Guide to Texas Grasses

3. Ecoregions of Texas

Anyone who has been in Texas understands that the state is huge! It is 801 mi (1,282 km) from the northernmost point in Dallam County to the southern tip of Cameron County, and 773 mi (1,237 km) from the Sabine River in Newton County to the Rio Grande in El Paso County. The state covers over 268,000 mi2 (694,120 km2 ) in surface area, which converts to roughly 172 million ac (69 million ha) (Poole et al. 2007). Texas has over 3,800 mi (6,080 km) of border, with over 2,011 mi (3,218 km) along the Rio Grande separating Texas from Mexico (Dallas Morning News 2008). The size of the state affords the opportunity for an amazing number of combinations of natural features referred to in various ways, such as vegetation types, natural regions, ecoregions, and major land resource areas. The term “ecoregions ” has been selected for use in this book. Ecoregions are typically large land (for our purposes) or water areas that have a homogenous set of natural features. Geologic, physiognomic , topographic, climatic, edaphic, and biotic (vegetative composition and structure in this case) features should be relatively similar across an ecoregion. Each of the natural features that contribute to ecoregions will be briefly discussed and illustrated at the state level to help visualize their relationship to the 12 ecoregions identified for Texas. Each ecoregion will then be discussed individually. Geology A curved line from Marathon around the big bend in the Rio Grande to Del Rio along Chapter 3 : ECOREGIONS OF TEXAS the Balcones Escarpment past San Antonio, Austin, Waco to Dallas, and extending to the Red River delineates the Gulf of Mexico coastline during the Cretaceous Period (60–100 million years ago [mya]) (fig. 3.1; Spearing 1991). Surface rocks east and south of that line are younger than those of the Cretaceous and become progressively more recent toward the Gulf. During the Cenozoic Era (66 mya) erosive forces have continued to deposit large amounts of sediments, extending the state farther and farther into the Gulf. Cretaceous-aged limestone dominates the central part of the state. Triassic rocks (245–208 mya) are exposed along the Caprock Escarpment. Permian deposits (286– 245 mya) cover the area between Amarillo to Abilene and the Guadalupe, Delaware, and Apache mountains. Pennsylvanian formations (320–286 mya) occur farther east toward Fort Worth. Most of the rocks in the western Panhandle on the Caprock represent materials eroded from the Rocky Mountains during the Pliocene, Miocene, and Oligocene epochs (35–2 mya) of the Cenozoic Era. Volcanism has been fairly restricted in Texas. Remnants of a small volcanic episode during the Cretaceous are visible around Austin and Uvalde. A much larger and more dramatic episode that occurred in the early Tertiary Period (around 65 mya) in West Texas formed lava fields and mountains (i.e., Davis and Chisos mountains). The most ancient of rocks (Precambrian, up to 1 billion years old) are found in the Llano area and several uplifts in West Texas Figure 3.1. Geologic ages in Texas (adapted from Geologic Map of Texas. Bureau of Economic Geology, University of Texas at Austin). 28 Chapter 3 (i.e.,Franklin Mountains). Holocene, or the most recent, deposits are represented by the sand dunes in the Coastal Bend region and in the upper Pecos River watershed. Physiography Figure 3.2 illustrates the complexity of Texas physiography, or the physical expression of landforms. As expected, most features are closely aligned with geology. For example, the slightly sloping Gulf Coastal Plains are composed of “recent” Cenozoic deposits. The Edwards , Stockton, and Comanche plateaus are hard Cretaceous limestones that have resisted erosion for millions of years. Pennsylvanian rocks occur in the Palo Pinto Basin. The Osage Plains or Red Bed Plains are all Permian in age and often have a distinctive reddish color. The Llano Estacado (High Plains) consists entirely of Tertiary-aged materials eroded from the Rocky Mountains and deposited to form a relatively flat and featureless plain. The Llano Basin (for the most part) is really an uplift of Precambrian-aged rocks. Recent Holocene deposits are the sandy lands in the Coastal Sand Plains and Toyah Basin. The Big Bend region of the state is most complex with pockets of nearly all rock ages and types. Topography Texas is also complex topographically. The Gulf Coastal Plains are relatively flat and gently tilted toward the Gulf of Mexico. Two...