Deserts & Desert Plants p. 478-482

Definition: Deserts have less than 10" of rain per year and low humidity.

Where are the deserts of the world located and why?

    1. Global air patterns

                The equator is hot and humid.  This hot air rises and then cools, releasing all its water vapor as rain.  The now dry air spreads out horizontally to the north and south and finally sinks at 30 degrees north and south latitude.  This where most of the worlds deserts lie. Click on the link below to see the major deserts of the world.

http://www.enchantedlearning.com/biomes/label/desert/labelanswers.shtml

    2.  Rain shadow effect

                This is similar to global air patterns but is due to mountain ranges.  When the prevailing winds hit the mountain range, the air rises and cools dumping all its moisture on one side of the mountain.  The other side becomes a desert.

Location: There are 4 deserts in North America. They are located between the Rocky Mountains on the east and the Sierra Nevadas on the west.

Here is a link to a physical map of North America.  Which direction do the geographic features run?  (North-South or East-West)?

Here is a link to a site about desert biomes. http://mbgnet.mobot.org/sets/desert/ofworld.htm

Be sure to get a map so you know where these deserts are located.

Phoenix averages about 7" of rainfall per year and 80+ days of over 100 degrees temperature.

With small amounts of year-to-year rainfall, there is greater year-to-year variation.

People feel cooler at low humidity because perspiration evaporates rapidly and keeps them cooler.

Of the 4 deserts of N. America, the Sonoran desert has the most abundant plant and animal life.  The Sonoran desert differs from the other 3 deserts of North America in two significant ways:

    1) Mild winter temperatures.  Besides dryness, hard winter freezes limit plant life in the other three deserts.  Although the Chihuahuan desert is south of the Sonoran, it is at higher elevations and freezing temperatures in winter are common.

    2) Rain is split between summer and winter.  Unlike the 3 other North American deserts, the Sonoran desert has two rainy seasons.  This allows more plants to survive.  It also allows a greater variety of plant life.  For example, spring annuals respond to the winter rains.

More strategies:

Succulence: Some plants such as cactus and yucca store water in their stems.

Barrel cactus

Small leaf size: This reduces evaporation

Mesquite tree

Photosynthetic stems: Some plants have no leaves at all

Euphorbia antisyphilitica

Waxy cuticles: Wax prevents water loss

Hair: Shades leaf surface

Brittlebush (Encelia farinosa) has leaves covered with hair which reflects light and gives them a grey color.

Spines: Shades plant surface.

Opuntia violacea has a beautiful, purple tone.  Note how much closer the spacing of the spines is on the edge vs. the flat surface. This provides extra shade to the part being struck by the sun.

Shallow roots: Many desert plants have shallow, spreading root systems allowing them to absorb water from small, brief rains.

Palo verde tree: Green bark, the state tree of Arizona

Ironwood trees: One of the heaviest native woods

Mormon tea: Genus Ephedra. Although the stimulant, ephedra, has been banned by the FDA as a dangerous stimulant, many of the desert species have little or none of this alkaloid. Ephedra is a gymnosperm.  These male flowers emerge directly from the stem.

Desert milkweed: Leafless. Some medicinal uses of glycosides.

Buckthorn cholla: joints break and form new plants

Mesquite tree: abundant uses by native Americans

Velvet mesquite

Teddy bear cholla: aereoles are modified axillary buds in cactus that produce hairs or spine. In the teddy bear, it produces lots of hair so it represents a fuzzy teddy bear.

Saguaro: state flower of Arizona. A plant that grows only in the Sonoran desert.

Agave: Also called Century plants, they bloom once and die.

Yucca: in the agave family. Fibers have many uses by native Americans like baskets and mats.

Ocotillo: Cuttings are sometimes made into a living fence by native Americans

Here are two Sonoran desert plants native to the Baja of California

Baja fairy duster: Hummingbirds love its bright, red flowers.

Boojum tree: In the same family as Ocotillo, its common name is based on a tree in a poem by Lewis Carroll (Alice in Wonderland author), "The Hunting of  the Snark".

Convergent evolution: Agaves and aloe are in different plant families. Yet, they have similar features such as succulent leaves with sharp edges. Similar adaptations allowed them to succeed in deserts a continent apart. When structures look alike and have similar function yet have different origins, this is called convergent evolution.

Another example of convergent evolution: the wings of a bird and bat appear similar. Yet, a bat is a mammal and a bird an egg layer with feathers. The wings perform the same function, yet have very different origins.

Pueblo Grande Museum:  How did the Hohokam people live in the Phoenix area for thousands of years?

Article Oct. 2009: Hohokam ax found at MCC

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