In your journeys around the Chihuahuan Desert, you may have noticed that our mountain ranges quite often have different vegetations even when pretty much equal in height and orientation. Often, this is due to the type of rocks making up the mountains. When all else is equal, mountains constructed of limestone tend to be much drier than those of other materials. The thing with limestone is that it dissolves under the influence of the mild acid produced by interaction of water and plant-root products. The natural fractures and joints intrinsic to limestones are thus slowly enlarged. As a result, precipitation quickly finds escape routes and flees the surface zone, leaving slope vegetation high and dry.
In mountains of other materials, lacking these mazes of dissolved
passageways, moisture moves slowly down slope, watering the plant life as it goes. This
difference shows up well in the Hueco Mountains east of El Paso. There, igneous syenite
porphyry intrudes into the mostly limestone mountains, forming the Hueco Tanks area.
And, on this igneous substratum, grow plants occurring nowhere else in these
mountains.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
>Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.
Limestone near the south end of the Franklin Mountains. The most prominent plant
here is Lechuguilla (Agave lechuguilla). Eastern El Paso is visible above the
crest, with the Hueco Mountains on the horizon. Photograph by A.H. Harris.