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Regional Trends of Biological Resources — Grasslands

Land Resources and Management


Prairie Wetlands

USGS photo: Prairie Wetlands  
     Fig. 7.   Prairie wetlands, showing the zonation of wetland plant
     communities.

The northern prairie contains numerous wetlands (Fig. 7), including the glaciated prairie pothole region (Fig. 8), the Nebraska Sandhills, and the Rainwater Basin (Krest et al. 1996; Mack 1996). The 770,000-square-kilometer prairie pothole region extends from Alberta, Saskatchewan, and Manitoba across northeastern Montana, then southeast through North Dakota and eastern South Dakota into western Minnesota and northwestern and central Iowa. This landscape is pock-marked with numerous small, shallow depressions that capture snowmelt and rainwater or are within reach of subsurface waters. Estimated losses of prairie pothole wetland range from 35% in South Dakota to 99% in Iowa with loss rates upwards of 1,300 hectares per year (Tiner 1984).

The Nebraska Sandhills is the largest dune area (5,260 square kilometers) in North America (Mack 1996). About 526,100 hectares of wetlands are scattered throughout the area. The rapid movement of groundwater creates a continuum among lakes, wetlands, and streams, thus an alteration in one area may easily affect vegetation and wetlands over a larger landscape. Wetlands in the sandhills range from shallow, extremely alkaline basins to deeper, freshwater lakes to spring-fed streams. They are economically valuable, particularly as a source of irrigation water.

Figure 8: Map showing prairie pothole region (Alberta, Saskatchewan, Manitoba, Montana, North Dakota, South Dakota, Minnesota). Map 5

Fig. 8.   The prairie pothole region (in green) of the northern Great Plains.

Another wetland area dependent on capturing water runoff is the Rainwater Basin of south-central Nebraska (6,720 square kilometers). Throughout the basin, rainwater is caught by scattered wind-excavated depressions underlain by an impermeable clay pan. Since the late 1800's efforts have been under way to drain Rainwater Basin; today, fewer than 400 depressions remain of an estimated 4,000, and they account for 22% of the former area. Other large, alkaline wetlands in Kansas include the Jamestown marsh, Talmo marsh, Lincoln salt marsh, Cheyenne Bottoms, Quivira National Wildlife Refuge (including Big and Little Salt marshes), and Slate Creek salt marsh. A similar alkaline lake is the Great Salt Plains Reservoir in Alfalfa County, Oklahoma.

Effects of collective water loss on the northern prairie range from significant declines in waterfowl breeding populations (Bethke and Nudds 1995) to elimination of the flood storage value of natural wetlands. About half of the continental waterfowl production comes from the prairie pothole region. Nebraska's Rainwater Basin is the major spring staging area for the buff-breasted sandpiper and the greater white-fronted goose, and it provides migratory habitat for endangered species such as the whooping crane and interior least tern. In addition, about 45% of North America's shorebird population east of the Rocky Mountains may stop at Cheyenne Bottoms during spring migration, including 90% of the North American populations of the white-rumped sandpiper, stilt sandpiper, Baird's sandpiper, long-billed dowitcher, and Wilson's phalarope, and over half of all pectoral sandpipers, marbled godwits, and Hudsonian godwits.

Interior wetiand from the edge of the prairie pothole region across the central Great Plains is associated with major river systems. The area has few natural lakes, the largest of which is Inman Lake (78 hectares) in central Kansas (Carlander et al. 1986). Climate and past events account for the interior wetland's habitat characteristics (Cross and Moss 1987). On the central Great Plains, such areas have been transformed from wide, unvegetated channels (Fig. 9) to the current extensive cottonwood-willow woodlands lining narrow channels (Johnson 1994). This transformation is a result of human alteration of natural flow regimes, cessation of prairie fires, and elimination of the bison (Currier 1982). Overall, the presettlement near-river mosaic of meadow, marsh, and drier upland grassland is now an open- to closed-canopy woodland. Major changes in the habitat mosaic are expected in the future because secondary succession of woody vegetation will lead to a climax forest of regenerating stands of the nonindigenous Russian-olive along the Platte River and elsewhere across the entire central and western United States (Olson and Knopf 1986).

Figure 9a: 1900 photo of travelers crossing water using horses and wagons. Figure 9b: 1990 photo of same area, now an agricultural grassland.
Fig. 9.   a.) Upper California Crossing, Oregon Trail, Fort Sedgwick (Ovid), Colorado, around 1900; b.) Same locale, town of Ovid, in 1990.

To the south are the Playa lakes. Upwards of 26,000 of these shallow, small wetlands dot the short-grass prairie in Texas and adjoining states, reaching northward to Colorado (Bolen et al. 1989). Because of evaporation, such lakes often are rimmed in a salty crust. Playa lakes have abundant populations of small, aquatic invertebrates (Carlander et al. 1986). At times, these lakes serve as vital waterfowl nesting and resting areas (U.S. Fish and Wildlife Service 1986) and as winter habitat for most North American sandhill cranes.


Aquifers and Waterways

The High Plains aquifer (formerly called the Ogallala aquifer) consists of one or more geological units connected belowground under the central Great Plains. The aquifer is essential to agricultural, urban, and environmental resources, containing about 20% of the irrigated farmland in the High Plains and about 30% of the water used for irrigation (Huntzinger 1996). Precipitation is the principal source of natural groundwater recharge, but recharge can also result from seepage loss from streams and lakes. Natural discharge occurs as evaporation from plants and soils where the water table is near the surface or as seepage to springs. Over the long term, natural recharge should compensate discharge.

The development of the High Plains aquifer for irrigation (1940-1980) is evident in an average area-weighted, water-level decline of 3 meters (0.07 meters per year; Dugan et al. 1994). Declines vary with locale, exceeding 30 meters in some parts of the central and southern High Plains; 6 meters in southwestern Kansas, east-central New Mexico, and the Oklahoma and Texas panhandles; and 3 to 6 meters in northeastern Colorado, northwestern Kansas, and southwestern Nebraska. Since 1980, water levels in such areas have continued to decline but at a slower rate, the result of greater than normal precipitation (10 to 15 centimeters annually), water conservation practices (particularly minimum tillage), and reduction in irrigated land area (about 540,000 hectares, 1979 to 1991). The extreme southern plains of Texas and eastern and east-central Nebraska have experienced water-level rises of 1 to 3 meters (Dugan et al. 1994).

Agricultural use of nitrogen fertilizers is the largest source of nitrates in near-surface aquifers in the midcontinent (Koplin et al. 1994). Over 100,000 metric tons of pesticides (herbicides, insecticides, and fungicides) were applied in the midcontinent in 1991, often to control nonindigenous plants and animals. In spring and summer 1991, concentrations of several herbicides exceeded U.S. Environmental Protection Agency standards in about half of the streams sampled in the upper Missouri River basin (Huntzinger 1996). Effects of these pollutants on the quality of human life and on the integrity of the ecological community are largely unknown. The U.S. Environmental Protection Agency has initiated an effort to develop stressor information to help recognize areas where urban development, agricultural nonpoint pollution (pesticides, toxic chemicals, nutrient pollution), and agricultural development may exacerbate ecological decline.

The rivers of the Great Plains flow from west to east; extreme turbidity is the key characteristic of the larger rivers. Whereas most water in these rivers originates from western mountains, sediments originate from thunderstorm runoff on the Great Plains. In small channels, fine particles held in suspension produced quicksands, which inhibited crossings by early travelers and caused extreme turbidity during low flows. In summer, open-river water temperatures often exceed 30°C, the salinity level is high because of salt- and gypsum-laden groundwaters, rates of evaporation are high, and the flow velocity is moderate.

The larger Great Plains rivers have been subjected to dewatering for irrigation, other consumptive uses, and reservoir construction (Cross and Moss 1987). In virtually all these river systems, such dewatering has altered the timing and extent of flows, downstream temperatures, levels of dissolved nutrients, sediment transport and deposition, and the structure of plant and animal communities. Few major interior rivers still exhibit the conditions evident before agricultural development and water management had occurred.

On the Missouri River, once a free-flowing river of more than 3,700 kilometers (from Three Forks, Montana, to St. Louis, Missouri), reservoir construction in Montana, North Dakota, and South Dakota has virtually stopped sediment transport below major reservoirs, as it also has on the Platte and Kansas rivers (Huntzinger 1996). Sediment deposition is part of reservoir design but remains a maintenance concern. Because of the value of surface waters, there is increasing interstate interest in surface waters such as those of the Missouri River. Such surface waters are important to wildlife, fish, and recreation, as well as to navigation and water-supply interests that rely on reservoir resources.

Ecologically, the effect of water management reaches far to the south because sediment deposits are needed to sustain the Mississippi Delta. Unchannelized reaches of the Missouri River are near Bismarck, North Dakota and in southeastern South Dakota and northeastern Nebraska. Such reaches host a number of pearlymussel, fish, and bird species that are federally listed or are candidates for listing under the Endangered Species Act of 1973.

The basic features of most Great Plains streams, such as flow and substrate, are unknown (Matthews 1988). In general, streams in the south are characterized by irregular flow, small particle size in substrates, and a distinct wet-dry cycle. Drought may be a more extreme disturbance than either the wet-dry cycle or floods. Streams in the northern prairies are more consistent in flow and tend to have cobble substrates, and winter precipitation (snow) is released with spring thaw. Great Plains streams fall into three categories: the shallow stream with shifting sand beds; clear brooks, ponds, and marshes supported by seeps and springs; and residual pools of intermittent streams (Cross and Moss 1987). All three stream types are affected by dewatering of channels and stabilized flows.


Soils

Grassland soils are fundamentally different from those found under a forest canopy (Simms 1988). Many factors—parent material, climate, time, and human intervention—influence grassland soil type and condition (Peterson and Cole 1996). For example, soil organic carbon, a significant influence on soil productivity, is greatest in the Northeast (cooler and wet) and lower in the Southwest (warmer and dry). Where evaporation is low, water is more likely to remain in the soil, increasing the rate of mineral weathering and allowing large amounts of nitrogen, phosphorus, and sulfur to accumulate in conjunction with carbon.

Managed agriculture began on the easternmost grasslands in the 1850's (Peterson and Cole 1996). Surface cover is reduced by agriculture, and soil structure is destabilized by reducing aggregate size—the result of mixing and grinding action by farm implements. Organic carbon loss is accelerated by agriculture, and cultivated crops (particularly in dry-land areas) return little carbon to the soil. Few agricultural practices of the early settlers captured or retained moisture. The black-dust storms of the 1930's resulted from exposing vast areas of cultivated prairie soil to wind action and drought (Sampson 1981).

Several straightforward and well-documented relationships exist between plant productivity and soil organic material after native sod is cultivated. For example, soil productivity (indexed by corn grain yield) declined 71 % and soil nitrogen 49% during a 28-year interval after cultivation began (Williams and Wolman 1986). Retention of organic matter—and thus the level of productivity—in grassland soils is only possible if the correct proportions of carbon, nitrogen, and phosphorus are present (Peterson and Cole 1996). Nitrogen fertilizers are used extensively to restore soil nitrogen levels, and more than 6.4 million metric tons of nitrogen fertilizers were applied to cropland in the Mississippi River basin in 1991 (Goolsby et al. 1993). In addition, removal of phosphorus in harvested plants and loss of organic phosphorus due to cultivation require fertilizer supplements to maintain productivity. Elevated concentrations of phosphorus may affect aquatic plant growth and reduce oxygen content in streams.

Soil formation is a slow, continuous process. About 2.5 centimeters of new topsoil is formed every 100 to 1,000 years, depending on climate, vegetation and other living organisms, topography, and the nature of the soil's parent material (Sampson 1981). Some soils, particularly where moisture is a limiting factor and growing seasons are short, may take 10,000 years to produce 2.5 centimeters of soil. On average, annual loss of topsoil in the United States is nearly three times greater than that being formed. Exact measurements of soil losses are difficult to obtain because soil eroded in one area is eventually deposited at another site (Peterson and Cole 1996). Smaller and lighter nutrient-rich organic soils are the most transportable, creating additional threat to the future productivity of prairie soils.


Prairie Processes

USGS photo by F.L. Knopf: controlled burning of grassland.  
Fig. 10.   Fire plays a major role in prairie dynamics.

Climate and fire (Fig. 10) are thought to be most important to the spread and maintenance of grasslands (Anderson 1990). The air mass originating in the Gulf of Mexico spreads high humidity and precipitation as it moves north. As it moves from west to east, the Pacific air mass passes over several mountain ranges, giving up most of its moisture and, as it meets the gulf air mass, creating a climate gradient across the Great Plains. The north-south gradient in temperature and moisture, largely influenced by snow cover to the north, is an effect of the polar air mass. Long-term response of vegetation to climate, particularly water availability, is illustrated by regional differences in species composition and height of native grasses: and western short-grass prairie, central mixed-grass prairie, and eastern tall-grass prairie. As documented in past droughts, grassland distribution is controlled by extremes of climate variability. These forces are evident in the changing eastern edge of the prairie peninsula and, to a great extent, the annual productivity of grasslands.

Grassland plants have growing points protected from fire beneath the soil surface. Frequent fire is essential to maintaining native species diversity, and it affects other components, including nutrient cycling and productivity (Collins and Wallace 1990). On tall-grass prairie, the relationship between total plant species richness and the number of times a site is burned is important and positive (Collins 1991), at least on a small scale. Small animals create gaps and edges that influence tall-grass plant community structure and composition (Reichman et al. 1993). Recently burned tallgrass prairie has also provided stopover habitat for many long-distance migrant birds such as the lesser golden-plover and the now-endangered and possibly extinct Eskimo curlew. In the past, grazing was localized in these burned areas because of their greater productivity and the nutritive value of their forage (Risser 1990). Thus, the movement and impact of grazing animals on tall-grass prairie grasslands were bound to the spatial distribution of burned patches. For mixed-grass prairie, discussions of community composition and individual species should be set in a similar context of species patterns of both past grazing animals (ranging from bison to ants) and current grazing animals (domestic livestock) in relation to disturbance events (Umbanhowar 1992).

Grazing has direct and indirect effects at landscape and regional scales, which, in turn, interact with other small-scale and large-scale factors to heighten temporal and spatial diversity in grasslands (Gibson and Hulbert 1987; Risser 1990). A recent comparison of grazing over a global range of environments, however, suggests grazing is a factor in the conversion of grasslands to less desirable shrublands (Milchunas and Lauenroth 1993). Moreover, primary production on grasslands, largely the production of plant material, does not necessarily change when plant species composition changes. Current species-based management criteria by land management agencies, therefore, may lead to erroneous conclusions about the ability and future of grasslands to sustain productivity. Adequate assessment of the effects of grazing on grasslands, as with the effects of climate and fire, must be multiscaled and match management inferences and applications (Steinauer and Collins 1996).

Interactions among other factors, aside from climate, grazing, and fire, also influence grasslands (Burke et al. 1991). In the cast, nitrogen normally restricts the annual production and composition of grasslands. In the semiarid west, the availability of nitrogen and water is important to composition and production. Long-term vegetative production on short-grass prairie is closely tied to precipitation (Lauenroth and Sala 1992). The most productive years are those when small precipitation events first stimulate nutrient availability, followed by large precipitation events that stimulate plant growth. Semiarid areas are thought of as especially variable in environmental conditions, particularly in precipitation, and the short-grass prairie is no exception. Effective grassland management requires understanding the effects of both the spatial and temporal patterns of precipitation on short-grass prairie.

In prairie wetlands, disruption of natural processes such as fire has led to domination by robust, emergent plants, particularly in the prairie pothole region. Cattail, once rare on the Great Plains, has spread across thousands of prairie wetlands, as has purple loosestrife, a species native to Europe which is now threatening waterways across the United States (U.S. Congress, Office of Technology Assessment 1993; Malecki and Blossey 1994). In the past, climate, fire, and grazing controlled the diversity and abundance of vegetation in northern prairie wetlands. As environmental conditions changed, some plant populations have declined and others have increased. Belowground seed reserves favor those species with seeds that germinate under a wide range of conditions, such as cattail, purple loosestrife, and other nonindigenous species.

More is known about the effects of grazing than fire. Nodal rooting, or underground branching, and unpalatability are evident evolutionary responses of wetland plants to grazing. Under certain conditions grazing can increase species diversity and the development of intricate patterns and sharp boundaries among prairie wetland plant communities (Bakker and Ruyter 1981).


Plant Assemblages

The Nature Conservancy, in a preliminary survey, has identified rare plant assemblages across the United States (Grossman et al. 1994). Of the 633 assemblages in the Great Plains, 107 (17%) are considered rare (Chaplin et al. 1996).

The 16 rare Great Plains forest assemblages are largely cottonwood and oak floodplain forests on the eastern and western edges of the plains (Grossman et al. 1994). The 20 rare canyon and mountain plant assemblages tend to be open pine, fir, and oak. The eight rare sparse woodland forests are primarily oak savannas on the eastern plains. The 19 rare shrubland assemblages include many sagebrush, hawthorn, and willow species.

Among 45 rare grassland assemblages on the Great Plains, 18 are found in tall-grass prairie, 13 in mixed-grass prairie, 7 in short-grass prairie, and 7 primarily in wetlands. Big bluestem is dominant in 9 of 18 rare tall-grass prairie communities, little bluestem in 3, and drop-seed species in 2. Similarly, little bluestem is common to 6 of 13 rare mixed-grass prairie communities, and sedges are important in 3. Buffalo grass, in part, distinguishes 5 of 8 rare short-grass prairie communities, and sedges are important to 2. The 7 remaining rare communities are dominated by forbs and embrace wetland plants.


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