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Ecoregions in Oklahoma
Mile for mile, Oklahoma offers the nation’s most diverse terrain. It’s one of only four states with more than 10 ecoregions, and has by far, the most per mile in America according to the EPA. Oklahoma’s ecoregions – or, terrains/subclimates – include everything from Rocky Mountain foothills to cypress swamps, tallgrass prairies, and hardwood forests to pine-covered mountains. Each is graced with wide blue lakes, rivers and streams.
General descriptions of each region follows (From EPA Region 6):
1. Western High Plains: (EPA #25) In Oklahoma, Ecoregion 25 consists of smooth to irregular, semiarid plains that are studded with playas and stock ponds, widely mantled by loess or sand, and underlain by semiconsolidated sand and gravel deposits. Elevations range from 2,400 to 4,800 feet, and are highest in the west. Overall, Ecoregion 25 is less dissected than the Southwestern Tablelands (26) and higher in elevation than the Central Great Plains (27). Precipitation increases eastward, averages only 17 to 20 inches annually,
and is erratic. Natural vegetation is mostly short grass prairie, but sagebrush–bluestem prairie is native on scattered sand plains and sand hills. Overall, natural
vegetation is distinct from the mixed grass and tall grass prairies of moister ecoregions to the east.
2. Southwestern Tablelands: (EPA #26) Ecoregion 26 is made up of dissected plains, hills, canyons, escarpments, plains, breaks, buttes, mesas, and terraces. It is more rugged than neighboring ecoregions. Rangeland and grassland occur but, unlike adjacent ecoregions, there is little cropland. In Oklahoma, mean annual rainfall ranges from 16 to 28 inches and is highly variable from year to year. Natural vegetation in this semiarid to dry-subhumid area is mostly short grass prairie; there is also mesquite-buffalograss in the southwest, sand sagebrush-bluestem prairie on dunes, and juniper-pinyon woodland in the western Panhandle. Small streams are often dry except after storms, but short, spring-fed runs occur. The diversity and richness of fish species is greater than in the High Plains (25), but less than in the Central Great Plains (27). Common fishes include the red shiner, sand shiner, suckermouth minnow, and plains killifish. Isolated populations of the Arkansas darter occur in a few spring-fed tributaries of the Cimarron River.
3. Central Great Plains: (EPA #27), The Central Great Plains in Oklahoma, are largely underlain by red, Permian-age sedimentary rocks and include scattered hills, breaks, salt plains, low mountains, gypsum karst, sandy flats, and sand dunes. Landform diversity is greater and elevations are lower than in the High Plains (25). Mean annual rainfall increases eastward, and varies from about 22 to 38 inches. Growing season increases towards the south. The upland natural vegetation in this dry-subhumid area is mostly mixed grass prairie, but mesquite-buffalograss and shinnery are native, respectively, to the south and to sandy areas; potential natural vegetation is distinct from the short grass prairie of the semiarid High Plains (25), the tall grass prairie of Ecoregions 28 and 40, and the oak savanna of the Cross Timbers (29). Riparian corridors can be wooded. Typically, after heavy rains, streams flow strongly. Flow stops or nearly stops in the summer, but scattered pools endure and serve as summer refuges for aquatic fauna. Salt or gypsum deposits and leaching produce high mineral concentrations in many streams and rivers. Numerous streams have been channelized and/or impounded resulting in the loss of riparian forest, unnatural flow regimes, entrenchment, bank erosion, substrate alteration, and fauna modification. In Ecoregion 27, the plains killifish occurs in large numbers in some streams. The most common minnows include the red shiner, sand shiner, suckermouth minnow, and the plains minnow; the endemic (and threatened) Arkansas River shiner also occurs. Slenderhead darters are widespread in Ecoregion 27. Freckled madtoms and isolated pockets of orangethroat and dusky darters also occur. The Red River pupfish is found in pools and backwaters of sandy-bottomed streams and rivers where temperature, salinity, and alkalinity are high.
4. Tall Grass Prairie: (EPA #28) Ecoregion 28 includes the western edge of tall grass prairie in Oklahoma; natural vegetation is unlike the oak savanna of the Cross Timbers (29) and the mixed grass prairie that dominates the Central Great Plains (27). Its grass-covered, open, low hills, cuestas, and plains are underlain by cherty limestone and shale. Ecoregion 28 is used for grazing. Cropland is restricted to river valleys and stone free uplands; it is far more limited than in Ecoregion 27. Mean annual precipitation is 38 to 42 inches. Springs are common enough to increase summer base flow in some streams. Tall grass Prairie and Crosstimbers share many of the same fish species.
5. Crosstimbers: (EPA #29) A mix of savanna, woodland, and prairie is native to the low hills, cuestas, ridges, and plains of Ecoregion 29, and separates the forests of eastern ecoregions from the prairies of drier, western ecoregions. The boundary between the Cross Timbers (29) and the nearly treeless Central Great Plains (27) coincides with the western limit of many mammals and insects. Post oak-blackjack oak woodland and savanna are native on porous, course-textured soils derived from sandstone; the percentage of blackjack oak increases westward. Tall grasses are native on fine-textured, moisture deficient soils derived from limestone, shale, or marl. Recent fire suppression has increased forest density and allowed eastern red cedar to invade many areas. Today, woodland, rangeland, pastureland, and several extensive, but declining, oil fields occur. Abandoned, depleted farmland is common. The remaining cropland is largely restricted to valleys near channelized streams whose degraded habitat supports very poor assemblages of aquatic fauna. Two types of streams are common. The first is characterized by a mixture of shaded riffles, runs, and pools that have gravel or cobble substrates. The second stream type has lower gradients and is found downstream of the first; it is characterized by wide, shallow, sand-choked channels. In the summer, surficial flow is often absent from wide, sandy, lower reaches. Erratic stream flow has led to the construction of many reservoirs. Generally, stream conditions in Ecoregion 29 are more stressful for fish than in eastern Oklahoma, but less rigorous than in the west. As a result, Ecoregion 29 lacks many sensitive eastern fish species as well as some river species. Other species are shared with adjacent regions. Common minnows include the red, sand, and redfin shiners and the suckermouth minnow. The redfin and orangethroat darters, smallmouth buffalo, river carpsucker, black and golden redhorses, and channel and flathead catfishes occur in many streams.
6. Caves and Prairie (Central Irregular Plains): (EPA #40) This Ecoregion in Oklahoma is a belt of prairie that separates the Cross Timbers (29) from the forests of the Boston Mountains (38) and Ozark Highlands (39). Interbedded Pennsylvanian-age shale, sandstone, limestone, and coal occur; the alternating hard-soft strata dip westward, forming nearly flat to irregular plains, low hills, and east-facing cuestas. The landform mosaic is distinct from the Flint Hills (28), Arkansas Valley (37), and Ozark Highlands (39). Natural vegetation is mostly tall grass prairie, but forests and woodlands, dominated by post oak, blackjack oak, and black hickory, are native on stony hilltops. Today, Ecoregion 40 is a mix of rangeland, grassland, woodland, floodplain forests, and farmland; cropland is most extensive on nearly level plains, and overall, is more common than in Ecoregions 29, 37, 38, or 39. Rivers and streams typically have low gradients, slowly moving water, muddy banks, and meander in wide valleys. Stream substrates and habitats vary from a high quality, variable mix of conditions to silt- and mud-choked channels. Runoff from bituminous coal mining has degraded water quality and affected aquatic biota in a few streams. The redfin shiner, suckermouth minnow, redfin and orangethroat darters, smallmouth buffalo, river carpsucker, black and golden redhorses, spotted suckers, yellow and black bullheads, and flathead catfish occur; diversity and richness of aquatic fauna is markedly lower than in Ecoregions 38 and 39.
7. Ozark Highlands: (EPA #39) In Oklahoma, Ecoregion 39 is a level to highly dissected plateau composed of flat-lying, cherty limestone. It is lithologically distinct from surrounding ecoregions and is less rugged than the Ouachita Mountains (36) and Boston Mountains (38). Karst features and clear, cool, Bank erosion has choked many channel reaches with cherty gravel, causing the reaches to become braided and dominated by unstable run habitat; in the process, many natural pools have been lost. In the Ozark Highlands, both habitat diversity and species richness are high and sensitive fish species are common. Minnows, sunfishes, and darters are plentiful. The banded sculpin and slender madtom occur in small streams, especially where aquatic macrophytes are present, and the southern redbelly dace inhabits headwaters. The shadow bass is nearly limited to the Ozark Highlands. Other common fishes include the orangethroat darter, stippled darter, greenside darter, fantail darter, northern hogsucker, white sucker, Ozark minnow, cardinal shiner, and bigeye shiner. The most important game species is the smallmouth bass.
8. Ozark Forest: (EPA #38) This Ecoregion type in Oklahoma is a deeply dissected, mountainous plateau composed of sandstone and shale, and naturally covered by oak-hickory forest; it is lithologically distinct from the limestone- or dolomite-dominated Ozark Highlands (39) to the north, and has different natural vegetation than regions to the south and west. Strata are much less deformed, and ridges are less defined, than in the Ouachita Mountains (36). Mean annual precipitation in this humid ecoregion varies from 44 to 51 inches, and increases eastward. Ecoregions 38 and 39 are dominated by many of the same fish species. Common fishes include minnows, darters, and sunfishes. Many sensitive species occur. Spring-fed streams are not as common as in Ecoregion 39. Summer flow in small streams is limited or nonexistent, but isolated, enduring pools and high quality aquatic communities may occur. During low flows, streams in both Ecoregions 36 and 38 usually run clear. However, during high flow conditions, turbidity in Ecoregion 38 tends to be greater than in Ecoregion 36.
9. Hardwood Forest: (EPA #37) The Arkansas Valley (37) separates the Ozark Plateau from the Ouachita Mountains. It is characteristically transitional and diverse. Plains, hills, floodplains, terraces, and scattered mountains all occur; the terrain is distinct from nearby ecoregions. A mix of oak savanna, prairie, oak-hickory-pine forest, and oak-hickory forest is native on uplands. Bottomland forest is native on floodplains and low terraces. Today, steep slopes are wooded and used for timber, woodland grazing, or recreation. Gently sloping uplands are used as pastureland or hayland. Cropland or pastureland occur on bottomlands. Other main land uses include poultry farming, coal mining, and natural gas production. Land use tends to be the primary factor influencing stream quality in Ecoregion 37. Turbidity, total suspended solids, total organic carbon, total phosphorus, and biochemical oxygen demand values are higher than in Ecoregions 36, 38, or 39, but mean stream gradients and dissolved oxygen levels are lower. Ecoregion 37 has the richest fish fauna in Oklahoma. Fish communities usually contain many sensitive species; a sunfish- and minnow-dominated community exists along with large numbers of darters and catfishes. Common fishes include the bigeye, steelcolor, and redfin shiners, the orangethroat and redfin darters, and suckers including the creek chubsucker, golden and black redhorses, river carpsucker, spotted sucker, and smallmouth buffalo. Summer flow in small streams is often limited or nonexistent.
10. Quachita Mountains: (EPA #36) The forested low mountains of Ecoregion 36 are characteristically underlain by folded, sedimentary rocks of Paleozoic age. In Oklahoma, mean annual rainfall in this humid ecoregion is 43 to 57 inches. Oak-hickory-shortleaf pine forest is native on uplands; it contrasts with the oak-hickory forest of Ecoregions 38 and 39 and the oak savanna or prairie of drier ecoregions to the west. Ecoregion 36 remains mostly forested, but pastureland and hayland occur in wider valleys. Logging and recreation are major land uses. Most streams have gravel, cobble, boulder, or bedrock substrates but a few have sandy bottoms. Stream gradients are steeper than in Ecoregion 35. Turbidity, total phosphorus, total suspended solids, and biological oxygen demand values are lower, and dissolved oxygen levels are higher, than in the streams of Ecoregions 35 and 37. Common fishes include the longear and green sunfishes, yellow bullhead, brook silverside, blackstripe and blackspotted topminnows, largemouth bass, smallmouth bass, redfin darter, suckers, and the bigeye, Ouachita Mountain, and ribbon shiners. Orangebelly darters, grass pickerels, and tadpole madtoms are also found in Ecoregion 36, but are absent from Ecoregions 38, 39, and 40.
11. Cypress Swamps & Forest: (EPA #35) This Ecoregion is an irregular, forested plain cut by shallow valleys and underlain by poorly-consolidated deposits; it is lithologically and physiographically distinct from Ecoregion 36. Mean annual rainfall in this humid region varies from 45 to 55 inches, and increases eastward. Ecoregion 35 occupies the edge of the southern coniferous forest belt; farther west, scrubby oak savanna and prairies occur. Natural vegetation is oak-hickory-pine forest on uplands and southern floodplain forest on bottomlands. Prairies once occurred on soils derived from limestone, marl, and calcareous shale. Today, uplands are largely pastureland or forest dominated by shortleaf pine, loblolly pine, oaks, and hickories. Poorly-drained floodplains support bottomland forests and wetlands. Cropland is most extensive along the Red River. Streams in forested watersheds typically have low concentrations of suspended solids whereas the Red River is continuously turbid. Summer flow in many small streams is limited or nonexistent, but enduring, deep pools usually occur. Species richness markedly increases towards the east as more fauna from the Mississippi Valley are encountered. In addition, downstream influences of the Ouachita Mountains on aquatic flora and fauna occur far into Ecoregion 35. Sunfishes, catfishes, gars, crappies, grass pickerels, orangebelly darters, and bigeye, ribbon, striped, and redfin shiners are common. Redhorses and creek chubsuckers are numerous in small and medium size streams. The smallmouth bass is an important game species. In Oklahoma, the dollar sunfish naturally occurs only in the South Central Plains (35), whereas the pirate perch is limited to the ecoregion's ponds, swamps, oxbows, and slowly moving streams. The lower Red River and its tributaries, the Muddy Boggy Creek and the Blue River, are the only uncontrolled rivers in Oklahoma that flow to the Gulf of Mexico; estuarine fish and the American eel reside in these waters.
12. East Central Texas Plain: (EPA #33) Ecoregion 33 is composed of plains with fine-textured soils and claypans. Substrates of large streams are typically composed of mud and very fine sand. Mean annual rainfall in this moist-subhumid region ranges from 42 to 45 inches. The Northern Post Oak Savanna ecoregion is characterized by level to rolling plains, extensive clay flats, and slowly to very slowly permeable soils that were derived from Cretaceous-age plastic shale, marl, limestone, sand, and gravel. Tall grass prairie and oak savanna are native, and contrast with the oak-hickory-pine forest of the neighboring South Central Plains (35). Cropland and pastureland are now common. Main crops are peanuts, soybeans, grain sorghum, small grains, hay, and cotton.