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"Playa" and its synonym "playa lake" are a couple of those vague terms like "swamp" or "marsh" that are generally used to describe some type of wetland. Playa is also a Spanish word with an English translation of shore or beach. The translation provides little help in describing a playa. If the titular question is asked relative to a particular geographic region, such as New Mexico, it becomes somewhat easier to answer, though the result is still not certain. When individuals use wetland terms like swamp, playa, or lake, their intended meaning generally aplies to a local region, but to someone from outside these local areas the terms may carry a different sense. A farmer in Wisconsin, for example, would likely form a much different mental image of "lake" than a West Texas farmer. The Texas farmer might envision a wet, shallow low spot in a field or pasture (such as the playa in fig. 1.1), whereas the Wisconsinite would likely see a deep fishing lake.
Many wetland ecologists also have little understanding of what a playa is. Mitsch and Gosselink in their book Wetlands define playa as a "term used in the southwestern United States (U.S.) for marshlike ponds similar to potholes but with a different geologic origin" (2000, 41). In the National Research Council report Wetlands characteristics and boundaries, a playa lake is defined as a "shallow depression similar to a prairie pothole, abundant on the Southern High Plains on a tableland south of the Canadian River in Texas and New Mexico, characterized by annual or multiyear cycles of drydown and filling" (1995, 288). Among other incorrect assumptions, both of these definitions make the naive assertion that playas are similar to prairie potholes. Although Mitsch and Gosselink were quite correct that playas and prairie potholes differ in their geologic formation and the National Research Council report was correct that the hydroperiod (the length of time a wetland has surface water) of a playa is erratic, playas, as this description will illustrate, are uniquely different from prairie potholes and any other wetland system. The need to equate playas to prairie potholes probably arose from the fact that more has been written about the latter, and most other wetlands in the United States, than about playas and that both of these wetland types are found in the Great Plains (fig. 1.2). Indeed, the paucity of literature on playas is likely because they occur primarily on private land in the more sparsely populated portions of the Great Plains, where there is little governmental ownership of wetlands. Thus, they have received less study than wetlands in more densely populated regions. Although one of the most endangered ecoregions in North America, the Great Plains itself is poorly understood relative to other U.S. ecoregions (Samson and Knopf 1996).
Geologists also have not reached a general consensus on the meaning of the term playa. Motts stated, "Most American geologists would probably consider a playa to have four characteristics: (1) an area occupying a basin or topographic valley of interior drainage, (2) a smooth barren surface that is extremely flat and has a low gradient, (3) an area infrequently containing water that occurs in a region of low rainfall where evaporation exceeds precipitation, and (4) an area of fairly large size (generally more than 2,000-3,000 feet [610-914 m] in diameter)" (1970, 9). Motts continued, "The barren surface, devoid of vegetation and abundant gravel. is a distinctive feature of a 'playa'. . . . Thousands of small, topographically enclosed areas ranging from a few feet to several hundred feet in diameter are scattered throughout western United States, yet one would hesitate to call them playas" (9). This definition would exclude most of the playas in the Great Plains, where playas are the most numerous.
More recently Rosen defined playas from a geologic perspective: "as an intracontinental basin where the water balance of the lake (all sources of precipitation, surface water flow, and groundwater flow minus evaporation and evapotranspiration) is negative for more than half the year, and the annual water balance is also negative" (1994, 1). He further stated: "The playa surface must act as a local or regional discharge zone. Evidence of evaporite minerals will generally be present in parts of the basin" (1). This definition also would exclude those playas of the Great Plains. Noting that this classification would not encompass playas in western Texas and eastern New Mexico (but not other portions of the Great Plains) he created a "special case," which he termed "recharge playa." As the special case name implies, these playas would not receive water (discharge) from ground sources (e.g., springs) but could supply water (recharge) to underground aquifers. This "special case" is true for the overwhelming majority of playas not just in Texas and New Mexico but for all the Great Plains. From a numbers and area perspective, the most abundant group of playas in the world, those of the Great Plains, should not be listed as a "special case"; rather, those that occur elsewhere covering less area probably should be the exception to the rule. Moreover, most geologists that have studied the playas in the Great Plains have not adopted Motts's or Rosen's definition (e.g., Osterkamp and Wood 1987; Gustavson et al. 1994; Reeves and Reeves 1996).
Regardless of these previously mentioned misunderstandings, the terms "playa" and "playa lake" have generally referred to various types of shallow wetlands in prairie, semiarid, or arid environments throughout the world (Neal 1975; Bolen et al. 1989; Rosen 1994). As noted above, their ecology, hydrology, and geology, however, varies greatly among geographic regions. Playas have even been hypothesized to have once existed on Mars (Hartmann 1998, 24, 26). However, for the most numerous group of playas, those found in the Great Plains, I define them as shallow, depressional recharge wetlands occurring in the Great Plains region that are formed through a combination of wind, wave, and dissolution processes with each wetland existing in its own watershed. As the words depressional and recharge imply, Great Plains playas only receive water from precipitation and runoff. Naturally water is only lost through evaporation, transpiration, and recharge.
Within the Great Plains, playas as thus defined, occur with highest densities in the High Plains portion of the Southern Great Plains of eastern New Mexico, western Texas, the Panhandle of Oklahoma, southeastern Colorado, and southwestern Kansas (fig. 1.2) but are also scattered throughout some northern portions of the Plains and western mountain states (Motts 1970; Osterkamp and Wood 1987; MacKay et al. 1990; Brough 1996; LaGrange 1997). Because the vast majority of playas occur in the Great Plains, from Wyoming and Nebraska to Texas and New Mexico, and most scientific study of playas has occurred there, the description of playas is focused on that geographic region.
Descriptions of Great Plains Playas
The most commonly used system to classify wetlands in the United States today is the Department of Interior's Cowardin et al. (1979) system. It is a hierarchical method similar to taxonomic classification with wetlands being categorized by system, class, plant community and substrate, water regime, and water chemistry. Other modifiers exist to note physical alterations to wetlands such as excavations and dikes. Following the Cowardin et al. classification, playas in the Great Plains are categorized as palustrine or lacustrine systems. Palustrine wetlands generally are dominated by woody plants or persistent emergent plants. (Rooted herbaceous plants that protrude above the water's surface are emergent vegetation.) Any nontidal wetland with more than 30% persistent emergent vegetation is palustrine. Palustrine wetlands that do not have greater than 30% persistent vegetation must be less than 8 hectares (20 ac), less than 2 meters (about 6.6 ft) in the deepest portion of the basin, and contain no active wave formed shoreline.
Lacustrine wetlands are generally larger than 8 hectares but this system cannot have persistent emergent plants exceeding 30% of the basin. This type of wetland can be placed in either littoral (less than 2 m water depth with no persistent plants) or limnetic (greater than 2 m deep) classes. These wetlands can be less than 8 hectares if an active wave-formed shoreline is part of the wetland boundary or if the deepest part of the basin exceeds 2 meters in water depth.
Because as Guthery and Bryant (1982) noted, the average area of a playa in the Southern Great Plains is 6.3 hectares (15.5 ac), and most playas are shallow ( 2 m), the majority of Great Plains playas are palustrine. Fewer are lacustrine littoral playas, and even fewer are lacustrine limnetic playas; but exact perentages have not been determined.
Most playas are further classified as "emergent vegetated wetland." Again, however, lacustrine littoral wetlands cannot have persistent emergent vegetation exceeding 30% of the basin, so their vegetation is necessarily classified as nonpersistent (e.g., annual). Some playas also may be classified as possessing "aquatic bed" vegetation, which refers to plants living completely in or on the water. Rooted submerged and/or other floating plants, including algae, are aquatic bed vegetation. Within the palustrine system, there are even a few playas that can be placed into the "scrub/shrub" or "forested" wetland class. Scrub/shrub are woody plants less than 2 meters (6.6 ft) high, whereas forested wetlands have trees taller than 2 meters. A few may not have any vegetation but have open water with "unconsolidated bottoms."
Today, the water regime in most playas is termed "temporarily" or "seasonally" flooded. Temporarily flooded playas may contain water for only a few weeks during the growing season while seasonally flooded playas have water present during extended periods during the growing season. Historically playas probably held water for longer periods than today; this will be discussed later. There are a few "semipermanently" flooded playas. These wetlands will have water in them throughout most years. Other commonly used modifiers for playas, in this classification system, are related to human-caused physical modifications such as "excavations" and "dikes." Indeed, for many playas, portions of the same wetland can be classified differently due to physical modifications, such as excavations.
Although the Cowardin et al. classification system allows various wetland types to be clearly categorized by function, and compared among regions, it does not greatly enhance the reader's ability to develop an accurate vision of a Great Plains playa. To recount, Great Plains playas are mainly freshwater wetlands, dependent on precipitation from storms (or irrigation runoff) for surface water, self-contained in their own closed watershed, and not recharged by elevated groundwater (e.g., figs. 1.1 and 1.3). Wetlands in the Great Plains that have springs or receive groundwater additions to their surface water are not generally considered to be playas. Because playa watersheds are not connected to one another and storms can be very localized in the Great Plains, a playa in one location may be full of water while only a short distance away other playas will be dry. They are shallow, usually only 1.5 meters (5 ft) deep, at most. Playas have erratic hydroperiods, drying and filling with water frequently within most years. These water fluctuations usually promote diverse herbaceous plant growth. However, whether the vegetation is annual or perennial, terrestrial or aquatic, depends on how long the playa has been with or without water (Guthery et al. 1982; Haukos and Smith 1997).
Playas are also small, with 87% being less than 12 hectares (30 ac) in area in the Southern Great Plains (Guthery and Bryant 1982). Although the average playa is small, the range in individual playa area is large with some less than a hectare to some more than 4 square kilometers (> 1 sq mi). Although scientists know little about the area of individual playas elsewhere in the Great Plains, the playas of southwest Nebraska are also small (LaGrange 1997). The remaining Rainwater Basin playas of south-central Nebraska may be a bit larger than those further south in the Great Plains.
Most playas in the Southern Great Plains appear almost perfectly circular (fig. 1.3). Indeed, Luo (1994) devised equations to calculate playa wetland area while studying sedimentation rates; the resulting formulas were very similar to the simple geometric equation used to calculate the area of a circle. North of the Southern Great Plains, playas often do not appear as circular and may be more irregularly shaped. This suggests playas in northern portions of the Great Plains possibly may have been formed through a combination of different processes than those in the south. Interestingly, though, some playas in the central and northern Plains share some unique physical features with those in the south. Many of the playas in the Rainwater Basin of Nebraska and the Southern Great Plains of Texas and New Mexico have small ridges or dunes on their east and south sides termed lunettes (Kuzila and Lewis 1993; Sabin and Holliday 1995). These are addressed in Chapter 2 when playa formation is discussed. Western Great Plains playas are also structurally simple in that there is generally a gentle slope starting from the edge of the hydric soil of the wetland to a level bottom, from which the elevation does not change (Luo et al. 1997; fig. 1.4). This is different from many other wetland types, such as riverine oxbows or prairie potholes, which have relatively irregular horizontal shapes and at least some elevational heterogeneity as one travels across the wetland basin.
Distribution and Numbers
Until relatively recently, few playas were thought to exist in the Great Plains outside the Southern Great Plains (USFWS 1988). The Southern Great Plains of southeastern Colorado, southwestern Kansas, western Texas, the Oklahoma Panhandle, and eastern New Mexico was traditionally referred to as the "Playa Lakes Region" by numerous agencies and in many scientific papers (e.g., Nelson et al. 1983; USFWS 1994). Although not occurring in the same high density as further south, playas do exist, essentially continuously, into northwestern Kansas, northeastern Colorado, eastern Wyoming, and western Nebraska (Holpp 1977; Osterkamp and Wood 1987; Brough 1996; LaGrange 1997). The majority of these Great Plains playas occur in the High Plains extending from western Nebraska and eastern Wyoming south to western Texas and eastern New Mexico. True to playa form, however, they cannot be categorized that easily because some playas also exist further east in the Low Plains. Indeed, the Rainwater Basin wetlands of south-central Nebraska were classified as playas by LaGrange (1997).
The highest density of playas occurs in the Southern High Plains, in an area south of the Canadian River known as the Llano Estacado (fig. 1.5). The largest plateau in North America at 82,000 square kilometers (32,000 sq mi), the Llano Estacado has been described as one of the largest featureless landscapes in the United States (Holliday 1991). The plateau is surrounded by relatively abrupt escarpments on the west, north, and east sides ranging from 50 to 200 meters (160-660 ft). On the south side, however, the plain gradually fades into the Permian Basin enough so that it is difficult to determine the Llano edge. Elevation of the Southern High Plains declines from approximately 1,500 meters (5,000 ft) in the northwest to 725 meters (2,300 ft) in the southeast. Playas are the most ubiquitous geomorphic and hydrological feature on the Llano Estacado (Sabin and Holliday 1995) with playa densities approaching 1 per 2.6 square kilometers (1/sq mi) (Guthery et al. 1981). It is difficult to imagine such a wetland density when one is at ground level, where the Llano is so flat that a person may not be able to see a playa that exists just a few hundred meters distant. From the air, however, especially after a rain when most playas have water, the sight is revealing (fig. 1.1).
Within the Southern High Plains, playa density and area varies as a result of differences in soil texture and annual precipitation. The average area of playas increases from the southwestern portion of the region to the northeast, which follows precipitation patterns (Grubb and Parks 1968; Allen et al. 1972). However, soil texture also varies across this gradient with the southern one-third of the Llano being coarse textured, the middle being medium textured, and the northern third being fine textured (Allen et al. 1972; Sabin and Holliday 1995). Although the size of playas is greatest in the northeast Llano, the density of playas is highest in the medium-textured soil zone according to Guthery et al. (1981) or the coarse soils according to Sabin and Holliday (1995). This contradiction in playa density between Guthery et al. and Sabin and Holliday might simply be related to the manner in which playa density and soil zones were delineated in the two studies.
Although a few other wetland types also occur on the Llano, including several riparian areas called "draws" and approximately 40 large "saline lakes" (Reeves 1976, 1990), they do not approach the numerical importance of playas. Sometimes saline lakes have been called playas, but they are not similar in hydrology, origin, or form to Great Plains playas and therefore are not typically considered to be playas by ecologists or regional geologists (Sabin and Holliday 1995).
The exact number of playas existing in the Great Plains is unknown but certainly exceeds 25,000. Most of the estimates (or guesses) of the number of playas have been made for the Southern Great Plains or that area traditionally called the Playa Lakes Region, as defined above. Osterkamp and Wood (1987) suggested there were about 30,000 whereas Curtis and Beierman (1980) counted 24,600, and Reddell (1965) listed 37,000. These suggestions did not include playas outside the Southern Great Plains, such as in northeastern Colorado, northwestern Kansas, eastern Wyoming, and Nebraska.
The estimate of 25,390 playas derived by Guthery and Bryant (1982) appears defendable for a portion of the so-called Playa Lakes Region because they actually counted playas in a 54-county region of the Southern Great Plains (table 1.1), using county soil-survey maps and other studies (Schwiesow 1965; Dvoracek and Black 1973), and conducted field checks to verify map information. These playas comprise an area of approximately 165,000 hectares (410,000 ac). Areas that had been mapped with Randall, Lofton, or Ness clays were included as were those designated as "intermittent lakes" on soils maps. Guthery and Bryant (1982) did not include some potential playas that had Randall fine sandy loam soils, or count playas in all counties of the Playa Lakes Region. Therefore, these estimates, because of their limited geographic coverage and other soil restrictions, should be considered conservative for the Southern Great Plains.
The conservative nature of this estimate is further supported by more recent estimates of playa abundance by Sabin and Holliday (1995, 300), who derived playa numbers from topographic maps. They suggested that 25,000 playas was a realistic estimate for the Southern High Plains, an area smaller than that surveyed by Guthery and Bryant (1982). But as Sabin and Holliday noted, playas continue to defy accurate estimation by scientists: "Not all playas appear on topographic maps and not all depressions are seasonally dry lake basins. Soil surveys are helpful because of the unique soils found in the playas, but not all regions of the Southern High Plains have reasonably current published surveys" (1995, 290). Further, the Department of Interior's National Wetlands Inventory, which has determined numbers and area of many of the different wetland types in the United States, has yet to complete the inventory of playas existing throughout the Great Plains.
If playas outside the traditionally defined Playa Lakes Region are included, estimates of playa numbers and area further increase. Four areas of playas are reported to exist in Nebraska: the Southwest Playas, Rainwater Basin, Todd Valley, and Central Table (LaGrange 1997; fig. 1.6). The area of Rainwater Basin playas remaining in south-central Nebraska is estimated at 13,807 hectares (34,103 ac; Raines et al. 1990) in approximately 400 relatively large wetlands and an unknown number of smaller basins (Schildman and Hurt 1984). In the Todd Valley and Central Table regions, there are approximately 716 hectares (1,769 ac) and 1,418 hectares (3,503 ac) remaining, respectively (LaGrange 1997, 13). The amount of playa habitat remaining in southwestern Nebraska is unknown, as is the area in northwestern Kansas, northeastern Colorado, and eastern Wyoming. Brough (1996), however, noted that there were at least 450 playas in the Powder River Basin of Wyoming. Regardless of the exact number and area, when one considers there are more than 25,000 playa wetlands, covering more than 180,000 hectares (445,000 ac), in a mostly semiarid to arid, highly agriculturalized environment with few other wetlands, it is obvious playas are a keystone ecosystem central to the ecological integrity of the entire Great Plains.
Most playas have been hydrologically and ecologically altered in some form or fashion in the Great Plains. Unlike playas in the Rainwater Basin of Nebraska (LaGrange 1997) and a large expanse of wetlands throughout the midwestern United States (Prince 1997), however, it has been difficult to tile or gravity-drain playas in the western Plains. Because the western Plains are so flat and distance to surface drainage features, such as creeks or draws so great, it has been difficult to effectively divert water completely away from these playas. However, terraces established in the watershed, in the name of soil conservation, have prevented significant amounts of water from entering playas. Most commonly, land managers wishing to eliminate playas have leveled some with soil or intentionally promoted soil erosion into others. The extent of this problem is just now being realized but the actual magnitude of wetland loss has not been quantified.
Throughout the Great Plains most playas less than 4 hectares (10 ac) are farmed when they are dry. Other playas have been hydrologically modified. Guthery and Bryant (1982) estimated that 33% of all playas in the Southern Great Plains had been modified. Playas have been trenched or filled for road construction, used for catchment of cattle feedlot runoff, and for urban wastewater and stormwater storage. They have even been used as dumps for municipal and agricultural trash. However, the most common form of hydrological modification has been pit excavation. Many of the larger playas have had deep pits dug into them to aid in irrigation of crops in the surrounding watershed (fig. 1.7). As noted below, irrigation agriculture is practiced extensively throughout the region, and initially many playas were integrated into a row-flood system (Bolen and Guthery 1982). Because playas are downhill from everything in the watershed, water collects there from precipitation and irrigation runoff (often termed "tailwater"). By constructing deep pits in playas, the typical surface area of the water is decreased, evaporation losses are thus minimized, and water pumping costs are lowered relative to the cost required to pump water from the regional aquifer. Pits also have been constructed to reduce the surface-water coverage of playas permitting easier cultivation of the basin or to provide more consistent water for livestock (fig. 1.8).
Of the playas larger than 4 hectares (10 ac) in the Southern Great Plains, 69% have had pits constructed in them (Guthery and Bryant 1982). The percentage of playas with pits in central and eastern Nebraska (also called "dugouts" in Nebraska) is also very high (LaGrange 1997) (fig. 1.9). Guthery et al. (1981) noted that most of the construction of pits in the Southern Great Plains had occurred from the mid-1960s through the 1970s. With changes in irrigation strategies, from traditional furrow flooding to the current use of center pivots and underground drip irrigation, the rate of construction of new pits has been greatly reduced, and the aconsequent need for maintenance (e.g., dredging out sediments) of pits already constructed has diminished. These pits and trenches, as described in subsequent chapters, have a great influence on not only the hydrology but also the fauna and flora depending on playas. The hydrology of a wetland shapes the entire ecosystem.
The majority of playas can be characterized by the presence of a specific hydric soil in their basin. In the Southern Great Plains most soils are Vertisol, with the Randall series occurring most frequently. However, the Lipan, Ness, Lofton, Stegall, and Pleasant series also indicate playa presence on county soil maps (Allen et al. 1972; Guthery and Bryant 1982; Nelson et al. 1983; Sabin and Holliday 1995; USDA 1996). These soils may not be substantially different from Randall but simply classified differently by different soil-survey teams. In Nebraska, the most common soils lining the basins of Todd Valley and Rainwater Basin playas are also nearly impervious clays in the Butler, Filmore, Scott, and Massie series (Gilbert 1989). The hydric soils of the Central Table and Southwest playas have been primarily listed as Scott and Filmore, although the taxonomy of some of these playas has been changed recently and are now listed in the Lodgepole series. Sometimes a playa exists on the landscape with the characteristic hydric soil but does not occur on the county soils map or occurs on the map without a soil designation. For example, in Baca County, Colorado playas often do not have a soils designation on the soils maps but are indicated merely as "intermittent lake."
The clay mineralogy of Southern Great Plains playa soils is similar to that existing in the surrounding watershed soils (Allen et al. 1972). The dominant clays of playa soils and those in the immediate watershed are montmorillonite and illite (Allen et al. 1972; Dudal and Eswaran 1988). The clay fraction of playas basin soils usually exceeds 50%, frequently >80% at the playa center. These clay soils often undergo gleying, indicating soil that has experienced frequent inundation (Haukos and Smith 1996). Gleying is a feature indicative of hydric playa soils. Redox concentrations, such as bodies of iron/manganese oxides, are also indicative of hydric soils (USDA 1996). These concentrations include soft masses, nodules, pore linings, and concretions formed under anaerobic conditions caused by flooding (USDA 1996, 26). Moreover, the clay of Southern Great Plains playa basins has a darker color easily distinguishable from soil in the surrounding uplands (Luo et al. 1999).
The high clay content of playa soils makes them only very slowly permeable relative to the soils of the surrounding watershed. Playas, therefore, have an excellent water-holding capacity (Bruns 1974). This hydrologic feature is what makes them so important from a landscape and ecological standpoint. As playa soils dry, they form large cracks, and eroded soil from the surrounding watershed slumps into the cracks. When the soil becomes wet again, the sediments become mixed with the subsoil. For example, in an attempt to reconstruct vegetation in playas from the past 120 years, Rhodes and Smith (unpublished data) surmised that different depths of sediments from soil cores could be aged using Cesium137. The different aged sediments could then be taken into a greenhouse and, because many upland and wetland plants have long-lived seeds (see, e.g., van der Valk and Davis 1979), the seeds could then be germinated. This would allow examination of recent historic changes in playa plant communities. However, the Cesium results were nonsensical. Because playa sediments mix so thoroughly during wind and flooding events, older sediments are not necessarily deeper than younger ones and there is no consistent pattern in the mixing.
The Great Plains Playa Setting
To comprehend playas and their ecology, one must understand their setting in the Great Plains landscape. Prior to European settlement, the Great Plains or prairie regions of the midcontinent of North America existed from east of the Mississippi River to the front range of the Rocky Mountains and from Saskatchewan and Manitoba to Central Texas (fig. 1.2). Today, the grasslands of the Great Plains are some of the most endangered ecosystems in North America (Samson and Knopf 1996), because of their conversion to one of the most intensively cultivated regions in the world (Samson and Knopf 1994).
Climate in the Plains where playas occur is temperate in the northeast, to semi-arid in the west, and dry steppe in the south. Given the great latitudinal variation in the range of playas in the Great Plains, it is to be expected that the climate varies accordingly (table 1.2). The major consistency throughout the entire region, however, is that precipitation comes mainly from thunderstorms, is typically highest in May and June, stays relatively high although variable through summer, and then drops off in October. Most often, therefore, playas usually fill with water in spring and summer. Average annual precipitation varies from a low in Midland, Texas, of 38 centimeters (15 in.) to a high in Grand Island, Nebraska, of 63 centimeters (25 in.), with amounts being higher in the eastern portions of the High Plains than in the west. However, in the Plains such "averages" are deceiving. Extremes in precipitation are the rule, average years uncommon, and droughts frequent in the Great Plains. Winters are relatively dry, although snow depths can often be substantial (> 25 cm; 1 ft). Potential evaporation rates vary from 284 centimeters (112 in.) in Midland, to 230 centimeters (90 in.) in Dodge City, to 165 centimeters (65 in.) in Grand Island. Evaporation and precipitation greatly influence the length of the playa hydroperiod, the native vegetation present, and the agricultural crops that are grown.
Prairie Vegetation and Recent Changes
Prior to cultivation by European settlers, the Great Plains was a large continuous grassland, the largest vegetative province in North America (Sampson and Knopf 1994). Its vastness was interrupted with woody plants only rarely along stream and river courses (Weaver 1968). The region also was endowed with abundant wetlands (e.g., Stewart and Kantrud 1972; Prince 1997), including playas (Bolen et al. 1989). Ecologists, geologists, and economists have divided the Great Plains into three general grassland zones. From east to west is the tallgrass, mixed-grass, and short-grass prairie. Although many individuals have identified these three grassland zones, there is little general agreement on their actual boundaries and extent. Researching various grassland publications for this description I examined at least 15 different vegetation maps for the Great Plains. All varied to some degree, especially in regards to the eastern and western boundaries of the tallgrass and short-grass prairies, respectively. For example, Steinauer and Collins (1996, 40) had the tallgrass prairie extending into Indiana but the short-grass prairie just barely touching eastern New Mexico, whereas Wright and Bailey (1982, 83) did not have the tallgrass prairie extending east of the Mississippi River but did include the short-grass prairie of eastern New Mexico. For purposes of this paper I have redrawn the three general grassland zones roughly following Küchler (1975) (fig. 1.2).
The area containing playas from western Nebraska to the southern end of the Llano Estacado is considered short-grass prairie. The uncultivated uplands and playa watersheds in this region are dominated by grasses such as gramas (Bouteloua spp.) and buffalo grass (Buchloë dactyloides) with scattered occurrence of wheatgrass (Agropyron spp.), three-awns (Aristida spp.), yucca (Yucca spp.), prickly-pear (Opuntia spp.), and various other forbs (Küchler 1975). Where sandy soils predominate, mixed stands of sandsage (Artemisia filifolia), sand shinnery oak (Quercus havardii), bluestems (Andropogon spp.), grama grasses, and yucca occur. Percent declines in native short-grass prairie, largely due to cultivation, have ranged from 80% in Texas to 20% in Wyoming (Samson and Knopf 1994, 419). The majority of the Rainwater Basin playas of south-central Nebraska occur in the mixed-grass prairie with uplands historically dominated by bluestems, wheatgrass, and needle grass (Stipa spp.) (Küchler 1975). Native mixed-grass prairie has declined by 77% in Nebraska and 30% in Texas (no data were available for Oklahoma) (Samson and Knopf 1994, 419). The eastern edge of the Rainwater Basin playas and the Todd Valley playas are in tallgrass prairie that was historically dominated by bluestems, switchgrass (Panicum virgatum), and Indian-grass (Sorghastrum nutans) (Küchler 1975). Samson and Knopf (1994, 418) estimated that 82-99% of the tallgrass prairie has been cultivated, a loss unsurpassed in any other major North American ecosystem.
The prairie grasses of the Great Plains evolved with fire and herbivory. However, with changes in fire frequencies, altered historic grazing (herbivory) regimes, and intentional plantings by land managers, exotics and native woody plants have encroached into the remaining prairie. Although climate is likely the overriding factor in creating the Great Plains grasslands, fire has historically played an important role in preventing woody plant encroachment and rejuvenation of the grassland (Wright and Bailey 1982, 82). Wright and Bailey (1982, 81) suggested that a natural (nonhuman-caused) fire frequency of 5 to 10 years was reasonable throughout the Plains grasslands. Many of these fires burned the numerous scattered wetlands, including playas. Herbivory by native large and small mammals was also important in the maintenance of Plains grasslands (Bragg and Steuter 1996; Weaver et al. 1996). Bison (Bison bison) are commonly listed as one of the dominant grazing forces affecting historic grassland vegetation and rightly so. But herbivory by elk (Cervus elaphus), prairie dogs (Cynomys spp.), pronghorn (Antilocapra americana), and insects, among other native species, was also substantial and influential on the composition and structure of prairie vegetation.
In the Southern High Plains, honey mesquite (Prosopis glandulosa) and junipers (Juniperus spp.) have moved into the grasslands. In the mixed- and tallgrass prairies, eastern red cedar (Juniperus virginiana), various oaks (Quercus spp.), and other woody plants have expanded into the prairie. Woody vegetation has also increased along riparian areas, which have had their flows drastically altered as a result of reservoir construction and withdrawals for human consumption and irrigation (Friedman et al. 1998). Native species such as cottonwood (Populus deltoides) and willow (Salix spp.) have often greatly expanded their coverage. But of much worse ecological consequence has been the spread of exotics like saltcedar (Tamarix pentandra) and Russian olive (Elaeagnus angustifolia). Although Russian olive is a serious threat to Plains ecosystem integrity (Olson and Knopf 1986), until recently it was included in shelterbelt and wildlife cover plantings by the U.S. Department of Agriculture and is still by some state forestry agencies. These same native and exotic species now also exist along the margins of many Great Plains wetlands where they were historically absent. Among the many exotic grasses and forbs (too many to list here) that have been introduced into the prairie are crested wheatgrass (Agropyron cristatum), Old World bluestem (Bothriochloa ischaemum), and Kentucky bluegrass (Poa pratensis). Many of these grasses were introduced to "improve" grazing by domestic livestock or to establish cover on previously farmed or eroded sites. The introduction and expansion of exotic grasses continues today, most recently through the Conservation Reserve Program (CRP) of the U.S. Department of Agriculture.
Current Agriculture and Population Trends
The dominant agricultural crops cultivated in the southern portion of the Southern High Plains are cotton, grain sorghum, and winter wheat (TDA 2001). Corn, grain sorghum, and winter wheat are grown in the northern and central Southern High Plains, with the area of cotton decreasing (TDA 2001). In the High Plains of Colorado, Kansas, and Oklahoma wheat and grain sorghum are predominant but some corn and sunflowers are also grown (CDA 2001; KDA 2001; ODA 2001). In southwestern Nebraska wheat and sorghum are important crops, but in some counties (e.g., Perkins) areas of corn are increasing (NDA 2001). Corn and soybeans are dominant throughout the Todd Valley and Rainwater Basin of Nebraska (NDA 2001). Essentially all grasslands in the Great Plains not enrolled in the CRP are grazed by domestic livestock.
As a result of Title XII of the 1985 Food Security Act, and its successors, a significant portion of the cultivated Great Plains was taken out of crop production for a period of at least 10 years through the CRP. In exchange for annual rental payments, landowners throughout the United States replaced annual crops on qualified highly erodible lands with perennial cover to reduce soil erosion and reduce surplus production to some areas. Because most of the Great Plains is highly erodible, from the forces of wind and water, these lands easily qualified for the CRP. Indeed, the highest density of CRP lands in the nation occur in the Great Plains (Licht 1997, 120). For example, over 900,000 hectares (> 2 mil. ac) alone were enrolled in the Southern High Plains of Texas (Berthelsen et al. 1989). On most of the enrolled land in the Great Plains annual crops were replaced with perennial grasses.
As subsequent requirements of the CRP were modified and 10-year contracts expired, some of the previously enrolled land was put back into crop production. The loss of some of this perennial grass cover came as a result of fluctuations in the agricultural economy (e.g., rising crop prices). In addition, the same previously established exotic perennial grass cover no longer completely qualified for the CRP. Unfortunately, in most of the Great Plains exotic grasses had been allowed to be planted under the 1985 program (Licht 1997, 121). As the initial contracts expired after 10 years, the CRP required that a higher percentage of native grasses to be planted in those original fields. By helping restore native species, this requirement was a positive revision to the program. In states, such as Kansas, that had the foresight to plant a higher percentage of native grasses to begin with, qualifying lands for reenrollment was not as great a problem as elsewhere in the Plains.
Most of the crops in the western Great Plains, where playas occur, depend on irrigation for successful production because of unpredictable and scarce precipitation (Nall 1990). Irrigation is possible because of the existence of the world's largest aquifer. The Ogallala Aquifer underlies much of the western Great Plains region from South Dakota to the Southern High Plains of Texas and New Mexico (Reeves and Reeves 1996). The presence of the aquifer, and, at least historically, inexpensive natural gas to allow pumping, encouraged much of the expanded cultivation of the Great Plains grasslands since the early 1900s (Nall 1990). Given the current rate of water use, however, much of the western Great Plains will likely see extreme shortages by the year 2020 potentially resulting in severe social and economic consequences (Luckey et al. 1988).
For example, that portion of the Ogallala Aquifer south of the Canadian River in Texas and New Mexico receives little recharge. The aquifer is essentially being mined to sustain crop production. At average pumping depths ranging from 30 to 200 meters (100-600 ft; Bolen et al. 1979), the Ogallala dropped by more than 15 meters (approx. 48 ft) in the Southern High Plains between 1930 and 1980 (Weeks 1986). The drought of the 1990s, the worst on record throughout much of the Southern High Plains, has resulted in continued high use of aquifer water. Between 1993 and 1997 there was an average decline of more than 2 meters (> 6 ft) throughout most of the region (Donnell 1998). In Lea County, New Mexico, the Associated Press (1998) reported that it would take 1,900 years to replace (recharge) the water pumped from the aquifer in the last half century. Even with widely hailed water-saving technological advances such as center-pivot and drip irrigation to replace traditional furrow flooding, and the enrollment of extensive CRP acreages, aquifer levels continue to decline. Indeed, center pivots have allowed irrigation to expand to sloping lands that did not allow traditional furrow irrigation, further decreasing aquifer levels (fig. 1.10). In some areas of the Southern High Plains, water levels have dropped so far that it is no longer economically feasible or hydrologically possible to irrigate. Landowners in such areas have had to revert to dryland farming, an economically much riskier proposition than irrigated agriculture (Nall 1990). Numerous farms have gone out of business because of the effects of the declining aquifer (Associated Press 1998).
In some areas north of the Southern High Plains, where the Ogallala historically has not been tapped at such a high rate, irrigated agriculture is expanding west. For example, in southwestern Nebraska the number of center pivot irrigation systems has been increasing. Irrigation permits the expansion of crops such as corn that require more water than do traditional crops like wheat. This agricultural shift is increasing soil erosion compared to dryland farming methods and causing a more highly fragmented prairie and playa environment.
Along with the difficulties faced by agriculture such as declining aquifer levels, increased energy costs, and recent low commodity prices, there has been a decline in the human population throughout the Great Plains. "Depopulation" has occurred throughout much of the Great Plains since the early 1900s, although the demographic declines have gone largely unnoticed by the remainder of the country (Nickels and Day 1997). In exemplifying what has occurred throughout the rural Great Plains, Nickels and Day (1997) reported human population trends in 67 counties in the Texas portion of the Great Plains since the early 1900s. From 1900 to 1930 the number of farms in the Texas Great Plains increased along with population in 64 of the 67 counties. However, since the 1930s and the Dust Bowl, the population in most of the 67 counties declined, and 54 of the 67 counties lost population in the 1980s. This trend persisted into the 1990s but slowed. Of the 67 counties studied by Nickels and Day (1997), 43 lost population in the 1990s (U.S. Bureau of the Census 2002). Depopulation has been mirrored in other western Great Plains states such as Nebraska and Oklahoma, which lost population in 50 of 52 and 22 of 23 Plains counties, respectively, during the 1980s (Popper 1992). The depopulation of the Plains of Texas has been due to many interrelated factors including younger people leaving rural environments, the agricultural economy, and, in many areas, declining water availability for agriculture (Nickels and Day 1997).
As a result of these changes, many of the Plains rural areas are becoming poverty-stricken because of fewer employment opportunities, aging populations, and subsequent declining tax bases (Davidson 1990). The declining tax base has resulted in fewer local governmental services. Fewer educational opportunities, reduced police and fire protection, and limited health care are notable. The declining population and human fortunes of the Great Plains has led some to propose reverting much of the private land to federal ownership and creating a large prairie-bison preserve (Matthews 1992; Popper 1992). This proposal has not been met with open arms by most of the private landowners in the western Great Plains (Licht 1997, 115), but, as suggested in the final chapter, modifications of the idea may offer some potentials for regional economies.