Images © by the Norton-Brown Herbarium, University of Maryland, unless otherwise credited.

BSCI 124 Lecture Notes

Department of Plant Biology, University of Maryland

LECTURE 35 - TERRESTRIAL BIOMES, Part 6 - Deserts
C. Warm Deserts: Sonoran Desert

XIIIc. Sonoran Desert:

A. Location: Coastal mountains in southwestern California (and the Pacific Ocean in Baja California) east to the Chihuahuan Desert of New Mexico (in the north) and the Sierra Madre Occidental of Mexico (in the south). A portion is often defined as the Colorado Desert

Ocean fog on Cerro Tecolote, Baja California, Mexico

B. Features:
- 1. Mountains mostly low (except Sierra Juarez and San Pedro Mártir in Baja California).
  2. Precipitation biseasonal: falls during Dec through Mar and Jul through Sep; mostly as rain, rarely as snow.
  3. Winter temperature average above freezing; summer temperatures above 100° F common.
  4. More than 200 days growing season.
Desert scrub near Rosarito, Baja California

C. Vegetation
- 1. Dominant shrub is creosote bush. Other common shrubs are crucifixion-thorn (Canotia holacantha), jojoba (Simmondsia chinensis - although originally discovered in California, the German botanist who named the plant thought it came from China), white bur-sage (Ambrosia dumosa), brittlebush (Encelia farinosa) and ocotillo (e.g., Fouquieria splendens; F. diguetii - palo adán).
  
  Fan palm near Santa Catavina, Baja California
  
  2. Low trees include palo verde (Parkinsonia microphylla - yellow palo verde; P. florida - blue palo verde; P. aculeata - Mexican palo verde), species of catclaw (e.g., Acacia greggii - long-flower catclaw), mesquite (e.g., Prosopis glandulosa var. torreyana - Torrey's honey mesquite; P. pubescens - American screw-bean), smoke tree (Dalea spinosa), tesota (Olneya tesota), all members of the pea family Fabaceae), elephant tree (Bursera microphylla, image) and several fan palms (Washingtonia filifera; W. robusta [image], Erythea armarta; E. brandegeei).
  3. Arborescent cacti: saguaro (Carnegiea gigantea), Organ Pipe cactus (Lemaireocereus thurberi), senita (Lophocereus schottii), desert night-blooming cereus (Peniocereus greggii), Emory's cereus (Bergerocactus emoryi) and cardón (Pachycereus pringlei).
  
  Boojum near Parador Punta Prieta, Baja California
  
  4. Most bizarre plant is the boojum tree (Fouquieria columnaris), a columnar trunk with leaves and a few branches at the very top some 50-65 ft (15-20 m) above the ground.
  - - But oh, beamish nephew, beware of the day,
      If your Snark be a Boojum! For then
      You will softly and suddenly vanish away,
      And never be met with again!
      - Lewis Carroll, The Hunting of the Snark,
        
        an Agony in Eight Fits
  5. Rich diversity of annuals and perennial herbs, especially succulent ones; numerous endemic species.
D. Animals:
- 1. Mammals: Deer, mountain lion, bobcat, mountain sheep, fox, coyote, and historically antelope. Feral goats and pigs are locally common and often destructive. Smaller mammals include the jackrabbit, badger and the ring-tail cat, a relative to the raccoon. Marine mammals include several species of sea lion and seals.
  2. Birds: Numerous shore and water fowl, hawks, resident song birds, large ravens, roadrunners, and numerous species of hummingbirds.
Saguaro on creosote bush on a recent lava flow, Arizona

E. History:
- 1. Desert established over last 30,000 years but in present position only about 4000 years.
  2. Pack rat (illustration) middens are a good source of historical record of desert vegetation over last 20,000 years.
  3. More grasses in the past than present.
F. Exploitation:
- 1. Overgrazing is significant problem causing loss of many species.
  2. Over collection of cacti, especially rare species, for commercial purposes.
  3. Water availability remains a significant problem.
  4. ORV activities.