For the most part, people of the Chihuahuan Desert are reasonably well fed. And for some, unreasonably overfed. Perhaps as a result, we tend to forget that managing energy is critical to most life. Generally organisms cannot make conscious choices as to the direction of their evolution—instead, natural selection decides the paths taken. Often, the pathway followed is determined by energy constraints.
Many trees are deciduous, meaning that they drop their leaves
seasonally. Other trees are evergreen, retaining green leaves the year around.
Sometimes both approaches are found in the same group of species, as among the oaks.
Manufacturing leaves is energy and material costly, so it might seem that keeping them
until they wear out is the only way to go. But leaves in winter tend to cause the plant
to lose water, and in cold climates, frozen ground prevents replenishing the lost
moisture. Furthermore, snow and ice tend to damage the leaves. The "choice",
then, is a balance between the costs of retaining leaves and the costs of losing and
having to regrow them.
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.
Fall scene in McKittrick Canyon, Guadalupe Mountains National Park, Texas. Deciduous
leaves have turned and are ready to fall. Photograph by Kodi Jeffery.