Biology Reference
In-Depth Information
root systems, succulence (abundant water-holding parenchyma cells), and
alleopathy. The latter is an adaptation reducing competition for the limited
water whereby roots or debris from the parent plant contain compounds
toxic to the growth of other plants, and this contributes to the spacing often
evident between desert plants (fi g. 2.6; also fi g. 2.22 below). The North
American deserts receive slightly more than 250 mm of rainfall (300-
355 mm, 10-14 in) annually and are moist deserts transitional to dry shrub-
land or woodland. The lowest point of land in North America is Death Valley
in the Mohave Desert at 86 m below sea level, and the highest temperature
for the continent was recorded there on 10 July 1913 (66°C, 134°F). The
vegetation includes
Larrea tridentata
(creosote bush, fi g. 2.6),
Franseria
dumosa
(bur sage),
Yucca brevifolia
(Joshua tree;
Y. carnerosana
, fi g. 2.22),
with creosote bush occupying the driest parts of the desert. A common
distribution pattern is for
Acacia
,
Prosopis julifl ora
(mesquite), and other
shrubs to grow near the base of hills where there is some moisture from
runoff, and creosote bush to grow farther out on the driest fl atlands.
Larrea
extends into the deserts of northern Mexico, then reappears as a disjunct,
along with numerous other plants of the northern deserts (e.g.,
Ephedra
,
Mormon tea;
Juniperus
) in the dry monte vegetation of northwestern Ar-
gentina 8000 km away.
The Sonoran Desert of Arizona and adjacent northwestern Mexico is
the most varied and the lowest in elevation of the American deserts (sea
level to about 900 m; avg. 600 m). Rainfall is from near 0 mm at places in
the west to 35 mm in the east, and it may reach 700 mm on some wind-
ward peaks. Temperatures range from winter lows of 20°C to a summer
maximum of 40°C, and on bare soils the daily temperature may fl uctuate
by 60°C. One of the signature plants is
Carnegiea gigantea
(saguaro; fi g. 2.6)
showing many of the adaptations to arid environments noted earlier. Large
columnar cacti are a feature of the Sonoran Desert.
The Chihuahuan Desert of southern New Mexico and west Texas is a
plain between 400 m elevation along the Río Grande to 1500 m toward the
south on the central Mexican Plateau. Precipitation is higher than in the
Sonoran Desert (to 400 mm), temperatures are 5°C-10°C lower, and there
is occasional winter frost. In addition to the widespread
Acacia
,
Larrea
, and
Prosopis
, there is
Agave
(Spanish bayonet, century plant), the source of hen-
equen and tequila, and
Yucca
(Joshua tree).
Between the lower limits of the western coniferous forest and the des-
ert, there is a transition zone of shrubland and woodland. Shrubland is a
plant formation of low, densely spaced trees, and if there is a conspicu-
ous component with spines, it is chaparral. The name for the protective
