Yucca schidigera



INTRODUCTORY


Br. Alfred Brousseau, Saint Mary's College

AUTHORSHIP AND CITATION:
Gucker, Corey L. 2006. Yucca schidigera. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

FEIS ABBREVIATION:
YUCSCH

NRCS PLANT CODE [87]:
YUSC2

COMMON NAMES:
Mojave yucca
Mohave yucca
Spanish dagger

TAXONOMY:
The scientific name of Mojave yucca is Yucca schidigera Roezl ex Ortgies (Agavaceae) [27,36,41,42,92]. Mojave yucca is part of the fleshy-fruited or Sarcocarpa section of the Agavaceae family [61].

Mojave yucca banana yucca (Y. baccata) hybrids that are intermediate to the parent forms occur in the Mojave Desert [49,91].

SYNONYMS:
Yucca macrocarpa Merriam, non Engelm. [41]
Yucca mohavensis Sarg. [41,53,54,85]

LIFE FORM:
Shrub-tree

FEDERAL LEGAL STATUS:
No special status

OTHER STATUS:
Information on state-level protected status of plants in the United States is available at Plants Database.


DISTRIBUTION AND OCCURRENCE

SPECIES: Yucca schidigera
GENERAL DISTRIBUTION:
Mojave yucca is a southwestern species, most common in the Mojave Desert. It occurs in the southernmost part of Nevada, Clark County, and in southwestern Utah's Washington County [42,91,92]. In Arizona, Mojave yucca is restricted to the northwestern part of the state [61]. Mojave yucca's southern limit is reached in Baja California Norte, and it is found as far west as the Pacific coast. The northern limit of Mojave yucca in California is San Bernardino County [31,91]. The Flora of North America provides a distributional map of Mojave yucca.

ECOSYSTEMS [32]:
FRES30 Desert shrub
FRES33 Southwestern shrubsteppe
FRES34 Chaparral-mountain shrub
FRES35 Pinyon-juniper
FRES40 Desert grasslands

STATES/PROVINCES: (key to state/province abbreviations)
UNITED STATES
AZ CA NV UT

MEXICO
B.C.N.

BLM PHYSIOGRAPHIC REGIONS [7]:
3 Southern Pacific Border
4 Sierra Mountains
7 Lower Basin and Range
12 Colorado Plateau

KUCHLER [46] PLANT ASSOCIATIONS:
K023 Juniper-pinyon woodland
K033 Chaparral
K039 Blackbrush
K041 Creosote bush
K042 Creosote bush-bur sage
K053 Grama-galleta steppe

SAF COVER TYPES [25]:
238 Western juniper
239 Pinyon-juniper

SRM (RANGELAND) COVER TYPES [78]:
206 Chamise chaparral
211 Creosote bush scrub
212 Blackbush
412 Juniper-pinyon woodland
502 Grama-galleta
506 Creosotebush-bursage
508 Creosotebush-tarbush

HABITAT TYPES AND PLANT COMMUNITIES:
Mojave yucca is rarely a community dominant in terms of abundance but may be the obvious visual dominant among low-growing associated species. The following vegetation classifications identify Mojave yucca as an important species:

Arizona:

California: Nevada: Mojave Desert:

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Yucca schidigera
GENERAL BOTANICAL CHARACTERISTICS:
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g. [27,36,42,65,66,92]).

Aboveground description: Mojave yucca is an evergreen shrub or small tree with a variable growth habit. Typically there are several stems, but clone shape ranges from symmetrical to ragged [54,61,91]. Webber [91] indicates that 4 to 7 stems per clump are common but up to 23 stems are reported. Stems are without branches or sparingly branched [42,61]. At the Deep Canyon Desert Research Center in California, stems did not branch until trunk diameter measured 4 inches (10 cm) or more, and the lowest branch point was 20 inches (0.4 m), although most were at or above 3 feet (1 m) [47]. Mojave yucca banana yucca hybrids in California's Valley Wells and Kessler Springs areas had stems on the ground with upturned ends that measured ~3 feet (1 m) tall [91]. Mature stem height commonly ranges from 2 to 20 feet (0.5-5 m) [54,92]. However, in the eastern Mojave Desert an unusually tall Mojave yucca was an estimated 30 feet (9 m) [64]. Trunk diameter ranges from 5.9 to 20 inches (15-50 cm) [61,91]. Mojave yucca wood is soft and spongy. Trunks may be covered with dead reflexed leaves to the ground or may be bare of leaves, revealing rough, furrowed bark [54,61].

Br. Alfred Brousseau, Saint Mary's College

1998 Tom Schweich. Used with permission, http://www.schweich.com

Mojave yucca is long-lived and grows slowly. Plant age can be estimated by number of vascular bundles, number of leaf blades, or height [89]. Mojave yucca produces 6 leaves at a time and produces 2 to 4 sets annually. Using leaf number to age plants in Nevada's Mercury Valley, the oldest Mojave yuccas were up to 200 years old [77]. Cardiff [15] reports that Mojave yucca increases in height just 0.4 inch (1 cm)/year. Rowlands [76] reports growth rates of 0.8 inch (2 cm)/year in Nevada. In the Rancho Santa Ana Botanic Garden, the average annual growth rate was 1 inch (2.6 cm) [91]. At the Deep Canyon Desert Research Center, about 50% of Mojave yucca clones were estimated at 500 or more years old using a 0.4 inch (1 cm)/year growth rate and a clonal new shoot production rate of 130 years. Few clones were younger than 200 years old [47].

Leaves occur in rosettes at the ends of branches. Typically leaves measure 12 to 59 inches (30-150 cm) long and 0.8 to 4.3 inches (2-11 cm) wide. Leaves have an expanded base attaching to the stem, stringy fibers along the margins, and spine tips that are 0.3 to 0.5 inch (7-12 mm) long [54,61,66,89,91]. Fibers along the margins are often used to distinguish Mojave yucca from Joshua tree [80]. Leaves are stiff and broadest at their midpoints [54,61,89,91]. Leaf rosettes of newly established stems are approximately 25% to 50% of parent size [91].

Round to bell-shaped flowers in a dense panicle occur at or above the longest leaves on a sturdy, 1.6- to 4.3-foot-tall (0.5-1.3 m) stalk [19,36,54,66,91,92]. Stalks produce flowers only once [79]. Webber [91] reports that 5- and 6-year-old Mojave yucca seedlings did not produce flowers. Rowlands [76] reports that Mojave yucca takes 35 years to reach maturity. At the Deep Canyon Desert Research Center, plants did not flower until the trunk was over 4 inches (10 cm) in diameter, and 1 foot (0.4 m) was the minimum height of flower-producing stems [47]. Mojave yucca produces an indehiscent, fleshy fruit (capsule). Capsules are cylindrical and measure 2 to 4.3 inches (5-11 cm) long and approximately 1.6 inches (4 cm) wide [19,36,42,44,66,92]. Seeds are thick, rough, and dull black with scattered shiny spots. Seed diameter is 0.2 to 0.4 inch (6-9 mm) [66,71,89]. From several southwestern collections, seed weight ranged from 80 to 100 mg [3]. From 2 populations in San Diego County, seed weight averaged 139 mg and number of seeds per locule averaged 20 [44].

Belowground description: Mojave yucca produces fleshy rhizomes with an expanded cortex. Rhizomes may reach 0.6 to 0.8 inch (1.5-2 cm) in diameter and 8 inches (20 cm) in length [3,66,92]. Webber [91] considered Mojave yucca rhizomes hardly more than a "modified basal sprout". Researchers found sparse roots at 30-inch (75 cm) depths at the Nevada Test Site that were capable of transferring water to shallow soils during the day while stomates were closed [97,98]. Rhizomes of potted Mojave yucca plants that were without water for over a month became shrunken and wrinkled. Within a few days of watering, rhizomes regained turgor [3].

RAUNKIAER [75] LIFE FORM:
Phanerophyte
Geophyte

REGENERATION PROCESSES:
Reproduction is sexual through seed production and asexual through sprouting and clonal growth [15]. Vegetative reproduction is predominant. At the Deep Canyon Desert Research Center, 59 of 66 monitored plants had 2 to 14 stems. The other 7 were single-stemmed and lacked evidence of vegetative reproduction. Careful searching of the study area revealed no seedlings. Based on the age of single-stemmed plants, the researcher estimated that the last successful seedling establishment occurred 40 or 50 years earlier [47].

Pollination: Mojave yucca maintains a mutualistic relationship with its pollinator, a yucca moth (Tegeticula yuccasella). Yucca moths collect pollen from several flowers, form it into a ball, carry it to other flowers, and force it down the stigma tube. Moths lay their eggs in the ovary after fertilizing flowers. Moth larvae feed on the developing seeds. It is unclear whether yucca moths are the only yucca (Yucca spp.) pollinators, and whether or not they achieve true cross-pollination by depositing pollen on a separate plant [91].

Investigations into the T. yuccasella species have caused many to refer to this as a complex of distinct species or subspecies [49,70]. Pellmyr [70] divided the complex into many different species based on morphological, molecular, and biological differences. He named the moth that pollinates Mojave yucca exclusively, T. mojavella. He also noted T. corruptrix, a "cheater" moth that oviposits in the developing seeds and fruits of Mojave yucca and other yuccas without pollinating.

Of 161 moths collected on Mojave yucca or banana yucca in an area where these species overlap, only 1 banana yucca moth species was found on Mojave yucca, and no Mojave yucca moth pollinators were found on banana yucca. On 7 likely Mojave yucca banana yucca hybrids, all moths collected were banana yucca moths [49].

See Seed Production for a discussion about fruit production without moth larvae and potential reasons for differences in yucca moth larvae production in yucca populations.

Breeding system: Mojave yucca produces perfect flowers [71].

Seed production: Fruit and seed production are variable. Production has been related to site conditions, plant maturity, plant size, moth larvae consumption, and predation by small mammals.

Of 25 Mojave yucca seeds collected in native southwestern habitats, viability averaged 80% [3]. However, McKelvey [61] reports that fruit production from "good-sized" plants ranges from abundant to scarce, and seed predation by small mammals is common [29].

Five- to six-year-old plants did not produce flowers but 15-year-old plants did, in the Rancho Santa Ana Botanic Garden. Sixty-three of the 116 Mojave yucca plants that were approximately 15.5 years old were flowering when observations were made on 30 March. Plants came from seed collected in Tecate, California, grew in heavy, gravelly loam soils, and had not been irrigated since planted [91].

On a 2.5-acre (1-ha) study area at the Deep Canyon Desert Research Center, no Mojave yucca plants flowered in 1976 or 1977, and only 2 stems flowered in 1978, yet fruit was not produced. The researcher noted that flowering and seed set were more frequent at elevations higher than the study area. Plants that flowered had trunks that measured at least 4 inches (10 cm) in diameter and were at least 1 foot (0.4 m) tall [47].

The percentage of seeds destroyed by Tegeticula larvae was low in 2 Mojave yucca populations in San Diego County. Just 3% of seeds were destroyed. About 65% of fruits had no larvae, and the number of larvae per fruit ranged from 0 to 4. Of all the yucca species studied, Mojave yucca fruits had the lowest number of larvae. This was true in a 2nd year of sampling as well. Close examination of Mojave yucca fruits revealed tissue scars across the ovary wall, suggesting that Tegeticula pollinated the flowers, oviposited in the ovary, but eggs or young larvae did not survive in developing fruits. Researchers suggested that the proportion of fruits without larvae may be a function of moth density or may be evidence of plants inhibiting Tegeticula egg hatching or survival [44].

Seed dispersal: Mojave yucca fruits and seeds are dispersed by mammals that consume and stash the fruits [71].

Seed banking: Information on Mojave yucca seed bank density or longevity is lacking. In the Granite Cove area of the eastern Mojave Desert, Mojave yucca was common but seed was not recovered from seed traps or soil collections in the area [73].

Germination: Many indicate that Mojave yucca seeds germinate readily given favorable conditions; however, the lack of Mojave yucca seedlings in the field suggests that favorable conditions are rare in native habitats [3,45,47]. Webber [91] found that seeds soaked in water for 24 hours and kept moist at 82 to 90 F (28-32 C) geminated in 6 days.

Seed collected from the Agave Hill study area at the Deep Canyon Desert Research Center had 80% germination in potting or native soils in greenhouse conditions, but none of the 50 seeds planted at the Agave Hill study area germinated in the field [47].

In an evaluation of seed heat tolerance, Keeley and Meyers [45] found that Mojave yucca seed did not tolerate long-duration, moderate-heat treatment or short-duration, high-heat treatment. Seed was collected from several coastal populations in southern California. Germination of seeds kept moist and not heat treated was 69%. Researchers suggested that Mojave yucca seeds are nonrefractory and germinate when moisture and temperature conditions are "adequate."

Duration 2 hours 5 minutes
Temperature (C) 80 90 90 100 110 120
Germination (%) 16b 0b 83a 54a 11b 0b
Germination percentages with superscript "b" are significantly (P<0.01) less than control (69%) and those
with superscript "a" are not.

Seedling establishment/growth: Mojave yucca seedlings are rarely observed in the field. In a study of the distribution of yucca species along an elevational gradient in the eastern Mojave Desert, researchers observed very few Mojave yucca seedlings [96]. In 4 years of field observations, Webber [91] found 1 Mojave yucca seedling in Arizona and 6 in southern California. Based on irrigation studies, Wallace [89] suggests that successful seedling establishment requires 3 to 5 consecutive years of favorable moisture conditions.

When twenty-five 6-month-old Mojave yucca seedlings were planted at the Deep Canyon Desert Research Center, all died even though heavy rainfall followed transplanting. Approximately half of the seedlings planted at the University of California Riverside Botanic Gardens survived in a nonirrigated area where conditions were described as "milder" than Deep Canyon [47].

Mojave yucca seedlings grown from seed collected in southern Nevada had the greatest root and shoot growth after 180 days at soil temperatures of 64 F (18 C). Root and shoot growth were lowest at soil temperatures of 50 F (10 C). Soil temperatures of 95 F (35 C) produced intermediate root and shoot growth. Researchers indicated that increasing soil temperature decreased the proportion of plant weight that was root [89,90].

Growth: Studies of populations in southern California indicate that Mojave yucca growth is limited to warm, moist periods. In the Rancho Santa Ana Botanical Garden, the amount of water received affected leaf length, sprout number, and main stem height. Leaf length averaged 26.6 inches (67.5 cm) when plants were not irrigated and 31.5 inches (80.1 cm) with irrigation. Main stem height and sprout number averaged 16 inches (41 cm) and 1.6, respectively, for plants without supplemental water and 33.5 inches (85 cm) and 3.9 for plants on irrigated sites [91].

At the Deep Canyon Desert Research Center, periods of maximum leaf growth coincided with warm to hot weather. If water was not limiting, temperature controlled new leaf production. The maximum number of leaves was produced after summer storms. Averages of 13.2 and 19.2 new leaves were produced on 15 stems in 2 years on nonirrigated and irrigated sites, respectively. Stem growth never exceeded 1.2 inches (3 cm)/year, and stems less than 1.6 feet (0.5 m) tall grew more slowly than taller stems. Growth was also slowed by flower production. A single nonirrigated stem that flowered produced only 12 leaves in the following 4 years [47].

Vegetative regeneration: Mojave yucca reproduces vegetatively through the production of sprouts from short rhizomes. Sprouts are adjacent or close to parent stems. Plants commonly support 1 to 5 sprouts but may have 10 or more [91]. Clones in Mojave yucca-buckhorn cholla vegetation in the eastern Mojave Desert averaged 5.1 stems [15]. In the Rancho Santa Ana Botanic Garden, Mojave yucca plants that were approximately 15.5 years old had 0 to 7 sprouts; sprout height averaged 0.6 inch (1.6 cm) [91]. Clonal growth is typically outward from the initial parent stem. Old central stems eventually senesce and die. Basal sprouting and central aging and dying lead to outward expansion [15].

At the Deep Canyon Desert Research Center, 59 of 66 monitored plants were clonal with 2 to 14 stems. Clones averaged 4.08 stems/plant in the 2.5-acre (1 ha) study area. Well-developed clones had tall, mature stems surrounding dead and decaying stems in the center. On flat surfaces, clones were largely circular. Using a growth rate of 0.4 inch (1 cm)/year and a clonal new shoot production rate of 130 years, clones averaged 300 to 600 years old, and approximately half were 500 or more years old [47].

Stem damage from insects may facilitate the separation of clones into individual plants. MacKay [58] reports that several varieties of Megathymus coloradensis lay their eggs on yucca sprouts. Larvae bore into tissues and sometimes break rhizome connections.

SITE CHARACTERISTICS:
Mojave yucca is most often described on dry rocky slopes, flats, or washes throughout its range [19,42,58,66,89,92].

Climate: Habitats occupied by Mojave yucca are hot and arid. The Mojave Desert where Mojave yucca is most common is considered the most arid of the North American deserts due to combined high temperatures and low rainfall levels. The little precipitation that the Mojave Desert receives comes in the winter and spring [37]. Mojave yucca habitats in San Diego County's Ysidro Mountains experience hot, dry summers with temperatures often above 100 F (38 C) and exceedingly high evapotranspiration rates [84]. In southern Nevada's Mercury Valley in the northern portion of the Mojave Desert, the 6-year average annual precipitation at an 1,100-foot (340 m) site was 6.2 inches (157.6 mm) [1].

At the Deep Canyon Desert Research Center, seasonal temperatures and precipitation levels were closely monitored for about 5 years. Winters were cool with night temperatures often below 50 F (10 C), and daytime temperatures mostly less than 68 F (20 C). In the spring, days were typically 59 to 86 F (15-30 C), and nighttime lows were 50 to 77 F (10-25 C). Summers were hot; daytime temperatures exceeded 86 F (30 C), and nighttime temperatures were above 68 F (20 C). Temperatures in the fall were variable; days were 59 to 86F (15-30 C), and nights were 50 to 68 F (10-20 C). Precipitation did not fit a seasonal pattern, and levels were erratic. Snow is rare in the area and occurs on average just once in 5 years. From 1976 to 1978, there were 3 successive years with annual precipitation levels that were double the long-term average. In 1975 there were just 15 days of measurable rainfall, and 3 storms produced 75% of the year's total precipitation. Patterns in precipitation delivery were evident. Winter precipitation was typically mild in intensity, long in duration, and widespread in area. Summers, however, produced localized, intense convection thunderstorms that often produced run-off and flooding [47].

Several studies highlight Mojave yucca's ability to cope with dry conditions. Potted Mojave yucca plants without water for over 1 month did not wilt or show any other adverse effects [3]. Mojave yucca frequency and density were equal or nearly so when compared in moist and drought years in southern Nevada's Cold Creek Canyon. The moist year received 5.8 inches (148.8 mm) of precipitation from December through February. The severe drought year, during the same time period, received approximately 0.5 inch (13.2 mm) of precipitation. Average monthly rainfall for this time period was 1.4 inches (35.6 mm), based on data collected for 61 years. Mojave yucca frequency was 65% in both the moist and drought years, and density was 1.9 plants/100 m and 2.1 plants/100 m in the moist and dry years, respectively [50].

Elevation: Mojave yucca is most typical from 0 to 5,900 feet (1,800 m) elevations [27,44]. Loik and others [55], however, report an upper elevation limit of 8,500 feet (2,600 m), and Yeaton and others [96] report that Mojave yucca is found only sporadically at 2,800 feet (850 m), and is not found above 5,000 feet (1,525 m). In the eastern Mojave Desert, Mojave yucca reaches its greatest height and density between 3,600 and 3,900 feet (1,100-1,200 m) [15]. Below are the reported elevational tolerances of Mojave yucca by state and/or region.

State/region Elevation (feet)
Arizona 1,000-3,500 [54]
California below 7,800 [66]
below 4,900 [19]
Nevada 3,100-5,000 [42]
3,000-3,600 [89]
Utah 3,000-4,900 [92]
up to 2,300 [71]
Colorado Desert mostly above 1,000 [80]
Mojave Desert below 5,000 [58]
Southwest sea level to 5,600 [61]
1,000-6,000 [91]

Soils: The soils in Mojave yucca habitats are often described as gravelly and calcareous [77,91]. In Nevada's Mercury Valley, Mojave yucca occurs in desert hard pan soils [89]. In creosotebush-white bursage vegetation in the Mojave Desert, Mojave yucca occurs on low-elevation bajadas with relatively shallow caliche zones [77]. However, Wallace and Romney [89] report that Mojave yucca seedlings grown with added CaCO3 had decreased shoot weights.

Differences in burned and adjacent unburned soils in southern Nevada blackbrush communities sampled 1 to 17 years following fire [51] are described in Fire Effects.

SUCCESSIONAL STATUS:
The concept of succession, in which community composition changes over time as a site is modified by past and present species, was developed in mesic eastern forests and is not appropriate for southwestern desert ecosystem dynamics. Desert plants have a limited effect on soil development, and late-seral vegetation that is well adapted to dry, stressful environments reestablishes following removal of the existing vegetation. Deserts do not experience native species composition changes over time as mesic systems do. Time required for complete "recovery" of a denuded desert site, however, may take centuries or millennia. Desert succession has been described as "parasuccession" [76].

Primary succession: Mojave yucca is important on dry washes of the Mill and Santa Anna creeks at the southern base of the San Bernardino Mountains. While water is present, willows (Salix spp.), mule's fat (Baccharis salicifolia), and Fremont cottonwood (Populus fremontii) are the typical associated vegetation. When the wash is dry, coastal sagebrush vegetation gradually colonizes. The pioneer is California broomsage (Lepidospartum squamatum). Time frame for colonization by Mojave yucca was not provided [20].

Secondary succession: Mojave yucca had not recolonized areas disturbed 5, 10, and 40 years earlier in the eastern Mojave Desert. Mojave yucca was absent from pits dug in a buckhorn cholla stand 5 years following the digging. Mojave yucca was also absent from sand and gravel operation sites in creosote scrub vegetation near Las Vegas, Nevada. Operations were abandoned 2 to over 10 years earlier [76].

Military camps in a creosotebush-white bursage vegetation type in the eastern Mojave Desert abandoned for 40 years were also without Mojave yucca. Sampled disturbed areas included graded dirt roads where the upper 4 to 8 inches (10-20 cm) of soil was removed, cleared tent areas where vegetation was cleared by hand and foot traffic was heavy, and parking areas where vegetation was cleared manually and vehicle use was extensive. Disturbed areas had significantly (P ≤0.05) more compacted surface soils to 12 inches (30 cm) than nearby, undisturbed sites. Mojave yucca density was 124 plants/ha and cover was 0.1% on undisturbed sites [74].

For information on Mojave yucca recovery on burned sites, see Fire Effects.

Recovery of Mojave yucca after harvesting depends on the amount of aboveground stem removed and extensiveness of postharvest predation. Following an illegal harvest in Mojave yucca-buckhorn cholla vegetation in the eastern Mojave Desert, about 26.5% of all Mojave yucca stems were removed and many clones were clearcut. There were 82 postharvest sprouts, but 66 suffered severe damage from small mammals [15]. Near Kingman, Arizona, researchers removed all Mojave yucca sprouts and cut main trunks at various distances from the ground. In the 1st postharvest year, the number of sprouts/cut plant ranged from 0 to 9, and sprouts were 3 to 9 inches (8-23 cm) tall. Plants with only leaf heads removed from the main stem produced a greater number of and more "vigorous" sprouts than plants with a portion of the main stem cut [91].

SEASONAL DEVELOPMENT:
Mojave yucca flowers in late winter or early spring throughout its range [27]. McKelvey [61] suggests that altitude may be more important than latitude in predicting flowering and fruiting dates. Below are flowering and fruiting dates by state and region.

State/region Flowering Fruiting
Arizona March-April fruit falls before winter [54]
California April-May [66]
Nevada March-May [42,89]
Nevada (Mercury Valley)* March May-July [1]
Mojave Desert April-May [58]
Southwest late March-early May [61]
*Based on 6 years of data collection.


FIRE ECOLOGY

SPECIES: Yucca schidigera
FIRE ECOLOGY OR ADAPTATIONS:
Fire adaptations: Mojave yucca sprouts following fire [19,56]. Vogl [88] describes postfire sprouting as "vigorous." Vegetative regeneration predominates following fire; seedlings are rarely observed [21,62,63].

Fire regimes: Tree species are rare in Mojave yucca habitats, making assessment of the fire regimes in these habitats difficult. Arrangement and loading of fuels in Mojave yucca habitats suggests that fires occurred primarily when a heavy crop of annual species was present and/or weather conditions were conducive to carrying fire. The increase in fire-tolerant nonnative grasses in Mojave yucca habitats, however, has the potential to increase fire frequency and alter fire behavior from that which likely existed in the past [12].

Early accounts of fire regimes in arid grassland and shrubland communities of the Mojave Desert were made by Humphrey [37]. Creosotebush-bursage (Ambrosia spp.) vegetation was sparse and required an abundance of annual species, which typically occur following heavy winter precipitation, to readily carry fire. Blackbrush-dominated vegetation lacked fine fuels but could burn successfully when temperatures were high, winds were strong, and humidity levels were low. Humphrey suggests that big galleta (Pleuraphis rigida) grasslands located near settlements or roads may have experienced repeated fire. Humphrey reported that Mojave yucca was "usually little harmed by fire" in big galleta communities, but quantitative data and field observations are lacking for Mojave yucca recovery on sites burned repeatedly. The invasion and successful colonization of arid Mojave and Great Basin deserts by nonnative annual grasses such red brome (Bromus rubens), cheatgrass (B. tectorum), and Mediterranean grasses (Schismus spp.) have fueled more frequent and larger fires by filling in the shrub interspaces that once limited fire spread in arid ecosystems [22,24]. A more detailed discussion on fire regimes in the Mojave Desert altered by nonnative grasses is provided in the Joshua tree Fire Ecology section.

The following table provides fire return intervals for plant communities and ecosystems where Mojave yucca 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)
California chaparral Adenostoma and/or Arctostaphylos spp. <35 to <100
grama-galleta steppe Bouteloua gracilis-Pleuraphis jamesii <35 to <100 [68]
cheatgrass Bromus tectorum <10 [72,94]
blackbrush Coleogyne ramosissima <35 to <100
western juniper Juniperus occidentalis 20-70
Rocky Mountain juniper J. scopulorum <35 [68]
creosotebush Larrea tridentata <35 to <100 [37,68]
pinyon-juniper Pinus-Juniperus spp. <35 [68]
Colorado pinyon Pinus edulis 10-400+ [28,34,43,68]
galleta-threeawn shrubsteppe Pleuraphis jamesii-Aristida purpurea <35 to <100 [68]

POSTFIRE REGENERATION STRATEGY [81]:
Tall shrub, adventitious bud/root crown
Rhizomatous shrub, rhizome in soil
Secondary colonizer (on-site or off-site seed sources)

FIRE EFFECTS

SPECIES: Yucca schidigera

IMMEDIATE FIRE EFFECT ON PLANT:
Mojave yucca is top-killed by fire. As of 2007, reports of aboveground tissue surviving fire were lacking.

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
No additional information is available on this topic.

PLANT RESPONSE TO FIRE:
Mojave yucca sprouts following fire [19,56]. Postfire sprouting is described as "vigorous" by Vogl [88]. First postfire sprouts have been observed as early as 2 months following fire [83,84] and as late as the 2nd postfire growing season [91]. Plants may have a "shrubby" appearance in the early postfire years [19]. Predation of postfire sprouts by small mammals can be severe and will affect postfire recovery. Predation from woodrats [91] and rabbits [83,84] can be heavy.

Seedlings are rarely observed in postfire communities [21,62,63]. Mojave yucca seeds collected from several coastal populations in southern California did not tolerate long-duration, moderate-heat treatments (180-190 F (80-90 C)) or short-duration, high-heat treatments (230-250 F (110-120 C)) [45]. For a summary of this study, see Germination.

First year postfire sprouts following the Hackberry Complex Fire
2006 Tom Schweich. Used with permission, http://www.schweich.com

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
Mojave yucca often survives fire; however, density and cover may be lower on burned than unburned sites. Based on the few studies reporting fire severity [51,83,84], severe fires produce the greatest and longest lasting reductions in Mojave yucca abundance [51]. Available literature (2007) does not address Mojave yucca recovery and survival following repeated fires at short intervals which would likely be more common in Mojave yucca habitats with increases in nonnative grasses that easily carry fire.

Sampling issues: It is important to note that the common methods used to estimate cover and density often perform poorly in arid regions and are rarely "adequate to detect important site differences" [60]. Methods are not described for the following summarized studies.

Early postfire response: The following studies focused on Mojave yucca recovery in the early postfire years. All results presented are within 3 years of burning.

Regeneration through sprout production was common in early postfire communities in southern California. In northern San Diego County, Mojave yucca sprouts occurred on burned sites following a December fire. Seedlings were rare. Burned sites were visited for the first 3 postfire seasons [62]. In burned chamise chaparral vegetation on the California and Baja California Norte border, only Mojave yucca sprouts were observed. There were no seedlings in the postfire community visited 1 to 3 years after the fire [63].

Webber [91] observed Mojave yucca sprouts in the 2nd postfire year but reported almost no green stems or sprouts in the 1st postfire year in a grassland (dominant grasses not identified) north of Stoddard Well in southern California. Fire season and severity were unknown. Burned areas were first visited in May 1943, likely the 1st postfire year, and revisited in May 1944. In 1944 one to four sprouts occurred at the base of burned Mojave yucca stems. The researcher estimated that the Mojave yucca stand would recover to be as dense or more dense than the prefire condition.

All burned Mojave yucca had root crown sprouts 1.5 years following a fire in Covington Flats, Joshua Tree National Park. The fire, ignited by lighting in July, consumed all vegetation on the valley floor and considerable vegetation from adjacent slopes. All blackbrush stems were consumed. Burned Mojave yucca clones averaged 6.2 (SD 3.3) sprouts [56].

The following study suggests that postfire weather and/or browsing may affect Mojave yucca sprout survival. Following a mid-July fire in the San Ysidro Mountains of San Diego County, Mojave yucca survival exceeded 90% on burned canyon and ridge sites. Air temperatures averaged ~80 F (30 C) when the fire burned, and recent precipitation was lacking. Most woody species were only partially consumed. Fire severity was lower in the canyon than on the ridge, and the unburned reference site was most similar to the ridge in terms of exposure and slope. One year following fire, Mojave yucca density was 30 plants/ha in the canyon, 20/ha on the ridge, and 20/ha on the unburned site. The number of sprouting plants and productivity of Mojave yucca decreased dramatically between the fall and winter season on the burned ridge site. A November snow storm 4 months following the fire brought cold temperatures and ~12 inches (300 mm) of snow, which may have affected sprout survival. High browsing levels were also reported for the fall season; see the section on Small mammals. A summary of Mojave yucca postfire sprouting and productivity is provided for the 2nd, 4th, 7th, and 10th postfire months [83,84]:

  Summer Fall Winter Spring
Time since fire (months) 2 4 7 10
 

number of sprouting plants/ha

Canyon 15 30 20 30
Ridge 5 50 20 20
 

productivity g/ha

Canyon 93 1,413 1,352 6,092
Ridge 2.5 3,284 1,464 9,681

In the 2nd postfire year, researchers noted a single Mojave yucca plant that produced 23 leaf clusters with some leaves extending over 3 feet (1 m). If all clusters became trunks, it would mean increased clone size.

Mojave yucca cover was lower on burned than control transects 9 months following a lightning-ignited August fire in the Little San Bernardino Mountains of Joshua Tree National Park. Mojave yucca cover was 1% on control and 0.1% on burned transects in the Johnny Lang Canyon. In the Juniper Flats area, Mojave yucca cover was 5.2% on control and 0.2% on burned transects. Fire severity was not described [48].

In coastal sage scrub dominated by brittle bush (Encelia farinosa) in San Bernardino County, Mojave yucca cover was 0.1% before and 0.1% in the 2nd postfire growing season after a mid-July fire described as "intense". The site had last burned 23 years earlier. At the time of burning, air temperature was 97 F (36 C) and relative humidity was 31%. Using a modeling program based on fire weather and fuel characteristics, fire intensity was an estimated 170 kcal/s/m, and total heat released from dead fuels after passage of the fire front was an estimated 3660 kcal/m. Researchers suggested that Mojave yucca would sprout following fires that exceed 170 kcal/s/m intensity [93].

Later postfire response: Very few studies compare burned and unburned sites beyond the 3rd postfire year. The few available studies suggest that burned and unburned comparisons made in the early postfire years may differ from those made in later postfire years.

Density of Mojave yucca was lower (0.13 plants/m) on nearly 10-year-old burned sites than on unburned sites (0.54 plants/m) when compared in a Joshua tree woodland in Joshua Tree National Monument [4].

Density of Mojave yucca was significantly greater (P≤0.05) on unburned than on 8- to 17-year-old severely burned sites in blackbrush communities of Clark County, Nevada. Differences in Mojave yucca density were not significant on burned and unburned sites 1 year following a moderate-severity fire in this area. Mojave yucca density on burned and unburned plots is provided below [51].

Site White Rock Springs Sandy Valley Bird Spring Blue Diamond
Month of fire June June July July
Time since fire
(years)
1 8 13 17
Fire severity moderate severe severe severe
B UB B UB B UB B UB
Mojave yucca density
(plants/100 m)
<0.01 0.02 0 1.9 0 2.9 <0.01 3.1

Regardless of site or fire severity, soil organic matter was significantly (P≤0.001) lower on burned than unburned plots. Soil water content, soil surface temperatures, and temperatures in the top 1 inch (3 cm) of soil were significantly (P ≤0.01) higher on burned than unburned plots. Soil pH, soil compaction, and soil bulk density were significantly (P≤ 0.05) higher on burned than unburned plots.

FIRE MANAGEMENT CONSIDERATIONS:
Information on Mojave yucca's recovery and survival following repeated fires is lacking. In the Mojave Desert, increases in nonnative grasses such as cheatgrass, red brome, and Mediterranean grasses will likely increase fire frequency and size in Mojave yucca habitats [22,24]. Without information on Mojave yucca's tolerance or intolerance of short-interval fire frequencies, effects of increased fire frequency on this species are difficult to predict.

McAuliffe [60] provides a log series survey method to quickly estimate plant cover and density in desert communities using 5,400 ft (500 m) plots. Common methods of estimating density and cover did not detect Mojave yucca, which was rare in the community near Las Vegas, but the log series survey method did.


MANAGEMENT CONSIDERATIONS

SPECIES: Yucca schidigera
IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Mojave yucca is important to a variety of desert wildlife species. Small mammals, birds, and reptiles utilize Mojave yucca for food, nest materials, nesting sites, and habitat. Mojave yucca is rarely utilized by livestock. Cattle reportedly consume yucca flowers [82], but evidence of cattle browsing is a sign of poor range condition [19].

Small mammals: Studies indicate that Merriam kangaroo rats, white-tailed antelope squirrels, woodrats, and rabbits all feed on Mojave yucca and utilize Mojave yucca for nest sites and materials. Postfire sprouts may be a preferred food source.

Mojave yucca frequency averaged 1.2% in 411 Merriam's kangaroo rat cheek pouches examined in Las Vegas Valley. Mojave yucca cover in the area was 0.2% [11]. White-tailed antelope squirrel diets were also examined in southern Nevada. Mojave yucca frequency was 13.6% in 154 cheek pouches and constituted 9.7% of 93 feeding observations [10]. In the Covington Flats area of the Joshua Tree National Monument, Mojave yucca frequency in desert woodrat caches was 10.6% to 13% and in stomachs was 2.9% to 7%. Mojave yucca was a smaller portion of caches and a larger portion of diets when desert woodrats were living side-by-side dusky-footed woodrats. Researchers suggested that Mojave yucca may be more important as a water source than a food source. Mojave yucca fibers were a large component of desert woodrat nests [13,14]. Nests were common at the base of Mojave yucca in southern Nevada [89].

Herbivory was monitored on burned sites in the San Ysidro Mountains of San Diego County. Mojave yucca was the most preferred spring browse species. Rabbits were considered the primary herbivore; Mojave yucca was avoided by bighorn sheep in all seasons on the ridges. The estimated productivity and number of sprouts browsed on burned canyon and ridge sites are presented below [83,84]. For information on Mojave yucca recovery following this fire, see Early postfire response.

  Summer Fall Winter Spring
Time since fire
(months)
2 4 7 10
 

total number of browsed sprouts/ha

Canyon 20 138 44 267
Ridge 0 430 15 5
 

estimated grams/ha consumed

Canyon 2 115 22 403
Ridge 0 178 trace 10

Other mammals: In the Queen Valley area of Joshua Tree National Park, bobcats utilized the plains areas, where Mojave yucca was noted as a dominant, extensively for hunting [99].

Birds: Mojave yucca is often among the tallest vegetation in its habitat and provides important hunting, roosting, and nesting sites for birds.

During a 2-year census in Nevada's Newberry Mountains, low desert shrub vegetation dominated by creosotebush and Mojave yucca was important habitat for ground insectivores, foliage insectivores, granivores, nectivores, and flycatchers. Northern mockingbirds and loggerhead shrikes bred only in the low desert shrub and not in the higher elevation California juniper woodlands [8]. Gullion [35] reports that the low desert shrub type in Nevada provides poor to good Gambel's quail habitat.

Bird surveys in portions of the Colorado, Mojave, and Great Basin deserts in California indicate that there are an average of 10 resident bird species in desert scrub communities, where a Mojave yucca overstory is common. Annual average density of birds in the desert scrub was 36.6 birds/40 ha. In the winter the number of resident species was 13 and bird density was 69.1/40 ha [23].

Effects of Mojave yucca harvesting on bird populations were studied in Mojave yucca-buckhorn cholla vegetation in the eastern Mojave Desert. Illegally harvested stands had reduced Mojave yucca stem density. The percentage of clones with 1 or more stems above 4 feet (1.2 m) tall was 60.1% in unharvested and 18.7% in harvested stands. Harvested stands supported fewer bird species and fewer individuals than unharvested stands. For winter bird species, there were 52% fewer species and 94% fewer individuals in harvested stands. Cactus wrens showed a significant (P-value not reported) decrease in harvested stands. Unharvested stands were not adjacent to harvested stands, but species composition and density were similar. Unharvested stands, however, were closer to water and harvested stands may have experienced heavier grazing. Mojave yucca was the tallest species in harvested and unharvested stands and was used for perching and nesting. Mojave yucca was also the sole nesting habitat for the following cavity nesters: American kestrels, screech owls, ladder-backed woodpeckers, and ash-throated fly catchers. Two ash-throated flycatcher nests were found in abnormally low positions (~2 feet (0.6 m) above ground) in harvested stands, suggesting that some birds may have settled for lower quality habitat in these stands [15].

Reptiles: Desert spiny lizards tend to be arboreal and are often collected on Mojave yucca in the Mojave Desert [77]. In the Piute Valley of southern Nevada, Mojave yucca is abundant in the overstory in a study area with a desert tortoise density of 50/km in 1979 and 72/km in 1983. Direct utilization of Mojave yucca and preference of Mojave yucca habitats was not reported [33].

Palatability/nutritional value: Mojave yucca has low livestock forage value, and cattle browsing of Mojave yucca signals a "severely degraded range condition" [19].

Cover value: The importance of Mojave yucca in the habitats of wildlife species has been integrated into the above sections. For additional information on the importance of Mojave yucca in wildlife habitats, see the species group of interest within Importance to Livestock and Wildlife.

VALUE FOR REHABILITATION OF DISTURBED SITES:
Mojave yucca plants salvaged from sites that will be disturbed in the future may be useful for revegetation or for ornamental use. In the eastern Mojave Desert, Mojave yucca plants were removed from a future mine site through hand digging. Plants survived well after being transplanted in a nursery. Plants were evaluated 1 to 2 years after transplanting. Of the 939 Mojave yucca plants, 66% were in excellent condition (no yellow leaves), 18% were dead (no green leaves), and 16% were in poor condition [30].

OTHER USES:
Mojave yucca was and remains a useful plant. Mojave yucca fibers, fruits, and roots were used extensively by early southwestern inhabitants. The tensile strength of Mojave yucca fibers is provided in [9]. Today, Mojave yucca is used in a variety of products and industries.

Past uses: Native people ate Mojave yucca fruits raw, cooked, and ground into a meal. Fibers were used in ropes, sandals, and baskets [80]. Mojave Desert natives made torches of dried Mojave yucca leaves [58]. The Chauilla people of southern California utilized almost all parts of the Mojave yucca plant. Fruits gathered in April or May were typically eaten roasted. Mojave yucca roots were scraped and the shavings mixed with water for washing. Fibers were used to make bowstrings, ropes, saddle blankets, shell money strings, basket materials, and body painting brushes. Whole leaves were used to secure house beams and poles. Seeds added to sticks became animal toys for children. Seeds also decorated necklaces [5]. Early inhabitants of Gypsum cave in southern Nevada used Mojave yucca fibers to make sandals and cords. Fur was twisted around Mojave yucca cord to make robes [6].

Current uses: The number of products and industries utilizing Mojave yucca is staggering. It is a common landscape plant in the Southwest [27]. In a review, Mojave yucca is noted as a flavoring and foaming agent in soft drinks, as a surfactant and preservative in cosmetics, and as a fertilizer for crops [59]. Yale [95] indicates that Mojave yucca extracts increase successful bacteria or plant growth under stressful growing conditions. Many review articles highlight the use of Mojave yucca extracts in confined feeding operations to decrease ammonia emissions and to increase livestock feeding efficiency and weight gain [16,17,40,59]. When Mojave yucca extract was added to pig feed, hydrogen sulfide and ammonia levels in the air in the confinement area were lowered [40]. Mojave yucca extracts have also decreased the fecal odor from dogs and cats when added to food. Researchers have also decreased the blood cholesterol of chickens and humans, increased the immune response in young piglets, increased vitamin and mineral absorption in animals, increased successful cattle reproduction, and decreased the number of swine stillbirths using Mojave yucca [16,17]. The antiplatelet properties of Mojave yucca phenolic compounds suggests a future use of Mojave yucca in the treatment of thrombosis and cardiovascular disease [67].

OTHER MANAGEMENT CONSIDERATIONS:
Mojave yucca harvesting: Given the number of Mojave yucca uses, it is not surprising that laws exist to prevent destructive Mojave yucca harvesting. In Arizona, Mojave yucca is listed as a species subject to theft and vandalism [2]. Guidelines for harvesting in Mojave yucca populations in Baja California are provided in [18].  In southern California (as of 1980), Mojave yucca harvesting regulations allowed trunks to be cut at ground level, leaving stumps and roots intact. Dead and live leaves were to be stripped from cut stems and used to cover the exposed stump, a step thought to aid plant recovery. Removal of entire clones or stems less than 6 feet (1.8 m) or over 12 feet (3.7 m) tall was illegal. However, stems that measure 6 to 12 feet (1.8-3.7 m) may comprise the bulk of reproductive plants. Some believe that these rules are inadequate for protection of Mojave yucca and suggest eliminating harvests, since Mojave yucca may grow only 0.4 inch (1 cm)/year and require 150 years to reach legal harvestable size [15]. The Mojave yucca and bird populations in an illegally harvested stand are summarized in Birds.

Herbicides: Mojave yucca was not affected when habitats in the northern Mojave desert were treated with a photosynthetic inhibitor to eliminate shrub cover. Coverage was greater on treated than untreated sites evaluated about 8 years following application. Residual of the chemical was present more than 8 years following application [38].

Climate change: In an attempt to anticipate changes in plant physiologies and eventually predict changes in plant distributions, researchers have studied Mojave yucca growth under elevated CO2 levels and increased temperatures. Huxman and others [39] conducted experiments on Mojave yucca growth and photosynthetic capabilities under elevated CO2 levels and increased temperatures. Loik and others [55] found that Mojave yucca's low temperature tolerance was increased by 2.5 F (1.4 C) when plants were grown in elevated CO2 environments. Increased temperatures and CO2 levels associated with climate change may allow Mojave yucca seedlings to establish at higher elevations and latitudes.


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