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 , 53-68. American Chemical Society (ACS) Symposium Series 330.
- One interesting consequence of this is an almost uniform spacing of adult plants, particularly shrubs.
- flying over many desert landscapes, the shrubs seem arranged almost like an orchard
- statistically, they may still reflect a random pattern of their original establishment but it is often biased in the direction of uniformity
- where water is locally abundant, though, as in a swale, you may see bunching or clustering of plants
- lucky you: there is a statistical technique for judging the distribution of points-in-space, such as desert plants, as uniform, random, or clustered. It's called nearest-neighbor analysis, and it is the subject of a lab!
Plants have a variety of ways of coping with moisture stress
- we've already considered exclusion of competitors in their root zone above
- another adaptation focusses on conserving water taken up by roots whenever it's available:
- as with chaparral and CSS, desert plants show some similar xerophytic adaptations: small leaves, sclerophyllous leaves, wool or hair to shade the plants and reflect sunlight
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succulence is commonly seen in desert plants, most famously in the case of cacti
- the problem with that approach is it makes you really a tempting target for any moisture-stressed animal out there
- so, cacti and many succulents are armed to discourage water plundering
- spines, or modified leaves or stipules, decorate cactus platyclades
- thorns, or modified twigs or branches, are seen on many desert plants (such as acacias)
- prickles, or sharp emergences from leaf or stem epidermis, as seen in roses and the teeth on oak leaves
- while common in many vegetation systems, this sort of aggression is highly developed in many desert plants
- another adaptation is taking up an extremely short but prolific life: ephemerals
.
- a particularly rainy winter or spring will wash away a lot of allelopathic chemicals
- a lot of soil moisture, of course, means a lot of temporary photosynthesis is possible
- so all kinds of annual forbs emerge, grow just tall enough to support flowering structures, and then invest almost all their photosynthetic production into budding, flowering, pollination, and setting seed
- as the short time of abundant soil moisture comes to an end, they die, but they die having pumped out a staggering number of seeds that can often remain viable for many years, until the next "perfect storm" of circumstances creates another "superbloom"
- still another adaptation is evolution of alternative photosynthetic systems
- C3 photosynthesis: regular photosynthesis that uses trapped electrons in sunlight to break down and re-arrange CO2 and H2O into sugar (e.g., C6H12O6 and O2 -- called that because it involves carbon dioxide interacting with rubisco to create two 3-carbon molecules. This is the most common photosynthesis (maybe 85% of land plants) and the most efficient. Unfortunately, it goes a little haywire in hot, dry conditions (as in deserts) and wastes some of the food it creates through photorespiration, where rubisco starts "fixing" oxygen instead of carbon dioxide like it's supposed to.
- C4 is an alternative photosynthesis that fixes carbon dioxide in daylight into a 4-carbon compound that is walled off from rubisco. To get to the inner cells that contain rubisco, the plant has to invest some ATP (energy currency) to push the 4-carbon compounds into an area free of oxygen and split the 4-carbon compound back into carbon dioxide that can then interact with rubisco away from oxygen to undergo regular photosynthesis with no chance of photorespiration. The process is a little wasteful itself, though, because it involves spending ATP to pump 4-C compounds into proximity to rubisco. If water and sun are abundant, C3 is the more efficient system; if it's hot and dry, there's a lot of waste through photorespireation, so C4 becomes the more efficient by not wasting rubisco. C4 photosynthesis is a spatially separated form of photosynthesis.
- CAM or crasulacean acid metabolism is another odd photosynthesis found in extremely hot and dry environments. This system creates a weird night-day activity pattern. At night, when the air is cooler and relative humidity is higher, the plants' stomata open up wide to take in carbon dioxide and turn it into malic acid. This drastically acidifies the plants' tissues (down to pH 4). In the daytime, the stomata shut down, so no water is lost from the plant, and then the malic acid is converted back into carbon dioxide and it then undergoes normal photosynthesis, the pH of the plant tissues rising as it does so. Such plants use way less water than C3 or C4 plants. CAM is a temporally separated form of photosynthesis.
- Deserts have higher proportions of CAM and C4 plants, compared with C3, than other vegetation systems, as these odd styles of photosynthesis have greater benefits than costs in harsh, dry, and hot conditions.
Desert vegetation, quite diverse in life forms present
- visually dominated by the shrub life form (creosote [Larrea divaricata], sagebrush [Artemisia tridentata], rabbit brush [Ericameria nauseosa])
- other life forms present and can be locally conspicuous
- trees (piñon [Pinus monophylla], juniper [Juniperus californica], Joshua tree [Yucca brevifolia], California fan palm [Washingtonia filifera]), especially on bajadas and swales and oases, less on desert floors
- cacti (chollas [such as Cylindropuntia bigelovii a so-called "jumping cholla, opuntias)
- forbs (desert ephemerals, including California poppies [Eschscholzia californica], lupines[Lupinus spp.], paint brush [Castilleja affinis], []coreopsis)
- grasses (desert needle grass [Stipa speciosa], Indian rice grass [Stipa hymenoides], and, now, a lot of exotic annuals, such as Bromus tectorum or cheatgrass)
- lichens, those odd symbioses of fungi with algae or cyanobacteria
- biological soil crusts, a fragile veneer of living organisms on soil, comprising lichens, mosses, microfungi, bacteria, and green algae.
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this veneer proects soil from erosion and any disturbance, like a bike track or footprint, can allow surface water and wind to remove soils -- the damage will be visible for years
- very importantly, these crusts hold up to ten times their volume in water and release it slowly into the soil for use by other plants
- some of their constituents, notably cyanobacteria, secrete chemicals that stimulate plant growth
- quite a bit of bare ground is visible between the larger plants -- rocks and gravelly alluvium -- but this may be occupied by a desert wildflower outgrowth for a short while in certain late springs
- in some areas, large dune fields can build up (ergs) and these are so unstable and constantly shifting that vegetation is quite limited to dune specialists
- in still others, a constant wind can severely limit vegetation and pull out all the fines in the soil, leaving behind only gravels and rocks, which are often jostled and vibrated into a tightly fitted surface called, evocatively enough, "desert pavement" (not to be confused with I-15!)
- despite desert pavement and dune fields, which may cover nearly 10% of deserts, most people not familiar with the desert are blown away by just how much vegetation is out there!
California high deserts
California low deserts
