South Texas Mixed Thornscrub

This thornscrub ecological system occurs throughout much of northeastern Mexico and southern Texas. It occurs on a variety of substrates and landforms. Dominant species include Acacia roemeriana, Leucophyllum frutescens, and Prosopis glandulosa. Other species present to codominant include Acacia berlandieri, Vachellia farnesiana, Amyris madrensis, Amyris texana, Celtis ehrenbergiana, Parkinsonia texana, and cacti such as Opuntia engelmannii var. lindheimeri. The herbaceous layer is not well-developed but Trichloris pluriflora, Setaria spp., and Malpighia glabra are present. This system generally occurs as a closed shrubland or low woodland, usually lacking a purely open herbaceous component. Soils are clays, clay loams, and clay flats and are often calcareous or alkaline to varying degrees. Some sites are highly saline, and these sites are occupied by Tamaulipan Saline Thornscrub (CES301.711), but transitions between the systems may be subtle.

Prosopis glandulosa is very often a conspicuous component of the canopy in stands of this system, sometimes reaching to 6 m in height. This canopy may be dense, but given the open nature of the canopy of individual Prosopis glandulosa, significant solar radiation reaches the lower strata. Vachellia farnesiana (= Acacia farnesiana), Celtis ehrenbergiana, Ebenopsis ebano, and Celtis laevigata may also be components of the canopy, but Prosopis glandulosa usually dominates. The overstory canopy may be open with only scattered emergent trees over a dense shrub layer at 1 to 3 m in height. Depending on land-use history, the shrub understory may be limited to a few species such as Opuntia engelmannii var. lindheimeri, Ziziphus obtusifolia, or Celtis ehrenbergiana (= Celtis pallida) on relatively recently cleared sites. On more mature sites, a diverse assemblage of species, such as Acacia rigidula, Castela erecta, Malpighia glabra, Opuntia engelmannii var. lindheimeri, Cylindropuntia leptocaulis, Ziziphus obtusifolia, Celtis ehrenbergiana, Lycium berlandieri, Forestiera angustifolia, Guaiacum angustifolium, Diospyros texana, Amyris texana, Karwinskia humboldtiana, Havardia pallens, Phaulothamnus spinescens, Schaefferia cuneifolia, Condalia hookeri, and Zanthoxylum fagara, may occur. Leucophyllum frutescens and Acacia berlandieri may be present, but occur as scattered individuals as opposed to dominating the aspect of the community as they sometimes do on some shallow-soiled calcareous sites. However, like some shallow-soiled calcareous sites, Acacia rigidula is the aspect dominant of the shrub layer. The herbaceous layer is usually fairly sparse. Currently, the herbaceous layer may actually be dense with the non-native grass Urochloa maxima. Other non-native species, such as Pennisetum ciliare, Cynodon dactylon, Bothriochloa ischaemum var. songarica, and Dichanthium annulatum, may also be present to dominant. Native grasses, such as Bothriochloa laguroides ssp. torreyana, Chloris spp. (= Trichloris spp.), and Pappophorum bicolor, may be present.

This system is well-represented on the Eocene Claiborne and Jackson groups and the Pleistocene Beaumont Formation, but is also found on various other formations. Its landforms are gently rolling to nearly level sites, sometimes interdigitated with calcareous ridges and low-lying drainages and bottomlands. Found on upland sites on tight soils deposited through alluvial processes associated with the Rio Grande, also occurs on uplands away from the delta on deeper soils. Clay, Clay Flat, and Clay Loam Ecological Sites are the typical soils for this system.

Fire plays a role in this system, occurring in situations adjacent to grasslands during dry conditions when fire would jump to the canopy and carry during wind events. Drought would influence fire occurring in the woodland and shrubland classes (Landfire 2007a).

This system was modeled by Landfire (2007a) using three classes: early-, mid- and late-seral. The early-seral (0-5 years) class is dominated by perennial grasses. This class was maintained on higher topographic positions somewhat longer because of slower shrub growth in more xeric situations. Frequent replacement fire (MFRI = 7 years) is the dominant disturbance type in this class (Landfire 2007a). Mid-seral class is dominated by shrubs (40-70% cover). In this class, mesquite is a component of the shrub layer along with the other shrubs. Drought is incorporated into the MFRI in that dry conditions would be required for fire to be carried in the canopy. Replacement fire (MFRI = 20 years) is the dominant disturbance type in this class (Landfire 2007a). The late-seral class has a shrub layer at a height of 2-4 m and 70-100% cover. Mesquite canopy is well-developed in this class. Shrub layer development is extensive forming an almost continuous layer. Replacement fire (MFRI = 30 years) is the dominant disturbance type in this class (Landfire 2007a).

Much of this system was decimated by development for agriculture early in the twentieth century (Crosswhite 1980). Grazing pressure removing native grasses, increase in invasive (introduced) grasses, and lack of fire threaten this system. Currently the non-native grasses Pennisetum ciliare and Urochloa maxima can serve as ladder fuel which increases the potential for fire in this system. Threats from development, including development for agriculture, overgrazing by livestock, and possibly energy development, continue to convert or degrade existing stands. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species. Persistent drought may result in loss of key species. Conversion of this type has commonly come from agricultural practices. Common stressors and threats include fragmentation from roads, agriculture and development, and non-native species invasion. Other stressors and threats include overgrazing/browsing by livestock, and possibly loss of pollinators.

According to Climate Wizard in 2050 global climate change model (using Medium A1B emission scenario and Ensemble Average general circulation model), the average annual temperature is predicted to rise approximately 5°F and average annual precipitation will not significantly change (TNC 2013). Seasonal shifts in precipitation predict increased fall (monsoon) moisture with similar levels of precipitation to current in the rest of the year (TNC 2013). Potential climate change effects on vegetation could include a shift to species adapted to a hotter, generally drier environment. While average precipitation amounts may remain similar or slightly decrease during the winter, spring and summer months, that, along with increased temperatures, may cause vegetation to experience less effective precipitation and more soil moisture deficit during much of the growing season reducing plant growth and increasing mortality from extreme events including exceptional drought. If the increased fall precipitation is from intense storms such as hurricanes, we can expect more disturbances from flooding and water erosion.

Occurs throughout much of northeastern Mexico and southern Texas.