Botanical Description

This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Identification keys are available (e.g., [155]). Some keys treat western and eastern Joshua trees as varieties rather than species (e.g., [86,89,119,179]) (see Taxonomy).

Joshua tree’s distinctive and iconic form inspires poetic descriptions, from a “delight to the eye and a fascinating feature of the western landscape” [109] to “the most repulsive tree in the vegetable kingdom” [60] with branches that reach “weirdly heavenwards” [183]. It has been likened to “a misshapen pirate with belt boots hands and teeth stuck full of daggers” such that “a landscape filled with Joshua trees has a nightmare effect even in broad daylight: at the witching hour it can be almost infernal” [41]. More specifically, Joshua tree is an evergreen monocot adapted to dry habitats, and its morphology varies depending on species and site characteristics [105,141,167].

Aboveground Characteristics

Photo Credit

National Park Service photo by Michael Faist.

Figure 4—Western Joshua tree in flower with tightly packed individual flowers along the peduncle. National Park Service photo.

Joshua tree is a tree-like succulent with dichotomous or pseudo-dichotomous branching [105]. Mature plants can have 50 to 200 branches [51]. Old leaves of both species are persistent and remain on the stem, reaching the ground when plants are young. As the plant matures, the trunk grows to 0.5 to 0.9 m in diameter with roughened bark [174], and old leaves fall from the trunk but remain along branches below new leaves.

Flowers are tightly packed on a ≈30-cm-long peduncle [44] (fig. 4). Joshua tree seeds are relatively large, flattened ovals [10] lacking wings [173], and they are enclosed in indehiscent pods (i.e., fruits) (fig. 5). Western and eastern Joshua trees differ in their vegetative and floral morphologies [164,167].

Western Joshua tree generally grows to 5 m to 12 m [86,179] or rarely to 15 m [11,48,119] tall. One specimen, reported by the New York Botanic Gardens in 1932, was apparently over 24 m in height with a circumference of nearly 3 m, although this individual was set on fire and destroyed [73]. Western Joshua tree first branches at 2 to 3 m above the ground, generally at first flowering [179]. Branching is not truly dichotomous [86], but rather is sympodial [167]. Leaves are typically 15 cm to 35 cm long [69,179]. Flowers are nearly spherical, with broadly egg-shaped, fleshy, cream-colored petals that are strongly curved inward and never fully expand. Ovaries are cone-shaped and expand from the base, and fruits are ovoid [105].

Photo Credit

Photo on left by Ashley Miller, with some rights reserved. Photo on right by Diane Etchison, iNaturalist, with some rights reserved.

Figure 5—Left: Joshua tree seeds. The top left seed was presumably damaged by yucca moth larvae (CC BY-NC 3.0 US). Right: A cluster of western Joshua tree fruits at the end of a branch (CC BY-NC 4.0).

Eastern Joshua tree typically grows to 3 to 6 m tall and first branches about 1 m above the ground [86,89,179]. Plants display true dichotomous branching when young [86] and initiate sympodial branching at first flowering [141]. Leaves are less than 22 cm long [70,179]. Flowers are narrowly bell-shaped, conspicuously swollen at the base, and somewhat constricted above. Their narrowly oblong petals are usually greenish and curved backward at the tips. Ovaries are narrowly egg-shaped, and fruits are ellipsoid [105].

Photo Credit

National Park Service photos by Rob Hannawacker.

Figure 6—Left: Exposed western Joshua tree roots, with roots originating from the bottom of the widened base. Right: Partially exposed base of a western Joshua tree. National Park Service photos.

Belowground Characteristics

Photo Credit

Photo from Webber (1953) [179].

Figure 7—Main stem, roots, and rhizomes of a western Joshua tree. Note that the main stem is located to the right and is dark against the dark background. 

Joshua trees have shallow, wide-spreading root systems [25,31,75,110]. The root system is laterally extensive for water uptake and plant stabilization [31]. Many long, tough, and cordlike roots [179] grow diagonally down and out in relatively straight lines [95], starting from inside the margin of the flat, distinctly lobed base of the plant [141,179] (fig. 6). The enlarged “elephant-foot-like” base of the trunk of mature trees [179] extends only about 0.3 m into the ground, suggesting Joshua trees are primarily supported by their roots. Joshua tree roots are capable of contracting along most of their length to pull the shoot farther down in the soil [121,141]. This helps anchor the plant, which likely has relatively high anchorage demands in the dry, shifting soils it typically occupies, due to its arborescent growth form and lack of a taproot [121]. In a blackbrush-eastern Joshua tree community, eastern Joshua tree roots were found in a soil pit dug 11 m from the nearest Joshua tree [25].

Rhizome production and clonal growth seem to be more common in western Joshua tree populations and populations at high elevations [141], which receive relatively more precipitation [133,179] (see Vegetative Reproduction and Regeneration). Joshua tree rhizomes are 1 to 5 cm in diameter [133], unbranched, succulent, and covered with bud scales [141]. Rhizomes grow horizontally in the soil several centimeters to nearly a meter below the soil surface and may produce clonal aboveground stems about 1 m from the parent plant [133] (fig. 7). After producing aboveground stems, rhizomes become woody and hard, lose their bud scales, and may produce lateral branches [141,179]. The bark-like periderm on these mature rhizomes is thin, dense, and hard—unlike the corky periderm on aboveground stems. Rhizomes can be 3 m long or more. Rhizome diameter is greatest at the base of aboveground stems, and roots commonly occur along the entire rhizome length. In rocky substrates, irregular rhizome growth is common [141].

Joshua trees may also sprout from primary root axes [115] or from nodules formed slightly above the soil surface [179]. Nodules and sprouts apparently do not produce roots and rarely, if ever, reach the size expected of an adult Joshua tree [179] (fig. 8).

Photo Credit

Photo from Barrios et al. (2017) [14].

Figure 8—A dead western Joshua tree stem and adjacent resprouts 18 years after a fire on Edwards Air Force Base. Resprouts from the base, such as these, may be vulnerable to toppling along with the dead stem [81], because they share the same root system.

Raunkiaer Life Form

• Phanerophyte [129]

Stand Structure

Joshua trees generally occur as a sparse to open emergent layer and as scattered individuals [120]. Across a 1,507,100-ha study area that included areas where it is not dominant, average density of western Joshua trees ranged from 4 to 7 individuals/ha [186]. In four studies that collectively surveyed 277 Joshua tree stands, density varied from about 4 to 450 individuals/ha (table 4) [53,133,147,153].

Population structure varies regionally. Populations outside of modeled climate refugia—especially those at the hot and dry southern and low-elevation extents of Joshua tree’s range—are generally comprised of mature individuals with limited recruitment [43,74]. For example, surveys in Joshua Tree National Park, at the trees' southern range limit, showed reduced seedling recruitment consistent with projected habitat losses [15,153]. In contrast, mature Joshua trees in the northern parts of its range are often surrounded by seedlings and saplings [43]. Where young Joshua trees are prevalent, they typically occur at the leading edges of established populations, and their abundance tapers off within a few hundred meters [56].

Joshua tree stand structure also varies locally. For example, at six sites in northern Los Angeles County, California, most western Joshua trees were 3 to 5 m tall, with fewer individuals in smaller size classes, indicating that even if mortality of young western Joshua trees is very low, there are not enough young individuals to replace older individuals [21]. Similarly, size-class data from three other California Joshua tree stands suggested that establishment had also declined in those areas, with more trees in the 2.5 to 3 m height class than in the smallest size classes [21]. In contrast, at a site in adjacent San Bernardino County, western Joshua trees showed a height-class distribution that would be expected for a sustainable or increasing population. Although relatively few Joshua trees were recorded in the smallest height class (0–0.5 m), possibly due to difficulties in detecting young plants or fewer establishment events in recent years, the largest number of individuals were in the 0.5 to 1 m height class (≈430 plants). Fewer than half as many individuals occurred in the 1 to 1.5 m and 1.5 to 2 m height classes (≈160–175 plants), and the number of individuals in taller height classes fluctuated at relatively low numbers (0–≈100 plants) up to 12.5 m. This suggests that there were enough young individuals to replace the oldest individuals and thus maintain the population size, despite higher mortality of young individuals [21]. Data from several National Park Service units in California and Nevada [53] and data aggregated from 11 sites in Kern County, California [21] also found stable levels of recruitment sufficient to maintain western Joshua tree populations in those areas. Similarly, 83% of western Joshua trees at a preserve near Red Rock Canyon State Park measured <1 m tall [21], indicating the population is stable or growing.

Stand structure differs between western and eastern Joshua tree populations. In populations of eastern Joshua trees, stands are generally contiguous with only small, scattered patches lacking eastern Joshua trees within the stand. In contrast, populations of western Joshua trees more commonly have gaps where western Joshua trees are not present, especially in the southwestern part of its range. This has been attributed in part to greater amounts of disturbance, such as human developments and fire, in western Joshua tree populations [56]. In the southwestern part of its range, western Joshua tree readily reproduces by rhizomes, forming dense clusters of clones (see Vegetative Reproduction and Regeneration).

Joshua tree stand density is typically higher at cooler, higher sites; a combination of lower seed germination, lower seedling recruitment, and higher drought mortality contribute to lower stand densities on hotter sites. Across 10 sites throughout the ranges of both species, Joshua tree density increased as the average temperature of the warmest month decreased from about 39 to 22 °C [147], although density varies widely, so it may not be appropriate to extrapolate this pattern throughout its range [167]. Similarly, across 20 sites throughout the ranges of both species, Joshua tree density trended higher with increasing elevation. Density was highest at two of the three sites over 1,750 m elevation (300 and 450 individuals/ha) [133].

See the U.S. Fish and Wildlife Service’s Species Status Assessment Report for Joshua Trees [167] for a detailed review of regional population density, structure, and demographics.

Seasonal Development

Joshua trees generally flower from March through May (table 5). Seeds mature in July [5] and are dispersed by rodents during the next 3 to 6 months (see Seed Dispersal). In Clark County, Nevada, planted seeds germinated in cohorts when relatively high soil moisture was accompanied by warm soil temperatures, which occurred in April and September. Eleven percent and 14% of available seeds germinated, respectively. A third cohort also germinated in January but only comprised 1% of available seeds. Seedlings had the best likelihood of surviving their first year when they germinated in September [130].

About 80% of annual growth occurs from January through May [145].

Regeneration Processes

In many parts of its range, Joshua tree reproduces primarily from seed. On some sites, it reproduces vegetatively from rhizomes, which may be the predominant method of regeneration in those areas. It may resprout following top-kill or injury, but the degree of resprouting varies, and survival of resprouts may be low.

Recruitment is limited by high rates of seed predation and low rates of germination and seedling establishment, which largely occur in episodic peaks in response to favorable climatic and microhabitat conditions [55,130]. Within a population, some individuals flower in most years; however, mass flowering occurs infrequently [18,194]. Many seeds are damaged by obligate pollinator moth larvae [22] or eaten by granivorous rodents and ants [173,195]. Germination is constrained to a narrow set of conditions that may not occur every year, and seed germinability decreases relatively rapidly [130]. Establishment of seedlings is further limited by herbivory by jackrabbits [55] and sparse availability of nurse plants [106].

Breeding System and Pollination

Joshua tree has perfect flowers [5,125] and is an obligate insect-pollinated species. Self-pollination is unlikely because anthers develop about 2 days later than the stigma in a given flower [158], and Joshua tree’s pollinator moths have only been observed pollinating the youngest flowers, in which the anthers were not yet developed [44].

Sexual reproduction in Joshua tree is dependent on flower pollination by two yucca moth species, with each Joshua tree species having one obligate moth pollinator. The western Joshua tree moth pollinates western Joshua tree, and the eastern Joshua tree moth pollinates eastern Joshua tree. These moths actively pollinate Joshua tree flowers then oviposit eggs within flower ovaries. Upon hatching, the moth larvae feed on developing seeds (fig. 5, left). Distributions of the two moth species do not overlap [124], except in the hybrid zone in Tikaboo Valley [134,151]. The two moth species do not hybridize [143].

Observations of active yucca moth pollination are very limited in the literature, with only Trelease (1893) having described western Joshua tree moth pollination of western Joshua tree [158] and Cole et al. (2017) having described eastern Joshua tree moth pollination of eastern Joshua tree [44]. Females of both yucca moth species collect pollen, carry it to another flower, actively transfer pollen to the stigma, and then oviposit eggs into the style, ensuring the production of seeds, many of which their larvae will consume [22,144] (see Seed Production and Predation).

It is not clear how often yucca moths move between flowers of the same plant or between different plants. Trelease (1893) noted that western Joshua tree moths were slow to take flight and sailed to the ground not far from the plant when they did, apparently restricting cross-pollination to flowers on the same plant to some degree. He also noted there “must be” frequent flights between plants in quiet weather [158].

Morphological differences between pollinator moths help explain the unidirectional gene flow between western and eastern Joshua trees [44,71,124] (see Taxonomy). Gene flow occurs from eastern Joshua trees to western Joshua trees because eastern Joshua tree moths can successfully pollinate western Joshua trees—its nonhost plant—and have lower host fidelity, but western Joshua tree moths apparently cannot oviposit or pollinate eastern Joshua trees and primarily visit their host plant [151]. Eastern Joshua tree moths produce fewer larvae on nonhost plants than can be explained if larval emergence was proportional to visitation by adult moths, suggesting that eastern Joshua tree moths have lower fitness when they lay eggs in nonhost plants [142].

Yucca moths have relatively limited dispersal distances. When Mojave yucca moth, a related yucca moth that pollinates Mojave yucca, was introduced into a moth-free area, moths dispersed as far as 117 m [104]. Yucca moths moved pollen of Adam’s needle 6 m to 293 m (mean: 118 m) from the pollen source plant, although a second study reported that yucca moths moved Adam’s needle pollen less than 51 m [104]. Further research is needed to determine how far Joshua tree’s yucca moths may travel when pollinating or dispersing.

Although it has not been observed in Joshua tree plant-pollinator relationships, collapse of obligate mutualisms with pollinators is possible. An apparent local population collapse of pollinators of the closely related banana yucca led to the complete failure of fruit production of thousands of plants surveyed [107].

Seed Production and Predation

Estimates of years to maturity for Joshua tree vary widely. Many sources suggest maturity is based on height thresholds, which are determined by growth rates influenced by differences in water availability and other climate factors (see Plant Growth and Mortality). Joshua trees typically flower for the first time when they are about 2 m tall [141,153] to 3 m tall [158]. Because of their slow growth rate, Joshua tree individuals may be 30 years old [55] to 90 years old (Sulenski 1972, cited in [132]) before they reach these heights and flower for the first time. A cohort of 30-year-old Joshua trees in Yucca Valley, Nevada, had not yet flowered [55]. The oldest individuals in a California botanical garden were 36 to 40 years old and 0.6 to 2.4 m tall and had never flowered. However, the earliest record of flowering occurred in the same botanical garden; four Joshua trees flowered when they were 19 years old and 1.2 to 1.8 m tall [57].

Flowering and seed production by Joshua tree are irregular and vary widely from year to year [53,124,173,184]. Flowering branches can have up to 200 flowers [44] and about 900 flowers/plant [107]. Flowering occasionally occurs en masse [135]—in most years, at least some Joshua tree individuals bloom, but only rarely do most bloom in the same year [18]. Climate models indicate Joshua trees flower slightly more often as variability of annual precipitation increases [194]. Observations and models also suggest that warming may increase flowering in Joshua tree [147] (see Management Under a Changing Climate). However, higher temperatures are also negatively correlated with stand density, suggesting that warmer temperatures may limit seedling establishment and recruitment (see Seedling Establishment and Mortality) [147].

Several possible explanations for years of high seed production (i.e., masting) in Joshua tree populations have been put forth in the literature, although none have been fully investigated. Masting may be an adaptation for pollinator attraction [67], and/or an adaptation for seed predator satiation [22,53,173,194]. Masting may be stimulated by stress, cold winter temperatures [135], greater than average precipitation [30,107,109,194], and/or accumulation of sufficient resources to support masting [22,194].

Information is limited about how disturbance affects flower and fruit production in Joshua tree, although they are likely to decrease on an area basis on sites where reproductive individuals are killed. One study in Beaver Dam Wash in southwestern Utah, found that flower and fruit production did not differ among individual eastern Joshua tree plants in unburned and burned edge plots. Eastern Joshua trees produced an average of 862 flowers/plant and 92 fruits/plant in unburned plots and an average of 956 flowers/plant and 111 fruits/plant in burned edge plots. On an area basis, fruit and flower production were similar in unburned and burned edge plots, despite lower plant cover and density on burned edges. In the interior of the burn, plant cover was significantly lower than unburned areas, and no fruiting or flowering occurred [108], possibly because surviving individuals were more likely to be resprouts not yet old enough to reproduce.

Yucca moth larvae consume Joshua tree seeds in the fruits, before seeds disperse. In a population near the southern California coast, 7% of seeds were destroyed by larvae. Most fruits contained larvae (61%); the remaining contained no larvae, likely because moth eggs failed to hatch [91]. In the San Bernardino Mountains of California, proportionately more seeds were damaged by yucca moth larvae in a year with low seed production compared to a year with high seed production. In the year with high seed production, 66% of inflorescences produced fruit, and yucca moth larvae infested 47% of fruits where they destroyed 20% of seeds in infested fruits. In the following year with low seed production, 46% of inflorescences produced fruit, and yucca moth larvae infested 83% of fruits where they destroyed 43% of seeds in infested fruits [22].

Rodents, yucca moth larvae, and ants consume many—sometimes nearly all—Joshua tree seeds produced in a given year [130,173,184,195]. When seeds were planted in the Piute Valley of southern Nevada in caged and uncaged plots, about 52% of seeds were apparently removed by rodents [130] (see Seed Banking). Went (1957) indicates that rodents or moth larvae consume “well over” 99% of Joshua tree seeds; however, experiments or observations supporting this assertion are not provided [184]. White-tailed antelope squirrels are widely-distributed and “wasteful” seed predators. As squirrels gnaw into brittle, dried fruits in the canopy of Joshua trees, remains of broken fruits and many seeds fall to the ground [104], where they are available to ground-foraging rodents and granivorous ants.

Seed Dispersal

Photo Credit

Photo by Forrest English and courtesy of iNaturalist with some rights reserved (CC BY-NC 4.0 License).

Figure 9—A white-tailed antelope squirrel harvests Joshua tree fruits. Photo by Forrest English with some rights reserved (CC BY-NC 4.0 License).

Joshua tree seeds are almost entirely dispersed by seed-caching rodents, who gather and open the fruits to release the enclosed seeds [104,170]. Most of the seeds are cached and later eaten, but some small proportion germinates and establishes seedlings from cached locations. Seeds not harvested by a seed-caching rodent may have virtually no chance of establishing as a seedling [170].

Rodents, such as white-tailed antelope squirrels, pinyon mice, and Merriam’s kangaroo rats, cache large numbers of Joshua tree seeds, often depositing them in single-seed caches beneath shrubs [22] and sometimes depositing multiple seeds in the same cache [170]. Rodents open Joshua tree fruits in the canopy [195] (fig. 9) or carry fruits to the ground to open them [22,173]. Pinyon mice open fruits and remove seeds while in the canopy, but it is unclear if they also remove entire fruits, and their activity is likely restricted to smaller trees (<5 m tall) [22]. White-tailed antelope squirrels removed 80% of fruits in the canopy on sites in Paiute Valley, Nevada, and Ivanpah Valley, California, in 86 days and 50 days, respectively [173]. At Potosi Wash in the Spring Mountains, Nevada, rodents removed all Joshua tree seeds in 1 or 2 days when scientists placed seeds on the ground near adult eastern Joshua trees. When scientists buried seeds about 3 cm deep, rodents removed 82% of seeds in 2 months [170]. In a high seed production year in the San Bernardino Mountains, rodents removed 60% of fruits on the ground after 50 days, and 100% after 120 days. Number of canopy fruits steeply declined after 120 days, and none were detected after 190 days. In a low seed production year (about 100 times fewer seeds produced), 100% of fruits on the ground were removed after 6 days, and canopy fruits disappeared after 10 days [22].

Information on Joshua tree seed dispersal distances is limited. In the Antelope Valley, California, juvenile plants were found as far away as 151 m from potential parent plants, with most juvenile plants within 90 m of potential parent plants. In Lanfair Valley, California, juvenile plants were found as far away as 251 m from potential parent plants, with most juvenile plants within 150 m of potential parent plants [104]. In Nevada, rodents cached seeds an average of 30 m from parent plants, although many seeds were removed from their original cache and probably recached, likely farther from parent plants [170].

Wind dispersal of Joshua tree fruits and seeds is unlikely because wind speeds required to move them across the soil surface are higher than those typically observed in the Mojave Desert [173]. However, some sources suggest that the relatively light, fragile fruits may be blown and broken open by high winds, allowing seeds to spread [141,158]. Two or more seedlings established at a single site may be evidence that they germinated from seeds that had remained enclosed together in a wind-dispersed fruit [104]. However, seed-caching rodents often cache multiple seeds in the same cache [170], which could result in multiple plants germinating from a single site.

Some authors suggest that dispersal of seeds may have been historically facilitated by extinct megafauna—perhaps the Shasta ground sloth [43] or North American elephant species [104]. Others suggest that an obligate relationship with an extinct, non-climbing herbivore is unlikely [170].

Seed Banking

Joshua tree has a short-term persistent soil seed bank [4,130]. In a semi-irrigated botanic garden in Riverside, California, seeds first germinated 1 or 2 weeks after planting and germinated continuously for the next 2 to 3 years [179]. Germinability of intact eastern Joshua tree seeds (no cracking, flaking, or holes) decreased with increasing soil residence time over 40 months in desert scrub habitat. Seeds showed similar germination patterns regardless of microhabitat: under creosotebush canopies or in open microsites (table 6) [130].

Granivorous rodents drive changes to Joshua tree’s soil seed bank. They add and remove Joshua tree seeds from the soil seed bank by caching them (see Seed Dispersal) and consuming them (see Seed Production and Predation), respectively. In Piute Valley, Nevada, scientists planted Joshua tree seeds in caged plots inaccessible to rodents and in uncaged plots accessible to rodents to monitor seed fate. In caged plots, 78% of seeds persisted after 12 months in the soil, and 81% of seeds persisted after 40 months in the soil. In uncaged plots, 26% of seeds persisted after 12 months in the soil. Persistence of seeds in uncaged plots after 40 months was not reported. Uncaged plots likely experienced similar seed losses to decomposition in the soil as those in caged plots (≈22%). Therefore, the remaining 52% of seeds that were unaccounted for were assumed to have been removed by granivorous rodents [130]. At Potosi Wash in the Spring Mountains, Nevada, rodents removed all Joshua tree seeds in 1 or 2 days when scientists placed seeds on the ground near adult Joshua trees. When scientists buried seeds about 3 cm deep, rodents removed 82% of seeds in 2 months and either consumed seeds or buried them in secondary caches [170].

Joshua tree seeds in the soil seed bank may be killed by high temperatures sustained under shrubs during wildfires (see Immediate Fire Effects).

Germination

Joshua tree seeds require prolonged shallow soil moisture for germination, and they appear to germinate most readily after summer monsoons, when warm temperatures accompany elevated soil moisture [31,183]. When seeds were planted in four cohorts over 11 months, the greatest proportion of seeds germinated after rainstorms that increased soil moisture in the warm months of September (14%) and April (11%). The smallest proportion germinated after a rainstorm in the cooler month of January (1%), regardless of planting month (January, July, October, and November 2009). Precipitation from each of these rainstorms exceeded 29 mm and resulted in increased soil moisture for 5 to 13 days after the storms. Daily soil temperature ranged from 20.9 to 34.8 °C preceding September germination, 6.2 to 18.2 °C preceding April germination, and 2.7 to 8.9 °C preceding January germination [130]. In the study area, conditions favorable for germination—more than 29 mm precipitation in a single month accompanied by several days of high soil moisture—occurred in about 68% of summers, suggesting germination may not be as uncommon as expected based on low seedling detection in the field [130]. In Joshua Tree National Park, western Joshua tree seed germination occurred in August and September, apparently after rain followed summer seed dispersal [183].

Joshua trees preferentially germinate in microhabitats under nurse plant canopies [170,173]. Nurse plants provide protection from herbivory and intense sunlight and moderate soil and air temperatures. Soil moisture, soil nitrogen [31], and soil phosphorus [25] also tend to be higher under nurse plants. However, competition for sunlight and summer soil moisture [130] from nurse plants may interfere with seedlings growing under them [31]. Western Joshua trees reached light saturation at approximately 25% of midday irradiation [145], so shading provided by nurse plants may not be a significant cost to seedlings. For seedlings of most desert species, higher soil moisture found below shrub canopies compensates for reduction in light (Callaway 1997, cited in [31]).

Joshua tree seeds germinate best at relatively high temperatures. Laboratory studies found high rates of germination (up to 100% in 1-5 days) at ≥20 °C [112,179,183], lower germination rates at 15 °C (24% in 8-10 days), and no germination at 10 °C [112].

Joshua tree seeds germinate readily under greenhouse conditions. Seed viability is often >90% without seed treatment [112,173,174], although germination rates may be much lower [10,53]. Germination rates decreased over time when stored in airtight glass jars on open shelves, from 98% 6 months after collection to 72% 18 months after collection [112]. Under warehouse storage conditions (no temperature or moisture control), seed viability decreased after 4 years, reaching 40% to 60% viability 5 years after collection. In sealed containers with moisture control, seed viability remained near or above 80% at three storage temperatures (-15 °C, 4 °C, or room temperature) for up to 10 years [88]. Because rodents quickly consume many seeds after maturation (see Seed Production and Predation), the window for germination in the field is limited [183].

Seedling Establishment and Mortality

Germination of Joshua tree seeds occurs readily under moist conditions and warm temperatures, but seedling establishment and recruitment require the alignment of favorable conditions for an extended period. Seedlings are most vulnerable to mortality for the first 3 to 5 years [174] and/or when they are <25 cm [55] or <1 m [49] tall. The presence of distinct cohorts of Joshua trees of similar ages suggests that successful seedling establishment only occurs a few times in a century [55].

Observations of Joshua tree seedling abundance in the field vary from many to few, but quantitative information is lacking. “Many” eastern Joshua tree seedlings grew at Cima Dome and the Mid Hills area in the eastern Mojave Desert [192]. In 4 years of fieldwork throughout the southwest, Webber (1953) observed “unlimited” Joshua tree seedlings, which were “plentiful” at higher elevations with relatively more rainfall, although very few seedlings were observed at lower elevations with less rainfall [179]. Although he did not specify the number of seedlings observed, Went (1948) remarked that in Joshua Tree National Park, western Joshua tree seeds seemed to germinate whenever rain fell immediately after seeds were mature (and dispersed) [183]. In contrast, on the Nevada National Security Site, young Joshua tree seedlings were “relatively sparse” [174], and over a 20-year study at three sites researchers only observed two seedlings, neither of which survived more than 2 years [46].

Since about the 1990s and possibly earlier, Joshua tree establishment has been less than necessary to maintain population sizes in parts of the species’ range, such as in southern California. Height and size-class data suggest decreased establishment of western Joshua trees since at least the 1990s in southern California [21,186], and likely since the 1950s in Los Angeles County and southwestern Inyo County [21] (see Stand Structure). Difficulties in detecting very small and young Joshua trees, which may be concealed by nurse plants [104], may skew results of demographic data [21]. Although the USFWS has aggregated demographic data [167], no study has obtained range-wide demographic data on Joshua tree, so range-wide patterns are difficult to discern and verify [194].

Seedling growth rates vary with age and are likely impacted by weather variation and resource availability. In Joshua Tree National Park, young, unbranched plants grew at an average rate of 7.6 cm/year for the first 10 years and an average of 3.8 cm/year thereafter [93]. Yucca seedlings grown from seed rapidly produced their first few leaves, then produced, on average, less than one new leaf every 2 months. Leaf size increased as leaf production rate decreased. Four Joshua tree seedlings had three to five leaves when they were nearly 6 months old [10]. In southern Nevada, 1-month-old seedlings averaged 3.5 cm in height (max: 6.9 cm), and generally consisted of a single cotyledon (73%) and occasionally one (27%) or two (<1%) primary leaves. One-year-old seedlings averaged 4.0 cm in height (max: 7.7 cm), with one (35%), two (49%), three (12%), or four (4%) leaves. Two-year-old seedlings averaged 6.4 cm in height (max: 10.2 cm) and had no more than four leaves, with some senescence of the cotyledon and earlier leaves apparent. Basal stem diameter of 2-year-old plants averaged 3.3 cm (min: 1.3 cm, max: 5.7 cm) [130]. In southern Nevada, growth was nearly undetectable in 3 of 4 years for a cohort of Joshua tree seedlings that were 32 cm tall (n = 23), and in 1 year, they grew 1.2 cm (±2.6 cm) on average [80].

Cold periods may improve seedling growth. Seedlings (3.5 years old) kept at 4 °C for 2 months had no new growth during the cold period but produced twice as many new leaves after the cold treatment as seedlings kept at warmer temperatures. When plants were 6 years old, individuals kept at warm temperatures (23–30 °C) produced 1 leaf/month, while plants kept at cool temperatures (4–10 °C) for 6 to 12 months produced 5 leaves/month after the cold period. Individuals kept at warm temperatures produced relatively short, downward-turned leaves, while individuals exposed to cold temperatures grew leaves that were twice as long and stood straight up [184,185].

Short day lengths result in more optimal seedling growth than long day lengths, suggesting higher growth rates in winter. After 1 year, Joshua tree seedlings grown from seeds collected in Arizona had the greatest number of leaves (15.1 leaves) and longest total leaf length (228.8 cm/plant) under the shortest simulated day length (10 hours light/14 hours dark). Seedlings grown under the longest simulated day length (16 hours light/8 hours dark) produced the fewest leaves (9.5 leaves) and shortest total leaf length (156.3 cm/plant). See McCleary (1973) [112] for complete results for other photoperiods.

Joshua tree seedlings are more likely to grow under nurse plants than in the open. Of 277 Joshua tree seedlings in the Sheep and Spring Mountain Ranges, nearly 93% grew under the canopy of a nurse plant. Joshua tree seedlings occurred more often than random under blackbrush, white bursage, littleleaf ratany, and spiny hopsage [31]. At Yucca Flat, Nevada, more young Joshua trees survived under nurse plant canopies (10 different species) than along the drip line or in interspaces, apparently due primarily to protection from herbivory [55]. In contrast, nurse plants increased the odds of Joshua tree seed germination but not seedling establishment near Searchlight, Nevada. Nurse plants (creosotebush) had no effect on establishment of September-emerging and January-emerging cohorts; however, an April-emerging cohort had lower survival rates under nurse plants than in the open, perhaps due to summer competition for soil moisture before seedlings could grow a more substantial root system [130].

Insufficient rainfall and herbivory constrain Joshua tree seedling establishment [174,179]. In southern Nevada, jackrabbits and possibly small rodents consumed 100 young plants of multiple species, including Joshua trees, within 2 weeks [174]. Black-tailed jackrabbits pull off the leaves and eat the growing tips of young Joshua trees [104], and they may consume postfire resprouts at high rates, particularly where they are adjacent to unburned habitat [148] (see Plant Response to Fire). Black-tailed jackrabbits consumed 62% (33 of 53) of 5- to 7-year-old plants over the course of 15 months at Yucca Flat, Nevada, a period of low precipitation [55]. Pocket gophers, white-tailed antelope squirrels, and woodrats also damage pre-reproductive Joshua trees, especially during drought years [55].

Plant Growth and Mortality

Joshua trees are long-lived and slow growing [46,62]. Based on an eastern Joshua tree population at Lytle Preserve in Utah, researchers predicted that 50% of individuals would survive 89 years, and 5% would survive 383 years. A more conservative estimate based on linear growth models indicated that the maximum age in that population was 138 years [62]. Bowers (1993) suggests that old individuals may live 200 years or more [24], and other estimates of maximum lifespan suggest that large individuals may live 600 to 800 years. The estimated age of the largest known Joshua tree was 1,000 years or more [87,179]. Many estimates of very old maximum ages stem from a few early resources and are difficult to verify. Joshua trees experienced a time of intensive use and vandalism in the early 1900s [179], which may have disproportionately affected old and large individuals and included the destruction of the largest known Joshua tree [73].

Because Joshua trees are monocots, they lack growth rings and are difficult to age [46,62,141]. Although Wallace (1972) indicates that Joshua trees have annual growth rings because of their “peculiar” cambium layer [174], no other sources support this observation. Age is sometimes estimated by height and growth rates or by sets of leaves produced annually. Two 21-year-old, minimally irrigated, unbranched western Joshua trees at a nursery in Riverside, California, stood 109 and 141 cm tall, with respective mean growth rates of 5.2 and 6.74 cm/year [179]. At a southern California botanical garden, the fastest growing cohort of eastern Joshua trees had reached up to 2.1 m tall at 6 to 10 years, while another cohort reached 2.4 m to 3.7 m tall at 20 years [57].

Growth rates vary among locations (table 7) and are positively associated with precipitation [55]. On the Nevada National Security Site, western Joshua trees produced about three sets of six leaves annually, on average. Plants produced six to eight sets of leaves in a particularly wet year and fewer than three sets of leaves in dry years [174]. Based on average leaf production, large Joshua trees at the Nevada National Security Site may be 200 years old [135].

Western and eastern Joshua trees exhibit different branching patterns. Western Joshua trees first branch after flowering or after injury by wind or insects. Flowering or injury kills the growing point, and plants respond by branching below the inflorescence or injury [24], where two or three buds typically develop into new branches. As few as one or as many as five branches may develop [141]. In contrast, young eastern Joshua trees display true dichotomous branching and branch before first flowering [86], typically beginning when the plant is <1 m tall. Sympodial branching is initiated when plants first flower, at which time they display the same growth habit as western Joshua trees after first flowering [141].

Adult Joshua trees have relatively low mortality rates. Seedlings have relatively high mortality rates, with larger unbranched individuals having a middling mortality rate [46]. Joshua trees are drought tolerant [77] but may be susceptible to mortality during extended drought. Pocket gophers chew into and hollow out stems in dry years, which may kill Joshua trees outright or weaken their stems and cause them to topple [49]. Adult Joshua trees may topple after fire if trunks and roots are weakened [80].

Vegetative Reproduction and Regeneration

Western and eastern Joshua trees can reproduce asexually by rhizomes, branch sprouts, and/or basal sprouts [95,141,179]. Vegetative reproduction by rhizomes is more pronounced in western Joshua trees than eastern Joshua trees, and vegetative reproduction by any means (i.e., basal sprouting, branch sprouting, or rhizome growth) occurs more frequently in both species at elevational extremes [67], especially at high elevations [51,133,141]. Basal buds may not develop into distinct rhizomes and instead grow new stems adjacent to the main stem as sprouts [141]. Vegetative regeneration may be stimulated by stem damage or stress [179].

Both western and eastern Joshua trees reproduce vegetatively via cloning, especially at elevational extremes. Cold temperatures, high winds, and abundant snowfall at high-elevation sites, and drought stress at low elevations may restrict seedling establishment and aboveground development and stimulate belowground development [67,133,141]. In Joshua Tree National Park, western Joshua trees reproduced almost exclusively by cloning at both extreme high and low elevations, where both seed production and seedling establishment were limited. Although western Joshua trees flowered at both extremes, no pollinating moths, fruit development, or viable seeds occurred at either extreme [67]. However, a broad overview about Joshua trees indicates that clonal growth may be less common in low-elevation dry areas than cooler, higher elevation areas; in low-elevation areas Joshua trees typically had one or two stems, while in higher, moister areas, two to three stems were more common, and some clumps (i.e., clonal individuals) were found [179].

Western Joshua tree populations in a relatively high-elevation band in southwestern California may exhibit extensive rhizome growth and a clonal form [26,179]. Plants with this growth form were once classified as a variety of Joshua tree [179] but are now recognized as a clonal growth form of western Joshua tree [82]. This clonal growth form appears to be most common, or perhaps exclusive to, western Joshua tree populations in the southwestern part of the species’ range (parts of “YUBR South” in fig. 2) [119], where precipitation averages are higher than in other parts of its range [179] (see Site Characteristics). Clones can be dense in some areas [179] and are typically shorter and have less branching than single-stemmed trees [141]. At the extreme northwestern end of the Sierra Pelona Mountains, for example, western Joshua trees form dense "impenetrable thickets," where even the largest individuals have relatively few branches, but clonal stems grow close together [26]. A single clone along Gorman Creek, California, in an area with high levels of winter snowfall occupied approximately 0.4 ha and was comprised of several hundred stems [141]. From the southern and western slopes of Tehachapi Mountains to at least Monolith, California, some western Joshua trees occur in clumps nearly 8 m in diameter, with 30 to 40 stems [179]. Fire has been suggested as an influential factor in the evolution of Joshua tree's clonal growth form [141,172], as fire may have been more frequent in the area where it is most common than in other parts of the species’ distribution because this area is prone to lightning strikes and receives relatively more precipitation [172] (see Fire Regimes).

Western and eastern Joshua trees resprout in response to injury or stress. It is common for dormant buds beneath the periderm to grow when old stems are bent or injured [141], such as by fire (see Plant Response to Fire). In Joshua Tree National Park, western Joshua trees resprouted in response to drought stress. Resprouting individuals had higher survival rates than non-resprouting individuals at some low-elevation sites [49].

Successional Status

Disturbances that initiate secondary succession were historically rare to infrequent in most desert ecosystems, including those in Joshua tree communities, but included wildfire and debris flows [100] (see Fire Regimes). In contemporary communities, disturbances from human developments (e.g., roads, agricultural fields, and military targets) also initiate secondary succession in Joshua tree communities. Joshua tree is slow to return following any type of severe stand disturbance [3,40,49,177,180], requiring extended periods to regain predisturbance abundance [3]. Western Joshua trees with clonal growth forms (see Vegetative Reproduction and Regeneration) may recover more rapidly. Joshua trees appear to be better able to recolonize by seedling establishment after disturbance in the northern portion of their range, where conditions are cooler and moister than in the southern part of its range [43].

Secondary succession proceeds very slowly in deserts [8], with changes occurring over a much longer period than in more mesic environments and more temperate regions [3], making patterns more difficult to discern and generalize. Native plant communities may require decades to centuries to reach predisturbance plant cover, community structure, species diversity, and species composition, or may transition to an alternative stable state. Many dominant species have infrequent and sporadic reproduction that depends on large-scale climatic patterns. For example, Joshua tree seedling establishment depends on the alignment of multiple factors (see Seedling Establishment and Mortality), which can prolong population recovery after disturbance [4].

Joshua tree stands are generally late-successional in nature, changing very slowly in the absence of disturbance (e.g., [46]). Because disturbances were historically rare and succession slow, the dynamics and underlying mechanisms of succession in Joshua tree communities are not well described. With the presence of nonnative invasive plants and changing climatic conditions, recovery to predisturbance community structure and composition may not be possible after stand-replacing disturbances in many contemporary desert communities, including Joshua tree communities [3] (see Management Under a Changing Climate).

Joshua trees are not likely to establish in early succession following stand-replacing disturbance, in part because of the lack of nurse plants. Although adult Joshua trees dominate late successional plant communities in full sun, Joshua tree seedlings and young plants require protection from herbivory and extreme temperatures, which is typically provided by a nurse plant [130]. While ruderal native shrubs may quickly establish cover after disturbance, Joshua trees generally require densely branched, low-lying perennial shrubs as nurse plants. Blackbrush is among the most important nurse plants for Joshua trees [31], but it is killed by fire, and blackbrush communities may take hundreds of years to recover compositionally after fire [4] or debris flows [42,178]. However, in at least one instance, blackbrush reestablished on a burned site within about 10 years after fire, allowing western Joshua trees to establish 12 to 15 years after fire [106].

Joshua trees are often killed or top-killed by fire, and postfire resprouting may be limited. Joshua tree woodlands are often replaced by short-lived, pioneer shrublands after fire. At low elevations these shrublands are dominated by species such as white brittlebush; at intermediate elevations by burrobrush, threadleaf snakeweed, Virgin River brittlebush, Mexican bladdersage, and rayless goldenhead; and above 1,400 m by Mexican bladdersage, burrobrush, big galleta, and desert needlegrass [115].

Joshua trees may recolonize some sites severely disturbed by agriculture and other anthropogenic disturbances but may fail to recolonize others. On plowed fields about 50 km northwest of Needles, California, that were abandoned for 70 years, most sites had 0.5 eastern Joshua trees/ha or less, although one disturbed site had similar average density as undisturbed sites (75 Joshua trees/ha), and another site had about 15 Joshua trees/ha [40]. No eastern Joshua trees grew on roads abandoned for 33 years in the ghost town of Wahmonie, Nevada, but they occurred at a density of 2 individuals/ha in the surrounding, less-disturbed area [181].