During the last segment of the recent ice age, the Wisconsinan age of the Pleistocene epoch, temperatures dropped and moisture available for plants increased in the Southwest. As a result, many kinds of plants now limited to the heights of sky islands scattered through our desert region descended to lower elevations. Indeed, many highland populations now isolated from others of their kinds were distributed continuously across the lowlands.
At the end of the ice age, climbing temperatures and decreasing amounts
of effective precipitation killed off the lowland dwellers, leaving relictual
populations behind that were able to survive only at higher elevations. Strangely
enough, one tree now a prominent member of these montane forests was nearly absent east
of the continental divide during the Wisconsinan age. Ponderosa Pine, almost ubiquitous
today in our higher mountains, is known as a fossil from only one site: from Rhodes
Canyon in the San Andres Mountains. We have yet to understand what conditions limited
its range during the height of the Wisconsinan and what conditions allowed its spread
at the end of the Pleistocene.
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.