ECOLOGY

What Is a Desert?

The American Heritage Dictionary defines a desert as “a region rendered barren or partially barren by environmental extremes.” Definitions are never that easy in the natural world; too many factors contribute to an ecosystem to allow such a simple declaration.

How then can we define a desert in more practical terminology? Early scientists described a desert merely as a place having an average of 10 inches or fewer of precipitation per year. This is also too simple; Tucson, Arizona, in the heart of the Sonoran Desert, averages more than 10 inches of rain annually, and some grasslands average fewer than 10 inches of annual precipitation.

A definition of what is a desert does revolve around water, but it is a complicated picture. How much precipitation falls in a year affects available water, which in turn is affected by the amount of yearly sunshine, soil types, desiccating winds, and seasonal precipitation.

One key to the amount of moisture available lies in the unpredictable nature of when, where, and how much precipitation falls. Moab averages 7 to 9 inches of precipitation per year, which usually arrives in winter snows when many plants cannot use the water. More rain falls in summer thunderstorms, when much of it washes away; and some comes as spring showers, when it has maximum absorption and use by plants. If it rained across the Colorado Plateau on a consistent, predictable basis, this landscape would be much greener.

A further clue lies in how temperature and sunshine influence water availability. Deserts are lands of extreme temperatures. The generally clear skies and sparse, widespread vegetation allow an estimated 90 percent of available sunlight to reach the earth’s surface, compared to 40 percent in more temperate zones. At night, the lack of moisture in the air reverses the process, and 90 percent of the heat escapes. These effects combine to create vast temperature fluctuations on both a daily and yearly basis. In the Moab area the yearly temperature can range from minus 10°F to 110°F. On a daily basis, 40° temperature differences are common.

These extremes affect how much moisture will escape back into the air from plant transpiration and soil evaporation. This process of evapotranspiration is so successful in the desert that areas receiving as little as 5 inches of precipitation may lose 120 inches of moisture back into the atmosphere each year. This places plants under extreme stress; thus, even when precipitation falls, plants may not be able to use it.

The inconsistent availability of water is the limiting factor in desert biological processes. Water determines where plants and animals live, when they reproduce, and what they consume. Observe the desert in springtime and watch the abundance of life that flourishes with spring rains and you will discover the importance and beauty of water.

Knowing how to define deserts doesn’t explain why they exist. Warm air absorbs and retains moisture. Cool air releases moisture. This simple phenomenon bears the primary responsibility for desert formation. Whether the air temperature is controlled by air currents, water currents, or mountain ranges, the effects are the same: Moisture is removed from a region, leaving a desert.

Global air circulation patterns produce a region of dryness between 15 and 35 degrees latitude, north and south of the equator. Warm air rises at the equator, cools and releases its moisture, and moves toward the poles. As it approaches the 30 degree parallels, the descending air begins to heat up and increases its ability to retain moisture; like a giant sponge, it sucks the earth dry. This phenomenon is the primary factor in producing the two largest deserts of the world: the Sahara and the Australian.

Global water circulation can also affect air temperatures. Cold ocean currents travel away from the poles alongside continents. As air moves over this water, it is cooled, decreasing its water retention ability. When the air reaches land, it may produce mist or fog but rarely rain. The driest spot on earth, the Atacama Desert on the Pacific coast of Chile, averages 0.5 inch of rain per year. The California current, off the coast of Baja California, produces a similar effect in Baja.

A third agent of change in air masses is the presence of mountain ranges. As warm, moisture-rich air encounters mountains, it rises and cools, leading to a subsequent release of rain or snow. By the time this air has climbed the peaks, it has lost most of its moisture and descends to lower elevations as warm, evaporative air. The Sierra Nevada blocks Pacific Ocean currents and produces the rain shadow responsible for the largest North American desert, the Great Basin Desert.

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Chert

Chert is a catch-all term that describes many varieties of cryptocrystalline (i.e., crystals can only be seen through a microscope) quartz. It is generally dense and smooth with fractures that resemble broken glass. Chert comes in a wide range of colors including white, red, yellow, black, and green. Varieties of cryptocrystalline quartz include jasper, chalcedony (kal-sed’n-e), agate, onyx, and flint. They cannot be distinguished in the field. Some geologists and archaeologists have proposed that flint should only be used in reference to artifacts such as arrowheads and spear points.

Chert forms in two ways. The first is when quartz is carried in solution in water and precipitated in rock cavities. This type is common, but not restricted, to limestone rocks. Chert also comes from the deposition of microscopic organisms such as radiolarians and diatoms that have quartz skeletons. Sedimentary rocks are the most common place to find chert formed by both methods.

Humans have used chert for thousands of years for tools and projectile points. Some of the earliest artifacts in the Southwest are from cultures that inhabited this region more than 10,000 years ago. Bones and other rocks were the primary tools used to make razor-sharp, long-lasting points.

Deserts are complex and beautiful environments. Plants and animals have evolved and adapted to fill particular niches, but their apparent hardiness belies their fragility. Limited water means that it is a resource more valuable than gold, and small changes can translate into large-scale effects.

This information provides broad details about deserts and desert formation, but if we take a closer look at the area around Moab, the picture becomes even more complicated. Canyon country is not all desert; several other ecosystems exist. These include the large-scale La Sal Mountain and Colorado River communities; the smaller scale Mill Creek, Pack Creek, and Negro Bill Canyon riparian communities; and the Scott M. Matheson Wetlands Preserve.

On the smallest scale are the micro-climates: hanging gardens, potholes, and seeps, to name a few. Each one is an important reservoir of increased biodiversity. Several species of shrimp, tadpoles, and beetles thrive in the ephemeral potholes. Douglas fir, aspen, and columbine grow in the protected hanging garden alcoves. These habitats also offer additional food, water, and shelter for other desert organisms.

All of these ecosystems are interrelated. The La Sals contribute additional moisture to the desert by creating summer thunderstorms, through stream runoff, and by recharging groundwater aquifers. The stream canyons provide important travel corridors for large mammals and birds. The recharging of the aquifers provides water for the wetlands, which in turn offer a haven for migrating bird species.

Biological Soil Crusts

Author note: Over the years, these crusts have had many names including cryptobiotic, cryptogamic, and microbiotic. Recently, biological soil crusts, or biocrusts, has become the preferred term, as the crusts are not hidden and they are full of living organisms.

Biocrusts are complex communities composed of cyanobacteria, green algae, lichens, fungi, and mosses that form a living cover on many ground surfaces. They grow in most semiarid and arid ecosystems on this planet, from hot deserts to polar regions. These living crusts are the topsoil of the desert. Although most people think of the crusts as dark and bumpy, they can be reddish or brown and smooth and are nearly invisible when they first develop.

Crusts perform several functions critical to productive desert environments. They stabilize the soil, increase the soil’s water absorption, aid in nutrient availability (particularly nitrogen) for vascular plants, and can enhance seedling establishment. Unfortunately, due to their extreme fragility, most of the crusts’ functions are severely curtailed when they are trampled. This problem is further exacerbated by the 15 to 250 years it takes for small areas to recover from being crushed. Large areas may take longer to recover.

Biological crusts are everywhere; therefore, you will damage them if you venture off the trail. If you must walk or ride off marked trails, hop from rock to rock or stick to slickrock and washes. Be creative and think of it as a game to “tiptoe around the crypto.” If you are in a group, do not spread out; walk in each other’s footprints. This still damages the crusts, so stay on trails.

Biological soil crusts stabilize soil by sending out a mass of filaments that wind their way both under and on the ground surface, leaving behind a sticky, mucilaginous trail that adheres to soil particles. This trail is produced from the sheaths and filaments of the crusts’ major constituent, cyanobacteria. Cyanobacteria, formerly known as blue-green algae, are one of the earth’s oldest known life forms. One species of cyanobacteria, Microcoleus vaginatus, represents up to 95 percent of the biomass of biological crusts in Utah deserts.

The sheaths and filaments wind their way through the soil, grabbing hold of rock and soil particles. These filaments are inactive when dry but become active soon after water reaches them. When moistened, numerous filaments spurt out from the Microcoleus sheath. As the ground surface dries, these living filaments secrete a polysaccharide material (the sheath) that sticks to the soil components and which, even when dry, firmly adheres to whatever it has encountered during the moisture cycle. Soil crusts build up thousands of these sheaths over time, all active in holding soil and rock particles together.

Another important function of sheaths is also water triggered. Sheaths can swell up to ten times their dry size when wet. Thus the sheaths act as sponges, sopping up and storing the desert’s limited rain. In one study at Arches National Park, water infiltration rates (the amount of water that soaks into the soil) were 90 percent less in trampled areas than in untrammeled areas. This results in less water for plants and increased erosion. The bumpy ground surface also restricts water movement, allowing more water to soak into the ground.

Although these sheaths provide excellent tensile strength (the ability to stretch) in holding the soil together, they cannot withstand the compressional strength (downward crushing) of feet, hooves, or tires. Think of the tubes of a bike frame, or of fiberglass sheaths. They have exceptional tensile strength; they can withstand thousands of miles of punishment. But if you put those tubes into a vise, it does not take much to crush the tubes to a useless mass.

Although plants need nitrogen to live, none can obtain it from the air. They need to obtain it from nitrogen-fixing lichen and bacteria, which remove nitrogen from the air and convert it to a usable form. Some plants have nitrogen-fixing nodules on their roots, but most plants on the Colorado Plateau do not. To obtain nitrogen, many plants rely on biological soil crusts.

A microscopic examination of a cross section of biological soil crust reveals numerous small air holes. These holes play a critical role in soil fertility by letting water and gases flow into the soil, providing space for microorganisms to live and facilitating root penetration. Without these holes, less light would reach the subsurface cyanobacteria, reducing photosynthesis within the soil. This in turn would result in fewer nutrients in an already nutrient-limited system. Without these crusts, most native flowers, shrubs, and trees would not thrive in the desert, and this region would quickly lose its plant diversity.

Crusts also function as a plant nursery for the desert. Seeds that fall into a crypto patch gain a roothold because the crusts hold moisture, block desiccating winds, and pull nutrients out of the atmosphere. In the words of one researcher, “[Crusts] are the glue holding this place together.”

Large-scale destruction of crusts may also lead to alteration of local weather patterns. Trampling by feet, hooves, and vehicles creates loose sand, which is then spread by winds, creating dunes. One small-scale example of dune replacement is the Sand Flats area near the Slickrock Bike Trail, where increased visitation has contributed to widespread crust destruction and a subsequent proliferation of dunes. This increases the surface reflectivity (sand is lighter colored than crusts), or albedo. Instead of the ground absorbing sunlight, it now reflects the sunlight back into the atmosphere. Rising columns of warm, dry air push clouds away, which in turn leads to fewer clouds and less localized rainfall.

This alteration of surface cover can also lead to less water infiltration. Water escapes from the ecosystem instead of remaining to benefit plants and animals. Less water soaks into the ground and less water is available for evaporation, which results in less water available for rainfall. Thus the cycle of land degradation builds on itself.

Some people think that they have seen crusts recover in as little as 3 years. Unfortunately, what they have seen is a minimal recovery of the top 2 to 4 millimeters of the crust. One to two species of cyanobacteria may be present, compared with fourteen to fifteen cyanobacteria species and many lichens, algae, and mosses in healthy crusts. And, more important, the underlying Microcoleus sheaths are destroyed and can no longer adhere to soil and rock particles or fix nitrogen.

Remember this when you hike, bike, or drive on biological soil crusts; you don’t kill them, but you severely restrict all processes and functions. The recovery process takes from 50 to 250 years. Once the crusts are disturbed, other people follow the tracks, areas start to erode, gullies begin to form, barren sand dunes take over the region, and a productive ecosystem is destroyed.

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Biological Soil Crusts

1500X YOUNG CRUSTS: 0 to 3 years Cyanobacteria float through air and fall to the ground, crawl across the surface, or are carried by bugs and animals. They remain on the surface to catch sunlight and photosynthesize for their life processes.

1500X MIDLIFE CRUSTS: 3 to 10 years Cyanobacteria secrete sticky sheaths that stick to sand particles. When buried by soil, the cyanobacteria move to the surface, shedding the sheaths below, which in time build up the soil depth. With frost heaving, sheaths form a contorted surface.

300X MATURE CRUSTS: 10+ years Lichens, mosses, fungi grow on surface; water debris and seeds become entrapped in pockets; seeds root, which further strengthens soil.