North American Deserts

Approximately 17% of the Earth’s terrestrial area is classified as arid, and one-tenth of that area can be attributed to the deserts of North America.

1. The main determinate of arid climates in western North America is the presence of local mountain ranges creating rain shadows in the Great Basin and Mojave Deserts (the Sierra Nevada-Cascades- and Rockies), the Sonoran Desert (the Peninsular Ranges and Sierra Madre Occidental), and the Chihuahuan Desert (The Sierra Madre Oriental and Sierra Madre Occidental).

a. In contrast, most of the world’s arid deserts are created by predictable descending high pressure systems at subtropical latitudes (Hadley cells). However, both the Chihuahuan and Sonoran deserts are influenced by the high pressure cells.

2. The 4 primary deserts of North America are the cold desert shrub and shrub steppe (Great Basin), and the warm deserts of the Mojave, the Sonoran, and the Chihuahuan.

3. As one moves west to east (Mojave to Chihuahuan, the ratio of winter to summer rainfall decreases.

a. The Mojave Desert and the Great Basin Desert receive primarily winter rainfall (spring growing season)

b. A majority of the Sonoran Desert has a bimodal rainfall regime (spring and summer growing seasons)

c. Chihuahuan Desert receives summer rainfall.

d. Although other factors are certainly involved, these differences in seasonal rainfall are sufficient to effect differences in vegetation structure and floristic composition.

4. No. American deserts range in elevation from -282’ to over 5,000’ (1,000’ elevation = about 300 miles in latitude towards the poles).

5. Great Basin

a. Large (409,000 km²), cold, temperate, semi-desert steppe region that extends from eastern Oregon and southern Idaho in the north (ca. 44 degrees) to central Nevada in the south (37 degrees), and from the Wasatch mountains of Utah in the east (112 degrees) to the Sierra Nevada/Cascades in the west (120 degrees).

b. Large internal drainage basin (a series of many parallel internal drainage basins).

i. Moisture received by mountains within this giant intermountain basin flows in washes and larger stream beds toward the center of each adjacent basin.

ii. Since the basins are without outlet, the water forms playas, becoming an ephemeral, shallow lake.

iii. The water evaporates, leaving its mineral burden in the dry playa, or to accumulate in the remaining lake waters (like the Great Salt Lake).

c. It was once a land of lakes, and 2 huge lakes formed in this region were Lakes Bonneville and Lahontan.

i. Lake Bonneville covered much of northwestern Utah (20,000 sq miles expanse).

ii. In western Great Basin, Lake Lahontan covered about 8,5000 sq. miles.

iii. About 12-15,000 years ago, the glaciers having retreated, the climate of the Great Basin became arid, and the lake waters began their lengthy evaporative shrinking.

iv. Today, only the small remnants of these lakes remain: the Great Salt Lake, the remnant of Lake Bonneville, is about 2,400 sq miles in extent. Highly mineralized: salt content is 8-10 x greater than that of the ocean. Outranked in salt content only by the Dead Sea, itself a desert-locked body of water. Only algae and brine shrimp can survive its salinity.

v. Pyramid Lake, in western Nevada, is approx. 200 sq miles is size.

d. Aridity is principally due to rainshadow effect of the Sierra/Cascade mountains to the west.

e. The elevation of the Great Basin is about 4,000’ and above.

f. Considerable amount of snow and hard frost in winter; prolonged freezing. Summers may only last 2-3 months.

g. Winter precipitation (60% of its precipitation is in winter) is derived from storms sweeping in off the Pacific Ocean.

6. Mojave Desert

a. Lies between the Great Basin to the north and the Sonoran Desert to the south.

b. Smallest of the 4 North American deserts (140,000 km2).

c. Extends northward to central Nevada (37 d), southward to Joshua Tree National Park (34 d), westward to Antelope Valley of southern CA (118 d), and eastward to the Colorado Plateau in AZ and UT (113 d).

d. Elevational range from below sea level (-86 m) in Death Valley to about 2,000 m at the northern transition with the Great Basin (between 2-4000’).

e. Basin-range topography (like the Great Basin), with numerous north-south-trending mountain ranges.

f. Like the Great Basin, the Mojave was once, at the end of the last ice age, a wetter, lake-dotted area, with major rivers draining into a series of lakes. Even today, populations of small pupfish remain in isolated small bodies of water in the Mojave as remnants of the populations that once resided in the plentiful waters (Mojave and Armagosa rivers).

g. Aridity is principally due to rainshadow effect of the Sierra mountains.

h. Rainfall is principally in the winter with winter storms coming from the Pacific Ocean. They are widespread, may last for several days, and are relatively mild in terms of amount of precipitation delivered, usually less than 20 mm in a 24-hour period.

1. Summer rainfall is from July to September. Precipitation occurs as strong, localized thunderstorms that enter the desert from the south and east. Moisture comes from the Gulf of Mexico and the Gulf of California, and is drawn into the desert by strong convectional currents. These storms can be very intense.

2. Much of these stormy precipitation events occur as run-off along the desert floor.

3. Native Americans have named winter and summer rains as male and female rains, respectively.

4. Precipitation decreases in the Mojave from west (5”/yr) to east (2”/yr).

5. Rainfall is very erratic.

7. Sonoran Desert

a. Intermediate-sized (275,000 km2), subtropical desert.

b. Centered in the state of Sonora in Mexico. Extends northward to central AZ (34N), southward to the southern tip of the Baja peninsula (23N), westward to Deep Canyon, CA (117W), and eastward to the foothills of the Sierra Madre Occidental in Sonora (109W).

c. Aridity is principally due to the presence of a subtropical high pressure cell (Hadley cells).

d. This is the hottest of our North American deserts, but a distinctly bimodal rainfall pattern produces a high biological

diversity. Not uncommon to have over 90 consecutive days of

temperatures above 100 F.

e. Represented within the Sonoran are more plant life forms, more

types of cacti, and more species of plants and animals than in

any of the other 3 North American deserts.

f. Freezing conditions can be expected for a few nights in winter.

8. Chihuahuan Desert

a. Largest of the North American warm deserts (453,000 km2), and exhibits the farthest range eastward. Extends northward to central New Mexico (34N), southward to San Luis Potosi (22N0, westward into eastern AZ (109W), and eastward into western Tamaulipas State in Mexico (99W). Three quarters of the Chihuahuan Desert is in Mexico.

b. Lies between the Sierra Madre Oriental and the Sierra Madre Occidental, most of it above 3,500’ elevation.

c. Aridity is principally due to the presence of a subtropical high pressure cell (Hadley cells).

d. Summer precipitation of 3-20”. One growing season: summer. (Due to summer rains and cold winter temperatures).

e. Summer temperatures are 10-20 F cooler than the adjacent Sonoran.

Abiotic Features

1. In arid regions, average annual precipitation ranges from near zero to 150 mm per year, and potential evapotranspiration (PET) ranges from 2000 to 4000 mm per year.

2. In regions with arid climates most of the water gained from precipitation is quickly lost thru evaporation.

3. An important difference between warm and cool deserts is that deep drainage rarely occurs on vegetated sites in the warm deserts, whereas it is common in the cold deserts. This helps explain the finding that deeper-rooted desert perennials may not utilize summer rains because they do not result in long-term soil moisture storage.

4. As one descends in elevation in desert regions, rainfall not only decreases, but variability in rainfall increases. In extremely wet seasons, significant amounts of rainfall can be lost via run-off in the arid deserts, resulting in a lower percentage of the total moisture stored in the soil. Dry seasons tend to be longer, and true climatic droughts (failure of predictable seasonal rains) become more prevalent as the climate becomes more arid.

5. The 4 deserts of North America can be demarcated by climate.

a. Great Basin is a distinctly cold desert, with monthly mean temperatures below zero from Dec thru Feb, and a mean annual temp of 9 C (48 F). This contrasts with a mean annual temperature of 20 C (68 F) for the 3 warm deserts.

b. Annual frost free season ranges from 80-150 days in Great Basin; 210-365 in Mojave and Sonoran Deserts, respectively. Cool temperatures during growing season are the second most important climatic factor (after water availability) influencing plant distribution and activity patterns in the cold deserts. Chihuahuan desert has a shorter frost free season than either Sonoran or Mojave, and mean temperatures that are intermediate between cold and other warm deserts. The Chihuahuan –Sonoran boundary corresponds to an increase in elevation and a corresponding steep gradient in temperature towards the Chihuahuan Desert, which limits the extension of subtropical taxa into that desert.

c. Northwestern deserts (Mojave and Great Basin) are characterized by winter-spring precipitation, and the southern deserts of Mexico are characterized by summer precipitation. The northern Sonoran receives predictable rainfall in both summer and winter.

d. Mojave Desert is an area of temperature extremes with mean minimum January temperatures ranging from -2.4 C (28 F) at Beatty, NV to 47 C (117 F) for a mean July maximum at Death Valley.

e. Diurnal fluctuations are greater in summer than in winter.

f. Cold air inversions – because colder air is heavier than warm air, it flows downslope during the night and settles in the bottoms of the basins with a layer of warm air on top. The basin floor may be 4-6 C colder than the bajada. Inversions are more important below 1,000m and are more intense during the winter.

g. Relative humidity is very low, and it is not unusual for humidities to be below 10%. During the day it is highest in the early morning, lowest in late afternoon, and increases again in the evening.

h. Winds – the winds of spring and winter are generally stronger than those of summer.

i. Evapotranspiration – Combination of high temperatures, low humidity, and wind produces high evaporation rates from moist surfaces and affects the rate of transpiration in plants.

Soils

1. High percentages of sand and coarse fragments,

2. Low organic matter,

3. Minimal soil horizon differentiation,

4. Concentrations of carbonates,

5. Sand and coarse fragments are found on bajadas (alluvium fans), plains and river washes.

6. Soils with high percentages of silt and clay are found on playas.

7. Soils with well-developed desert pavement covered with desert varnish may be found on alluvial fans.

8. Desert soils with organic matter concentrations are found mostly in localized areas under shrub canopies or at higher elevations with greater amounts of precipitation and vegetation.

9. Soil under shrub canopies may also have higher concentration of essential nutrients (N) and moisture than intershrub soil (fertile islands).

Geomorphology and Soils

1. Bajadas, desert pavement, arroyos (coarse-textured, incised channels that carry flash floods down the slopes and bajadas during heavy rainfall).

2. Dune fields are areas where Aeolian materials accumulate over extended periods of time.

3. Dry lakes and playas occur at bottoms of internal drainage basins.

a. They accumulate run-on water in the wet season, leaving behind the fine-textured sediments and salts washed down from the surrounding bajadas.

b. Soil aeration is also low here, and it is believed that many species are unable to colonize playas because of low soil oxygen rather than salt salinity.

4. Desert soils are immature with weakly developed profiles;

a. dry for most of the year;

b. contain little organic material and are poor in N and P;

c. are saline, with the exception of sands;

d. rich in inorganic ions such as carbonates and gypsum, which form lime and gypsum hardpans or crusts;

e. are alkaline.

5. Slope to playa (coarse-grained soils to progressively finer-grained material), texture is important because coarse-grained soils have higher water infiltration rates, but lower water holding capacity.

6. Propensity for shallow-rooted perennials dominate loamy bajadas (less air spaces), whereas deeper-rooted trees and shrubs dominate coarse-textured washes and upper bajadas.

7. In arid climates, perennial vegetation reaches its highest cover and biomass on coarse-textured sands and gravels.

8. In semiarid climates, fine-textured loams exhibit the highest plant cover and biomass due to greater soil moisture storage capacity after heavy rains.

9. Nest to moisture availability, salinity may be the most important factor that affects the distribution of plants in desert regions. Accumulation of salts in fine-textured basin soils causes surface crusts after rainfall and subsequent evaporation events.

a. Can form salt hardpans in some areas.

b. High salt content causes water and ionic stress in glycophytic plants

c. Salt inhibits nitrification while increasing ammonia volatilization.

10. Desert soils tend to be deficient in micro- and macro-nutrients. Due, in part, to slow decomposition rates, low soil moisture content, and soil alkalinity.

11. Soil nutrient content varies by topographic position:

a. bajadas and arroyos tend to have the lowest N concentrations,

b. dunes the lowest P concentrations,

c. playas have the highest overall nutrients.

Spatial patterns

1. When viewed from a low, oblique angle, plant cover seems high, but when the canopies are projected vertically down, only 10-25% of the ground typically lies beneath perennial cover.

2. Above ground biomass is generally low, about what it is in the tundra (700 g/m2), leaf area is very low. For much of the year plants are either absent, leafless, dormant, or functioning at a low level – over the course of a year the fraction of solar radiation trapped in photosynthesis is very low. Net productivity, then, as a function of area is generally low.