Plant Ecology of the Sonoran Desert Region - Design with the Desert: Conservation and Sustainable Development - page 158

In contrast to succulents that can take up water only from nearly saturated soil, drought
tolerant plants can absorb water from much drier soil. A creosote bush can obtain water
from soil that seems dust-dry to the touch. Similarly these plants can continue to photo-
synthesize with low leaf moisture contents that would be fatal to most plants.
Some plants in this adaptive group are notoriously difficult to cultivate, especially in
containers. It seems paradoxical that desert ferns and creosote bushes, among the most
drought-tolerant of desert plants, can be kept alive in containers only if they're never
allowed to dry out. The reason is that these plants can survive drought only if they dry out
slowly and have time to make gradual physiological adjustments. If a potted plant misses
a watering, the small soil volume dries out too rapidly to allow the plant to prepare for
dormancy, so it dies. Researchers showed that some spike mosses ( Selaginella spp.) must
dehydrate over a 5-7 day period. If they dry more rapidly they lack time to adjust, and if
drying takes longer than a week they exhaust their energy reserves and starve to death.
Selaginella lepidophylla from the Chihuahuan Desert is widely sold as a novelty under the
name “resurrection fern.” Rehydration and resumption of active life takes only a few hours.
8.4.6 Drought Evasion
The stretch of Interstate 40 from Barstow to Needles, California traverses some of the emp-
tiest land in the West. It dashes as straight as it can through 130 miles of dry valleys that
are almost devoid of human settlements. The vegetation is simple, mostly widely scat-
tered creosote bushes. It's difficult to tell if you're driving through the Mohave or Sonoran
desert. The small, rocky mountain ranges interrupting the valleys beckon to true desert
lovers, but the drive is just plain bleak to the clueless. The exits on this freeway average 10
miles apart and connect to two-lane roads that shoot straight over the distant horizon with
no visible destinations. You rarely see a vehicle on any of them.
Frequent travelers on this freeway become accustomed to its monotony until they think
they know what to expect. The creosote bush may turn if there's been a rain; ocotillo
( Fouquieria splendens ) always flowers in April; most of the time it's just brown gravel and
brown bushes. Then one spring travelers are astonished to discover the ground between
the bushes literally carpeted with flowers. It happened in March 1998, when for 3 weeks
the freeway bisected a nearly unbroken blanket of desert sunflowers 40 miles long and 10
miles wide (Figure 8.5). At every exit-to-nowhere several cars and trucks had pulled off and
people were wandering through the two-foot-deep sea of yellow. Those with a long mem-
ory may have recalled that the same thing happened in 1978. They should have wondered
where these flowers came from, and where were they during the intervening 20 years.
Those desert sunflowers ( Geraea canescens ) were annual wildflowers, plants that escape
unfavorable conditions by “not existing” during such periods. They complete their life
cycle during a brief wet season, then die after channeling all of their life energy into
producing seeds instead of reserving some for continued survival. Seeds are dormant
propagules with almost no metabolism and great resistance to environmental extremes.
(A propagule is any part of a plant that can separate from the parent and grow a new
plant, such as seeds, agave aerial plantlets, and cholla joints.) They wait out adverse
environmental conditions, sometimes for decades, and will germinate and grow only
when specific requirements are met.
Wildflower spectacles like the one described earlier are rare events. Mass germination
and prolific growth depend on rains that are both earlier and more plentiful than normal.
The dazzling displays featured in photographic journals and on postcards occur about
once a decade in a given place. In the six decades between 1940 and 1998 there have been