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Texas Blackland Prairie

EVT 7422Texas Blackland Tallgrass Prairie

CES205.684GNRHerbGrassland

Summary

This grassland system is found primarily in the Blackland Prairie region of Texas but may range into southern Oklahoma. It is typified by the presence of dark alkaline Vertisol soils over calcareous parent material, although substantial belts of acidic, sandy clay loam Alfisols and loamy or clay loam Mollisols also occur. These soil types relate directly to the underlying surface geology. Microtopography such as gilgai occurs over Vertisols, and mima mounds occur over Alfisols. These create important microhabitats that increase plant diversity in this system. Schizachyrium scoparium, Sorghastrum nutans, and Andropogon gerardii are the most common dominants. Tripsacum dactyloides and Panicum virgatum are common associates on the Vertisol soils, especially on the gilgai microtopography. Fire, drought, and possibly grazing were the major natural dynamics influencing this system.

Source: NatureServe Explorer

Vegetation

Currently, only remnants of this system exist, with most of the historical distribution replaced by crop production or improved pasture. Schizachyrium scoparium is the most ubiquitous component of occurrences of this system. Andropogon gerardii and Sorghastrum nutans are also common dominants. Other species commonly encountered include Bouteloua curtipendula, Carex microdonta, Sporobolus compositus, Nassella leucotricha, Bothriochloa laguroides ssp. torreyana, Eriochloa sericea, Paspalum floridanum, and Tridens strictus. Forbs commonly encountered in this system include Symphyotrichum ericoides, Stenaria nigricans var. nigricans, Helianthus maximiliani, Rudbeckia hirta, Bifora americana, Acacia angustissima var. hirta, Desmanthus illinoensis, and many more. Perhaps more commonly encountered species include Croton monanthogynus, Amphiachyris dracunculoides, and Asclepias spp. Lowland sites and swales are often dominated by Tripsacum dactyloides and Panicum virgatum (Elliott 2011). Heavy grazing has allowed species such as Bouteloua dactyloides (= Buchloe dactyloides) and Bouteloua rigidiseta to increase. Tripsacum dactyloides and Panicum virgatum are common associates on the Vertisols, especially on the gilgai microtopography.

Source: NatureServe Explorer

Environment

This system is typified by the presence of dark alkaline Vertisol soils over calcareous parent material interspersed with patches of acidic, sandy loam Alfisols and Mollisols. The detailed geology includes Cretaceous shales, marls and limestones, such as those of the Pecan Gap Chalk, Marlbrook Marl, Eagle Ford, Gober Chalk, Annona Chalk, and Austin Chalk formations, and Taylor and Navarro groups, as well as portions of the Eocene Midway Group and Wilcox Formation. Also, Miocene formations (Fleming and Oakville Sandstone formations) underlie the southern outlier of Blackland prairie recognized as the Fayette Prairie. Landforms are flat to gently rolling and dissected by drainages, with the most significant ridges associated with harder chalk formations. Microtopography such as gilgai and mima mounds can occur and are important microhabitats that lead to an increase in plant diversity in this system (Diamond and Smeins 1993). Soils are typically Vertisols, but this system may occupy Mollisols or Alfisols with the latter more common. The system generally occurs on calcareous clays, but may also occur on loams, clay loams, or even sandy clay loams or silt loams. Annual rainfall averages 890mm, wettest seasons are spring and fall (Harmel et al. 2003).

Source: NatureServe Explorer

Dynamics

Fire, drought and possibly and grazing constitute the major natural dynamics influencing this system. Frequent fires (mean fire-return interval of 2.5 years) prevent woody species from establishing and favor grassland species adapted to fire for reproduction and vigor (Landfire 2007a) prevent woody species from establishing and favor grassland species adapted to fire for reproduction and vigor. Bison and other ungulates possibly played an important role in the vegetation composition and structure of this system (Eidson and Smeins 1999). Fire suppression and overgrazing have allowed woody species to invade. Heavy grazing has also altered the floristic composition by allowing species such as Bouteloua dactyloides and Bouteloua rigidiseta to invade. This system is important for a suite of wildlife, many of which are declining, that are dependent on native grasslands (TPWD 2012a).

Source: NatureServe Explorer

Threats

Historic descriptions of the Blackland Prairie region by early travelers indicate the region was dominated by a tallgrass prairie. Forests were limited to stream valleys, and trees and shrubs sometimes occurred as scattered individuals and clumps in a vast sea of grasses and wildflowers (Diggs et al. 1999). Today, only small remnants (occupying <1% of the original extent) remain (Riskind and Collins 1975, Diggs et al. 1999, Eidson and Smeins 1999). Threats to the remaining remnants include elimination of the landscape-level processes that maintained the system such as fire and native grazers, introduction of exotic species (Bothriochloa ischaemum, Dichanthium sericeum, Lolium arundinaceum (= Schedonorus arundinaceus)), woody plant encroachment, overgrazing by livestock, urban and rural development, and infrastructure development (Eidson and Smeins 1999).

Source: NatureServe Explorer

Distribution

This system is restricted to the Blackland Prairie region, part of the Crosstimbers and Southern Tallgrass Prairie Ecoregion, in Texas and possibly adjacent southern Oklahoma.

Source: NatureServe Explorer

Ecologically Associated Plant Species

Plant species that characterize this ecosystem type, organized by vegetation stratum. These are species ecologically associated with the ecosystem, not confirmed present in any specific area.

Herb (field)

Andropogon gerardi, Bifora americana, Bouteloua rigidiseta, Buchloe dactyloides, Hedeoma reverchonii var. reverchonii, Mimosa quadrivalvis var. platycarpa, Panicum virgatum, Schizachyrium scoparium, Sorghastrum nutans, Tripsacum dactyloides

Source: NatureServe Ecological System assessment

Ecologically Associated Animals (4)

Animal species ecologically associated with this ecosystem type based on NatureServe assessment. These are species whose habitat requirements overlap with this ecosystem, not confirmed present in any specific roadless area.

Mammals (4)

Common Name	Scientific Name	G-Rank
Hispid Pocket Mouse	Chaetodipus hispidus	G5
Fulvous Harvest Mouse	Reithrodontomys fulvescens	G5
Plains Harvest Mouse	Reithrodontomys montanus	G5
Hispid Cotton Rat	Sigmodon hispidus	G5

Source: NatureServe Ecological System assessment

Component Associations (8)

Plant community associations that occur within this ecological system. Associations are the finest level of the U.S. National Vegetation Classification (USNVC) and describe specific, repeating assemblages of plant species. Each association represents a distinct community type that may be found where this ecosystem occurs.

Name	G-Rank
Schizachyrium scoparium - Andropogon gerardii - Sorghastrum nutans - Bifora americana Mollisol Grassland	G1	NatureServe
Schizachyrium scoparium - Paspalum plicatulum - Sorghastrum nutans - Dichanthelium oligosanthes - Symphyotrichum pratense Alfisol Grassland	G1	NatureServe
Schizachyrium scoparium - Sorghastrum nutans - Andropogon gerardii - Bifora americana Vertisol Grassland	G1	NatureServe
Schizachyrium scoparium - Sorghastrum nutans - Bifora americana Alfisol Grassland	G1	NatureServe
Sporobolus silveanus - Carex meadii Grassland	G1	NatureServe
Sporobolus silveanus - Tridens strictus Grassland	G2	NatureServe
Tripsacum dactyloides - Panicum virgatum - Sorghastrum nutans - Helianthus maximiliani Grassland	G1	NatureServe
Tripsacum dactyloides - Sporobolus compositus var. compositus Grassland	G1	NatureServe

State Conservation Ranks (1)

Subnational conservation status ranks (S-ranks) assigned by Natural Heritage Programs in each state where this ecosystem occurs. S1 indicates critically imperiled at the state level, S2 imperiled, S3 vulnerable, S4 apparently secure, and S5 secure. An ecosystem may be globally secure but imperiled in specific states at the edge of its range.

State	S-Rank
TX	SNR

Methodology and Data Sources

Ecosystem classification: Ecosystems are classified using the LANDFIRE 2024 Existing Vegetation Type (EVT) layer, mapped to NatureServe Terrestrial Ecological Systems via a curated crosswalk. Each EVT is linked to the USNVC (U.S. National Vegetation Classification) hierarchy through pixel-level co-occurrence analysis of LANDFIRE EVT and NatureServe IVC Group rasters across all roadless areas.

Vegetation coverage: Coverage percentages and hectares are derived from zonal statistics of the LANDFIRE 2024 EVT raster intersected with roadless area boundaries.

Ecosystem narratives and community species: Sourced from the NatureServe Explorer API, representing professional ecological assessments of vegetation composition, environmental setting, dynamics, threats, and characteristic species assemblages.

IVC hierarchy: The International Vegetation Classification hierarchy is sourced from the USNVC v3.0 Catalog, providing the full classification from Biome through Association levels.

Component associations: Plant community associations listed as components of each NatureServe Ecological System. Association data from the NatureServe Explorer API.

State ranks: Conservation status ranks assigned by NatureServe member programs in each state where the ecosystem occurs.