Plants Outwit Drought and Heat
Few plants can deal with the combination of heat, cold, and dehydration. Large areas in the shifting dune fields have only a handful of plants struggling to grow fast enough to avoid burial. In the low-lying areas that once held great lakes, only salt-tolerant halophytes can detoxify concentrations of salt and minerals that would cause the cells of normal plants to rupture. Any desert plant must have evolved ways to cope with the heat and dryness. Some put down deep, broad root systems. Some grow only in sheltered areas. Many germinate within days of a good rain and sprint through their life cycle in a race against heat and dehydration, living in a frenzy to scatter seeds that can await the next big rain. The scattered volcanic mountain ranges harbor survivors from earlier, wetter times, since the upper reaches of an 11,000-foot (3,352 m) mountain may have conditions today similar to the conditions that existed near the base of the mountain 10,000 years ago. Woody Sahara plants among these ice age survivors include species of the olive, cypress, and mastic trees.
When ancient, underground water comes to the surface in an oasis, other hardy desert survivors like doum palm, oleander, date palm, thyme, and acacia seek out a survivor's foothold. So do salt-tolerant plants like tamarisk, or salt cedar, which people brought from Africa to plant in the American Southwest to use as windbreaks a century ago. Since then, the salt cedar has spread throughout waterways in the American desert to become a major pest. Hardened in the Sahara Desert, the salt cedar has proved formidable competition along the degraded riparian areas of the American West.