South Texas Limestone Thornscrub

This xeric thornscrub ecological system is restricted to limestone and calcareous sandstone hills and caliche substrates such as along the Bordas Scarp in southern Texas and northeastern Mexico. Soils are shallow, alkaline, strongly calcareous and underlain by bedrock or a caliche layer. It has a shorter, more open shrub canopy (usually less than 2 m) when compared to more typical thornscrub growing on more favorable sites. However, shrub cover is generally greater than 70% and often greater than 85%. Dominant species include Leucophyllum frutescens, Acacia berlandieri, and Acacia farnesiana with many other shrub species that may be locally dominant. The sparse to moderately dense herbaceous layer is dominated by perennial graminoids.

Sites are most frequently dominated by shrubs between 0.5 and 2 m in height. Shrub canopy can be dense (to about 90% cover), or sparser where rocky exposures reduce substrate for rooting. A sparse overstory, usually <4 m in height, may be present and composed of species such as Prosopis glandulosa and, in the south, Ebenopsis ebano, Cordia boissieri, and/or Helietta parvifolia. Quercus fusiformis may form a relatively open canopy in areas in the northeastern part of the South Texas Plains. The shrub layer may be heavily dominated by Leucophyllum frutescens, Acacia berlandieri, and/or Acacia rigidula. More commonly, a diverse array of shrubs is present, including these three in addition to several of the following species: Acacia schaffneri, Aloysia macrostachya, Amyris madrensis, Amyris texana, Bernardia myricifolia, Castela erecta ssp. texana, Celtis ehrenbergiana, Condalia spathulata, Croton incanus, Diospyros texana, Ephedra antisyphilitica, Eysenhardtia texana, Forestiera angustifolia, Guaiacum angustifolium, Helietta parvifolia, Jatropha dioica, Karwinskia humboldtiana, Koeberlinia spinosa, Krameria ramosissima, Mahonia trifoliolata, Cylindropuntia leptocaulis, Parkinsonia texana var. macra, Salvia ballotiflora, Sideroxylon celastrinum, Sophora secundiflora, Yucca treculeana, and others. More southerly occurrences may also contain Lippia graveolens, Helietta parvifolia, Gochnatia hypoleuca, Croton humilis, Ebenopsis ebano, and/or Mortonia greggii. The herbaceous layer may be somewhat well-developed, but often bare rock is easily visible through the layer. Many sites are now dominated by non-native grasses, particularly Bothriochloa ischaemum var. songarica and/or Pennisetum ciliare. Other grasses are often short grasses, with species such as Bouteloua rigidiseta, Bouteloua hirsuta, Bouteloua dactyloides, Hilaria belangeri, Aristida purpurea, Bouteloua curtipendula, and Setaria leucopila present. Forbs and subshrubs are conspicuous in the herbaceous layer and include species such as Tiquilia canescens, Thamnosma texana, Galphimia angustifolia, Polygala alba, Cordia podocephala, Acourtia runcinata, Dalea aurea, Calliandra conferta, Chamaecrista greggii, Heliotropium torreyi, Melampodium cinereum, Hymenopappus scabiosaeus, Desmanthus velutinus, Calylophus hartwegii, Simsia calva, Hermannia texana, Mandevilla macrosiphon (= Macrosiphonia lanuginosa var. macrosiphon), Viguiera stenoloba, Stenaria nigricans, Thymophylla pentachaeta, Wedelia acapulcensis var. hispida (= Wedelia hispida), and Meximalva filipes (Elliott 2011). Downslope from these sites, soil development increases, soils tend to be tight, a more well-developed overstory of Prosopis glandulosa becomes prominent, and species such as Castela erecta and Ziziphus obtusifolia increase in cover relative to other species.

This system is restricted to xeric, rocky hills, rolling or level plateaus, and ridges composed of limestone and calcareous sandstone, as well as caliche substrates such as of the Goliad Formation or Uvalde gravel along the Bordas Scarp in southern Texas and northeastern Mexico. Soils are thin, alkaline, strongly calcareous and underlain by bedrock or a caliche layer. These are Shallow, Shallow Ridge or Gravelly Ridge Ecological Sites.

Erosion occurs on these sites, creating gullies, but not causing a shift in the community. Fire played little to no role in this system, though may have spread into the margins of stands during drought and high wind conditions (Landfire 2007a).

This system was modeled by Landfire (2007a) using a single class. Dense shrubland, generally 40-90% cover with sparse cover from emergent overstory species. Little natural disturbance affects this shrubland. Low fine fuel loadings make fire spread minimal except under extreme windy and dry conditions when fire may spread into it from surrounding sites. Species are drought-resistant. However, this system occurs in large patch to matrix scale and marginal fires likely spread little into the interior portions of occurrences.

Threats from development, including overgrazing by livestock, mining, and 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 effective brush eradication using mechanical, chemical or prescribed burning method. Common stressors and threats include fragmentation from roads, non-native species invasion (Landfire 2007a), and development, mining, agriculture. 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.

Restricted to limestone and calcareous sandstone hills and caliche substrates such as along the Bordas Scarp in southern Texas and northeastern Mexico.