A Journey To The Desert's Edge, Part Five

Note: this is the fifth in a series of occasionally appearing entries focusing on deserts in general and the drylands of West Texas in particular

 

Six or seven hours after leaving Houston in the rearview, headed west on I-10, we've left the Hill Country and San Antonio behind.  Now we've reached the outer edges of the Concho Valley, provided we remembered to dog-leg northwestward at Junction.  The watershed of the valley's namesake river and its tributaries is remote country but not particularly large by Texas standards.  Massachusetts, Rhode Island, and a tiny chunk of Connecticut occupy about the same amount of space...

Texas, on the other hand, is a large state and Texans prefer others not forget the fact...

This humorous vintage postcard fosters two misconceptions
about Texas: in reality, giant saguaro cacti are not native to
the western part of the state and the jackrabbits are, in fact,
much larger than this specimen which appears to be of a stunted
New Mexico species

Until Alaska came along to add a 49th star to the American flag, Texas occupied more land area in the United States than any other state-- some 268, 620 square miles of the 3, 119, 884. 69 square miles of earth and water comprising the lower 48.  From north to south, a traveler might cover 801 miles, according to the Texas Almanac.  The distance from east to west is considerably shorter, a mere 773 miles...

Such great spaces led Texans to subdivide the state into vaguely defined areas such as East Texas and West Texas for conversational convenience.  Where West Texas lies is a matter of individual definition.  Not entirely tongue in cheek, we say it is the part of the erstwhile Lone Star Republic made up of Panhandle and Desert (or Panhandle, Desert, and Serious Desert)... 

Far Eastern Texas: a cypress swamp in the Big Thicket National Preserve

Boundaries for these two (or three) places are tied to both climate and the economic usefulness of the land.  The Panhandle, a Texan might say, is good country to farm or ranch except for that 75% of the year when it is too hot, cold or dry.  The Desert stays hostile towards man and beast for 10 or 11 months annually.  Its residents will say the land is hot, dry, and windy but do not like to admit to themselves that these traits are character marks of a desert.  Serious Desert country is universally acknowledged to be worthless ground but any true Texan knows it can be tamed by gruff, two-fisted empire builders of the sort John Wayne played in movies...

 

Vast distances (plus the fact easternmost Texas can see over 50" of rain yearly while westernmost Texas may see under 10" during that same time) make for a plethora of geological regions and ecoregions...

Far Western Texas: the Rio Grande river bed in the Big Bend

An online chart from Texas Parks and Wildlife says there are 18 ecoregions in the state.  The chart is based on Robert G Bailey's lengthy analysis of a 1976 map published by the Ogden, Utah, branch office of the U S Forestry Service where the geographer once worked.  His tome has basically guided the agency's efforts to scientifically manage our ecosystems since the mid-1990s...

As a person who has traveled throughout most of Texas, I can say what grows along the Oklahoma border by the Red River does not necessarily thrive on the northeast bank of the Pecos.  Average annual differences of 16 or more inches of precipitation between the two locations practically guarantees this. 

Robert G Bailey, Geographer

Yet both locations belong to the Rolling Plains ecoregion according to Bailey who used an algorithmic approach to determine its boundaries .  In practical terms, this means he divided the United States into very large regions based on climate and subdivided each region step by step, considering the plant life that could theoretically occur naturally in an area, geological surface features, and soil characteristics...

Other ecologists approach the subject a bit differently.  Among them is James Omernik who began his career in the 1960s at the Defense Intelligence Agency.  Shifting bureaucratic loyalties in 1972, he spent three decades analyzing the nation's ecology for the Environmental Protection Agency.  Omernik seems a bit of a workaholic.  Officially retired, he still consults part time for the US Geological Survey...

Bailey's Ecoregions of Texas

The EPA-Omernik collaboration produced a four-tier analysis of US ecoregions based on "biotic and abiotic" factors, simply meaning it considers both living and non-living characteristics when determining ecoregion boundaries.  These factors include geology, physiography, vegetation, climate, soils, land use, wildlife, and hydrology. According to an EPA description of Mr Omernik's holistic approach, "the relative importance of each characteristic varies from one ecological region to another"...

For an amateur naturalist such as myself, the EPA analysis is particularly useful since my interest in the subject of ecoregions grew out of my interest in learning more about the plant and animal life of the western Concho Valley and Trans-Pecos regions of Texas...

The EPA Level-IV analysis of Tom Green County, sees it as a meeting place for the Red Prairie, Limestone Plains, Edwards Plateau Woodland, and Semi-Arid Edward Plateau ecoregions...

Epa Level IV Ecoregions for the area enclosed by a rectangle
in the illustration of Bailey's ecoregions.  30d designates the
Semi-Arid Edwards Plateau with its vegetation dominated by
mesquite and redberry juniper

This last ecoregion lies west of the 100th meridian where the strong winds and lack of precipitation do much to shape the landscape.  Rounded hills, not rare in central Texas, turn into mesas and buttes at the city limits of San Angelo.  Streams flow sporadically.  Arid land shrubs are common.  Here we find mesquite, lotebush, desert sumac, agarita, javelina bush, white brush, catclaw acacia, mimosa, pencil cactus, prickly pear, ephedra, sotol, agave, yucca (giant and otherwise)...

Consider the above mentioned mesquite shrub...

Archeological evidence in the form of excavated Native American campsites establishes the presence of mesquite in the Semi-Arid Edwards Plateau region long before the first Spanish explorers came to the land of three rivers in Tom Green County.  When, in the late spring of 1849, Army Lieutenant Frances T Bryan, surveyed the Lipan Flats area to the east of the future city of San Angelo, he chanced upon scattered mesquites which were not young shrubs...

Then men introduced cattle to the outer edge of the desert in the years following the Civil War.  Cows are particularly fond of mesquite beans.  And bovines moving from place to place spread mesquite beans from place to place...

Rangeland, Tom Green County, 1901

Rangeland, particularly after it has been fenced and heavily grazed for several decades, tends to become less capable of supporting either native or invasive plants and animals.  Damage is not immediately apparent and, in fact, may not become obvious for dozens of years... 

By 1880, after over a decade of overgrazing, the desert grasslands of the Semi-Arid Edwards Plateau appeared slightly sparser but otherwise no worse for the wear.  Twenty years or so later, the number of mesquite shrubs seemed to be increasing.  But there weren't that many of them, not enough to cause alarm...

Rangeland, Tom Green County, 2010, in a photograph taken by
the author in the same area depicted in the 1901 illustration 

Mesquite root systems can branch out laterally (as far as fifty feet away from the shrub itself) to capture any available surface moisture.  This characteristic allows them to be quite aggressive if not checked by the occasional wildfire.  Other native plants decline and gradually decrease in numbers as mesquites spread by wandering cows prosper...

Around 1960, it seemed a hundred mesquites sprouted up overnight on the Semi-Arid Western Plateau for every one that had been growing the night before... 

West Texas versus East Texas

Note: All images located through Google Images without source information except as follows: EPA Level Ecoregions of Texas from United States Environmental Protection Agency; Bailey Ecoregions of Texas, Vegetation Types of Texas from Texas Department of Parks and Wildlife; Tom Green County Rangeland 1901 from National Archives, Washington DC; Robert G Bailey from USDA Forest Service, Washington DC; Tom Green County Rangeland 2010 by Louis R Nugent