Fire adaptations: Soaptree yucca can sprout from the stem after fire, even when some leaves are burnt. If the apical meristem is damaged the stem may branch, but more commonly it dies and is replaced by sprouts from rhizomes and the root crown [40,43,48].
FIRE REGIMES: Although soaptree yucca can tolerate fire [20,40,42,43], it has increased with the suppression of fires [43]. Thomas and Goodson [40] report that an average fire return interval of 3-10 years on semidesert grasslands would support soaptree yucca (and other succulents), but at a lower density than at present.
Fire return intervals for plant communities in which soaptree yucca occurs are listed below. Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica 5-100 plains grasslands Bouteloua spp. grama-galleta steppe B. gracilis-P. jamesii blue grama-tobosa prairie B. g.-P. mutica paloverde-cactus shrub Cercidium microphyllum/Opuntia spp. creosotebush Larrea tridentata Ceniza shrub L. t.-Leucophyllum frutescens-Prosopis glandulosa galleta-threeawn shrubsteppe Pleuraphis jamesii-Aristida purpurea mesquite Prosopis glandulosa mesquite-buffalo grass P. g.-Buchloe dactyloides Texas savanna P. g. var. glandulosa 34]It is reasonable to expect 25% mortality of soaptree yucca following fire [20,43]. This may be problematic where fire frequently occurs [43]: young regrowth would likely be less fire tolerant because a larger proportion of stems would be shorter and exposed to lethal temperatures. Though long-term studies of different FIRE REGIMES' effects on soaptree yucca populations have not been conducted, it has been suggested that soaptree yucca would survive in the historic 3-10 year fire return interval of semidesert grasslands; however, in many areas its density would be lower than at present [43].
Managers often seek to eliminate undesirable shrubs by burning in high fuel years (after wet growing seasons or 1-2 years without grazing) to achieve maximum crown burning. This practice may result in high mortality of succulents including soaptree yucca. Damage to succulents is reduced by low-severity fires that kill shrub seedlings and scorch some crowns Although reduction of shrub canopy cover and seed production is not as great as that achieved with crown fires, greater coverage of soaptree yucca and other succulents is retained [38].
Soaptree yucca grows on a wide range of sites but prefers coarse soils [8,9,17]. In the Guadalupe Mountains of New Mexico, soaptree yucca is most prevalent on gypsum dunes, but also present on sandhills derived from quartz [9]. In the Davis Mountains of Texas and Coahuila, Mexico, soaptree yucca is common on gentle to moderate slopes with coarse soils derived from igneous materials [17]. In the Chisos Mountains of the Rio Grande area of Texas, soaptree yucca "reached best development" on eroded rocky slopes [30].
The climate of the Jornada Experimental Range is typical of the semidesert grassland communities where soaptree yucca is prevalent. At this site there is an average of 8.33 inches (215 mm) precipitation per year, with 64% occurring in May through September. Mean annual temperature is 59 degrees Fahrenheit (15 oC) [6].
In southwestern Texas and central and southern Arizona soaptree yucca grows between 1,500 and 6,000 feet (450- 1,220 m) in elevation, Verdi yucca grows in central Arizona between 3,000 and 6,000 feet (910-1,220 m) in elevation [24].
Soaptree yucca is present in many desert scrub
communities and is also an important component of semi-desert grasslands. In the
San Simon Valley of southeastern Arizona, soaptree yucca, Mariola (Parthenium incanum),
and cacti (Opuntia spp.) were subdominant in communities where
creosotebush (Larrea tridentata), tarbush (Flourensia cernua),
mesquite (Prosopis spp.), or acacia (Acacia spp.) were dominant
[10]. Similar shrub communities were described in the Huachuca Mountains of the
southern border of Arizona below 4,500 feet (1,370 m). Grass species present here were
black grama (Bouteloua eriopoda), crowfoot grama (B. rothrockii), burrograss (Sclerpogon
brevifolius), fluffgrass (Tridens pulchellus), bush muhly (Muhlenbergia porteri),
and threeawns (Aristida spp.). Desertholly (Perezia nana) and burrowweed (Isocoma
tenuisecta) were present in disturbed areas [45]. Soaptree yucca is also subdominant
on the Jornada Experimental Range near Las Cruces, New Mexico, and similar communities
occur there [3].
Soaptree yucca occurs in an irregular,
clumped distribution in grasslands dominated by
gramas (Bouteloua spp.), threeawns, tobosagrass (Pleuraphis mutica), and dropseeds
(Sporobolus spp.) [6,17,32]. On the Jornada Experimental Range, soaptree yucca
is described as a "structural dominant:" 1 of few shrubs growing in
black grama grasslands [25].
Soaptree yucca, creosotebush, and fourwing saltbush (Atriplex canescens) are the predominant shrubs in the large area of dune fields
known as the
Mesilla Basin, New Mexico and adjacent Chihuahua, Mexico [29].
Soaptree yucca is relatively resistant to browsing pressure in the short term because of frequent
clonal reproduction. Because of this reproductive strategy, an early
source stated that leaving seed trees is unnecessary except to provide shade [8].
However, a more recent study cited several reasons for reducing
browsing pressure on soaptree yucca inflorescences. The authors stated that
overconsumption of inflorescences may: 1) lead to local declines in the
flight-limited yucca moth, reducing pollination in subsequent years, 2) reduce
mobilization of nutrients and carbohydrates from aborted fruit to other parts of
the plant, leading to decreased ability
to produce new caudices, 3) reduce recruitment of seedlings that maintain
genetic diversity in populations and provide a means of colonizing new habitats,
and 4) reduce local biodiversity, especially birds and insects because of loss
of species that depend on the inflorescence and fruit [25]. The study concluded that in
soaptree yucca populations where some members are tall enough (>1.7
m) to escape fruit consumption by livestock, tall individuals are
important refugia for yucca moths and help maintain genetic diversity. In populations where no individuals are tall enough to escape
browsing, management should reduce grazing in spring to avoid complete
consumption of inflorescences [25].
Grazing may favor soaptree yucca if more palatable forage is available. Some sources have cited
grazing as a cause of increase in soaptree yucca cover, but the data are not
conclusive. Two studies were conducted on the New Mexico State University
College Ranch; 1 found shrub cover in general was consistently higher in grazed
areas compared to ungrazed areas, but cover of soaptree yucca was significantly higher (p
<0.05) in grazed areas only on
some sample locations and times [19]. In the 2nd study, there was a trend for
soaptree yucca to have more cover on good condition than excellent condition range, but the
trend was not statistically significant [40].
Desert grasslands have declined due to cultivation, urbanization,
and shrub invasion [19,20,32,36,40,45]. Causes of shrub increase include fire suppression and grazing.
Desert grasslands are important for biodiversity, as
many avifauna use both soaptree yucca-black grama communities in the Chihuahua Desert and shortgrass prairies elsewhere [25,36]. Raitt and
Pimm [36] conclude that "lowland grasslands in the Chihuahua Desert
region... are relevant beyond immediate, local considerations, however
important. The future of a number of continental bird populations may
depend upon their success." Fire and grazing
management strategies are used by managers to maintain or restore desert grasslands that have
become shrub-dominated [20].
Livestock browse soaptree yucca leaves during winter and spring months when
young regrowth is available and grasses are least productive [8,16,25]. Soaptree yucca is palatable to cattle [25].
Cattle on the
Jornada Experimental Range consumed soaptree yucca leaves in March (14% of diet) and December (17%) [16]. Similarly, in
southern New Mexico soaptree yucca leaves comprise up to 20% of
cattle diets in winter and spring [18]. Campbell and Keller [8]
commented that livestock use soaptree
yucca infrequently on "conservatively grazed range, supporting the black grama climax."
On
the New Mexico State University College Ranch, cattle browsed soaptree yucca
more heavily than any other shrub present. Grasses, primarily gramas and dropseeds, were 86% of the winter diet, soaptree yucca was approximately
9% of the diet, and other shrubs were 2% [50].
Soaptree yucca inflorescences are consumed heavily by livestock
[25,35] and wildlife including mule deer, pronghorn, black-tailed jackrabbit, and several types of
woodrats. In cattle exclosures on the New Mexico State University
College Ranch, woodrats consumed a mean of 15% (n= 10, range = 0-42%) of
soaptree yucca floral production. Up to 98% of
inflorescences were consumed on similar sites with cattle (density = 50 cows/ha) [25].
Soaptree yucca produces from seed and by sprouting. Soaptree yucca has an obligate mutualistic relation to its pollinator the yucca moth (Tegeticula yuccsella) [21]. The yucca moth pollinates its flowers and moth larvae feed on the developing fruit, decreasing viable seed production by up to 20% [1,25]. Soaptree yucca aborts many fruits, killing larvae therein; this process is thought to help maintain the mutualistic relationship [22]. Fruits produce many small, windblown seeds [1,25]. Little information is published regarding seed longevity and seed banking.
In most cases, reproduction is primarily clonal [8,8,35,42,43]. Sprouts originate from meristems on rhizomes and the root crown. Soaptree yucca may also resume growth from unburned portions of the stem [40,43,48,48]. Seedling establishment is not thought to be a reliable means of regeneration for soaptree yucca, primarily because germination and seedling establishment are controlled by a number of factors including, most importantly, adequate soil moisture and facilitation by other shrubs [42,43].
The scientific name of soaptree yucca is Yucca elata (Engelm.) Engelm. (Agavaceae)
[4,23,24]. Three varieties of soaptree yucca are
recognized [23]:
Yucca elata var. elata soaptree yucca
Yucca elata var. utahensis (McKelvey) Reveal Utah yucca
Yucca elata var. verdiensis (McKelvey) Reveal Verdi yucca
Most literature regarding the soaptree yucca does not specify which subspecies is studied.
However, most research has focused on populations of New Mexico, Arizona, and
the Chihuahuan Desert of Texas and adjacent Mexico; these populations are either
the typical variety (Y. e. var. elata) or Verdi yucca.
In southwestern Utah and southeastern Nevada, Utah yucca ×
narrow-leaved yucca (Y. angustissima) hybrids are common [48]. Soaptree yucca × soapweed yucca (Y. glauca)
hybrids have been reported in
northern Arizona and New Mexico [47,48]. Soaptree yucca reportedly hybridizes with Buckley yucca (Y.
constricta) in Pecos and Howard Counties, Texas,
and in southeastern New Mexico [48].
Because soaptree yucca is 1 of few long-lived woody plants on many sites, it is of value for long-term soil quality. Its litter increases soil organic matter and helps retain soil water [8]. Soaptree yucca helps stabilize sand in dune areas [29]. The stems often produce adventitious roots that increase sand stability [48].
Soaptree yucca is difficult to transplant. Campbell and Keller [8] reported that only 25% of soaptree yucca transplants survived due to taproot breakage. Soaptree yucca has been transplanted to revegetate highway rights-of way, but there was great expense in removing entire roots, as is required for successful planting. Successful transplanting of yuccas (an unspecified amount of which were soaptree yucca) has been done; plants were removed with as little root damage as possible and immediately watered when replanted [48].
Yucca elata is a perennial plant, with common names that include soaptree, soaptree yucca, soapweed, and palmella.[3][4] It is native to southwestern North America, in the Sonoran Desert and Chihuahuan Desert in the United States (western Texas, New Mexico, Arizona), southern Nevada, southwestern Utah, and northern Mexico (Chihuahua, Coahuila, Sonora, Nuevo León).[5][6] Yucca elata is widely distributed, although its population appears to be decreasing.[1]
This plant grows from 1.2 to 4.5 m tall, with a sparsely branched trunk. The trunk is brown, cylindrical in shape and has a small diameter and often has holes drilled by escaping yucca moth larvae. The leaves are arranged in a dense spiral whorl at the apex of the stems, each leaf 25–95 cm long and very slender, 0.2-1.3 cm broad. The white, bell-shaped flowers grow in a dense cluster on a slender stem at the apex of the stem, each flower 32–57 mm long, creamy white, often tinged pinkish or greenish.[7][8][9]
The soaptree yucca's fruit is a capsule 4–8 cm long and 2–4 cm broad, maturing brown in summer, when it splits into three sections to release the black seeds. They do not flower every year.[6]
These plants fare best in dry, semi-desert conditions. They are very cold-hardy, but need much sunlight.[10]
There are three subordinate taxa are sometimes recognized, although sources differ as to whether these should be considered varieties or subspecies:[6][11]
Native Americans used the fiber of the soaptree yucca's leaves to make sandals, belts, cloth, baskets, cords, and mats, among other items; they also ate the flowers.[12] Inside the trunk and roots of the plant is a soapy substance high in saponins. In the past, this substance was commonly used as soap and shampoo, which was used to treat dandruff and hairloss. At least one tribe, the Zuni, used a mixture of soap made from yucca sap and ground aster to wash newborn babies to stimulate hair growth. The Apaches also use yucca leaf fibers to make dental floss and rope. In times of drought ranchers have used the plant as an emergency food supply for their cattle; the chopped trunk and leaves can be eaten.[12]
Flowers of the genus Yucca are the state flowers of US state of New Mexico.[13] No species name is given in the statute citation,[14] however the New Mexico Centennial Blue Book from 2012 references the soaptree yucca (Yucca elata) as one of the more widespread species in New Mexico.[15]
Yucca elata is a perennial plant, with common names that include soaptree, soaptree yucca, soapweed, and palmella. It is native to southwestern North America, in the Sonoran Desert and Chihuahuan Desert in the United States (western Texas, New Mexico, Arizona), southern Nevada, southwestern Utah, and northern Mexico (Chihuahua, Coahuila, Sonora, Nuevo León). Yucca elata is widely distributed, although its population appears to be decreasing.
