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Desert Diary
Ecology/Mountain Islands


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Islands are bits of land surrounded by water, right? Well, it depends on your perspective. Desert people look at the higher mountains scattered through their region and see islands—islands of green woodlands and forests such as the Organ Mountains, surrounded not by water, but by hot, arid lowlands.

Two questions cry for attention: How are forests able to survive surrounded by desert, and how did they get there in the first place?

Elevation is the answer to the first—as you climb, the temperature drops, slowing evaporation and allowing the waters of summer rains and melting winter snows to sink deeply into the soil. Moreover, mountains force wind to climb over them; surging upward, even moderately moist air cools and can no longer hold its allotment of water. This orographic precipitation and the cool temperatures are all that are needed to support the highland vegetation.

But how did the trees get there? Many invaded from the north, during the cool, wet Pleistocene, more than 10,000 years ago—and were then marooned on their islands as a sea of aridity engulfed the lowlands.
pen and ink

Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.


Contributor: Arthur H. Harris, Laboratory for Environmental Biology, 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.

Aerial view of Organ Mts.

Aerial view of the Organ Mountains from the south. Photograph by Scott M. Cutler.



Van Devender, T. R. 1990. Late Quaternary vegetation and climate of the Chihuahuan Desert, United States and Mexico. Pp. 104-133, in Packrat middens, the last 40,000 years of biotic change. Betancourt, J. L., T. R. Van Devender, and P. S. Martin (eds.). University of Arizona Press, Tucson, 467 pp.

Van Devender, T. R., and W. G. Spaulding. 1979. Development of vegetation and climate in the southwestern United States. Science, 204:701-710.