The Sonoran Desert

2/12/12

Saguaro cactus

     North America has four deserts:  Great Basin, Mohave, Chihuahuan and Sonoran.  I live in the Sonoran Desert that runs from the state of Sonora, Mexico up through southern Arizona to the Phoenix area, and west to sourtheastern California through Baja California.  The Sonoran Desert covers 100,000 (260,00 sq km).

     The Sonoran Desert differs from the three other North American deserts in that it has mild winters.  Frosts are few or rare in some parts of the Sonoran Desert.  My area had six nights of  below freezing weather last winter, but we have had only two nights below freezing so far this year.  Another defining characteristic of the Sonoran Desert is the bi-seasonal rainfall pattern.  The area gets gentle rains from December to March from storms originating in the North Pacific.  From July to mid-September, the summer monsoon brings violent thunderstorms and localized deluges.

Organ Pipe cactus

         There are six subdivisions of the Sonoran Desert based on distinctive vegetation.  I live in the hottest and driest subdivision called the Lower Colorado River Valley area (although the Colorado River is 180 miles from where I live).  Summer highs can exceed 120 F (49 C), and surface temperatures can near 180 F (82 C) from the intense solar radiation from cloudless skies.  My area of the Sonoran Desert gets the least amount of rain, from three to eight inches a year.  Last year we had four inches of rain.  The terrain consists mostly of broad, flat valleys with widely-scattered, small mountain ranges of almost barren rock.  There are many areas of loose sand.

Velvet Mesquite tree

     I have read that this desert supports a surprising 2,000 species of plants.  Two species distinguish the Sonoran Desert from other North American deserts:  legume tress and columnar cactus.  Examples of columnar cactus include the Organ Pipe cactus and the Saguaro cactus shown above.  The Sonoran Desert is the only place in the world where the Saguaro cactus grows in the wild.  Legume trees that grow here include the mesquite and acacia.  Trees grow only along the larger dry washes, even though the wash may carry water for only a few hours or days a year.





Creosote bush





  











White Bursage bush

     The valleys are dominated by low shrubs, primarily Creosote bush and White Bursage.  These are the two most drought-tolerant plants in North America.

Desert Globemallow

Indian Paintbrush

      We do have some spring wildflowers, such as Desert Globemallow and Indian Paintbrush.  Though desert plants must cope with scarce water, it is a misconception that they struggle to survive.  The native species are adapted to and thrive under desert conditions.

What Do We Mean By "Native"?

2/5/12

Palo Verde tree, native to southern Arizona

     I don't know any gardeners that have only native plants in their yards or gardens, although there are gardeners probably striving to grow only natives.  But defining what we mean by native is challenging.

     Many gardeners accept only indigenous plants as native.  Gray's Anatomy of Botany says that to be indigenous there must be scientific evidence that the species has inhabited an area for a great length of time, that it's an integral part of local, evolutionary relationships with other plants and animals.

Saguaro cactus, native to southern Arizona

     But what is a great length of time?  Thousands of years? Hundreds of years?  For example, there were plants that were present in an area of the globe before a glacial period, different plants that were present during a glacial period and different plants after a glacial period.  Can all be considered native?  When European colonists arrived in the Americas hundreds of years ago and introduced species that live on unassisted today, are these species considered native?  If a plant has  become naturalized as a member of the wild flora, needing no human assistance in self-perpetuating, can that plant then be called native?  What of an indigenous plant that migrates to a new area and becomes naturalized without assistance?  Is native a function of time and place?

Lantana, not native to southern Arizona

      Situations like this occur all over the world.  If native status is a function not only of time but also of place, defining place can be a perplexing.  There can be dramatic differences in habitat within a relatively small area.  In my own state of Arizona, within a twenty mile distance, you can go from desert habitat to tundra.  And the plants in the tundra habitat in Arizona are the same as the plants in the tundra of Alaska.

     And consider what we call invasive plants.  They would not exist unless they were better adapted to current conditions than so-called native species.  As Richard Darke says in the book The New American Landscape, " ...most natives are no more than earlier arrivals that established themselves because, at the time, they had a competitive advantage."

gazania, not native to southern Arizona

     Non-native plants fulfill roles that can't be accomplished by natives.  Many food plants grown in North American gardens do not have their origins here.  Many flowers that gardeners grow are not indigenous to their area, but provide beauty, fragrance, color and pleasure, such as the lantana and gazania pictured above that I grow in my yard.. 

     Most gardeners realize that no garden or landscape remains static, that local and global ecologies change.  In the end, I think most gardeners strive for balance among all their plants.