Friday, November 22, 2013


Journey To The Desert's Edge, Part 10

Note: this is the tenth in a series of occasionally appearing entries focusing on deserts in general and the drylands of West Texas in particular
“At the foot of the Panhandle lies the vast western portion of Texas, an area of the state that epitomizes the isolation and beauty of the American desert.  From the edge of the metropolitan area of El Paso, the West Texas desert stretches over four hundred miles (640 km) into the heart of the state.”-- Gary Reyes, Texas, Mallard Press, New York, 1991


Defining deserts is a tricky endeavor and defining their boundaries can be even trickier business…

Readers of previous columns in this series about the dry country remember we have settled on a fairly simple definition for a desert-- an area where the potential water loss through evapotranspiration significantly (and consistently) exceeds that area’s precipitation.  Most scientists who study deserts agree on this definition…


The Chihuahuan Desert, broadly outlined in this map, is both
North America's largest and most biologically diverse desert.
It is also one of the continent's least explored wilderness areas.
We’ve also discussed the lack of universal scholarly agreement as to what “significantly exceeds” means.  In 1937, H C Trumble provided empirical answers to part of this dispute by demonstrating water need for a month exceeds water supply when computed evaporation is more than three times precipitation.  His work, cited over a decade later by C W Thornthwaite in a landmark analysis of climate, dealt only in facts and not in the nuances of language.  Thus, scholars continue to debate exactly what makes a desert a desert…

Does a region become a desert when this “three times precipitation” threshold is met-- or must the area be even more arid?  If it must be even drier, at what point is the land desert dry-- when computed evaporation is five times the precipitation, nine times the precipitation, twenty times?  Just what does “significantly exceeds” mean ...

One school of thought embraces a very wide stretch of country as desert and includes all of Wladimir Koeppen’s “B” series of dry climates.  Another school excludes those areas Koeppen defined as steppes but includes ones he called deserts.  (A problem with this approach is that Tucson, Arizona, and most of the Big Bend area of Texas would be among the places not considered deserts-- an omission that most visitors or residents of these areas might strenuously dispute.)… 

Near the author's home in western Tom Green County
Some scholars also say there are no true deserts in North America.  They limit deserts to exceedingly arid regions like the Sahara of northern Africa and the Atacama in Chile.  The latter may be the driest place on Earth.  Researchers believe parts of this vast desolation haven’t received measurable rainfall for four hundred years.  Average annual precipitation for the Atacama as a whole hovers around an estimated 0.004 inches.  Placing that figure in perspective, it could be somewhere around the year 2263 before we add a single inch to this desert’s sparse total…

A majority of laymen (and probably most scientists) say there are deserts in the United States.  There’s also general agreement four large areas-- the Mojave, the Sonoran, the Chihuahuan, and the Great Basin-- can be so designated.  Fixing their boundaries is a difficult and probably impossible task though…

Rough outlines of deserts have been established by studying plants native to a region.  The same can be said for other ecosystems such as grasslands or rain forests.  When Wladimir Koeppen first conceived the notion for developing a world-wide classification scheme, he drew upon both his training as a botanist and his childhood travels to botanically diverse regions through the Russian empire to create the broad outlines of his system. Later, as Peggy Larson commented in a study of the southwestern deserts, the botanist Forrest Shreve took this same understanding that plants define the natural character of an area and used it to decode the landscape of “the North American desert into four individual deserts… based on their distinctive vegetation”…


Ocotillo, Fouquieria splendens, as photographed by the author.
For more photographs of this fascinating desert shrub, visit the
Lady Bird Johnson Wildflower at the link provided in the text.
Shreve, who became director of Desert Investigations in 1928 for the Carnegie Institute, published his desert boundary outlines in a 1942 Botanical Review article.  Since then, subsequent researchers have either confirmed his descriptions, contracted the territory included in them, or expanded the areas he considered to be desert based on factors they consider relevant...

The largest disagreements over the boundaries of any desert focus on North America’s easternmost desert.  Mountain ranges basically limit the maximum possible boundaries for three of our four deserts.  But, for the Chihuahuan, those boundaries are ill-defined products of heat, wind, and potential evapotranspiration rates of dry west Texas plains.  Complicating the problem is the fact these boundaries more likely lie in places more resembling short grass prairies dotted with xeric shrubs than in the harsh landscapes known to us from cowboy movies...

Where those eastern boundaries might be can be seen on the map of Texas shown at the end of this essay…

First, we should explain what the map shows…

Colors are used to denote the presence of various plants in the map.  Counties shaded in brown contain populations of Agave lechuguilla and/or Flourensia cernua, populations documented on range maps developed and maintained by the United States Department of Agriculture (USDA) and/ or the Biota of North America Program (BONAP).  One should note that range maps are not always complete.  The reader will notice that Irion County, between Tom Green County and Reagan County, lacks a USDA or BONAP documented population of Flourensia cernua.  But such a population does exist...
 
Spindly tarbush, Flourensia cernua, like Agave lechuguilla roughly
outlines the borders of the Chihuahuan Desert in the United States


Red counties have USDA or BONAP documented populations of Larrea tridentata, a widely distributed desert shrub, but lack similar records for either Agave lechuguilla or Flourensia cernua having been found inside the county.  Yellow means Condalia ericoides is present with no documented populations of the three other plant species.  Gray documents Tiquilia canescens...

Note: Readers who are interested in learning more about the plants briefly mentioned above and discussed in the following paragraphs are encouraged visit the Lady Bird Johnson Wildflower Center of Austin, Texas at https://www.wildflower.org/explore/.  The site contains descriptive information as well as photographs for more than 7000 native species.

Now, we need to explain what the map means…

Botanists like Shreve consider certain plants as unique to certain regions-- giant Saguaro cacti belong to the Sonoran Desert and the oddly shaped Joshua Tree yucca to the Mojave…

Unfortunately for those who love majestic desert beauty, two plants used to define the Chihuahuan don’t tower above arid landscapes as magnificently as do their cousins in the drylands of the west… 

But tarbush (Flourensia cernua) and lechuguilla (Agave lechuguilla) are like the Joshua Tree and the Saguaro in needing a hard land with poor soil, sporadic rains, and month after month of searing heat if they hope to flourish…


Saltbush, Atriplex canescens, seen here at the San Angelo State Park in Texas,
is one of the most widely distributed xeric shrubs in North America and is not
unique to any specific desert region.


Related to century plants but generally more compact in size, lechuguilla enjoys a bit of notoriety as the source of tequila. Sharp tipped leaves, slender and spiny enough to impale a man unlucky enough to fall on it, give the plant another common name-- shin dagger.  Like other agaves, it may take twenty-five years for lechuguilla to flower.  When it finally does, it shoots a spectacular plume of tiny blooms skyward from the center of the plant and then dies.  The life span of any lechuguilla is tied to the amount of rain it receives and the soil in which it lives-- the wetter the weather and the richer the soil, the sooner the plant dies…

Counties in red may lack documented populations of lechuguilla or tarbush but they do provide a home to creosote bush.  Known to scientists as Larrea tridentata, creosote bush often grows in association with tarbush in the desert country of Texas and New Mexico and southeastern Arizona.  Both are short spindly shrubs with tiny leaves.  Both exude odors highly reminiscent of petroleum-derived products.  Both thrive on alluvial soils derived from limestone.  But creosote bush can be found in more places than tarbush-- it and a tall wand-like plant with flaming red flowers known as ocotillo vie for the title of most widely distributed plant found in the “hot” deserts of North America…

(As the reader may suspect, creosote bush and ocotillo also grow, more often than not, in the brown counties where lechuguilla and tarbush can be found.  This association is strongest in the barren Trans-Pecos where almost every county is home to all four plant species.)

In the western Concho Valley of Texas, the association of these four species becomes sporadic.  Only lechuguilla has been found east of the San Angelo area.  Creosote bush makes limited appearances outside Reagan and Crockett counties and is difficult to find.  A Mesquite-Juniper-Creosote-Live Oak brush community can be found in southern Tom Green County but the numbers of Mesquite, Juniper, and Live Oak are far greater than the number of creosote bushes.  Texas Handbook Online articles anecdotally note the shrub's presence in the countryside near San Angelo, particularly around the hardscrabble country surrounding the Devil’s Courthouse Peak, known to most folk in the area as Devil’s Mountain, but one will find more Desert Sumac (Rhus microphylla) or White Brush (Aloysia gratissima) in a region that is also home to giant yuccas ... 


Javelina bush, Condalia ericoides, growing near the somewhat misnamed
Water Valley settlement in western Tom Green County.  Other photographs
of this plant, including some by the author, can be found at the Lady Bird
Johnson Wildflower Center website by visiting the link provided elsewhere in
this blog.
The spiny and twisted javelina bush can be found in Irion County, at one time part of Tom Green County.  Known to the botanical world as Condalia ericoides, javelina bush has a distribution much akin to that of tarbush and can be found growing in all counties west of the Pecos.  It is one of the many dry country species that the biologist Vernon Bailey noted as growing near San Angelo when he passed through the Concho Valley in May, 1899.  He described the area as a “genuinely arid region…with great stretches of smooth surfaces with only short grasses and little desert plants…(and) a scattered growth of small mesquites”…

One of those little desert plants mentioned by Bailey is Tiquilia canescens. Commonly known as “dog ear” due to the shape of its small, grayish leaves, this sub-shrub strays but very little outside desert regions.  Its presence, marked in gray on the map, is an indicator of land which receives erratic rainfall rapidly lost to wind and sun…

This map with color-shaded counties leaves us with much to ponder and topics to be explored in detail at a later date…

It suggests a large portion of Texas is exceptionally and naturally dry.  Detailed climate studies confirm that roughly 80% of the Lone Star State can be described as subhumid, semi-arid, or arid.  This implies very real challenges to both agricultural activities and human populations heavily dependent on water for survival, challenges highlighted and made more urgent by the proximity of large cities kissing the desert’s edge-- cities like Austin in Travis County and San Antonio in Bexar County…





  



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CREDITS

Note: Information for this essay was obtained primarily through readily available sources such as Wikipedia and the Encyclopedia Britannica except as noted:  Forrest Shreve and desert boundaries from The Deserts of the Southwest by Peggy Larson (Sierra Club Books, San Francisco, 1977); biographical information about Forrest Shreve from http://people.wku.edu/charles.smith/chronob/SHRE1878.htm; Flourensia cernua and Agave lechuguilla as indicator plants of Chihuahuan Desert region from Deserts by James A MacMahon (Knopfm New York, 1997) and Trees and Shrubs of the Trans-Pecos and Adjacent Regions by A Michael Powell (University of Texas Press, Austin, 1998) plant distribution data from range maps developed by the United States Department of Agriculture and the Biota of North America Program; information about creosote-juniper-live oak plant association from Distribution, Natural History, And Biogeographic Relations On The Edwards Plateau Of Texas by Jim Goetze (doctoral dissertation, Texas Tech University, 1995); Vernon Bailey quotes from Texas Natural History: A Century of Change by David J Schmidley (Texas Tech University Press, Lubbock, 2002) All photographs for this essay were located through Google Images or Wikipedia, without authoritative source or ownership information except as noted: map of Chihuahuan Desert region from the Centennial Museum, University of Texas at El Paso; blank county level map used to outline Texas desert species distribution from United States Census Bureau; all other illustrations by Louis R Nugent




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