Lower Texas Coast Clay Dune Shrubland

This ecological system occurs on well-drained portions of clay dunes (lomas) rising above surrounding coastal tidal flats. It is a xeric, subtropical shrubland dominated by thorny evergreen shrubs, generally 2-4 m tall. Composition of this system is extremely variable, and there is usually no clear dominant, except locally. Local dominants may include Citharexylum berlandieri, Leucophyllum frutescens, Havardia pallens (= Pithecellobium pallens), and Ebenopsis ebano. While there is often no clear dominant, Yucca treculeana is a constant and conspicuous emergent in many occurrences. Some lomas may be flooded by the sea during severe storm events. Vegetation in this system is sometimes influenced by salt spray, high winds, limited rooting depth, saline water table, and extreme xeric conditions.

These are typically fairly dense to extremely dense shrublands, often 2-4 m in height. While there is often no clear dominant, Yucca treculeana is a constant and conspicuous emergent in many occurrences (Johnston 1952). Other dominants may include Acacia rigidula, Castela erecta, Celtis ehrenbergiana, Citharexylum berlandieri, Ebenopsis ebano, Forestiera angustifolia, Guaiacum angustifolium, Jatropha dioica, Karwinskia humboldtiana, Leucophyllum frutescens, Opuntia engelmannii var. lindheimeri, Phaulothamnus spinescens, Prosopis glandulosa, Sideroxylon celastrinum, Zanthoxylum fagara, and Ziziphus obtusifolia. There may be scattered emergent trees of Ebenopsis ebano and Prosopis glandulosa forming a sparse woodland. Within these shrublands, the herbaceous layer is typically not well-developed, however, the non-native Urochloa maxima may be conspicuous. A grassland, often dominated by Sporobolus wrightii occupies the margins of these clay dunes, as they grade downslope into the surrounding salty flats. These margins may also contain Sporobolus pyramidatus, Monanthochloe littoralis, and Spartina spartinae. Other somewhat halophytic species, such as Maytenus phyllanthoides and Prosopis reptans, may also occupy these dunes. The proximity of many of these dunes to active tidal fluctuations and salt spray also influences species composition at these sites.

This system occupies well-drained portions of clay dunes (lomas) along the lower Texas coast (and somewhat inland) and adjacent Mexico. These rise above surrounding coastal tidal flats and often develop from deposition of windblown fine sediments, resulting in elevated landforms within a matrix of tidal flats (Elliott 2011). At the time of formation, lomas were located on the leeward side of irregularly flooded lagoons and tidal flats that when dry provided the source for the windblown clayey sediments. The geology consists of Quaternary windblown deposits identified as clay dunes (Qcd). Landforms are round, elliptic, or crescent-shaped topographic highs, often within a matrix of low flats influenced by wind-driven tides. Soils include Point Isabel clay loam and Lalinda fine sandy loam, which are often associated with the Coastal Ridge Ecological Site. Lomas are characterized as wind-formed clay dunes on or near the coast, often surrounded by flats containing halophytic vegetation, coastal grasslands, or unvegetated wind-tidal flats. They usually occur as topographic highs in the surrounding level landscape, sometimes to 10 m above the surrounding plain and are a small-patch occurrence.

From Landfire (2007a): Hurricanes and tropical storms can affect these sites through tidal surge causing influx of saline waters. Saltwater inundation would be restricted temporally to the period during storm surge and would not likely significantly affect shrub mortality. Also, high-intensity storms may completely eliminate these sites through erosion. Erosional processes would tend to completely eliminate sites rather than causing changes in the system structure. Fire is not a process important to this system and does not or rarely occurs. Tidal flat islands are important for wildlife such as migratory birds, mollusks and fish (USACE 2013).

This system occurs as small-patch sites ranging from 10s to a few 100 acres. Disturbances tend to have local effects or completely eliminate a site. This BpS occurs as a stable system.

Key threats are potential changes in weather patterns that would increase the number and power of high-intensity storms such as hurricanes that can completely eliminate these sites through erosion of these islands in the tidal flats where they occur. Rising sea level may cause long-term influx of saline water that would cause mortality of shrubs that stabilize these tidal flat islands (lomas). Other threats from poor coastal management, including land development, dredging, levee construction, invasive species, chemical pollution and other human impacts, continue to degrade existing stands. Persistent drought may result in loss of key species. Conversion of this type has commonly come from land development and dredging. Common stressors and threats include coastal management actions such as levee construction and maintenance, development and invasion of non-native species.

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.

This coastal system is known from Aransas County, Texas, south into Mexico.