Plant Ecology of the Sonoran Desert Region - Design with the Desert: Conservation and Sustainable Development - page 162

8.4.7 Combined Drought Adaptations
These three basic drought coping strategies are not exclusive categories. Ocotillo behaves
as if it were a CAM-succulent, drought deciduous shrub, but it is neither CAM nor succu-
lent. The genus Portulaca contains species that are succulent annuals. The seeds may wait
for a wet spell to germinate, but the resulting plants can tolerate a moderate drought. The
semisucculent yuccas have some water storage capacity but rely on deep roots to obtain
most of their water. Mesquite trees are often phreatophytes (plants with their roots in the
water table), but mesquite and some other species can also grow as stunted shrubs on drier
sites where ground water is beyond their reach.
8.5 Adaptations to Other Desert Conditions
Water scarcity is the most important but not the only environmental challenge to desert
organisms. The aridity allows the sun to shine unfiltered through the clear atmosphere
continuously from sunrise to sunset. This intense solar radiation produces very high sum-
mer temperatures that are lethal to nonadapted plants. At night much of the accumulated
heat radiates through the same clear atmosphere and the temperature drops dramatically.
Daily fluctuations of 40°F (22°C) are not uncommon when the humidity is very low.
Microphyllary (the trait of having small leaves) is primarily an adaptation to avoid over-
heating; it also reduces water loss. A broader surface has a deeper boundary layer of stag-
nant air at its surface, which impedes convective heat exchange. A leaf up to 10 mm across
can stay below the lethal tissue temperature of about 115°F (46°C) on a calm day with its
stomates closed. A larger leaf requires transpiration through open stomates for evapo-
rative cooling. Since the hottest time of year is also the driest, water is not available for
transpiration. Large-leafed plants in the desert environment would overheat and be killed.
Desert gardeners know that tomatoes will burn in full desert sun even if well watered;
their leaves are just too big to stay cool. Desert plants that do have large leaves produce
them only during the cool or rainy season or else live in shaded microhabitats. There are
a few mysterious exceptions such as Jimson weed ( Datura wrightii ) and desert milkweed
( Asclepias erosa ). Perhaps their large tuberous roots provide enough water for transpiration
even when the soil is dry.
Leaf or stem color, orientation, and self-shading are still more ways to adapt to intense
light and heat. Desert foliage comes in many shades, but rarely in typical leaf-green. More
often leaves are gray-green, blue-green, gray, or even white (Figure 8.7). The light color
is usually due to a dense covering of trichomes (hairlike scales), but is sometimes from a
waxy secretion on the leaf or stem surface. Lighter colors reflect more light (=heat) and thus
remain cooler than dark green leaves. Brittlebush and white bursage ( Ambrosia dumosa )
leaves show no green through their trichomes during the dry season, while desert agave
( A. deserti ) is light gray due to its thick, waxy cuticle. Other plants have leaves or stems
with vertical orientations; two common examples are jojoba ( Simmondsia chinensis ) and
prickly pear cactus ( Opuntia spp.). This orientation results in the photosynthetic surface
facing the sun most directly in morning and late afternoon. Photosynthesis is more effi-
cient during these cooler times of day. Prickly pear pads will burn in summer if their flat
surface faces upward. Some cacti create their own shade with a dense armament of spines;
teddy bear cholla ( Opuntia bigelovii ) is one of the most striking examples.