Deserts

Deserts

Desert definition: an arid landscape in the tropics and mid-latitudes

• high temperatures seasonally or all year
• little precipitation
• much greater potential evapo-transpiration than precipitation

Arid climates can be subdivided into four basic types (with Köppen classes included):

• tropical arid with average temperatures (day and night, all 12 months) hotter than 18° C (~64° F), such as the Sahara (BWh)
• mid-latitude arid with average temperatures less than 18° C (BWk), such as the Gobi Desert
• tropical semi-arid with average temperatures hotter than 18° C but with more precipitation (BSh), such as the Sahel in Africa just south of the Sahara
• mid-latitude semi-arid with average temperatures less than 18° C and more precipitation (BSk), such as the Great Plains
• California boasts all four, as you can see in this Köppen climate map of California
• focussing on the deserts of California in the context of deserts in the American West, we contain three of the four or five types recognized in North America:
  • most of the Mojave
  • a part of the Basin-and-Range
  • the northern part of the Sonoran
  • none of the Chihuahuan
  • none of the Colorado Plateau (though sometimes a part of the Sonoran desert running along the Colorado River is called the Colorado Desert)
  • here is a rough map
• focussing on the deserts, in Southern California, we often differentiate them as:
  • high desert, meaning mid-latitude desert, which is generally at higher elevations, such as the Mojave Desert and the Basin-and-Range Desert
  • low desert, meaning tropical desert, which is at lower elevations, often called the Sonoran Desert or, sometimes, the Colorado Desert (not to be confused with the Colorado Plateau Desert in the Four Corners region)

California deserts and steppes

• tropical deserts include Imperial Valley, Palm Springs, the Colorado River borderlands, Death Valley and Panamint Valley graben, southern San Joaquin Valley
• mid-latitude deserts include the Mojave Desert, the Basin-and-Range Deserts north of Death Valley, and the southwestern San Joaquin Valley
• tropical semi-arid climate can be found in the Inland Empire, the Los Angeles Basin inland from the coast in L.A. County and the OC, and in smaller areas bordering tropical desert, as in the southeastern San Joaquin
• mid-latitude semi-arid climate dominates most of the San Joaquin, much of the Southern California coastal strip from about LAX down to Baja, and in smaller patches bordering the true deserts

Deserts (and even semi-arid areas) are very challenging for plants

• low precipitation means high moisture stress on plants
• couple that with the much higher potential evapo-transpiration and the struggle for enough water to engage in photosynthesis and respire can be pretty terrific
• as a result, there is high competition for water among plants
  • any seedlings establishing themselves in their root zones are a threat to their survival
  • plants are active players in the landscape, and they have evolved all kinds of nasty techniques to minimize this threat
    • sometimes, this can entail the development of dense, shallow root systems
      • this puts roots right on the spot to take up water nearly instantly in the rare event of a brief rainstorm
      • the sheer density of roots can preclude seedling establishment, as young plants of other or the same species, with their tiny root systems, cannot outcompete the dense network of roots around them
    • one of these is allelopathy, or chemical warfare among plants, or, more technically, harmful effects of one plant on others through chemicals they release into the environment around them. Some such chemicals include a variety of:
      • terpenes, hydrocarbon volaltile oils, which produce strong scents, such as terpentine, garlic, orange oil
      • phenolic compounds, also often aromatic, such as salicylic acid (aspirin and wintergreen), juglone (walnuts), tannins (oaks)
      • alkaloids
    • Desert plants, especially those in the Asteraceae (composites) and Lamiaceae (mints and sages), pump these out in several ways
      • fresh or dried leaves and stems may take the chemicals to the ground during rare rain events or just by accumulating under the plants as these parts age and fall off to form a litter
      • sometimes the chemicals will be secreted by roots
      • production of these chemicals tends to go up as the plants are stressed by inadequate water or by lack of access to some other essential (e.g., trace nutrient) or by disturbance by grazing or browsing animals
      • desert conditions can cause faint releases of allelopathic chemicals to build up over time, such as low rates of leaching into the subsoil because of the lack of rain or suppression of detritivory by microörganisms
      • as a result, concentrations of the chemicals can vary a lot in space and time, producing obvious suppression in one area and hardly any, even in the same plant communities
      • a classic article on this is Friedman, Jacob. 1987. Allelopathy in desert ecosystems. Ch. 6 of