A desert is a biome that receives no more than 25 centimeters (10
inches) of precipitation per year. Deserts are found in both
temperate and tropical areas. The largest deserts are found at
about 30° north or south latitude due to the dry air masses over
these latitudes. Deserts also occur in rain shadows. A rain shadow
is a dry region on the leeward side of a mountain range (see Lesson
16.1). Examples of rain shadow deserts include Death Valley and the
Mojave Desert, both partly in California. The dry air in deserts
leads to extreme temperature variations from day to night. Without
water vapor in the air, there are no clouds to block sunlight
during the day or hold in heat at night. Desert soil is usually
very poor. They tend to be sandy or rocky and lack organic content.
Because of the low precipitation, minerals are not leached out and
may become too concentrated for plants to tolerate. Plant cover is
very sparse, so most of the soil is exposed and easily eroded by
wind. The occasional rain tends to be brief but heavy, causing
runoff and more erosion. Most desert plants have evolved
adaptations to the extreme dryness. For example: Many plants have
special water-storing tissues in leaves, stems, or roots. Some
plants have very long taproots that can reach down to the water
table. Some plants have wide-spreading roots that can absorb water
over a large area. Plants may have small, spiny leaves that help
reduce water loss. Most desert animals have adaptations to the
extreme heat and bright sunlight. For example: Many small animals
stay underground in burrows during the day and come out only at
night. Most animals that are active in daytime spend as much time
as possible in the shade of rocks or plants. Some animals have very
large ears or other appendages, which help them lose heat to the
environment, keeping them cooler. Many animals are light in color,
which helps them reflect sunlight and stay cooler.
