments 100% of the landscape. Fragmentation reduces habitat quality and quantity, impedes the fire regime, and is severely detrimental to biodiversity.
Water Quality and Quantity
There are water quality and quantity issues in the mixed-grass prairie region. Pollution from agriculture, CAFOS, oil development, and other sources impairs water quality. Reservoirs, channelization, impoundments, and withdrawal threaten flow regimes and riparian habitats. Cattle grazing contributes to riparian area degradation. Groundwater depletion and withdrawal in the area alters morphology and hydrology.
Grazing Management Practices
Mixed-grass prairie is one of the most important range types for livestock production. About 1/3 of the area is used for livestock grazing. Domestic livestock grazing started with the arrival of the Spaniards in the 17th century. Continuous grazing is superior to specialized systems in terms of both individual livestock performance and vegetative productivity.
Cow calf and stocker operations are the dominant types of livestock operations in the mixed-grass prairie. When corn prices are high cattle tend to graze the range longer, resulting in higher stocker weights. Common use grazing is also utilized. Common use grazing combines cattle, sheep, goats, or deer to increase animal production per unit area and to improve rangelands.
Not enough is known to predict responses on long time scales or overall restoration project successes in the mixed-grass prairie. For any restoration project, it is recommended that: a diverse mixture of native grasses and forbs be used; there be an understanding of the nature of the landscape; and, attention be paid to the inherent role of grazing on the prairie. Reseeded areas have a 30-50 year recovery period and require inputs to achieve necessary amounts of organic matter, carbon, and nitrogen. In these projects, it is important to consider spatial heterogeneity to determine which species to plant and/or seed.
Cultivation takes place on about 2/3 of the land with the widespread use of center-pivot irrigation. The major crops are corn, soybeans, wheat, alfalfa, and grain sorghum.
Commercial Hunting and Recreation
In some areas of the mixed-grass prairie, commercial hunting can generate more income than livestock. The principal game animals are white-tailed deer, mule deer, raccoon, squirrel, quail, mourning dove, pheasant, turkey, prairie chicken, and rabbit. Other recreational uses include wildlife tourism, fishing, hiking, bird watching, photography, and canoeing/kayaking.