Western Great Plains Shortgrass Prairie

This ecological system is found primarily in the western half of the Western Great Plains Division in the rainshadow of the Rocky Mountains and ranges from the Nebraska Panhandle south into Texas and New Mexico, although grazing-impacted areas appearing as this type may reach as far north as southern Canada where it is a component of Northwestern Great Plains Mixedgrass Prairie (CES303.674). This system occurs primarily on flat to rolling uplands with loamy, ustic soils ranging from sandy to clayey. In much of its range, this system forms the matrix system with Bouteloua gracilis dominating. Associated graminoids may include Aristida purpurea, Bouteloua curtipendula, Bouteloua hirsuta, Bouteloua dactyloides, Carex filifolia, Carex inops ssp. heliophila, Hesperostipa comata, Hesperostipa neomexicana, Koeleria macrantha, Pascopyrum smithii, Pleuraphis jamesii, Sporobolus airoides, and Sporobolus cryptandrus. Although mid-height grass species may be present, especially on more mesic land positions and soils, they are secondary in importance to the sod-forming short grasses. Sandy soils have higher cover of Hesperostipa comata, and Sporobolus cryptandrus. Scattered shrub and dwarf-shrub species such as Artemisia filifolia, Artemisia frigida, Artemisia tridentata, Atriplex canescens, Eriogonum effusum, Gutierrezia sarothrae, Lycium pallidum, and Yucca glauca may also be present. Also, because this system spans a wide range, there can be some differences in the relative dominance of some species from north to south and from east to west. Large-scale processes such as climate, fire and grazing influence this system. High variation in the amount and timing of annual precipitation impacts the relative cover of cool- and warm-season herbaceous species.

In contrast to other prairie systems, fire is less important, especially in the western range of this system, because the often dry and xeric climate conditions can decrease the fuel load and thus the relative fire frequency within the system. However, historically, fires that did occur were often very extensive. Currently, fire suppression and more extensive livestock grazing in the region have likely decreased the fire frequency even more, and it is unlikely that these processes could occur at a natural scale. A large part of the range for this system (especially in the east and near rivers) has been converted to agriculture. Areas of the central and western range have been impacted by the unsuccessful attempts to develop dryland cultivation during the Dust Bowl of the 1930s. The short grasses that dominate this system are extremely drought- and grazing-tolerant. These species evolved with drought and large herbivores and, because of their stature, are relatively resistant to overgrazing. This system in combination with the associated wetland systems represents one of the richest areas for mammals and birds. The endemic bird species of the shortgrass system may constitute one of the fastest declining bird populations in North America.

This system spans a wide range and thus there can be some differences in the relative dominance of some species from north to south and from east to west. This system is primarily dominated by Bouteloua gracilis and Bouteloua dactyloides (= Buchloe dactyloides) throughout its range with various associated graminoid species depending on precipitation, soils and management. Associated graminoids may include Achnatherum hymenoides, Aristida purpurea, Bouteloua curtipendula, Bouteloua hirsuta, Bouteloua dactyloides, Carex filifolia, Hesperostipa comata, Koeleria macrantha (= Koeleria cristata), Muhlenbergia torreyana, Pascopyrum smithii (= Agropyron smithii), Pleuraphis jamesii, Sporobolus airoides, and Sporobolus cryptandrus. In southern examples of this system (Texas), Bouteloua dactyloides and Bouteloua hirsuta may dominate (especially where soils are rocky) in addition to Bouteloua gracilis. In addition, Bothriochloa laguroides ssp. Torreyana, Bouteloua rigidiseta, Erioneuron pilosum, Hilaria belangeri, Hordeum pusillum, Pleuraphis mutica, and Scleropogon brevifolius may occur in Texas examples (Elliott 2011). Although mid-height grass species may be present especially on more mesic land positions and soils, they are secondary in importance to the sod-forming short grasses. Sandy soils have higher cover of Hesperostipa comata, Sporobolus cryptandrus, and Yucca elata. Scattered shrub and dwarf-shrub species such as Artemisia filifolia, Artemisia frigida, Artemisia tridentata, Atriplex canescens, Eriogonum effusum, Gutierrezia sarothrae, and Lycium pallidum may also be present. In Texas examples, shrub cover is generally low but may include species such as Acacia greggii, Rhus microphylla, Rhus trilobata, Dalea formosa, Mahonia trifoliolata, Juniperus sp., and Prosopis glandulosa. Forbs such as Calylophus sp., Melampodium leucanthum, Krameria lanceolata, Ratibida columnifera, Psoralidium tenuiflorum, and others are often present. Gutierrezia sarothrae may be present with significant cover, especially on sites with intense and continuous grazing (Elliott 2011). High annual variation in amount and timing of precipitation impacts relative cover of herbaceous species. Cover of cool-season grasses is dependent on winter and early spring precipitation. The vegetation description is based on several other references, including Shaw et al. (1989), Hazlett (1998), and Schiebout et al. (2008).

This system forms the matrix grassland in the southwest half of the Great Plains and largely occurs in the rainshadow of the Rocky Mountains. This system occurs on various geologic formations, primarily on flat to rolling uplands. Soils typically are loamy and ustic (bordering on aridic) but can range from sandy to clayey (Scifres 1980, Shiflet 1994).

Climate: Climate is temperate, semi-arid, and continental with mean annual precipitation generally about 300 mm ranging to 500 mm to the east. Annual precipitation has a bimodal distribution occurring mostly before the growing season in winter and early spring and then during summer as monsoon thunderstorms (Sims et al. 1978). In most years, rates of evaporation are greater than precipitation for this system. Most of the annual precipitation occurs during the growing season as thunderstorms. Precipitation events are mostly <10 cm with occasional larger events (Sala and Lauenroth 1982). High variation in amount and timing of annual precipitation impacts the relative cover of cool- and warm-season herbaceous species. This is the driest of the Great Plains grasslands ecosystems. Average daily temperature in July varies from 27°C in the southeast to 21°C in the northwest and along the foothills of the Rocky Mountains. Average daily temperature in January varies from 3°C in the south to -6°C in the northwest.

Physiography/landform: Stands occur on primarily flat to rolling uplands and to a lesser extent mesatops and plateaus.

Soil/substrate/hydrology: Soils are typically well-drained, shallow to moderately deep, loamy and ustic and range from sandy to clayey (Scifres 1980, Shiflet 1994). In the southeasternmost expression of the system in Texas, it occurs on sites with soils providing relatively dry conditions such as Rough Breaks, Shallow Clay, Very Shallow, Very Shallow Clay, Moderately Alkaline Deep Hardland, and Hardland Ecological Sites (Elliott 2013).

Large-scale processes such as climate, fire and grazing constitute the primary processes impacting this system. The short grasses that dominate this system are extremely drought- and grazing-tolerant (Lauenroth and Milchunas 1992, Lauenroth et al. 1994a). These species evolved with large herbivores and drought (Milchunas and Lauenroth 2008) and adapted to historical heavy grazing with their low stature making them relatively resistant to overgrazing (Lauenroth et al. 1994a). The return intervals for grazing varied with areas distant from water sources likely grazed less heavily as those near water. However, the shortgrass prairie is probably the system with the highest intensity of grazing than other systems historically (Lauenroth et al. 1994a, Milchunas 2006). This is a drought-tolerant system. Many shortgrass species are drought-tolerant and have root systems that extend up near the soil surface where they can utilize low precipitation events (Salas and Lauenroth 1982). If blue grama is eliminated from an area by extended drought (3-4 years) or disturbance such as plowing, regeneration is slow because of very slow tillering rates (Samuel 1985), low and variable seed production (Coffin and Lauenroth 1992), minimal seed storage in soil (Coffin and Lauenroth 1989) and limited seedling germination and establishment due to particular temperature and extended soil moisture requirements for successful seedling establishment (Hyder et al. 1971, Briske and Wilson 1978, 1980).

In contrast to other prairie systems, fire is less frequent, especially in the western range of this system, because the often dry and xeric climate conditions can decrease the fuel load and reduce lightning events, and thus the relative fire frequency within the system. However, historically, fires that did occur were often very extensive. Wright and Bailey (1982c) suggest that in semiarid areas, big prairie fires usually occurred during drought years that followed one to three years of above average precipitation, because of the abundant and continuous fuel. Consequently, these wildfires could travel far when the winds and air temperatures were high and relative humidity was low. There is debate as to the mean fire-return interval (MFRI) for this shortgrass system. Because of the lack of long-lived trees, and trees that do exist are in relatively productive sites, there is absolutely no way to reconstruct a reliable historic fire-return interval. All estimates of historic fire-return intervals must be based on those for surrounding vegetation types that do have means for reconstruction, and then extrapolating based on differences in primary production and herbivore removal of fuel loads. Therefore, there is no means to directly obtain the estimate, and the range is varied. It depends on many factors: portions will be drier, and portions will vary in frequency over time and there will be decadal variation. Anderson (2003) reports a broad fire-return interval (FRI) of <35 years for shortgrass prairie. There is a wide variability of MFRI across this system, based on precipitation, fuel and ignition sources (LANDFIRE 2007a).

LANDFIRE developed a VDDT model for this system which has three classes (LANDFIRE 2007a, BpS 2711490).

A) Mid Development 1 Open (20% of type in this stage): Instead of calling the classes early, mid and late, which do not actually apply in shortgrass prairie and the different stages that we are describing, we are calling all stages "mid-development." The stages of the grassland are created and/or maintained by disturbances or lack thereof. Class A is the low biomass (0-1" based on the Robel pole density / visual obstruction method), heavy disturbance-dependent community. It combines 2 types of communities. One consists of the high cover blue grama-buffalo grass sod that looks like a golf course (high cover in patches). The other is the low cover bare soil, Aristida, and forb stage, which could have taller grasses than the sod, but they are spaced apart due to bare soil between. See biomass in Milchunas and Lauenroth (1989) and Milchunas et al. (1994) and basal cover for sod class by point frame in Milchunas et al. (1989). Please note that this system should be distinguished by on-the-ground biomass and not cover, since the cover in class A actually varies from a low, mosaic-bare-ground cover to a high sod-cover, which includes litter too. Due to mapping constraints, we are defining dropdown boxes on cover; however, this stage could go up to 70% cover, including litter, with very low biomass. Basal cover for high cover sod is approximately 45% or higher if including litter. Basal cover for low cover prairie dog areas is approximately 20-25% cover. On the ground, this class should be distinguished by biomass. There are relatively few cool-season grasses in this stage. There is always blue grama in this stage, as in the others. Cactus is present (and could even be a dominant in the class A sod depending on soil type). Aristida is present, which increases with prairie dog colonies. Annual grasses - sixweeks fescue, red three-awn, ragweed, annual forbs. [Currently, you would see non-native annuals in this class such as cheatgrass and kochia - only in the high biomass type. Annuals and exotics are actually less abundant in the sod type than any other class (Milchunas et al. 1989, Milchunas and Lauenroth 1989, Milchunas et al. 1988); the landscape might also have non-natives of bindweed on prairie dog towns today, but not historically.] On loamier or sandier sites, there is sand dropseed. For the southern, New Mexico version, other indicator species are lemonweed, showy goldeneye, and verbena.

B) Mid Development 2 Closed (60% of type in this stage): Instead of calling the classes early, mid and late, which do not actually apply in shortgrass prairie and the different stages that we are describing, we are calling all stages "mid-development." The stages of the grassland are created and/or maintained by disturbances or lack thereof. Class B is the mid biomass (2-4" based on the Robel pole density / visual obstruction method), mid cover stage. See biomass in Milchunas and Lauenroth (1989) and Milchunas et al. (1994). This stage again consists of blue grama. Cactus is often present and could even be the second dominant depending on soil type. There is less needle-and-thread and western wheatgrass than in class C. This also includes the "historic climax plant community" with blue grama, buffalograss, and western wheatgrass, galleta grass, green needle grass (not in New Mexico), fringed sage, and New Mexico feather grass in the south. Historically, there would have been more midgrasses (Harvey Sprock et al. pers. comm.). In New Mexico, there would be scatterings of black grama, vine-mesquite on heavier soils. Fire does occur in this stage. If there is 1-2 years of no grazing or 4-10 years of no fire, then 4-10 years post-fire, this class would transition to the high biomass class C stage. This was modeled as "alternate succession" occurring as a probability of 0.05, for modeling purposes. Prairie dogs could occur in this stage. If they do, the long-term prairie dog grazing causes a transition to class A.

C) Mid Development 3 Closed (20% of type in this stage): Instead of calling the classes early, mid and late, which do not actually apply in shortgrass prairie and the different stages that we are describing, we are calling all stages "mid-development." The stages of the grassland are created and/or maintained by disturbances or lack thereof. Class C is the high biomass (4+" based on the Robel pole density / visual obstruction method), high cover stage. See biomass in Milchunas and Lauenroth (1989) and Milchunas et al. (1994) and basal cover in Milchunas et al. (1989). The same grasses are present as the previous. However, there are also more C3 perennial cool-season grasses. (However, some have questioned the increase in cool-season grasses with succession as being speculative. There are definite edaphic differences. Gravelly sites in New Mexico often support Hesperostipa neomexicana even under intense grazing regimes.) Blue grama is still present and dominant. Needle-and-thread, galleta grass and western wheatgrass are more prominent. Note also that more annuals and exotics occur in the ungrazed than in the heavily grazed sod class (Milchunas et al. 1989, Milchunas et al. 1992). This stage is arrived at through lack of fire and grazing, although while already in this stage, fire would be more likely to occur due to the increased biomass. Fire does occur in this stage. If there is fire and then grazing, this will over time transition to class B, and with long-term heavy grazing to class A. Fire alone may not cause a transition but can especially on coarser textured soils and also when fire occurs with heavy grazing. Regular grazing can just move the class to class B. Prairie dogs are unlikely to occur in this class, but when they do, they will occur as a patch within the matrix and will cause a transition.

During LANDFIRE modeling workshops, some experts suggest that the MFRI was historically approximately 25-35 years with small fires at times so fire-return interval at one spot was longer than expected, i.e., a fire can burn somewhere on the landscape often, but it may not necessarily return to the same spot for 25-50 years or more (LANDFIRE 2007a). However, other experts thought MFRI was shorter, between 5-20 years, dependent on the precipitation gradient east to west with shorter FRI (~5 years) occurring in the more mesic eastern extent of the shortgrass prairie (LANDFIRE 2007a). A proposed precipitation gradient between drier versus wetter of approximately 350-375 mm annual precipitation to delineate a change in fuels and fire behavior across the west to east gradient in precipitation / above-ground primary productivity. The western portion would have a MFRI of 15-20 years and in the eastern portion, it would be shorter (5-10 years) (LANDFIRE 2007a). A MFRI of 5 years is similar to mixed and tallgrass prairies (Bragg and Hulbert 1976, Bragg 1986, Umbanhowar 1996, LANDFIRE 2007a).

Black-tailed prairie dogs are an ecologically important component of the grazing regime in shortgrass prairie and would have occurred extensively (Lauenroth and Milchunas 1992, Milchunas and Lauenroth 2008). There were some very large towns, but there were also areas without any towns. Quantitative historical estimates of black-tailed prairie dogs abundance are difficult to obtain, but the U.S. Fish and Wildlife Service estimated that about 160 million ha (395 million acres) of potential habitat historically existed in the U.S., and about 20% was occupied at any one time (Gober 2000). Shortgrass has most of the suitable soil types for prairie dogs; in general, they need loamy or clay soil. In historic times, there was frequent and broad-scale grazing by bison and pronghorn antelope. Through the growing season, bison might have been there for relatively short periods in some years and longer in other years. There were also resident herds of bison in areas of Colorado (LANDFIRE 2007a). Historically, such areas would also have been populated by bison in sufficient numbers to support populations of wolves. Bamforth (1987) suggested that bison herds under relatively undisturbed conditions (prior to 1846) most often ranged in size from several hundred to several thousand. Shaw and Lee (1997) reviewed diaries of European travels in the southern Great Plains from 1806 to 1857. Organized by historical period and biome type, the authors suggest populations of three major large herbivores (bison, elk and pronghorn) changed in the first half of the nineteenth century; bison were most numerous on the shortgrass prairie prior to 1821, pronghorn were most abundant on the shortgrass prairie between 1806 and 1820, again in the 1850s (LANDFIRE 2007a). The dry half of the Great Plains has high interannual rainfall variability, so historically, the population declined faster in dry years (LANDFIRE 2007a). This resulted in a time lag or temporal variability, in which density could be reduced greatly. Bison historically moved nomadically in response to vegetation changes associated with rainfall, fire and prairie dog colonies (LANDFIRE 2007a). The time lag for return movements provided deferment during the regrowth period, which according to both historic and archeological records, may have ranged from 1 to 8 years (Malainey and Sherriff 1996). If there was a series of droughts followed by a wetter year, there would have been little grazing pressure, which would then result in a higher severity or frequency of fire. Drought and grazing were probably most important disturbances historically and greatly influenced fire frequency and extent. Insects such as grasshoppers, range caterpillars, and Mormon crickets were also a natural disturbance agent on the landscape (LANDFIRE 2007a).

Biological soils crusts (BSC) are important for soil fertility, soil moisture, and soil stability in semi-arid ecosystems such as the drier portions of the shortgrass prairie (Belnap and Lange 2003). Cyanobacteria (especially Nostoc) fix large amounts of soil nitrogen and carbon (Evans and Belnap 1999, Belnap and Lange 2003). Generally, BSC are more important on sites with more exposed soil surface and less herbaceous and litter cover; however, cover varies locally with site characteristics, especially disturbance (Belnap et al. 2001, Belnap and Lange 2003).

Historically, fires were often very expansive, especially after a series of years with above-average precipitation when litter/fine fuels built up. Currently, fire suppression, fragmentation of landscapes, and more extensive grazing in the region have likely decreased the fire frequency even more, and it is unlikely that these processes could occur at a natural scale. Heavy continuous livestock grazing, military training, invasive non-native species, altered fire regime (fire suppression), conversion to agriculture, fragmentation from roads and development such as exurban and urban development, and more recently gas and oil exploration and extraction stress the shortgrass prairie ecosystem. Of these, altered grazing and fire regimes stressors are prevalent throughout the range. Cultivation for row crop agriculture has been widespread and extensive in the higher precipitation parts of the range, where more conducive soil moisture conditions exist, or where irrigation is possible. Habitat fragmentation from roads is common throughout the range, probably less in the drier parts of the range where large ranches are more common, but none the less, still at levels limiting natural fire regimes through the range. Stressors related to urban and suburban development and military training affect a relatively small proportion of the range of this system, but where they occur, impacts are often severe.

Conversion to agriculture and pastureland with subsequent irrigation has degraded and extirpated this system in approximately 40% of its range (Samson and Knopf 1994). Conversion of this type has commonly come from dryland wheat cultivation in the less xeric portion in eastern Colorado and western Kansas and from all types of irrigated agriculture typically near rivers such as the Platte and Arkansas basins. Historically, areas of the central and western range have been impacted by the unsuccessful attempts to develop dryland cultivation during the Dust Bowl of the 1930s (CNHP 2010). Urban and exurban development along the Front Range and water developments/reservoirs are also significant. Locally, mechanical disturbance (roads, mechanized military training, ORVs, sacrifice areas surrounding livestock tanks, etc.) may eliminate cover of blue grama and other grasses that are slow to recover. Conversion to invasive non-native species is generally not a widespread or significant problem on dry upland sites. Invasion and conversion to woodlands by native trees Juniperus spp. and Prosopis glandulosa (in southern extent) is an issue where alteration of natural fire regime has permitted woodland expansion into former grasslands.

Common stressors and threats include fragmentation, altered fire regime, overgrazing by livestock, and invasive species (in the less xeric regions). Fire suppression and certain grazing patterns such as continuous heavy grazing in the region have likely decreased the fire frequency even more, and it is unlikely that these processes could occur at a natural scale. The short grasses that dominate this system are extremely drought- and grazing-tolerant although continuous heavy grazing and extended drought (3-4 years) will reduce cover of dominant species.

This system is found primarily in the western half of the Western Great Plains Division east of the Rocky Mountains and ranges from the Nebraska Panhandle south into the panhandles of Oklahoma and Texas and New Mexico, although some examples may reach as far north as southern Canada where it grades into Northwestern Great Plains Mixedgrass Prairie (CES303.674).