The Blackland Prairie ecoregion spans approximately 6.1 million hectares from the Red River on the north to near San Antonio in south Texas. It is part of a tallgrass prairie continuum that stretches from Manitoba to the Texas Coast. The region consists of the main belt, covering 4.3 million hectares, and two smaller islands of prairie, southeast of the main belt. These are the Fayette prairie, 1.7 million hectares, and the San Antonio prairie, .7 million hectares. A significant part of the main belt is bounded by oak woodland and savanna on the north, east and northwest. The San Antonio and Fayette prairies are imbedded in and are separated from the main belt by the oak woodlands of the East Central Texas Forests.
The main belt of the Blackland Prairie is divided into four narrow, geomorphic areas aligned in a north-south direction. These include-- from west to east-- the Eagle Ford Prairie, the White Rock Cuesta, the Taylor Black Prairie, and the Eastern Marginal Prairie (Montgomery, 1993). The soils of the Eagle Ford and Taylor Black Prairies are primarily clays of the order vertisol, while the soils of the White Rock Cuesta are mollisols and the Eastern Marginal Prairie of the order alfisol. Alfisols are the important soil order in the San Antonio prairie, while both Alfisols and Vertisols are important in the Fayette prairie. Microtopography such as gilgai on vertisols and mima mounds on alfisols are important microhabitats. Gilgai are shallow microdepressions 1 to several meters across formed by pedoturbation of montmorillonitic clays. Mima mounds are small circular hills which are variable in size but may be more than a meter high and 1 to 14 meters across. The origins of mima mounds are not clear and are probably of variable origin (Diamond and Smeins 1993). The climate is warm temperate to subtropical and humid. Precipitation ranges from 762 mm on the western edge to 1,016 mm on the east.
The natural vegetation of the region was dominated by tallgrass prairie on uplands . Deciduous bottomland woodland and forest were common along rivers and creeks (Diamond and Smeins 1993). The Blackland Prairie is characterized by a high degree of plant community diversity. This diversity, which is in part represented by four major prairie community types, is attributable to the ecoregion’s variety of soil orders and their variation in texture and soil pH (Diamond, Riskind, and Orzell 1987; Diamond and Smeins 1985). Little bluestem (Schizachyrium scoparium), and Indiangrass (Sorghastrum nutans) are frequently dominants on Blackland Prairie alfisols and vertisols. Big bluestem (Andropogon gerardii) is of variable importance on vertisols and is frequently a dominant on Blackland Prairie mollisols. Gamagrass-switchgrass (Tripsacum dactyloides-Panicum virgatum) prairies are associated with bottomland sites throughout the region, and are also found on upland sites of the northern main belt vertisols where they are especially associated with gilgai microtopography. Silveanus dropseed- mead’s sedge (Sporobolus silveanus-Carex meadii) prairies are found over low pH soils of the northern main belt. Little bluestem-brownseed paspalum (S. scoparium-Paspalum plicatulum) prairie is associated with Fayette Prairie alfisols. Each community differs further in secondary florae. For example, eastern forb species such as Liatris pycnostachya and Coreopsis grandiflora are largely limited to the alfisols of the Eastern Marginal prairies, while grasses such as Bouteloua hirsuta and Muhlenbergia reverchonii, as well as a diversity of species in the genus Dalea are generally found on the mollisols of the White Rock Cuesta.
The Blackland Prairie was a disturbance maintained system. Prior to European settlement (pre-1825 for the southern and pre-1845 for the northern half) important natural landscape-scale disturbances included fire and periodic grazing by large herbivores, primarily bison and to a lesser extent pronghorn antelope. Fire and infrequent but intense, short duration grazing suppressed woody and invigorated herbaceous prairie species. The latter were adapted to fire and grazing by virtue of maintaining perenniating tissues below ground It has been suggested that second only to climate, fire has been the most important determinant of the spread and maintenance of grasslands (Anderson, 1990). Fire frequency in the pre-settlement Blackland Prairie is unclear, but may have occurred at intervals of 5 to 10 years (Wright and Bailey 1982). Both natural (i.e. lightning strike) and anthropogenic ignition sources are recognized. Bison herds, though reported for the Blackland Prairie, were far smaller than those found further west in the mixed and shortgrass prairies (Strickland and Fox, 1993). Their impact was probably local with long intervals between grazing episodes. Bison were probably extirpated from the region by the 1850’s.
Compared to its northern tallgrass counterpart, the original community diversity of the Blackland Prairie was greater, by virtue of its greater representation of soil orders. Among the four plant community series, six major plant associations have been identified. On Vertisols and Alfisols, microhabitat processes are important. Mima mounds, circular mounds several meters across and tens of centimeters to more than a meter tall, occur on all undisturbed Alfisols of the region (Diamond and Smeins, 1993). Three distinct vegetation zones and as many as four different habitats can occur within a single mound (Collins, 1975). Gilgai microtopography is associated with the dramatic shrink-swell capacity characteristic of Vertisols. On level ground, this pedoturbation creates microdepressions often more than 40 centimeters deep. On sloping topography trough-like depressions of up to 20 centimeters in depth run perpendicular to the contour of the slope (Miller and Smeins, 1988). Xeric, mesic and hydric moisture regimes correspond to microtopographic position. The Blackland Prairie is habitat for more than 500 native faunal taxa including 327 species of bird (Schmidly et al., 1993). 7 reptile and 15 bird species are considered imperiled (ranging from state "watch listed" to federally endangered). Most officially listed mammals have been extirpated from the Blacklands.
Habitat Loss and Degradation
By the second half of the nineteenth century, row crop agriculture was well established in the Blackland Prairie. By the middle 1920’s more than 80% of the original vegetation had been lost to cultivation (Bland and Jones, 1993). In the second half of the century urbanization continued to reduce the remaining prairie. Today less than 1% of the original vegetation of the Blackland Prairie remains, in scattered parcels across the region (Smeins and Diamond, 1983). Most of the remaining Blackland Prairie survived by virtue of the value of the cattle forage that it produced. Over the past century, annual hay crops were taken from a majority of the prairie remnants. This practice continues today. Annual mowing, with and without the addition of fire provided valuable disturbance. However, it is possible that long-term mowing at the same time of year, while not shifting overall composition, has changed the numeric relationships of the species. The greatest portion of remnant prairie is found in the northern part of the main belt, where hay production was an important agricultural business.
Remaining Blocks of Intact Habitat
The following sites are protected or under a volunteer registry program. Voluntarily protected lands are identified only by area and county to protect the privacy of the owners.
Protected Lands (land under conservation easement, public land, or ownership by conservation organization)(all areas found within eastern Texas)
•Clymer Meadow, Hunt County - 106 ha
•Tridens Prairie, Lamar County - 41 ha
•Leonhardt Prairie, Falls County - 16 ha
•County Line Prairie, Hunt County - 8 ha
•Mathews Prairie, Hunt County - 41 ha
•Parkhill Prairie, Collin County - 21 ha
•Kachina Prairie, Ellis County - 16 ha
•Rosehill Prairie, Dallas County - 30 ha
•Flagpole Hill Prairie, Dallas County - ha
•Cedar Hill State Park, Dallas County - 8 ha
Voluntarily Protected Lands (listed by county)
•Hunt, 5 tracts totalling 159 ha
•Lamar, 2 tracts totalling 970 ha
•Franklin,1 tract totalling 41 ha
•Van Zandt, 2 tracts totalling 117 ha
•Kaufman, 1 tract totalling 81 ha
•Rockwall, 1 tract totalling 16 ha
•Denton Co, 1 tract totalling 4 ha
Degree of Fragmentation
Fragmentation is extreme due to agricultural development and urbanization. Few patches greater than 16 ha remain.
Degree of Protection
Almost all of the remaining Blackland Prairie is under private ownership. Approximately 12% of the remaining Blackland Prairie is currently under protection by The Nature Conservancy of Texas, the Texas Chapter of The Nature Conservancy, a private, non-profit conservation organization. Almost 60% is voluntarily protected under a private land registry program administered by that organization.
Types and Severity of Threats
Primary threats to prairie remnants are urbanization, row crop agriculture, invasion by exotic plant species, fragmentation, and loss of landscape-scale processes, especially fire and grazing by large native herbivores. The most threatened community type is the Schizachyrium-Andropogon-Sorghastrum association typical of the mollisol prairie of the white rock cuesta. In this case, the single greatest threat is urbanization, as this narrow, elongated strip follows a line of urban development stretching between Dallas, Austin and San Antonio.
Suite of Priority Activities to Enhance Biodiversity Conservation
The Blackland Prairie remains today as small islands of native diversity in a matrix of primarily oldfield, tame pasture, and urban development. Over the past 40 years much of the former cropland has reverted to marginally productive oldfield. Land use in these rural portions of the region has begun to shift to small scale cow-calf operations and low density residential development. Customarily, these marginally productive oldfields are lightly stocked or are "improved" with exotic forages requiring fertilization to realize maximum productivity. Concurrently, one of the greatest limitations in conservation of the Blackland Prairie is lack of adequate remnant size to re-establish landscape scale processes such as fire and grazing of bison. One site conservation strategy under development by The Nature Conservancy of Texas and other partners, particularly Texas A&M University, is the expansion of existing native grasslands and reconnection of prairie fragments by the introduction of a planted native forage system based on genetic materials obtained from remnant prairies. Forage quality produced by such systems may be comparable to that produced by exotic systems but without chemical management and associated environmental impacts. In order to implement this strategy, the following activities will be undertaken:
•complete/update inventory of remaining prairie.
•identify/prioritize sites by threats, ecological condition and potential for long-term viability.
•fully protect critical tracts
•evaluate opportunities for landscape-scale restoration by identification of linkages between remnant patches.
•develop sustainable planted native forage systems using locally obtained native genetic material
•by identifying profitability relative to exotic systems, and developing establishment technologies.
•identify and develop markets for prairie products (eg. seed and forage)
•introduce native forage grazing strategies to private partner grazing lands within critical ecological sites.
•avail native genetic material to CRP and other revegetation programs with critical ecological sites.
•identify appropriate management strategies for planted native forage systems
•Collin County Open Space Program
•The Nature Conservancy of Texas
•Natural Areas Preservation Association
•Native Prairies Association of Texas
•Texas A&M University
•Texas Parks and Wildlife Department
Relationship to other classification schemes
The boundary of the Texas Blackland Prairie is taken from Omernik (1995). It generally corresponds with K üchler’s (1985) bluestem prairie (68).
Prepared by: J Eidson and F.E. Smeins