Plant Ecology of the Sonoran Desert Region - Design with the Desert: Conservation and Sustainable Development - page 153

synthesized from this first one. About 96% of all plant species use C3, but there are two
specialized variations.
One variant is called C4 photosynthesis because the first product is a four-carbon
sugar. C4 plants actively transport carbon dioxide to localized bundles of photosynthetic
tissue. This process offers improved efficiencies under hot, sunny conditions. C4 plants
use carbon dioxide more efficiently (by bypassing photorespiration) and lose less water
through transpiration (water evaporated from inside plants) per unit of carbohydrate made.
The overall result is that C4 plants can grow much faster under high temperatures than
most C3 plants. The majority of summer-growing grasses in warm climates are C4. So are
many of the summer-growing plants, especially weeds (invasive pioneer plants) that seem
to spring up overnight such as pigweed ( Amaranthus spp.), summer spurges ( Euphorbia
hyssopifolia and others), devil's claw ( Proboscidea spp.), and many saltbushes ( Atriplex spp.).
Only 0.4% of all the Earth's plant species are C4, but a number of them are vital crops,
such as corn ( Zea mays ), sorghum ( Sorghum spp.), sugar beets ( Beta vulgaris ), and sugar cane
( Saccharum officinarum). ). Another variant of photosynthesis, crassulacean acid metabolism
(CAM), is discussed in Section 8.4.1.
8.4 Coping with Desert Climate
The impression that the desert environment is hostile is strictly an outsider's viewpoint.
Adaptation enables indigenous organisms not merely to survive here, but to thrive most
of the time. Furthermore, specialized adaptations often result in a requirement for the
seasonal drought and heat. For example, the saguaro ( Carnegiea gigantea ), well adapted
to its subtropical desert habitat, cannot survive in a rain forest or any other biome, not
even a cold desert. In these other places it would drown, freeze, or be shaded out by faster
growing plants.
Aridity is the major—and almost the only—environmental factor that creates a desert,
and it is this functional water deficit that serves as the primary limitation to which desert
organisms must adapt. Desert plants survive the long rainless periods with three main
adaptive strategies: succulence, drought tolerance, and drought evasion. Each of these is a
different but effective suite of adaptations for prospering under conditions that would kill
plants from other regions.
8.4.1 Succulence
As a group, succulents are the most picturesque desert plants (Figure 8.2). They capture
our attention because they look nothing like the familiar plants of the temperate zone
where most people live. Their vernacular names suggest how they command our attention:
elephant tree ( Bursera microphylla ), boojum ( Idria columnaris ), jumping cholla ( Opuntia
fulgida ), creeping devil ( Stenocereus eruca ), and shindagger ( Agave schottii ). Spanish names
translate into such as dragon's blood, child-killer, and old man's head. Even some scientific
names are inspired by the plants' characteristics: Ferocactus (as in ferocious), Opuntia molesta
(the molesting-spined cactus), Opuntia invicta (the inflicting one), and Agave jaiboli (as in
highball, because liquor is made from it) (Figure 8.3).
Succulent plants store water in fleshy leaves, stems, or roots in compounds or cells where
it is not easily lost. All cacti are succulents, as are such non-cactus desert dwellers as agaves,