Environmental Engineering Reference
In-Depth Information
full growth in 24-48 h after a rain. Agaves have visible new roots just 5 h after a rain.
A dormant nonsucculent shrub takes a couple of weeks to resume maximum metabolic
activity. Therefore, succulents can take rapid and maximum advantage of the soil moisture
that quickly evaporates after a summer rain. The combination of shallow roots and CAM-
idling with its rapid response enables succulents to respond to and benefit from less than
a quarter-inch (6 mm) of rain.
8.4.4 Protection
Stored water in an arid environment requires protection from thirsty animals. Most
succulent plants are spiny, bitter, or toxic, often all three. Some unarmed, nontoxic species
are restricted to inaccessible locations. Smooth prickly pear ( Opuntia phaeacantha var. laevis )
and live-forever ( Dudleya spp.) grow on vertical cliffs or within the canopies of armored
plants. Still others rely on camouflage; Arizona night blooming cereus ( Peniocereus greggii )
closely resembles the dry stems of the shrubs in which it grows.
These adaptations are all deterrents that are never completely effective. Evolution is a
continuous process in which some animals develop new inheritable behaviors to avoid
spines or new metabolic pathways to neutralize the toxins of certain species. In response
the plants are continually improving their defenses. For example, pack rats can handle even
the spiniest chollas and rarely get stuck. They also eat prickly pear for water and manage
to excrete the oxalates that would clog the kidneys of most other animals. Toxin-tolerant
insects often incorporate their host plant's toxins into their own tissues for protection
against their predators.
8.4.5 Drought Tolerance
Drought-tolerant plants often appear to be dead or dying during the dry seasons. They're
just bundles of dry sticks with brown or absent foliage, reinforcing the myth that desert
organisms are engaged in a perpetual struggle for survival. They're simply waiting for
rain in their own way, and are usually not suffering or dying any more than a napping dog
is near death (Figure 8.4).
Drought tolerance or drought dormancy refers to desert plants' ability to withstand
desiccation. A tomato plant will wilt and die within days after its soil dries out. But many
nonsucculent desert plants survive months or even years with no rain. During the dry
season the stems of brittlebush ( Encelia farinosa ) and bursage ( Ambrosia ) are so dehydrated
that they can be used as kindling wood, yet they are alive. Drought tolerant plants often
shed leaves during dry periods and enter a deep dormancy analogous to torpor (a drastic
lowering of metabolism) in animals. Dropping leaves reduces the surface area of the
plant and thus reduces transpiration. Some plants that usually retain their leaves through
droughts have resinous or waxy coatings that retard water loss (e.g., creosote bush [ Larrea
tridentata ]).
The roots of desert shrubs and trees are more extensive than those of plants of the same
size in wetter climates. They extend laterally two to three times the diameter of the canopy.
Most also exploit the soil at greater depth than the roots of succulents. The large expanses
of exposed ground between plants in deserts are probably not empty. Dig a hole almost
anywhere except in active sand dunes or the most barren desert pavement and you're
likely to find roots.
Rooting depth controls opportunities for growth cycles. In contrast to the succulents'
shallow-rooted, rapid-response strategy, a substantial rain is required to wet the deeper
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