|© 1998 Christopher L. Christie|
for Yucca brevifolia var. brevifolia:
Yucca brevifolia var. herbertii (Webber) Munz 
The following varieties are recognized, although not consistently:
Yucca brevifolia var. brevifolia Engelm.
Yucca brevifolia var. jaegeriana McKelvey [49,50]
In the Tickapoo Valley of southern Nevada, the distributions of Y. b. var. brevifolia and Y. b. var. jaegeriana overlap, and hybridization may occur .
The scientific names given above will be used when discussing varieties in this review.LIFE FORM:
In the northern part of the Mojave Desert, Joshua tree is associated with Great Basin species, and in the southern Mojave, Joshua tree is found with characteristic Mojave and Sonoran desert species . Often, perennial grasses are the dominants in Joshua tree stands. In the western Mojave, desert needlegrass (Achnatherum speciosum) and Indian ricegrass (A. hymenoides) are common dominant associates. In the eastern Mojave Desert, western Arizona, and Joshua Tree National Park, big galleta (Pleuraphis rigida) and black grama (Bouteloua eriopoda) are dominant in Joshua tree stands, while galleta (P. jamesii) and blue grama (B. gracilis) dominate Joshua tree stands in northern parts of the eastern Mojave and in the Great Basin . Other common Joshua tree associates in Joshua Tree National Park include California juniper (Juniperus californica), singleleaf pinyon (Pinus monophylla), shrub live oak (Quercus turbinella), blackbrush, green ephedra (Ephedra viridis), eastern Mojave buckwheat (Eriogonum fasciculatum), white burrobrush (Hymenoclea salsola), bladdersage (Salazaria mexicana), and Mojave desertrue (Thamnosma montana) .
The following vegetation classifications identify Joshua tree as an important species:California:
California juniper-Joshua tree woodlands in the San Bernardino Mountains 
desert vegetation type in the Tehachapi Mountains 
Joshua tree series of the desert scrub vegetation type in the Liebre Mountains where California juniper, big sagebrush (Artemisia tridentata), and rubber rabbitbrush (Chrysothamnus nauseosus) are typical 
Joshua tree-Great Basin sage scrub vegetation in the San Bernardino Mountains 
Joshua tree grasslands dominated by grama (Bouteloua spp.) and galleta (Pleuraphis spp.) in the eastern Mojave Desert in southern California 
Mojave mixed woody scrub
Mojave mixed steppe
Mojavean California juniper woodland and scrub types 
succulent woodlands found from San Bernardino to Inyo county 
Joshua tree/big galleta 
Joshua tree-Utah juniper (Juniperus osteosperma)/big sagebrush 
southern desert shrub vegetation 
tree-yucca (Yucca spp.) scrub and tree-yucca scrub steppe communities 
|© 2004 James M. Andre|
|© Br. Alfred Brousseau, Saint Mary's College|
Aboveground description: Maxwell  describes Joshua tree as a "delight to the eye and a fascinating feature of the western landscape." More specifically, however, Joshua tree is a 20- to 70-foot (5-20 m) tall, evergreen, tree-like plant. Trees exceeding 40 feet (10 m) are rare, and height is easily overestimated [51,62,72,76,77]. Tree size and growth form often vary with site and climate conditions [37,68,92]. Typically trees have 1 main stout stem that measures 1 to 3 feet (0.3-0.9 m) in diameter and have an expanded base [21,50,56,105,107]. Growth forms with several large stems are noted as well [92,107,110]. Trunks are fibrous, and the bark or periderm is "soft and cork like" [55,66,92]. Mature tree trunks typically measure 1 to 3 feet (0.3-0.9 m) in diameter. Bark plates measure 3 to 6 inches (7.5-15 cm) long by 1 to 2 inches (2.5-5 cm) in thickness .
Branching is often extensive on old plants, and rounded open crowns are common [37,50,62,92]. Young trees typically lack branches and are covered with persistent reflexed leaves . Trees normally reach 3 to 9 feet (0.9-3 m) tall before branching . Johnson  describes Joshua tree branching as "grotesque" and random. However, branching is formally referred to as dichotomous or almost dichotomous. Branches are formed following terminal bud death due to flowering or insect damage [50,66,68,88]. Branches are often 7 to 20 feet (2-5 m) or longer and fork at 2- to 3-foot (0.6-0.9 m) intervals. Inner branches are typically erect, and outer branches can be horizontal or drooping [21,50,62,72,110].
Joshua tree is slow growing and long lived [22,62]. Wallace and Romney  indicate that height, growth rings, or number of leaf blades may be used to age Joshua tree, but they caution that height may not accurately age "very mature" plants. Webber  reports that 21-year-old Joshua trees were unbranched, and the average annual growth rate was 5.9 cm/year. Other Y. b. var. jaegeriana plants grew an average of 11.7 cm/year. Johnson  indicates that large trees can be 300 years old, and Keith  suggests that Joshua tree has an average life span of 150 years. Little  suggests that Joshua tree is among the among the desert's "oldest living plants." An approximately 60-foot-tall (20 m) tree in California was an estimated 1,000 years old .
Leaves are clustered in rosettes at the branch ends. Clusters are commonly 1 to 5 feet (0.3-1.5 m) long and 1 to 2 feet (0.3-0.5 m) in diameter. Leaves are linear, needle shaped and measure 5.9 to 14 inches (15-35 cm) long by 0.3 to 0.6 inch (0.7-1.5 cm) wide. Enlarged bases attach the leaves to the branch. Leaf shape is slightly triangular and leaf margins are lined with small teeth. Spines measuring 0.3 to 0.5 inch (7-12 mm) occur at the leaf tips [6,21,37,50,51,62,76,77,107,110]. Leaf clusters are longer (3 to 5 feet (1-1.5 m)) on juvenile plants than on mature plants (1-3 feet (0.3-1 m)) . Outer leaf layers are thick and waxy to reduce water loss . Dead leaves are persistent and fold down, covering the branches and coating the trunks of young trees .
Joshua tree flowers occur in dense, heavy panicles that measure 8 to 20 inches (20-40 cm) long. Individual flowers are round to egg shaped and measure 1 to 2 inches (2.5-5 cm) by 0.4 to 0.8 inch (1-2 cm) wide [21,37,47,51,62,76,77,110]. Fruits are indehiscent capsules, which become spongy and dry with age. Egg-shaped capsules are 2 to 4 inches (6-10 cm) long and approximately 2 inches (5 cm) in diameter. Fruits develop at the base of the inflorescence while the upper portion is still in flower. Mature fruits contain 30 to 50 seeds, which are flat to thickened with smooth to undulate surfaces. Seeds are 0.3 to 0.4 inch (7-11 mm) long [3,21,47,50,61,62,72,76,77,107]. Fruit clusters often weigh over 9 pounds (4 kg), while a single capsule frequently weighs over 8.8 ounces (250 g). Fruits borne on erect branches are not easily detached . Average individual seed weight ranged from 0.0025 to 0.0035 ounce (0.07-0.1 g) based on several seed collections in the Southwest . In Los Angeles County, California, the average fruit length was 2.7 inches (69 mm), the number of seeds per locule averaged 26, and individual seed weight averaged 99 mg .
Belowground description: The Joshua tree root system is described as deep and extensive [11,22]. The enlarged trunk base of mature trees can be almost 4 feet (1.2 m) in diameter but extends only about 1 foot (0.3 m) into the ground, suggesting that Joshua trees are supported mainly by their roots and rhizomes . A large number of small fibrous roots penetrate down and horizontally . In southern Utah, Joshua tree roots were found in an excavation pit in a blackbrush community when the nearest Joshua tree was 36 feet (11 m) away .
Not all Joshua trees produce rhizomes. Rhizome production and clonal growth are more common at high elevations . See Asexual regeneration for a discussion of possible reasons for rhizome presence or absence.
Newly produced rhizomes are unbranched, succulent, and covered with bud scales. Young rhizomes grow and produce aerial stems quickly. After producing aerial stems, rhizomes become woody and hard, lose their bud scales, and may produce lateral branches [92,107]. The periderm on mature rhizomes is thin, dense, and hard: not at all corky like the periderm on aboveground stems . Rhizomes are typically 0.4 to 2 inches (1-5 cm) in diameter and grow horizontally approximately 3 feet (1 m) from the parent plant before sending up aerial stems . Simpson  reports that rhizomes can be as long as 10 feet (3 m) or more. Rhizome diameter is greatest at the base of aboveground shoots, and roots commonly occur along the entire rhizome length. In rocky substrates, irregular rhizome growth is common .
Variability: Yucca b. var. jaegeriana is often generically referred to as dwarf Joshua tree and displays true dichotomous branching. This variety is often smaller (10 to 20 feet (3-6 m) tall), with shorter leaves (<8.7 inches (22 cm)) and shorter branches (2-3 feet (0.7-1 m)), than Y. b. var. brevifolia [50,51,72,88,92]. Yucca b. var. brevifolia is less stocky, often 20 to 40 feet (5-12 m) tall, with longer leaves (7.5-15 inches (19-37 cm)) and higher branches (7-10 feet (2-3 m) above ground) than Y. b. var. jaegeriana. Yucca b. var. brevifolia branching is not truly dichotomous [50,88]. Simpson  indicates that Y. b. var. brevifolia trees taller than 70 feet (20 m) have been recorded. Growth forms of both varieties may vary with elevation. Joshua trees growing below 4,000 feet (1,200 m) are often single-stemmed trees, but when growing above 4,000 feet (1,200 m), plants often have many stems connected by long, thin, horizontal rhizomes . For more information about clonal Joshua tree growth forms, see Asexual regeneration.RAUNKIAER  LIFE FORM:
Pollination: Flowers are pollinated by a single species of moth. The yucca moth, Tegeticula synthetica, is commonly considered Joshua tree's pollinator . Researchers have discovered another Joshua tree pollinator, Tegeticula antithetica, in the eastern and northeastern parts of the Mojave Desert where Y. b. var. jaegeriana occurs. Distributions of the 2 moth species are not thought to overlap .
Many refer to and describe the pollination process, but experiments and true observations are lacking as of this writing (2006). It is generally accepted that a female moth emerges from her pupa near a Joshua tree plant, mates in a flower, and flies to a freshly opened flower. Using specialized mouth parts sometimes referred to as "tentacles," she scrapes pollen from the anthers, forms it into a ball, and carries it between her tentacles and thorax to another flower. Whether or not the receiving flower is on the same inflorescence or tree is often speculated, but direct observation is lacking. The female moth penetrates the ovary wall and deposits 1 or more eggs in a locule. All eggs may be put in 1 locule or eggs may be distributed among several locules. She then pushes the pollen ball into the stigmatic tube. Moth larvae feed on the developing Joshua tree seeds .
When the Joshua tree moth pollinator was introduced into a moth-free area, Lenz  found that moths dispersed as far as 384 feet (117 m). Force and Thompson  found that Tegeticula synthetica is susceptible to parasitism by endo- and ectoparasites. Parasitized larvae in Joshua tree fruit and fruit stalks collected in California's San Gabriel Mountains ranged from 12.5% to 82.4% .
Breeding system: Joshua tree is chiefly monoecious, but some perfect flowers are produced . Some suggest that a period of cold temperatures is necessary for flower production .
Seed production: Seed production is most often described as periodic or rare [56,68,112]. "Wet years" are suggested as best for flowering and fruit production [56,68].
In Los Angeles County, researchers evaluated seed predation by Tegeticula moth larvae (see Pollination). The average number of Tegeticula larvae per fruit was 1.4 in 155 examined fruits. Just 7% of seeds were destroyed. The range of larvae per fruit was 0 to 6; 39% of Joshua tree fruit had no larvae. Researchers suggested several potential reasons for fruit production without the presence of larvae. Flowers may have been self pollinated or pollinated by a vector other than Tegeticula, which researchers considered unlikely. Moths may have pollinated the flower but failed to oviposit, or moths laid their eggs, but the larvae died .
Seed production may also be affected by small mammal predation. California ground squirrels climb Joshua trees and consume the fleshy fruits and seeds, thus destroying some of the seed crop . Went  predicts that 99% of Joshua tree seeds are consumed by rodents or moth larvae; however, experiments or observations leading to this assertion are not described.
Seed dispersal: Joshua tree seeds are dispersed by mammals and wind. As fruits become overmature, skins crack and moisture is released, making fruits lighter and more easily wind dispersed [61,92]. Finding clumps of 2 or more seedlings is likely evidence that the dried fruits were wind dispersed .
White-tailed antelope squirrels collect overripe dry fruits and crack the coating, consuming some seeds and allowing others to fall to the ground. It is common to find broken fruits and seeds at the bases of Joshua trees. It has been suggested that the large effort in fruit production by Joshua tree without a specialized dispersal agent may indicate that this type of fruit production is an evolutionarily old trait designed to attract a now extinct megaherbivore dispersal agent. The researcher suggests elephants. However, with current dispersal means, young or juvenile plants have been found as far as 495 feet (151 m) from a seed-producing plant in Los Angeles County. A maximum dispersal distance of 823 feet (251 m) was recorded in San Bernardino County .
Seed banking: Longevity of seed in the soil seed bank is unknown. Joshua tree seeds collected in Arizona were stored under artificial conditions, and germination was 98% and 72% after 6 months and 1.5 years of storage, respectively .
Germination: Joshua tree seeds germinate readily in the laboratory and do not require any pretreatment [1,105]. Joshua tree seeds may germinate any time after being shed and receiving moisture. Because of seed predation by rodents, Went  suggests that the best chance for successful germination is immediately after falling.
Seeds collected in Joshua Tree National Park germinated well in the laboratory at 68°F (20°C) and 77 °F (25 °C) . Controlled seed germination experiments from collections made at the Nevada Test Site indicate that germination was best at 64 °F (18 °C) compared to germination at 50 °F (10 °C) and 95 °F (35 °C) . Viability of 25 Joshua tree seeds collected from native habitats was 96%, and of 50 seeds kept on moist filter paper, 24% produced seedlings . Joshua tree seeds, collected in Arizona and kept in a dark laboratory, showed 0% initial germination at 50 °F (10 °C); 24% after 8 to 10 days at 59 °F (15 °C); 100% after 2 to 5 days at 68 °F (20 °C); and 100% after 1 to 2 days at 77 °F (25 °C) .
Short durations of hot temperatures may increase Joshua tree germination. Germination percentages of seed, collected from several Joshua tree populations in the Mojave Desert and subjected to heat treatments, are provided below. Germination percentages for seed kept at 190 °F (90°C) for 5 minutes were significantly higher (p<0.01) than seed under control conditions .
|2 hours||5 minutes|
Seedling establishment/growth: Quantity of Joshua tree seedlings observed in the field varies. Yeaton and others  report numerous Joshua tree seedlings in the eastern Mojave. Went  observed seedlings in Joshua Tree National Park, but abundance was not reported. Wallace and Romney  report few seedlings at the Nevada Test Site, and population structure observations suggested that successful seedling establishment may occur only a few times each century in the area. These differences may be related to site variation, observation timing, climate differences, and/or observation effort. Seedlings are easily concealed by nurse plants , and seedling predation by black-tailed jackrabbits is common in Lanfair Valley, California (Griffith, personal communication cited in ), suggesting that effort and timing may be crucial to finding Joshua tree seedlings.
Seedling growth rates and production vary with age, temperature, and photoperiod. In Joshua Tree National Park, unbranched seedlings grew at an average rate of 3 inches (7.6 cm)/year for the first 10 years and an average of 1.5 inches (3.8 cm)/year thereafter . Yucca (Yucca spp.) seedlings grown from seed collected from several native populations produced their 1st few leaves rapidly, then produced, on average, 1 new leaf every 2 months. Along with a decreased rate of leaf production was an increase in leaf size. Twenty-two-day-old seedlings had an average of 4 leaves . Based on studies at the Nevada Test Site, Wallace  indicated that Joshua tree seedlings that survive 3 to 5 years are established.
Went [112,113] suggests that cold periods are required for optimal seedling growth. Seedlings (3.5 years of age) kept at 40 °F (4 °C) for 2 months produced twice as many new leaves as seedlings without the cold treatment, even though there was no new growth produced during the 2-month cold period [112,113].
McCleary  found that day length affected the growth of seedlings grown from seed collected in Arizona. Seedlings grown with 10 hours of daylight and 14 hours of dark produced on average the longest and most leaves (x =15.1); seedlings grown in 16 hours of daylight and 8 hours of dark produced the shortest and fewest leaves (x =9.5). Other photoperiods were tested; see  for complete results.
Nurse plants: In the Spring and Sheep mountain ranges of southern Nevada, shrub species, especially blackbrush, provide important Joshua tree seedling habitat. Joshua tree stands on study sites were between 3,000 and 7,000 feet (1,000-2,000 m). In sixteen 100 × 50 m-sites, researchers located a total of 277 seedlings. Of these, 257 grew under the canopy of some other shrub, even though shrub coverage averaged just 20.1 % in the area. The majority of seedlings occurred at 5,200 foot (1,600 m) elevation, and 71% of all canopy seedlings grew under blackbrush. White bursage (Ambrosia dumosa), spiny hopsage (Grayia spinosa), and range ratany (Krameria parvifolia) also had more Joshua tree seedlings beneath their canopy than expected based on available canopy area or density. Researchers suggested that seedlings growing under shrub canopies experience increased soil moisture, decreased insolation, reduced soil temperatures, decreased evapotranspiration, increased nutrients, decreased herbivory, and/or lower wind desiccation. For more on the effects of site aspect as related to nurse plants, see .
Asexual regeneration: Some Joshua trees reproduce asexually by rhizomes, branch sprouts, and/or basal sprouts [56,92]. Stem damage, as well as certain environmental conditions, may encourage rhizome production and clonal growth . It is common for dormant buds beneath the periderm to grow when old stems bend or stems are injured. Joshua trees with extensive rhizome growth and clonal form are typically shorter and have less branching than single-stemmed trees. In some cases basal buds do not develop into distinct rhizomes, and stems grow adjacent to the main stem as sprouts .
Several Joshua tree populations in southern California are clonal [12,107]. Joshua trees in the Leibre Mountains form dense "impenetrable thickets" . From the southern and western slopes of Tehachapi Mountains to at least Monolith, California, some Joshua trees occur in clumps nearly 30 feet (8 m) in diameter, with 30 to 40 trunk-like stems. Plants with this growth form were once classified as Y. b. var. herbertii . Simpson  found a single clone in Gorman Creek, California, that occupied approximately 1 acre (0.4 ha) and was comprised of several hundred stems. Rowlands  and Simpson  report that the extent of cloning increases with increased elevation. Webber  indicates that in low-elevation dry areas Joshua tree rarely forms more than 1 or 2 stems, but 2 to 3 stems are common, and some clumps are found, in higher, moister areas. Cold temperatures, high winds, and abundant snowfall, common at high-elevation sites, may "restrict aerial development" thereby "necessitating elaboration of underground portions," according to Simpson . The extensive clone in Gorman Creek occurred at approximately 3,000 feet (910 m) in "montane" weather conditions with high levels of winter snowfall. Fire has also been suggested as a possible factor in the evolution of Joshua tree's clonal growth .SITE CHARACTERISTICS:
Climate: Joshua tree survives in areas with cold winters, hot summers, and little precipitation. Several researchers indicate that Joshua tree is restricted to areas with cold winter temperatures [25,89]. A dormant period is considered "essential" for Joshua tree . Leaves collected from Joshua trees in Joshua Tree National Park survived minimum and maximum temperatures of 12 °F (-11 °C) and 140 °F (59 °C), respectively . Lenz  reports that plants tolerate temperatures of -13 °F (-25 °C) to 120 °F (51°C) and annual precipitation ranges of 3.9 to 10.6 inches (98-268 mm). Hughes  reports that summer temperatures often reach 120 °F (51°C), annual precipitation ranges from 3 inches (80 mm) in dry years to 14 inches (360 mm) in El Niño years, and droughts are typical from May to July in the Mojave Desert. Joshua tree woodlands in southern California receive 6 to 15 inches (150-380 mm) of precipitation annually, and some comes in the summer months .
In Joshua tree-blackbrush communities in Utah's Washington County, the number of days with temperatures above 105 °F (40.5 °C) was a low of 8 in 1970 and a high of 15 in 1971 over the course of a 3-year study (1969-1971). The lowest temperature recorded was 4.5 °F (-15.3 °C). Most precipitation came from November through March. In 23 years the average annual precipitation ranged from a low of 3.7 inches (93 mm) to a high of 16.9 inches (428 mm) . Welsh and others  indicate that Joshua tree grows successfully as far north as Salt Lake City, Utah.
Elevation: Lower elevational limits increase with Joshua tree's more northerly distribution. This phenomenon is likely due to a complex interaction of precipitation and evapotranspiration . Joshua tree occurs within the following elevation ranges:
|Arizona||up to 3,600 |
|Death Valley, California||above 5,600 |
below 3,600 
|Intermountain West||2,800-7,200 |
|Mojave Desert||2,000-6,600 [66,89]|
Soils: Soils in Joshua tree habitats are silts, loams, and/or sands described as fine, loose, well drained, and/or gravelly [37,55,89,99,101]. Joshua tree tolerates alkaline and saline soils .
In a study of Joshua tree-blackbrush communities in Utah's Washington County, soils were predominantly shallow sandy loams. The pH was approximately 8, and soils had low organic matter. Based on a 3-year period, soil temperatures ranged from a high of 110 °F (46 °C) at a 2 inch (5 cm) depth in June, to approximately 39 °F (4 °C) in winter .
In the Great Basin-Mojave desert ecotone in Washington County, Utah, researchers studied a gradient (100-200 feet (30-50 m) elevation) from floodplain to ridgetop. Joshua tree occurred at all positions, but had greatest coverage on the floodplain and 2nd greatest coverage at the ridgetop. On the ridgetop soil depth was lowest, and there were large areas of exposed rock. On the floodplain soil depth was greatest, and there were large areas of bare ground. Shrub cover was greatest on the slopes .SUCCESSIONAL STATUS:
Based on the few studies that compare pre- and postdisturbance communities in Joshua tree habitats, it is generally true that Joshua tree abundance is often less in disturbed than undisturbed communities. In a Nevada study, old roads of Wahmonie ghost town had not been traveled for 33 years. Joshua tree was absent from old roads and occurred only on less disturbed adjacent sites . Joshua tree density was much greater on undisturbed sites than on old-field sites on eastern Mojave Desert uplands. Fields were abandoned approximately 65 years ago, and of 10 old-field sites, just 1 had Joshua tree density that was not significantly (p<0.01) lower than undisturbed sites. On another old-field site, Joshua tree density was nearly 20% of that of undisturbed sites, but for all other old-field sites Joshua tree density did not exceed 0.5 plant/ha. Density of Joshua tree on undisturbed sites averaged 75 plants/ha .
In California, Brooks and Matchett  compared burned and unburned sites in blackbrush communities in the Mohave Desert that had burned 6 to 14 years prior to the study period. Cover of woody perennials was 60% lower on burned than unburned sites, and annual forb cover was 266% greater on burned than unburned sites. Researchers noted that there were some changes in species composition but predominantly changes were in dominance. Joshua tree was present on unburned and 6-, 8-, and 14-year-old burned sites ; however, absolute Joshua tree coverage was not reported.
Joshua tree grows best in full sun conditions  and likely does not increase with browsing pressure .SEASONAL DEVELOPMENT:
Fire regimes: Presettlement fire history in the Mojave Desert is largely unknown . Many researchers have speculated on the frequency or occurrence of fire in desert ecosystems based on vegetation patterns and fuel structure. In creosotebush-white bursage communities, the open stand structure does not carry fire well except when high annual herbaceous production follows remarkably heavy winter rainfall. The ordinarily low forb and grass cover in blackbrush communities suggests that high temperatures and wind speeds and low relative humidity are necessary for burning . In 1930 Bauer  reported that fire was probably not an important influence in desert vegetation of California's Tehachapi Mountains, where Joshua tree is often important.
Leary  suggests 3 reasons that fires were historically rarer in southwestern deserts than in other ecosystems. Vegetation spacing in the deserts did not promote fire spread. Litter and fuel levels were low in the deserts, and lastly, deserts were sparsely populated and had a reduced chance of human-caused fires. However, invasive species have changed the fuel and litter loads (see Changes in fire frequency and size with nonnatives), and human-caused fires have become more common (see study in Discussion and Qualification of Plant Response by ). Loik and others  report that the current fire return interval for singleleaf pinyon-California juniper communities of Quail Mountain in Joshua Tree National Park is approximately 15 years.
Changes in fire frequency and size with nonnatives: It is well documented that increases in herbaceous nonnative vegetation, namely cheatgrass (Bromus tectorum) and red brome (B. madritensis), have facilitated increased fire incidence and fire size in the Mojave and Great Basin deserts since the mid 20th century. At the Nevada Test Site, researchers have been surveying plants since 1957. Red brome and cheatgrass have increased in density and frequency since 1957. The density of cheatgrass or red brome reached 1,000 plants/m² by 1988 . Following the very wet winter of 2004 to 2005 in Nevada's Delamar Valley, Joshua tree on unburned sites grew in dense cheatgrass up to 20 inches (60 cm) tall. In the Mohave-Great Basin desert transition zone, cheatgrass and red brome promote previously uncharacteristic large fires by filling in the shrub interspaces that once retarded fire spread in arid ecosystems .
In the western Mojave desert of California, nonnative annual grasses (red brome, cheatgrass, and Mediterranean grasses (Schismus spp.)) and forbs (chiefly, cutleaf filaree (Erodium cicutarium)) may comprise over 50% of the biomass. Fires are more frequent, since these nonnative species have altered the fuel structure and subsequent fire behavior in what was a relatively fire-resistant landscape. Nonnative annual grass stems are persistent, and nonnative litter decomposes slowly, providing fuel for frequent fires. Red brome contributed to substantial increases in fire frequency in the Mojave and Colorado deserts of California since the 1970s. From 1980 to 1995, 77% of the total BLM-managed Mojave Desert areas burned. Approximately 25% of the fires were started by lightning, while the other 75% were human caused. Most fires burned in the summer (May-September), and most fires in BLM-managed areas of the Colorado Desert burned along the Mojave Desert ecotone near Joshua Tree National Park [14,15].
Fires were rare in Joshua Tree National Park until about 1965. Since the establishment of red brome and cheatgrass, fires have become more frequent and more severe. Before 1965 most lightning fires burned less than 0.25 acre (0.1 ha). In 1979 the Quail Mountain Fire burned 6,000 acres. In 1995, the Covington Fire burned 5,158 acres (2,087 ha), and 4 years later 13,894 acres (5,623 ha) of Joshua Tree National Park burned .
The following table provides fire return intervals for plant communities and ecosystems where Joshua tree is important. For further information, see the FEIS review of the dominant species listed below.
|Community or Ecosystem||Dominant Species||Fire Return Interval Range (years)|
|basin big sagebrush||Artemisia tridentata var. tridentata||12-43 |
|Wyoming big sagebrush||Artemisia tridentata var. wyomingensis||10-70 (x=40) [103,116]|
|saltbush-greasewood||Atriplex confertifolia-Sarcobatus vermiculatus||<35 to <100|
|desert grasslands||Bouteloua eriopoda and/or Pleuraphis mutica||<35 to <100|
|grama-galleta steppe||Bouteloua gracilis-Pleuraphis jamesii||<35 to <100|
|blue grama-tobosa prairie||Bouteloua gracilis-Pleuraphis mutica||<35 to <100 |
|cheatgrass||Bromus tectorum||<10 [84,114]|
|blackbrush||Coleogyne ramosissima||<35 to <100|
|western juniper||Juniperus occidentalis||20-70|
|Rocky Mountain juniper||Juniperus scopulorum||<35 |
|creosotebush||Larrea tridentata||<35 to <100 [42,80]|
|pinyon-juniper||Pinus-Juniperus spp.||<35 |
|Colorado pinyon||Pinus edulis||10-400+ [30,35,52,80]|
|galleta-threeawn shrubsteppe||Pleuraphis jamesii-Aristida purpurea||<35 to <100 |
|mesquite||Prosopis glandulosa||<35 to <100 [37,80]|
|oak-juniper woodland (Southwest)||Quercus-Juniperus spp.||<35 to <200 |
© 2005 Steven Perkins
|© 2003 Monty Rickard|
IMMEDIATE FIRE EFFECT ON PLANT:
Apical meristems growing high above the ground and fire-resistant bark on mature Joshua tree trees may allow Joshua tree to survive fire in some vegetation types [18,43]. Vogl  suggests that Joshua tree becomes more fire resistant once the dead leaves that encourage fire spread into the crown are shed from its trunk. However, plants may be top-killed  or killed by fire . Fires that burn into Joshua tree crowns often kill the plant .
DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
No additional information is available on this topic.
PLANT RESPONSE TO FIRE:
Joshua tree may sprout from the root crown, rhizomes, or branches following fire [18,18,34,43,64,64,92,104]. Some have described postfire sprouting as "vigorous" . Others suggest that postfire sprouting may be linked to plant size , fire temperature , or differences in Joshua tree varieties .
If killed by fire, Joshua tree recolonization depends on off-site seed sources . The current available literature (2006) does not address effects of fire on Joshua tree seed. In the laboratory, however, germination of Joshua tree seeds collected from several Mojave Desert populations was tested following heat treatments. Germination after 5 minutes at 190 °F (90 °C) was significantly higher (p<0.01) than for untreated seeds . For a more complete summary of this study, see Germination. In Joshua Tree National Park, researchers did not find Joshua tree seedlings on burned sites but found many young Joshua tree plants within blackbrush canopies on unburned sites. Researchers suggested that recovery of blackbrush may be necessary for Joshua tree seedling establishment . Lenz  reports that new Joshua tree seedlings are easily concealed in nurse plants, which may be an important consideration in postfire sampling. The recovery of Joshua tree woodlands to prefire conditions may take decades or centuries .
Sampling issues: In many cases, Joshua tree abundance is low on both burned and unburned sites, and small quadrat understory sampling does not allow for accurate estimations of Joshua tree abundance . Joshua tree is often missed with small quadrat sampling (see results presented in ). In stands with high Joshua tree density (over 150 trees/ha), 0.1 to 0.2 ha is an adequate sampling area; in areas with low Joshua tree density (<50 trees/ha) a 0.5- to 1.0-ha sampling area is recommended. Accurate estimates of Joshua tree cover or density typically require a sampling area of at least 0.2 acre (0.1 ha) .
Postfire sprouting: According to Emming , Y. b. var. jaegeriana, which is distributed at the upper limits of Joshua tree's range, often sprouts following fire. Yucca b. var. brevifolia, found at lower latitudes than Y. b. var. jaegeriana, normally establishes by seed on burned sites .
On the Nevada test sites, rhizome and root crown sprouts were most common on mid-size burned trees. Sprouting percentages were lower for smaller- and larger-sized trees. The number of sprouts on burned Joshua tree plants was evaluated in postfire year 1 after a 20 July lightning fire. Survival of sprouts beyond postfire year 1 was not reported. These findings suggest that long-term fire protection in Joshua tree stands may affect postfire regeneration strategies. Recurrent fires and a complete removal of fire may both be detrimental to Joshua tree stands. A summary of postfire sprouting is provided below :
|Tree height (m)||Number of trees||Percentage with sprouts|
Baldwin  also reports that plant size affected sprouting following an August fire in Joshua Tree National Park. A more complete summary of this study is provided in the discussion below.DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
Two days after a late June fire in California's San Bernardino County, researchers found that some aboveground stems had survived and that rhizomes approximately 1 foot (0.3 m) below ground were uninjured. Fire characteristics were not described .
Joshua tree sprouts were observed 1 year following an August fire in Joshua Tree National Park. High winds were reported but other fire characteristics were not. Three to four sprouts sometimes occurred where a single tree had burned. Sprouts were much more common from small Joshua trees (under 10 feet (3 m)) tall that were approximately 5 to 50 years old than from large burned trees. The researcher suggested that large trees had more area covered with fire-fueling dead leaves and may have burned at a higher temperature than small trees. Mortality was greater for large than small trees .
Researchers found Joshua tree root crown sprouts and canopy sprouts following a July 1995 lightning fire in the Lower Covington Flats area of Joshua Tree National Park. Twenty-eight percent of Joshua tree plants along five 1,000-m² belt transects had root crown sprouts 16 months following fire. Canopy sprouts occurred in 4% of the plants, but no plant had both rhizome or root crown and canopy sprouts. Researchers speculated that root crown sprouting was likely related to extent of tissue death, which was determined by fire temperature .
Joshua tree was classified as an increaser following fire when burned and unburned sites were compared in the Joshua tree woodland-singleleaf pinyon-California juniper ecotone in the Victor Valley of the southwestern Mojave Desert. Six burned and adjacent unburned sites were evaluated. Sites had burned approximately 3 to 13 years earlier in June, July, or August. No other fire characteristics were reported. Joshua tree density was determined using 0.025-acre quadrats. Density averaged 71 individuals/acre on unburned sites and 142.7 individuals/acre on burned sites. Frequency of Joshua tree was slightly lower on burned (29.2%) than on unburned (36.3%) sites. The clumped nature of postfire sprouts explained the increases in density and decreases in frequency. Methods for delineating individual plants for density estimates were not reported. Mature trees without shaggy dead leaves at the base of the plant had outer periderm exposed, which reduced the chance of fire in the crowns and increased the chance of apical meristem survival .
Researchers found that Joshua tree height and basal diameter generally increased with increased time since fire in a study of burned sites in Joshua Tree National Park. Visited sites had burned 1 to 28 years previously. Sites with longer recovery time had the most Joshua trees growing independently of a nurse plant and the fewest fire surviving plants (identified as sprouts in the table). Sites burned 9 to 12 years earlier were a mixture of Joshua tree sprouts and seedlings (mostly emerging from the canopy of nearby vegetation), while sites burned 1 year earlier were populated only by sprouting Joshua trees. Study results are summarized below :
|Time since fire (years)||Number of trees measured||Independent||Canopy emergent||Sprout (based on fire scars)|
|Lost Horse Valley||28||22||77.2||9.1||13.6|
|Lost Horse Mine||9||12||16.6||41.6||41.6|
|Lower Covington Flats||1||37||0||0||100|
In Joshua Tree National Park, Joshua tree was absent from early postfire communities, but density on burned and unburned sites was equal 10 years following fire. Joshua tree was only found on unburned sites following an August lightning fire in the western part of Joshua Tree National Park. Burned areas were visited 3 months, 6 months, and 8 months following the fire. Fire characteristics were not reported . In other studies, Joshua tree density was equal on 10-year-old burned and unburned sites in Joshua Tree National Park. No information about the fire was provided . Joshua tree was present on 3 burned sites in the Mojave Desert. Sites had burned 6, 8, and 14 years earlier. Neither absolute coverage percentages on burned and unburned sites nor fire characteristics were reported .
Joshua tree density was greater on unburned than burned sites in southern Nevada's Spring Mountain area. Joshua tree was absent from sites burned 8 and 13 years before the study, but present on sites burned 17 years earlier. Differences between burned and unburned densities were greatest on severely burned sites. Generally, burned soils had higher soil temperatures but lower organic matter and moisture contents than unburned soils. The table below summarizes fire characteristics and Joshua tree density on burned and unburned plots :
Hughes  provides purely descriptive studies of burned and unburned sites within the Big Hole grazing allotment in northwestern Arizona. Sites were visited in late 1990s or early 2000s. In 1 area, the prefire community was dominated by blackbrush, creosotebush, and Joshua tree. The area burned in the 1940s, and most of the burned area was dominated by unidentified annual species. Hughes  reported that Joshua tree and other shrubs were returning to the burned sites. In another area burned between 1970 and 1980, annuals were dominating burned sites, and Joshua tree was conspicuous only on unburned sites .FIRE MANAGEMENT CONSIDERATIONS:
Removal of fire from Joshua tree habitats, while likely impossible, may also not be prudent. As reported above , postfire sprouting is most common in mid-sized Joshua trees, which suggests that recurrent fires and complete removal of fire may harm Joshua tree stands.Several threatened and endangered species that are associated with Joshua tree woodlands in Joshua Tree National Park need to be considered in fire management decisions for this area .
Small mammals: Squirrels, woodrats, jackrabbits, kangaroo rats, and mice utilize Joshua tree habitats and/or feed on Joshua tree fruits. In a review, McKelvey  reports that Mexican woodrats have been observed climbing Joshua tree to cut its spiny leaves, which they use to protect burrow entrances. Merriam's kangaroo rats and southern grasshopper mice are considered "diagnostic" of Joshua tree woodlands in the San Gabriel Mountains of California. The Panamint kangaroo rat is also common in Joshua tree woodlands. Coyotes are the dominant carnivore . Joshua tree has also been recovered from macrofossil woodrat middens in the western Mojave Desert  and in Death Valley .
Antelope squirrels cache Joshua tree seeds . California ground squirrels climb Joshua tree and consume fruits and seeds, and white-tailed antelope squirrels collect over mature dry fruits. The mature fruit coating is cracked and some seeds are consumed, while others fall to ground and are dispersed by wind . The seedlings are a food source for black-tailed jackrabbits in Lanfair Valley, California (Griffith, personal communication in ).
In the Coso area of California's Inyo County, Mohave and white-tailed antelope squirrels were observed in June and July feeding on Joshua tree fruits. Of 22 individually sighted Mohave ground squirrels, 20 were observed harvesting Joshua tree seeds. Mohave ground squirrels worked in the tops of Joshua trees nearly continuously from 3 hours after sunrise to 1 hour before sunset. One Mohave ground squirrel was observed for 4 hours working on clumps of Joshua tree fruits. Every 15 to 20 minutes the ground squirrel made trips back to a Joshua tree, where the seeds were cached in a burrow near the base of the trunk. Joshua trees were apparently a preferred Mohave ground squirrel food. There were 16 Mohave ground squirrels and 21 white-tailed antelope squirrels observed at Joshua tree fruit clusters in an approximately 0.4 km² area on 3 July from 2:45 to 3:30 p.m. Trees never had more than 1 Mohave ground squirrel, but there were as many as 7 white-tailed antelope squirrels in a single tree. Mohave and white-tailed antelope squirrels occurred together in Joshua trees, but aggressive behavior was only avoided in large trees with 2 or more widely spaced fruit clusters .
Birds: Numerous bird species utilize Joshua tree and Joshua tree habitats in the Mojave Desert. Twenty-five bird species use Joshua tree as a nesting tree. Scott's orioles nest in the crown; ladder-backed woodpeckers and northern flickers nest in trunk or limb holes. American kestrels and loggerhead shrikes use Joshua tree as a perch when hunting. Many bird species feed on Joshua tree blossoms .
A bird survey in Joshua Tree National Park concentrated on populations occupying habitats with cliffs and those without. Joshua tree occurred only on sites without cliffs. Fourteen species were found in noncliff habitats. American kestrels, common nighthawks, ash-throated flycatchers, cactus wrens, northern mockingbirds, loggerhead shrikes, and orange-crowned warblers were exclusive to noncliff sites . A bird census of the Great Basin-Mojave desert ecotone found that the ecotone provided habitat to 22 resident bird species. Thirteen species were encountered along the Mojave Desert transect. Ladder-backed woodpeckers were found solely on Joshua tree .
Herptiles: Joshua tree provides protection and feeding sites for some Mojave Desert lizards. The small desert night lizard is often found in Joshua tree bark and clusters of dead leaves , as are desert spiny lizards . A night lizard that was not identified to species is considered dependent on Joshua tree. Joshua tree bark provides protection and shelter, while the night lizard feeds on insects . In southwestern Utah, Joshua tree is common in desert tortoise habitats, but specific use of Joshua tree was not reported .
Arthropods: Spiders, scorpions, beetles, and white ants utilize dead Joshua tree leaves and fallen branches as homes in the Mojave Desert . The Navaho yucca borer lays eggs in young Joshua tree stems produced from rhizomes but avoids stems produced from seed .
Palatability/nutritional value: Few studies report on the palatability or nutritional composition of Joshua tree. Dittberner and Olson  rate the palatability of Joshua tree as poor for cattle, domestic sheep, horses, pronghorn, elk, mule deer, and small mammals. In Los Angeles County, Joshua tree fruits collected in early June had an average sugar content of 11.6%. In early July, the average was 14.5%. When Joshua tree fruits are fully ripe the sugar content may exceed 20% .
The composition of Joshua tree leaf blades collected in Yucca Flat, Nevada is presented below. Researchers indicated that phosphorus and potassium contents decreased with age, while sodium calcium, silicon, iron, boron, aluminum, and titanium increased with age .
In a review, Webber  reports the chemical composition of Joshua tree seed pods as follows:
Cover value: See the species of interest above in Importance to Livestock and Wildlife for information on Joshua tree's use as cover. Dittberner and Olson  report that Joshua tree provides poor cover for livestock and native ungulates.VALUE FOR REHABILITATION OF DISTURBED SITES:
In the eastern part of the Mojave Desert, Joshua trees between 3 and 8 feet (0.9-2 m) tall with just a few branches were salvaged from a future gold mine site. Plants were grown close together in rows and given supplemental water. After 2 years, just 9% of the 1,447 trees had died; 36% were in excellent health (no yellow leaves), and 56% were in poor health. The study indicated that large plants (up to 50 years old) could be salvaged from future mine sites for later revegetation of the disturbed areas .OTHER USES:
Native people of the Mojave Desert used Joshua tree for food and in construction. Cahuilla people of southern California used Joshua tree fibers to make sandals and nets and consumed Joshua tree blossoms . Red Joshua tree rootlets were utilized as a dye for baskets and blankets [2,55], and sweet Joshua tree flowers were roasted and eaten by Native people . Joshua tree seeds were eaten raw or ground into a mash and cooked by southern California Natives . In a review, Webber  reports that Joshua tree beams and timber have been found in ancient cliff dwellings.OTHER MANAGEMENT CONSIDERATIONS:
Climate change: Numerous studies have investigated the potential changes in Joshua tree growth and distribution based on climate change and elevated CO2 levels. Huxman and others  conducted experiments on Joshua tree growth and photosynthetic capabilities under elevated CO2 levels and increased temperatures. Dole and others  modeled changes in Joshua tree's distribution based on climate change and increased CO2 levels. Based on work by Loik and others , the lethal low temperature tolerance for Joshua tree seedlings is lowered by 2.9 °F (1.6 °C) under doubled CO2 concentrations. The model predicted that a considerable portion of Joshua tree's current range would become unsuitable, but that some new habitat would be made suitable. However, occupation of new habitats would depend on successful recruitment and availability of the new habitats. For maps of the future distribution of Joshua tree with climate change, increased CO2 levels, and/or altered freezing tolerance, see .
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