About the Shortgrass Steppe
The SGS-LTER site encompassed a large portion of the Colorado Piedmont Section of the western Great Plains. The site was bounded on the west by the Central Plains Experimental Range (CPER), located 61 km east of Fort Collins and 40 km northeast of Greeley in Weld County, Colorado. The research site also encompassed the entire Pawnee National Grassland (PNG). The CPER is a contiguous 6280 ha block of land. The PNG is 78,100 ha of public land that lies within an approximate 50 by 100 km checkerboard of public, private and State of Colorado lands, just east of the CPER.
The PNG is discontinuously distributed as a result of private land acquisition that began in the Dust Bowl era of the 1930s. This distribution resulted in the broad variation in land use and cultural diversity associated with a diverse set of land users and land managers, and contributed to the importance and relevance of having an ecological research program in the shortgrass steppe region.
The climate of the SGS-LTER is typical of mid-continental semiarid temperate zones, but is somewhat drier because of a strong rain shadow effect of the Rocky Mountains to the west. Annual precipitation, and its seasonal distribution, profoundly influences this semiarid grassland. Average annual precipitation across the SGS-LTER is 320 mm over the past 6 decades, with approximately 70% of the precipitation occurring between April and September. In spring, long-lasting storms provide soil-penetrating rains, while summer brings intermittent afternoon thundershowers that can be locally heavy. Inter-annual variation is dramatic, with spring or summer rain failing in any year. Precipitation-induced changes cascade through the ecosystem, causing fluctuations in vegetative structure, the abundance and species composition of animal communities, and ecosystem functions such as net primary productivity, N mineralization and trace gas flux. Mean annual temperature is 8.6 °C and average monthly temperatures range from -4 °C in winter to 22 °C in summer.
Natural disturbances are responsible for some of the spatial variability in the shprtgrass steppe ecosystem. Most disturbances are considered small, at approximately 0.1 m2 to several ha in size. The most frequent disturbances are digging and burrowing by small mammals such as badgers (Taxidea taxus taxus), ground squirrels (Spermophilus tridecemlineatus), pocket gophers (Thomomys talpoides and Geomys bursarius) and prairie dogs (Cynomys ludovicianus), outbreaks of root-feeding invertebrates, and nest-building by harvester ants (Pogonomyrex occidentalis). Bison grazing was an important part of the evolutionary history of the shortgrass steppe ecosystem, such that we do not consider herbivory to be a disturbance. Disturbances related to soil erosion and deposition occur over longer time and larger spatial scales and are often linked to regional shifts in climate.
The biotic communities of the shortgrass steppe ecosystem are particularly well-adapted for drought, with vegetative species such as blue grama (Bouteloua gracilis) and prickly-pear cactus (Opuntia polyacantha), large herbivores such as cattle (and previously, bison), and burrowing animals such as the black-tailed prairie dog (Cynomys ludovicianus) playing dominant roles in ecosystem function and maintenance.
The main natural plant communities are shortgrass steppe, floodplain shrubland, and salt meadow. The ecosystem is dominated by short grasses (64%), succulents (21%) and dwarf shrubs (8%). Blue grama predominates and contributes 60 to 80% percent of plant cover, biomass, and net primary productivity. Long-lived C4 grasses such as blue grama dominate under the characteristically dry conditions of the shortgrass steppe by efficiently accessing available water. Other important plants include buffalo grass (Buchloe dactyloides), prickly pear cactus , rabbitbrush (Chrysothamnus nauseosa) and saltbush (Atriplex canescens). The shortgrass steppe stores most biomass and resources belowground, so that aboveground disturbances do not drastically alter the vegetative community.
Pronghorn (Antilocapra americana) are among the most common wildlife species seen on shortgrass steppe. Black-tailed prairie dogs (Cynomys ludovicianus), through their clipping and burrowing activities, are also conspicuous, and create habitat for a number of other invertebrate and vertebrate animals, including horned larks (Eremophila alpestris), mountain plovers (Charadrius montanus), and burrowing owls (Athene cunicularia). Other species of special conservation interest or concern include the swift fox (Vulpes velox) and the lark bunting (Calamospiza melanocorys), the state bird of Colorado.
The topography of the western portion of the shortgrass steppe ecosystem, including the CPER, is characterized by gently rolling hills, broad ephemeral stream courses and low flat-topped terraces. The western shortgrass steppe is underlain by late Cretaceous shales and interbedded sandstones of the Laramie Formation that are visible as occasional outcrops of low relief. The eastern portion of the shortgrass steppe is topographically similar to the western, but contains occasional outcrops of Tertiary-age remnants of the Brule, Chadron and Ogallala Formations expressed as buttes and bluffs. Shortgrass steppe soil parent materials are principally derived from surficial deposits of alluvium and wind-reworked sediments eroded from local sedimentary rock and Rocky Mountain sources and are Holocene and late Pleistocene in age. The dominant soils of the PNG are classified as Aridic Argiustolls and Ustic Haplargids. Ustic Torriorthents and Ustic Torrifluvents occur in areas of more recent alluvial and eolian activity, particularly along modern drainages.
Aspects of physiography that regulate the shortgrass steppe ecosystem include landscape position, soil age, water holding capacity, soil depth and surface texture which, in turn, determine such properties as soil moisture storage, net primary productivity and the distribution of small mammals such as prairie dogs and pocket gophers.
The shortgrass steppe is unique among North American grasslands for its long history of grazing by large herbivores and periodic drought. Over time, intense selection by grazing and drought has created an ecosystem that is well adapted to both, with low-standing vegetation and below ground concentration of biological activity and organic matter. Currently, grazing by domestic livestock is the primary land use of native grassland, which occupies about 55% of the land area of the shortgrass steppe.
Since the late 1800s, the dominant land use regime included livestock grazing and farming. Farming systems include both dryland and irrigated row-crop agriculture. Approximately forty percent of the shortgrass steppe ecosystem is currently cultivated, and conversion to cropland drastically alters biological diversity, biogeochemical dynamics, soil organic matter and plant biomass. In recent decades, the western portion of the shortgrass steppe has been subjected to very rapid population growth. Urbanization and exurban development is now more common. While land use formerly varied at relatively large spatial extents, humans continue to fragment the shortgrass steppe, with privately owned lands rapidly being subdivided from large ranges and farms to smaller “horse properties” for commuters to the cities. Energy development is also occurring for oil and gas deposits in the Niobrara shale formation.