Joshua Tree National Park - Geology

The geologic landscape of Joshua Tree has long fascinated visitors to this desert. Geologists believe the face of our modern landscape was born more than 100 million years ago. Molten liquid, heated by the continuous movement of Earth’s crust, oozed upward and cooled while still below the surface. These plutonic intrusions are a granitic rock called monzogranite.

The monzogranite developed a system of rectangular joints. One set, oriented roughly horizontally, resulted from the removal—by erosion—of the miles of overlying rock, called gniess (pronounced “nice”). Another set of joints is oriented vertically, roughly paralleling the contact of the monzogranite with its surrounding rocks. The third set is also vertical but cuts the second set at high angles. The resulting system of joints tended to develop rectangular blocks. (figure 1) Good examples of the joint system may be seen at Jumbo Rocks, Wonderland of Rocks, and Split Rock.

As ground water percolated down through the monzogranite’s joint fractures, it began to transform some hard mineral grains along its path into soft clay, while it loosened and freed grains resistant to solution. Rectangular stones slowly weathered to spheres of hard rock surrounded by soft clay containing loose mineral grains. Imagine holding an ice cube under the faucet. The cube rounds away at the corners first, because that is the part most exposed to the force of the water. A similar thing happened here but over millions of years, on a grand scale, and during a much wetter climate. (figure 2)

After the arrival of the arid climate of recent times, flash floods began washing away the protective ground surface. As they were exposed, the huge eroded boulders settled one on top of another, creating those impressive rock piles we see today. (figure 3)

Visitors also wonder about the “broken terrace walls” laced throughout the boulders. These are naturally occurring formations called dikes. Younger than the surrounding monzogranite, dikes were formed when molten rock was pushed into existing joint fractures. Light-colored aplite, pegmatite, and andesite dikes formed as a mixture of quartz and potassium minerals cooled in these tight spaces. Suggesting the work of a stonemason, they broke into uniform blocks when they were exposed to the surface.

Of the dynamic processes that erode rock material, water, even in arid environments, is the most important. Wind action is also important, but the long-range effects of wind are small compared to the action of water.

The erosion and weathering processes operating in the arid conditions of the present are only partially responsible for the spectacular sculpturing of the rocks. The present landscape is essentially a collection of relict features inherited from earlier times of higher rainfall and lower temperatures.

MOUNTAINS
Within the park there are six distinct mountain ranges: the Little San Bernardino Mountains in the southwestern part; the Cottonwood, Hexie, and Pinto Mountains in the center; and the Eagle and Coxcomb Mountains in the eastern part. Both the southern and northern margins of the park are marked by steep escarpments that rise abruptly from the lower desert areas. Much of the park lies at elevations above 4,000 feet.

Between the park’s numerous mountain ranges, there are valleys, which are classified according to their method of formation. Queen Valley and Lost Horse Valley were formed by a difference in the rate of erosion between the rock underlying the valley itself and the rock composing the surrounding mountains. The mountainous rock is more resistant to erosion and therefore rises above the valleys. Pleasant Valley, on the other hand, was formed by down-dropped motion along faults that formed basins (called grabens). Some valleys have playas: lakes that may contain water a few weeks a year during the rainy season.

GEOLOGIC FAULTS
North America is moving westward over the Pacific Plate at one or two inches per year. When tension builds in rocks from this collision, the strain is released along faults in the form of earthquakes, as the rocks finally break.

Joshua Tree is crisscrossed with hundreds of faults, and is a great place to see raw rocks and the effects of earthquakes. The famous San Andreas Fault bounds the south side of the park, and can be observed from Keys View. Blue Cut Fault in the center of the park can be seen from the hilltop behind Lost Horse Mine. The fault forms the straight, abrupt base of the Hexie Mountains east of Queen Valley.

Fault zones are important factors in localizing natural springs. Movement by faults causes impervious zones of shattered rock fragments to form an underground dam forcing ground water to rise. The Oasis of Mara at the visitor center in Twentynine Palms marks the Pinto Mountain fault. The park has four other fault-caused oases that support the native palm tree, Washingtonia filifera. These oases supply food and water to a wide variety of wildlife and point to the connection between the park’s geology and its wildlife habitat.

Joshua Tree National Park - Ecology

Joshua Tree National Park is located in southern California on the eastern end of the broad mountainous belt called the Transverse Ranges, which stretch from Point Arguello (50 miles west of Santa Barbara) eastward nearly 300 miles to the Eagle Mountains in the Mojave Desert. Unlike most mountain ranges in North America that run north-south, the Transverse Ranges lie on an east-west axis.

RAINSHADOW DESERT
The area of southeastern California is a rain shadow desert. The rain shadow effect is produced by the high mountains on the west, which block the movement of wet winter storms. Coastal storms moving east collide with Mount San Jacinto (10,804 ft.) and Mount San Gorgonio (11,502 ft.) dropping most of their moisture on the west sides of these mountains. Land on the east side receives much less rain, which results in a desert environment.

During late August or September occasional tropical storms move into southern California from the south. These storms end up on the east side of the Peninsular Ranges and can dump a considerable amount of water in a short time. Some five to 10 inches of rain may fall in a few hours, representing a large portion of Joshua Tree’s annual precipitation.

Except for the occasional spectacular wildflower bloom, the desert appears to the casual visitor as an unchanging landscape. In reality it is a dynamic, constantly shifting ecosystem shaped primarily by wind and rain.

Desert Defined
Some sources define a desert as an area receiving no more than ten inches of precipitation annually. However, many areas receiving this amount of precipitation are not deserts. This definition is not complete. Both the timing and type of precipitation determine the environment established.

In a desert, rain isn’t evenly distributed throughout the year. Weather patterns often create short, violent downpours that produce flash floods. Much of the water runs off before it can soak into the soil. A lot of moisture is also lost to evaporation.

Many deserts lie in areas of high pressure systems where there is little cloud cover. At least 90 percent of the sun’s rays reaches Earth’s surface, producing seasonal hot temperatures. (For comparison, the surface of more humid lands, covered with more vegetation, receives only 40 percent of possible solar radiation.) The hot, dry air causes any available water to evaporate quickly.

When temperatures are extremely hot, rain can evaporate before it reaches Earth. The conditions producing high daytime temperatures reverse the process after sundown. Approximately 90 percent of the day’s accumulated heat radiates back toward the sky. (In moister climates only about 50 percent of this heat is lost.) These conditions produce the wide range of daily temperatures characteristic of deserts. This range is often 50 degrees or more.

The rapid heating and cooling of air create another characteristic of most deserts: strong winds. These winds, circulating air that is often hot and dry, increase the already high rate of evaporation. Evaporation in American deserts ranges from 70 to 160 inches per year.

A desert then is not so simply defined. Several characteristics: seasonal, high temperatures; low, sporadic rainfall; a high rate of evaporation; wide temperature ranges; and strong winds are part of the definition.

With 700 species of vascular plants, Joshua Tree is renowned for its plant diversity. No wonder that when the area was first proposed for preservation in the early 1930s, the name suggested was Desert Plants National Park.

Plant communities, or what we call “associations,” describe groupings of various plant species, and are often dependent upon latitude, soil characteristics, and elevation. Using these descriptions makes it easier to understand why certain plants only grow in certain places; it also helps to identify plants in unfamiliar terrain.

Plant associations within the park are divided into tree-dominated, shrub-dominated, herbaceous-dominated, and sparse/non-vegetated. Each association is named after the most conspicuous plant in the landscape.

Tree-dominated plant associations in the park include: California juniper, singleleaf pinyon, Joshua tree, desert willow, California fan palm, blue palo verde, smoketree, Gooding willow, Freemont cottonwood, and mesquite.

Shrub-dominated associations are the most diverse group, numbering 35. California Mormon tea, creosote bush, creosote bush/white bursage, blackbrush, brittlebush, bigberry manzanita, cheesebush, Mojave yucca, teddy-bear cholla, and desert almond are just a few examples.

Herbaceous-dominated associations are those communities that are mostly comprised of species like perennial bunch grasses or annual grasslands. The main associations are big galleta grass and cheatgrass.

Sparse associations include non-vegetated areas (e.g. desert pavement, rock outcrops, dunes, playas, washes, and disturbed areas) and areas with less than two percent shrub cover. These areas may be dominated by annual wildflowers during moist years, but normally appear devoid of vegetation.

CACTI / DESERT SUCCULENTS
Hot temperatures pose special problems for cacti. Most leafy plants cool themselves during the day by opening their pores. The movement of water from stem to leaf to air keeps leaf temperatures from rising too high. Because their pores open only at night, cacti cannot take advantage of such transpirational cooling. They adapt to desert heat by internal mechanisms. Teddybear cholla, for instance, can withstand an air temperature of 138 degrees F. Most other plants would literally cook at this temperature, but teddybear cholla often rises 59 degrees F or more above the air temperature!

TREES AND SHRUBS
Unlike desert wildflowers, which germinate and bloom during the brief rainy seasons and leave their seeds to endure the hottest and dryest times, desert trees and shrubs must adapt to the the climate year-round. Their modifications are varied, but whether like the tiny, drought-deciduous leaves characteristic of paloverde, or the furry, gray leaves of brittlebush, they are equally effective.

Known as the park namesake, the Joshua tree, Yucca brevifolia, is a giant member of the lily family. Like the California fan palm, Washingtonia filifera, the Joshua tree is a monocot, in the subgroup of flowering plants that also includes grasses and orchids.

FAN PALM OASES
In an otherwise hot and sparse environment, palm oases are a luxuriant gift of shade and solace. The verdant display requires a constant supply of water so oases often occur along fault lines, where uplifted layers of hard impermeable rock forces underground water to the surface. There are only 158 desert fan palm oases in North America. Five are located in Joshua Tree National Park.

A California Native: the desert fan palm, Washingtonia filifera, is native to the low hot deserts of southern California where it can live for 80 to 90 years. Towering up to 75 feet, the desert fan palm is among the tallest of North American palms. It is definitely the heaviest; a mature desert fan palm can weigh as much as three tons. Its distinctive leaves are shaped like a fan and folded like an accordion. They measure up to six feet in length and are nearly as wide. Looking much like "petticoats," the fan palm’s dead leaves remain attached to its trunk until removed by fire, wind, or flood.

Fire is beneficial: Fire rarely kills an adult. In palms the vascular bundles, those tubes that transport water and nutrients, are scattered throughout the trunk. This arrangement provides insulation from the heat of a fire. By contrast, trees such as oaks have all their vascular tissue in a ring just beneath the bark. Fire does kill young palms, but it also removes competitors and opens up space for palm seeds to germinate. In fact, desert fan palms increase seed production immediately after fires. A healthy palm can produce as many as 350,000 seeds.

Indians cultivated fan palms: People have been attracted to palm oases since prehistoric times. American Indians ate palm fruit and used the fronds to build waterproof dwellings. The Cahuillas (pronounced: Ka-wee-yahs) periodically set fire to oases in order to increase fruit production and to remove the sharp-edged palm fronds littering the oasis floor. The Cahuillas also planted palm seeds in promising locations.

Water is a mixed blessing: Water is a necessity. Desert fan palms suck up water using a mass of pencil-wide rootlets so dense that the roots of other plant species cannot penetrate. This mass may extend as far as 20 feet from the trunk in all directions. But water, in the form of flash floods, is also the most common cause of death for desert fan palms living in narrow canyons.

Palm oases provide habitat: Water also draws animals to oases: bighorn sheep, Gambel’s quail, coyotes. Coyotes help spread palms by eating palm fruit at one location and depositing the undigested seeds at another. The cool shade of an oasis provides habitat for animals that live nowhere else. After dark, a rush of air may be caused by the passing of a western yellow bat, they roost only in palms. During the day, a flash of yellow-orange might be a hooded oriole preparing to build its woven sack-like nest under the large green leaves of a desert fan palm.

Beetles keep palm groves healthy: The dime-sized holes seen in the trunks of palms are exit holes of the two-inch, blue-black, giant palm-boring beetle, Dinapate wrightii, who lives exclusively in palm oases. The larvae of the Dinapate beetle spend about five years chewing tunnels within the trunks of desert fan palms. The chewing is so loud that flickers use the noise to locate the larvae. Successful larva pupate within the trunk then chew their way out.

Because their rear end is wider than their front end, they exit going backwards to avoid getting stuck. Emerging in June, males and females mate and then die within a few weeks. Eventually these beetles can kill a palm, but they only inhabit older trees. Giant palm-boring beetles keep the palm population young and vibrant. The presence of these beetles is actually a sign of a healthy oasis.

CRYPTOBIOTIC SOIL
Cryptobiotic soil is found throughout the world. In arid regions, these living soil crusts are dominated by cyanobacteria, and also include soil lichens, mosses, green algae, microfungi and bacteria. These crusts play an important role in the ecosystems in which they occur. In the high deserts of the Colorado Plateau (which includes parts of Utah, Arizona, Colorado and New Mexico), these knobby black crusts are extraordinarily well-developed, and may represent 70 to 80 percent of the living ground cover.

What Are Cyanobacteria?
Cyanobacteria, previously called blue-green algae, are one of the oldest known life forms. It is thought that these organisms were among the first land colonizers of the Earth's early land masses, and played an integral role in the formation and stabilization of early soils. The earliest cyanobacteria fossils found are called stromatolites, which date back more than 3.5 billion years. Extremely thick mats of these organisms converted the earth's original carbon dioxide rich atmosphere into one rich in oxygen and capable of sustaining life.

Cyanobacteria occur as single cells or as filaments. The most common form found in Colorado Plateau soils are the filamentous type, which are usually surrounded by sticky, mucilaginous sheaths.

When moistened, cyanobacteria become active, moving through the soil and leaving a trail of sticky material behind. The sheath material sticks to surfaces such as rock or soil particles, forming an intricate web of fibers throughout the soil. In this way, loose soil particles are joined together, and an otherwise unstable surface becomes very resistant to both wind and water erosion.

The soil-binding action is not dependent on the presence of living filaments. Layers of abandoned sheaths, built up over long periods of time, can still be found clinging tenaciously to soil particles, providing cohesion and stability in sandy soils at depths up to 10cm.

Nitrogen fixation is another significant capability of cyanobacteria. Vascular plants are unable to utilize nitrogen as it occurs in the atmosphere. Cyanobacteria are able to convert atmospheric nitrogen to a form plants can use. This is especially important in desert ecosystems, where nitrogen levels are low and often limiting to plant productivity.

The sheaths have other functions as well. When moistened, they swell up to ten times their dry size. This ability to intercept and store water benefits both the crustal organisms as well as vascular plants, especially in arid regions with sporadic rainfall.

Sheaths, and the organisms they surround, also contribute organic matter and help make essintial nutrients available to vascular plants. Negatively charged clay particles, often found clinging to the sheaths, bind positively charged nutrients, preventing them from being leached out of the upper soil horizons or becoming bound in a form unavailable to plants. Like soil stability, this function is not dependent on the presence of living filaments, but only the presence of sheath material.

Environmental Impacts
Unfortunately, many human activities are incompatible with the presence and well-being of cryptobiotic soils. The fibers that confer such tensile strength to these crusts are no match for the compressional stress placed on them by footprints or machinery, especially when the crusts are dry and brittle.
Air pollutants, both from urban areas and coal-fired power plants, also harm these crusts.

Tracks in continuous strips, such as those produced by vehicles or bicycles, are especially damaging, creating areas that are highly vulnerable to wind and water erosion. Rainfall carries away loose material, often creating channels along these tracks, especially when they occur on slopes.

Wind not only blows pieces of the pulverized crust away, thereby preventing reattachment to disturbed areas, but also disturbs the underlying loose soil, often covering nearby crusts. Since crustal organisms need light to photosynthesize, burial can mean death. When large sandy areas are impacted during dry periods, previously stable areas can become a series of shifting sand dunes in just a few years.

Impacted areas may never fully recover. Under the best circumstances, a thin veneer of cryptobiotic soil may return in five to seven years. Damage done to the sheath material, and the accompanying loss of soil nutrients, is repaired slowly during up to 50 years of cyanobacterial growth. Lichens and mosses may take even longer to recover.

Joshua Tree National Park - Wildlife

Despite the impression that the desert is lifeless, many animals make their homes in deserts. Birds, lizards, and ground squirrels are most likely to be seen because they are largely active during the day. However, it is at night that desert animals come out to roam. Mostly nocturnal animals include: snakes, bighorn sheep, kangaroo rats, coyotes, and black-tailed jack rabbits. Dusk and dawn are good times for viewing many kinds of animals, both those just going to bed and those just getting up.

Animals that thrive in desert environments often have special adaptations for dealing with limited water and high summer temperatures. One does not walk far in the desert without seeing a multitude of burrow openings. The smaller mammals and all reptiles take refuge from the heat underground.

Desert mammals make more efficient use of their bodies’ water supply than does the human body. Reptiles are physiologically adapted to getting along with little water, and birds can fly to water sources when they need a drink. Nevertheless, the springs and seeps in the park are necessary to the survival of many animals.

Most of the reptiles and many small rodents and insects go into an inactive state of hibernation during the winter. However, winter is the time of greatest bird concentrations in the park, because of the presence of many migrant species.

MAMMALS
The chief obstacles to survival in the desert are lack of water, shortage of food, and extreme temperatures. Mammals, including humans have the ability to maintain a constant body temperature regardless of external conditions. This has advantages and disadvantages in the desert. Mammals can endure a large range of air temperatures, but are unable to tolerate even a small change in body temperature without encountering problems.

When most mammals get hot they perspire, and the evaporation of this water cools them down and helps maintain a constant body temperature. Some mammals use panting to produce the same effect. Both methods work well, but they have an important drawback for life in the desert. They involve substantial loss of water. Where water is in short supply, animals must minimize water loss. Thus, few desert mammals use perspiration or panting as their main method of keeping cool.

Because scarcity of food in the desert limits the number of large mammals that can be supported, most desert mammals are small. Joshua Tree National Park is home to 52 species of mammals. Of these, 24 are small rodents. Being small has its advantages and disadvantages. Rodents can burrow into the ground or hide in rocky crevices to avoid the mid-day heat. But their small body size means that they can gain or lose body heat rapidly. Many of them plug the entrance to their burrows to keep out the hot, desiccating air.

Most small mammals make the most of the positive side of being small, spending the day in burrows and emerging at night when the temperature drops to a more comfortable level. The larger mammals, such as mule deer and mountain sheep stay close enough to springs to be able to drink daily.

A few desert mammals, such as the round-tailed ground squirrel, a diurnal rodent, enter a state of aestivation when the days become too hot and the vegetation too dry. They sleep away the hottest part of the summer. They also hibernate in winter to avoid the cold.

Many of our Joshua Tree mammals are paler in color than their relatives in more moderate environments. Pale colors not only ensure that the animal will absorb less heat from the environment, but help make it less conspicuous to predators in the bright, pallid landscape.

Most desert mammals are herbivores and derive water directly from the plants they eat. Some, like kangaroo rats, have extreme adaptations enabling them to live without ever drinking water. They have super efficient kidneys that extract most of the water from their urine and return it to the blood. And much of the water that would be lost in breathing is recaptured in the nasal cavities by specialized organs. If that weren’t enough, kangaroo rats actually manufacture water metabolically from the digestion of dry seeds!

The Desert bighorn, Ovis canadensis nelsoni, ranges through the dry, desert mountains of eastern California, much of Nevada, northwestern Arizona, and southern Utah (range map). The total population of this sheep is about 13,000. Two hundred fifty or so live in Joshua Tree National Park.

REPTILES
Reptiles are closely associated with the desert in many peoples minds. This seems to be based partly on reality and partly on perception. Reptiles do form a very conspicuous part of the vertebrate fauna of warm deserts such as are found in Joshua Tree National Park. There may not be any larger number of reptiles in the desert than in neighboring less arid areas, but the lack of dense vegetation on the desert certainly makes them easier to see. Many of the lizards are especially conspicuous as they bask atop boulders or other elevated sites.

Reptiles are better adapted to life in arid lands than are most birds and mammals. Being ectotherms (obtaining their body heat solely from the external environment), reptiles have a much lower cost of living than do birds and mammals which produce their own body heat using a great deal of food in the metabolic process. In desert lands, where primary productivity (plant growth) is low, reptiles are thus able to maintain larger populations on the limited food supplies than is possible for birds and mammals.

The most limiting factor for life on the desert is drinking water. Reptiles are pre-adapted to such arid conditions. They do not need water for cooling because they do not perspire or pant. They just crawl into a cool hole in the heat of the day. Their scales also greatly retard water loss through the skin.

In addition, reptiles do not need water for excretion; they produce no urine. Their nitrogenous wastes are excreted as a solid: uric acid. Reptiles can get all the water they need from the food they eat. Although desert tortoises and probably most other reptiles will drink water when it appears after summer rains, many lizards and snakes probably go their whole lives without a drink of water.

The reptiles of Joshua Tree National Park include one tortoise, 18 lizards, and 25 varieties of snakes.

BIRDS
With over 250 kinds of birds recorded from Joshua Tree National Park, it is understandable that the park affords a rewarding place to study them. This is especially true during the winter months when migrants abound.

The vast majority of our recorded bird species are migrants and vagrants. Lying astride the inland portion of the Pacific flyway, the park serves as a rest stop for many migrants. The aquatic areas of Barker Dam and the Desert Queen Ranch attract many types of waterfowl on their way to the Salton Sea, birds that would not otherwise be seen in the desert. Rest stops are important for most migratory birds for purposes of water intake and for metabolism of fat reserves, which may not keep pace with energy use while they are actually in flight. Many of our migrants are actually residents of the nearby mountains, from which they fly to escape the heavy winter snows.

Although most birds require drinking water almost every day, this is not such a limiting factor as might be supposed. There are many springs and seeps in the park, which are readily accessible to animals that can fly to them. The chief limiting factor for birds in the desert is food. Birds require relatively large amounts of food daily, especially during the breeding season. Thus, it is understandable that there are only 78 species of birds known to nest and raise young in the park.

The park is an attractive place to sight and watch birds. The lack of dense vegetation makes birds much easier to see here than in most national parks. Golden eagles hunt in the park regularly. The roadrunner, of cartoon fame, is an easily recognized resident. And the call of Gambel’s quail is a noteworthy sound of the desert.

AMPHIBIANS
Frogs are probably the last thing that people expect to see when they visit the desert. However, some frogs and toads have adapted to life in arid lands. True, they still need water, but they seek it out when it is available.

Amphibians are animals that have two life stages: a larval, aquatic form and an adult, terrestrial form. This is the difference between a tadpole and a toad.

Breeding and toad choruses occur in spring following winter rains or after the monsoon storms of summer. Male tree frogs and toads do the vocalizing. Gelatin-covered eggs are laid by the females at the bottom of a pool and hatch in a few days. Then, in the case of toads, it is a race to finish the tadpole stage before the pool dries up.

Three amphibians are found in Joshua Tree National Park:
The California tree frog, Hyla cadaverina, is found only in southern California and is listed as a Species of Special Concern. It is found in the rocky, permanent water sources created by the Pinto Fault along the northern edge of the park. This species reaches the eastern edge of its range here.

The red-spotted toad, Bufo punctatus, is a true denizen of the desert, where it spends most of its life underground. Found from one end of the park to the other, it appears after good, soaking rains. This toad lays its eggs in potholes, springs, and the intermittent streams found in rocky canyons after heavy rains.

The California toad, Bufo halophilus, has been reported from the Oasis of Mara. It may be established around watered areas in the urban parts of the Morongo Basin. The nearest natural population is in Little Morongo Canyon.

Joshua Tree National Park - Flora & Fauna

With 700 species of vascular plants, Joshua Tree is renowned for its plant diversity. No wonder that when the area was first proposed for preservation in the early 1930s, the name suggested was Desert Plants National Park.

Plant communities, or what we call “associations,” describe groupings of various plant species, and are often dependent upon latitude, soil characteristics, and elevation. Using these descriptions makes it easier to understand why certain plants only grow in certain places; it also helps to identify plants in unfamiliar terrain.

Plant associations within the park are divided into tree-dominated, shrub-dominated, herbaceous-dominated, and sparse/non-vegetated. Each association is named after the most conspicuous plant in the landscape.

Tree-dominated plant associations in the park include: California juniper, singleleaf pinyon, Joshua tree, desert willow, California fan palm, blue palo verde, smoketree, Gooding willow, Freemont cottonwood, and mesquite.

Shrub-dominated associations are the most diverse group, numbering 35. California Mormon tea, creosote bush, creosote bush/white bursage, blackbrush, brittlebush, bigberry manzanita, cheesebush, Mojave yucca, teddy-bear cholla, and desert almond are just a few examples.

Herbaceous-dominated associations are those communities that are mostly comprised of species like perennial bunch grasses or annual grasslands. The main associations are big galleta grass and cheatgrass.

Sparse associations include non-vegetated areas (e.g. desert pavement, rock outcrops, dunes, playas, washes, and disturbed areas) and areas with less than two percent shrub cover. These areas may be dominated by annual wildflowers during moist years, but normally appear devoid of vegetation.

CACTI / DESERT SUCCULENTS
Hot temperatures pose special problems for cacti. Most leafy plants cool themselves during the day by opening their pores. The movement of water from stem to leaf to air keeps leaf temperatures from rising too high. Because their pores open only at night, cacti cannot take advantage of such transpirational cooling. They adapt to desert heat by internal mechanisms. Teddybear cholla, for instance, can withstand an air temperature of 138 degrees F. Most other plants would literally cook at this temperature, but teddybear cholla often rises 59 degrees F or more above the air temperature!

TREES AND SHRUBS
Unlike desert wildflowers, which germinate and bloom during the brief rainy seasons and leave their seeds to endure the hottest and dryest times, desert trees and shrubs must adapt to the the climate year-round. Their modifications are varied, but whether like the tiny, drought-deciduous leaves characteristic of paloverde, or the furry, gray leaves of brittlebush, they are equally effective.

Known as the park namesake, the Joshua tree, Yucca brevifolia, is a giant member of the lily family. Like the California fan palm, Washingtonia filifera, the Joshua tree is a monocot, in the subgroup of flowering plants that also includes grasses and orchids.

FAN PALM OASES
In an otherwise hot and sparse environment, palm oases are a luxuriant gift of shade and solace. The verdant display requires a constant supply of water so oases often occur along fault lines, where uplifted layers of hard impermeable rock forces underground water to the surface. There are only 158 desert fan palm oases in North America. Five are located in Joshua Tree National Park.

A California Native: the desert fan palm, Washingtonia filifera, is native to the low hot deserts of southern California where it can live for 80 to 90 years. Towering up to 75 feet, the desert fan palm is among the tallest of North American palms. It is definitely the heaviest; a mature desert fan palm can weigh as much as three tons. Its distinctive leaves are shaped like a fan and folded like an accordion. They measure up to six feet in length and are nearly as wide. Looking much like "petticoats," the fan palm’s dead leaves remain attached to its trunk until removed by fire, wind, or flood.

Fire is beneficial: Fire rarely kills an adult. In palms the vascular bundles, those tubes that transport water and nutrients, are scattered throughout the trunk. This arrangement provides insulation from the heat of a fire. By contrast, trees such as oaks have all their vascular tissue in a ring just beneath the bark. Fire does kill young palms, but it also removes competitors and opens up space for palm seeds to germinate. In fact, desert fan palms increase seed production immediately after fires. A healthy palm can produce as many as 350,000 seeds.

Indians cultivated fan palms: People have been attracted to palm oases since prehistoric times. American Indians ate palm fruit and used the fronds to build waterproof dwellings. The Cahuillas (pronounced: Ka-wee-yahs) periodically set fire to oases in order to increase fruit production and to remove the sharp-edged palm fronds littering the oasis floor. The Cahuillas also planted palm seeds in promising locations.

Water is a mixed blessing: Water is a necessity. Desert fan palms suck up water using a mass of pencil-wide rootlets so dense that the roots of other plant species cannot penetrate. This mass may extend as far as 20 feet from the trunk in all directions. But water, in the form of flash floods, is also the most common cause of death for desert fan palms living in narrow canyons.

Palm oases provide habitat: Water also draws animals to oases: bighorn sheep, Gambel’s quail, coyotes. Coyotes help spread palms by eating palm fruit at one location and depositing the undigested seeds at another. The cool shade of an oasis provides habitat for animals that live nowhere else. After dark, a rush of air may be caused by the passing of a western yellow bat, they roost only in palms. During the day, a flash of yellow-orange might be a hooded oriole preparing to build its woven sack-like nest under the large green leaves of a desert fan palm.

Beetles keep palm groves healthy: The dime-sized holes seen in the trunks of palms are exit holes of the two-inch, blue-black, giant palm-boring beetle, Dinapate wrightii, who lives exclusively in palm oases. The larvae of the Dinapate beetle spend about five years chewing tunnels within the trunks of desert fan palms. The chewing is so loud that flickers use the noise to locate the larvae. Successful larva pupate within the trunk then chew their way out. Because their rear end is wider than their front end, they exit going backwards to avoid getting stuck. Emerging in June, males and females mate and then die within a few weeks. Eventually these beetles can kill a palm, but they only inhabit older trees. Giant palm-boring beetles keep the palm population young and vibrant. The presence of these beetles is actually a sign of a healthy oasis.

CRYPTOBIOTIC SOIL
Cryptobiotic soil is found throughout the world. In arid regions, these living soil crusts are dominated by cyanobacteria, and also include soil lichens, mosses, green algae, microfungi and bacteria. These crusts play an important role in the ecosystems in which they occur. In the high deserts of the Colorado Plateau (which includes parts of Utah, Arizona, Colorado and New Mexico), these knobby black crusts are extraordinarily well-developed, and may represent 70 to 80 percent of the living ground cover.

What Are Cyanobacteria?
Cyanobacteria, previously called blue-green algae, are one of the oldest known life forms. It is thought that these organisms were among the first land colonizers of the Earth's early land masses, and played an integral role in the formation and stabilization of early soils. The earliest cyanobacteria fossils found are called stromatolites, which date back more than 3.5 billion years. Extremely thick mats of these organisms converted the earth's original carbon dioxide rich atmosphere into one rich in oxygen and capable of sustaining life.

Cyanobacteria occur as single cells or as filaments. The most common form found in Colorado Plateau soils are the filamentous type, which are usually surrounded by sticky, mucilaginous sheaths.

When moistened, cyanobacteria become active, moving through the soil and leaving a trail of sticky material behind. The sheath material sticks to surfaces such as rock or soil particles, forming an intricate web of fibers throughout the soil. In this way, loose soil particles are joined together, and an otherwise unstable surface becomes very resistant to both wind and water erosion.

The soil-binding action is not dependent on the presence of living filaments. Layers of abandoned sheaths, built up over long periods of time, can still be found clinging tenaciously to soil particles, providing cohesion and stability in sandy soils at depths up to 10cm.

Nitrogen fixation is another significant capability of cyanobacteria. Vascular plants are unable to utilize nitrogen as it occurs in the atmosphere. Cyanobacteria are able to convert atmospheric nitrogen to a form plants can use. This is especially important in desert ecosystems, where nitrogen levels are low and often limiting to plant productivity.

The sheaths have other functions as well. When moistened, they swell up to ten times their dry size. This ability to intercept and store water benefits both the crustal organisms as well as vascular plants, especially in arid regions with sporadic rainfall.

Sheaths, and the organisms they surround, also contribute organic matter and help make essintial nutrients available to vascular plants. Negatively charged clay particles, often found clinging to the sheaths, bind positively charged nutrients, preventing them from being leached out of the upper soil horizons or becoming bound in a form unavailable to plants. Like soil stability, this function is not dependent on the presence of living filaments, but only the presence of sheath material.

Environmental Impacts
Unfortunately, many human activities are incompatible with the presence and well-being of cryptobiotic soils. The fibers that confer such tensile strength to these crusts are no match for the compressional stress placed on them by footprints or machinery, especially when the crusts are dry and brittle.
Air pollutants, both from urban areas and coal-fired power plants, also harm these crusts.

Tracks in continuous strips, such as those produced by vehicles or bicycles, are especially damaging, creating areas that are highly vulnerable to wind and water erosion. Rainfall carries away loose material, often creating channels along these tracks, especially when they occur on slopes.

Wind not only blows pieces of the pulverized crust away, thereby preventing reattachment to disturbed areas, but also disturbs the underlying loose soil, often covering nearby crusts. Since crustal organisms need light to photosynthesize, burial can mean death. When large sandy areas are impacted during dry periods, previously stable areas can become a series of shifting sand dunes in just a few years.

Impacted areas may never fully recover. Under the best circumstances, a thin veneer of cryptobiotic soil may return in five to seven years. Damage done to the sheath material, and the accompanying loss of soil nutrients, is repaired slowly during up to 50 years of cyanobacterial growth. Lichens and mosses may take even longer to recover.

Joshua Tree National Park - Contact

Joshua Tree National Park
74485 National Park Drive
Twentynine Palms, CA 92277-3597

Visitor Information 760-367-5500
Education Programs 760-367-3011
Park Headquarters 760-367-5502
Special-use Permits 760-367-5545

Fax: 760-367-6392

Oasis Visitor Center
Open All Year
8 a.m. to 5 p.m.
Located at park headquarters in Twentynine Palms, at the junction of Utah Trail and National Park Drive.

Joshua Tree Visitor Center
Open All Year
8 a.m. to 5 p.m.
Located one block south of Hwy 62 (Twentynine Palms Highway) on Park Boulevard in Joshua Tree Village.

Cottonwood Visitor Center
Open All Year
9 a.m. to 3 p.m.
Located eight miles north of Interstate 10 at Cottonwood Spring.

Black Rock Nature Center
Open October through May
8 a.m. to 4 p.m. except on Friday
Noon to 8 p.m. on Friday
Located in Black Rock Campground.