Biology Reference
In-Depth Information
leather leggings called chaps is derived from chaparral. Among the trees of
the western shrubland are
Quercus turbinella
(scrub oak) and
Cercocarpus
ledifolius
(mountain mahogany). If the crowns of the trees do not touch, the
vegetation is called a woodland, the piñon pine-juniper woodland being an
example (fi g. 2.5). Savanna is a grassland with widely scattered trees such
as
Acacia
. Savanna is not extensively developed as a natural plant formation
in North America north of Mexico, and it is most familiar from African
scenes showing herds of large carnivores and grazing animals where the
scattered trees are often
Acacia
.
The North American dry vegetation is the result of several interacting
factors. The equatorial regions receive the direct rays of the sun for the
longest period of the year and are warmed more than the temperate regions
to the north and south. As the warm air rises it cools, the water condenses
and falls as rain. Thus, many parts of the equatorial regions are character-
ized by warm temperatures, low pressure systems (rising air), high rainfall,
low evapotranspiration, and tropical rain forest. The cooled air descends at
about 30°N and 30°S as a mountainous column of water-depleted air that
warms as it approaches the Earth's surface. Thus, midlatitudes are often
characterized by high pressure systems (descending air), low rainfall, fewer
clouds, high temperatures, strong winds, high evapotranspiration, and des-
ert vegetation. This circulation cell is known as the Hadley regime.
Another factor in the development of deserts is continentality; that is,
the tendency for the interior parts of continents to be drier than the coasts
because cool, moisture-laden winds off the oceans lose water as they move
across the warmer land. A third factor is orographic defl ection, that is,
mountains barring moisture to the lee side. Edaphic conditions represent
a fourth factor, because if the soil is coarse, any rain that falls percolates
through before it can be absorbed by the roots. Last are the interrelated
factors of slope, allowing surface moisture to rapidly drain away, and expo-
sure, because in the Northern Hemisphere south-facing slopes are warmed
more than north-facing ones. These elements combine at different locali-
ties and in various ways to produce dry vegetation ranging from shrubland/
chaparral, to woodland, savanna, and desert.
Study of the history of arid-land vegetation is diffi cult because the en-
vironment is not conducive to the accumulation, transport, and preserva-
tion of organic material. Exceptions are petrifi ed wood (I, 128-32), and
the leaves, wood, fruits, seeds, and adhering pollen brought into protective
rock shelters and used as nesting material by species of the packrat
Neotoma
(fi g. 2.8). Packrats forage within a distance of about 50 m, so over the years
the sequence of accumulating plant debris in the middens refl ects from
