Desert
plants, since they are rooted in place, must cope
with extremes in temperature, water availability and
solar radiation physiologically rather than behaviorally.
In fact, surface temperatures in direct sunlight are
commonly 25 to 50 degrees F warmer than the air temperature
six feet above. Most desert plant adaptations seem
to be geared towards minimizing water loss: a difficult
task since plants must "breathe" (collecting
Carbon Dioxide from the air) in order to photosynthesize,
losing body water to the atmosphere in the process.
Drought
Escapers
Drought escapers are plants that make use of favorable
growing conditions when they exist. These plants
are usually annuals and complete their life cycles
in a matter of days or weeks when water is plentiful
enough for them to do so. Seeds may lie dormant
for years if conditions are not favorable. Most
grasses are "escapers," as are the spring
wildflowers that sometimes bloom during April and
May.
Drought
Resistors
Drought resistors are typically perennials. Many
perennials have small, spiny leaves which reduce
the impact of solar radiation; others may drop their
leaves when water is unavailable. Spines and hairs
on leaves act as a buffer against warm air currents,
limiting the amount of water lost to evaporation.
Plants also use "solar tracking" to regulate
their exposure to the sun. Cacti store water within
their bodies and have extensive, shallow root systems
that are able to soak up rainwater quickly. Yucca
have extensive tap roots that are able to use water
beyond the reach of other plants. Moss, a plant
not commonly associated with deserts, thrives because
it can tolerate complete dehydration: when rains
finally return, the plant greens up almost immediately.
Another extreme adaptation can be found in the utah
juniper tree, one of the most common plants in the
southwest. During a drought, junipers can self-prune,
shutting off water flow to one or more their branches
in order to conserve enough water for the rest of
the tree to survive.
Drought
Evaders
Drought evaders take advantage of wetter "micro
climates" found in the desert. Monkey flower,
columbine, easter flower, and ferns are found in
well-shaded alcoves near seeps or dripping springs.
Cottonwood, willow and cattail all require lots
of water, and only grow in riparian areas where
their roots can reach the water table easily.
