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 [17].

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 [109]. 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 [20].

In California, Brooks and Matchett [16] 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 [16]; however, absolute Joshua tree coverage was not reported.

Joshua tree grows best in full sun conditions [37] and likely does not increase with browsing pressure [41].

FIRE ECOLOGY

• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

Fire regimes: Presettlement fire history in the Mojave Desert is largely unknown [42]. 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 [42]. In 1930 Bauer [7] reported that fire was probably not an important influence in desert vegetation of California's Tehachapi Mountains, where Joshua tree is often important.

Leary [58] 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 [60]). Loik and others [64] 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 [44]. 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 [26].

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 [34].

The following table provides fire return intervals for plant communities and ecosystems where Joshua tree is important. Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

FIRE EFFECTS

IMMEDIATE FIRE EFFECT ON PLANT DISCUSSION AND QUALIFICATION OF FIRE EFFECT PLANT RESPONSE TO FIRE DISCUSSION AND QUALIFICATION OF PLANT RESPONSE FIRE MANAGEMENT CONSIDERATIONS
	© 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 [105] 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 [34] or killed by fire [14]. Fires that burn into Joshua tree crowns often kill the plant [26].

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 [18,18,34,43,64,64,92,105]. Some have described postfire sprouting as "vigorous" [34]. Others suggest that postfire sprouting may be linked to plant size [43], fire temperature [64], or differences in Joshua tree varieties [26].

If killed by fire, Joshua tree recolonization depends on off-site seed sources [18]. 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 [54]. 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 [64]. Lenz [61] 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 [26].

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 [88]. Joshua tree is often missed with small quadrat sampling (see results presented in [64]). 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) [88].

Postfire sprouting: According to Emming [26], 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 [26].

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 [43]:

Baldwin [5] 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.

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 [92].

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 [5].

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 [64].

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 [96].

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 [64]:

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 [59]. 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 [5]. 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 [16].

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 [60]:

Hughes [40] 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 [40] 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 [40].

Removal of fire from Joshua tree habitats, while likely impossible, may also not be prudent. As reported above [43], 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.

MANAGEMENT CONSIDERATIONS

• IMPORTANCE TO LIVESTOCK AND WILDLIFE
• VALUE FOR REHABILITATION OF DISTURBED SITES
• OTHER USES
• OTHER MANAGEMENT CONSIDERATIONS

Small mammals: Squirrels, woodrats, jackrabbits, kangaroo rats, and mice utilize Joshua tree habitats and/or feed on Joshua tree fruits. In a review, McKelvey [72] 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 [103]. Joshua tree has also been recovered from macrofossil woodrat middens in the western Mojave Desert [70] and in Death Valley [110].

Antelope squirrels cache Joshua tree seeds [55]. 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 [61]. The seedlings are a food source for black-tailed jackrabbits in Lanfair Valley, California (Griffith, personal communication in [61]).

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 [118].

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 [55].

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 [19]. 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 [107].

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 [72], as are desert spiny lizards [89]. 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 [68]. In southwestern Utah, Joshua tree is common in desert tortoise habitats, but specific use of Joshua tree was not reported [69].

Arthropods: Spiders, scorpions, beetles, and white ants utilize dead Joshua tree leaves and fallen branches as homes in the Mojave Desert [72]. The Navaho yucca borer lays eggs in young Joshua tree stems produced from rhizomes but avoids stems produced from seed [68].

Palatability/nutritional value: Few studies report on the palatability or nutritional composition of Joshua tree. Dittberner and Olson [24] 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% [61].

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 [106].

In a review, Webber [108] 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 [24] report that Joshua tree provides poor cover for livestock and native ungulates.

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 [32].

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 [8]. 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 [55]. Joshua tree seeds were eaten raw or ground into a mash and cooked by southern California Natives [79]. In a review, Webber [108] reports that Joshua tree beams and timber have been found in ancient cliff dwellings.

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 [45] conducted experiments on Joshua tree growth and photosynthetic capabilities under elevated CO₂ levels and increased temperatures. Dole and others [25] modeled changes in Joshua tree's distribution based on climate change and increased CO₂ levels. Based on work by Loik and others [63], 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 [25].

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