South Central Semi-Arid Prairies Level III Ecoregions
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South Central Semi-Arid Prairies Level III Ecoregions 25 (9.4.1) High Plains The High Plains is an area of vast flatlands and gently rolling hills that historically was a short and midgrass prairie. This region covers a large latitudinal extent, from southeastern Wyoming, western Nebraska, eastern Colorado, western Kansas, through the panhandles of Oklahoma and Texas, and into eastern New Mexico. In Texas it is a relatively level high plateau separated from the Rolling Plains to the east by the Caprock Escarpment. The High Plains developed on sediments that originated in the Rocky Mountains to the west. The Rocky Mountains were formed by deformations of the earth’s crust which lasted from approximately 66 to 1.6 million years ago. During that time the Rockies were eroded by wind and water. Streams flowing eastward out of the Rocky Mountains were full of sand, gravel, silt, and other rock debris. Over millions of years, this mass of eroded material filled the stream valleys and eventually covered the hills, creating a huge, gently sloping floodplain. The southern portion of this region is also known as the “Llano Estacado” or “Staked Plains”. It is believed the first European settlers drove stakes into the ground to help guide them across this featureless region. These early pioneers found a vast short grass prairie dominated by of blue grama and buffalo grass. Today, the High Plains are mostly irrigated cropland and rangeland. Streams are mostly intermittent or ephemeral; however there are a few larger rivers that originate in the Southern Rockies that cross the region. Although the southern portion has few to no streams, there are thousands of playa lakes (seasonal depressional wetlands) that occur. These serve as recharge areas for the important Ogallala Aquifer. Water withdrawals from the aquifer usually exceed recharge, however. These playa lakes are also essential for waterfowl during their yearly migration along the Central Flyway of North America. Oil and gas production occurs in many parts of the region. 26 (9.4.3) Southwestern Tablelands The Southwestern Tablelands flank the High Plains with red hued canyons, mesas, badlands, and dissected river breaks. Most of the region's vegetation is that of the shortgrass to midgrass prairie with juniper-scrub oak-grass savanna on some escarpment bluffs. Riparian areas also wooded. During the Permian Period several thousand feet of brick-red shales, siltstone, sandstones, and gypsum were deposited in this region. Erosion has exposed these deposits giving the region its characteristic butte and mesa appearance. This ecoregion includes the areas in the trough of the Canadian River and the channels of its major tributaries that are incised below the plane of the level Llano Estacado. Water is generally scarce; streams are mostly ephemeral and intermittent. A few perennial rivers that originate in the Southern Rockies cross the region. Unlike most adjacent Great Plains ecological regions, little of the Southwestern Tablelands is in cropland; most is semiarid rangeland, with ranching and livestock grazing as the dominant land uses. There is some oil and gas production in the southern part of the Texas portion. 27 (9.4.2) Central Great Plains The Central Great Plains is slightly lower, receives more precipitation, and are more irregular than the High Plains to the west. Soils in this region are generally deep with shallow soils on ridges and breaks. The ecological region was once grassland, a mixed or transitional prairie from the tallgrass in the east to shortgrass farther west. Today, brushy species have invaded the Central Great Plains as a result of overgrazing, fire suppression, soil erosion, declining ground water levels, and the fragmentation of native grassland cover. Few perennial streams are found although some larger rivers cross the region, typically with braided, sandy channels, and often turbid water. Some springs occur but few natural lakes. Most of the ecoregion is now cropland or rangeland. The eastern boundary of the region marks the eastern limits of the major winter wheat growing area of the United States. Prairie dog colonies once covered tens of thousands of square miles of prairie and were a plentiful food source for predators. Their extensive system of underground burrows housed other animals, such as kit fox, burrowing owl, and horned lizard. 28 (9.4.4) Flint Hills The Flint Hills is the smallest grassland ecoregion in North America stretching from eastern Kansas to northeastern Oklahoma. It is a region of rolling hills with relatively narrow steep valleys, and is composed of shale and cherty limestone with thin rocky soils. It can be distinguished from other grassland associations by its less diverse tallgrass biota. These flinty beds of limestone, from which the name of this ecoregion is derived, rendered large areas unsuitable for corn or wheat farming. Today, the Flint Hills is an anomaly–an essentially unplowed (although heavily grazed) remnant of the tallgrass prairie; the last large piece in the world. This ecoregion offers the best opportunity for restoration of tallgrass prairie in the United States. Part of the region is now in national preserve land and other conservation land. In contrast to surrounding ecological regions that are mostly in cropland, most of the Flint Hills region is grazed by beef cattle. Intermittent and perennial streams are found here with springs increasing summer base flow in some streams. Few lakes are present in the region. 29 (9.4.5) Cross Timbers The Cross Timbers is a transitional area between the prairie and the forested low mountains or hills of eastern Oklahoma. The region stretches from southern Kansas into central Texas, and contains irregular plains with some low hills and tablelands. It is a mosaic of forest, woodland, savanna, and prairie. The dominant trees are post oak and blackjack oak with an understory of shrubs and grasses. The prairie openings historically contained taller grasses. Early settlers described the Cross Timbers as “an immense natural hedge” or belt of thick impenetrable forest. “The forest was so impenetrable that we could not pass through on horseback without cutting down some trees with axes and knives. We advanced seven leagues with great difficulty, arriving in the afternoon in an open spot, that God had placed there for us to rest after such a painful journey. I shall not easily forget the mortal toil, and the vexations of flesh and spirit, that we underwent occasionally, in our wanderings through the Cross Timber. It was struggling through forests of cast iron.” The transitional natural vegetation of little bluestem grassland with scattered blackjack oak and post oak trees is today used mostly for rangeland and pastureland. The savanna character of the Cross Timbers has been reduced by fire suppression and heavy grazing. The widely-spaced oaks growing over prairie grass has been replaced by thickets of young trees, shrubs, and vines with occasional prairie openings. Intermittent and perennial streams in addition to several large rivers cross the region. Some reservoirs are present. Oil production has been a major activity in this region for over eighty years. 30 (9.4.6) Edwards Plateau This ecoregion is largely a dissected limestone plateau characterized by rolling hills and intervening broad flat valleys that is hillier to the south and east where the edge of the uplifted limestone shield has been eroded forming the Balcones Canyonlands. Covered by juniper-oak savanna and mesquite-oak savanna, most of the region is used for grazing cattle. In the absence of fire, Ashe juniper has increased in some areas, reducing the extent of grassy savannas. Within the Edwards Plateau you find the Llano Uplift. This region gets its name from a mass of magma pushed upward in the Tertiary period that hardened into granite. The granite was subsequently exposed when the overlying limestones were eroded away. These rocks are some of the oldest rocks in Texas at more than one billion years old. Where grasslands still exist, short grass species grow on ridge tops and heavily grazed areas while taller species cover slopes and moist depressions. Ranchers have conducted juniper eradication efforts for decades in an attempt to reclaim land and create more forage for livestock. However, in many areas, soil has been lost from steep hill slopes, making it difficult to provide a seed bed for grasses. In some areas, where restoration of grassland has been successful, dry springs have begun to flow again. Rain water percolates slowly through grass cover and then through the limestone substrate. In contrast, precipitation runs off the bare rocky soil under juniper thickets. The region contains a sparse network of perennial streams, but they are relatively clear and cool compared to those of surrounding areas, and are often spring-fed. The region has a system of underground fissures and caverns that fill with groundwater to create aquifers. 32 (9.4.7) TEXAS BLACKLAND PRAIRIES The Texas Blackland Prairies is distinguished from surrounding regions by fine-textured, clayey soils that are derived from Cretaceous age material. The rolling to nearly level plains of the Northern Blackland Prairie stretches over 300 miles from Sherman in the north to San Antonio in the south and historically was a vast expanse of tallgrass prairie. “I can sit on the porch before my door and see miles of the most beautiful prairie interwoven with groves of timber, surpassing, in my mind, the beauties of the sea. Think of seeing a tract of land on a slight incline covered with flowers and rich meadow grass for 12 to 20 miles…” – John Brooke, an early settler in the tallgrass prairies of Texas, 1849. Prairie fires burned intensely hot and extensively, stopped only by a river or creek break, change in topography or soils, or lack of dry fuel.