|FEIS Home Page|
AUTHORSHIP AND CITATION:
Steinberg, Peter. 2001. Yucca elata. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.fed.us /database/feis/ .
Yucca utahensis McKelvey 
= Y. elata var. utahensis (McKelvey) Reveal 
NRCS PLANT CODE :
The scientific name of soaptree yucca is Yucca elata (Engelm.) Engelm. (Agavaceae) [4,23,24]. Three varieties of soaptree yucca are recognized :
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 . 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 .
FEDERAL LEGAL STATUS:
No special status
Soaptree yucca occurs in central Arizona, southern New Mexico, western Texas [2,8], and Coahuila and Chihuahua, Mexico . The PLANTS database provides a distributional map of soaptree yucca in the United States. The typical variety occurs from central Arizona east to southwestern Texas; it is the only variety native to Mexico . Utah yucca occurs from Nevada east to southwestern Arizona and south to north-central Arizona. Verdi yucca is known only in central Arizona [4,24,49].
KUCHLER  PLANT ASSOCIATIONS:
K041 Creosote bush
K042 Creosote bush-bursage
K043 Paloverde-cactus shrub
K044 Creosote bush-tarbush
K045 Ceniza shrub
K046 Desert: vegetation largely lacking
K053 Grama-galleta steppe
K054 Grama-tobosa prairie
K058 Grama-tobosa shrubsteppe
K059 Trans-Pecos shrub savanna
K060 Mesquite savanna
K061 Mesquite-acacia savanna
K065 Grama-buffalo grass
K076 Blackland prairie
SRM (RANGELAND) COVER TYPES :
211 Creosote bush scrub
503 Arizona chaparral
505 Grama-tobosa shrub
507 Palo verde-cactus
701 Alkali sacaton-tobosagrass
702 Black grama-alkali sacaton
703 Black grama-sideoats grama
705 Blue grama-galleta
706 Blue grama-sideoats grama
707 Blue grama-sideoats grama-black grama
712 Galleta-alkali sacaton
715 Grama-buffalo grass
725 Vine mesquite-alkali sacaton
HABITAT TYPES AND PLANT COMMUNITIES:
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 . 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 . Soaptree yucca is also subdominant on the Jornada Experimental Range near Las Cruces, New Mexico, and similar communities occur there .
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 .
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 .
IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Livestock use soaptree yucca leaves as a secondary or seasonal source of feed. A number of wildlife species also use the species as browse, but quantitative data are lacking. The following table summarizes soaptree yucca use by birds and mammals [8,11,23,25,46,51].
|white-throated woodrat||leaves are up to 33% of diet (by volume) , nests built at base of stems |
|southern plains woodrat||leaves are up to 16% of diet |
|black-tailed jackrabbit||leaves a small part of diet (mean = 2%) when within reach [25,46]|
|desert cottontail||leaves part of diet when within reach [8,25]|
|kangaroo rats||seeds are minor part of diet |
|desert mule deer||leaves, inflorescence stalks part of diet [25,26]|
|pronghorn||inflorescences consumed |
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 . Cattle on the Jornada Experimental Range consumed soaptree yucca leaves in March (14% of diet) and December (17%) . Similarly, in southern New Mexico soaptree yucca leaves comprise up to 20% of cattle diets in winter and spring . Campbell and Keller  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% .
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) .
Soaptree yucca foliage, particularly actively growing apices, is low in acid-detergent lignin . The consumption of the nutritious apex is limited by larger fibrous leaves with pointed tips below [8,25]. The nutritional value of soaptree yucca leaves, collected on the New Mexico State University College Ranch during active cattle browsing, was analyzed 5 times. The following shows nutritional content expressed as percentages of dry weight :
|Sample date||Protein||Acid-detergent fiber||Acid-detergent lignin||Ash||Ca||P||K|
The inflorescences are high in moisture and protein. The nutritional value of inflorescences collected on the New Mexico State University College Ranch in 1991was analyzed as follows. Data are means and 1 standard deviation .
|Floral part||Mass (g)||Moisture (%)||Neutral- detergent fiber (%, n=3)||Acid- detergent fiber (%, n=3)||Crude protein (%, n=3)||Ash (%, n=3)|
|young stalk||88.1 (49.5, n=5)||65.4 (4.4, n=5)||28.3, (2.9)||23.6 (2.3)||21.7 (1.3)||6.4 (0.7)|
|mature inflorescence||1160.3 (638.6, n=5)||73.0 (2.9, n=5)||44.1 (4.8)||37.7 (3.7)||16.9 (1.5)||6.2 (0.4)|
|flowers||600.3 (385.7, n=5)||78.7 (3.5, n=5)||13.7 (0.8)||14.3 (1.4)||26.5 (3.4)||8.6 (0.4)|
|young leaves||82.6 (30.1, n=4)||70.1 (3.3, n=4)||55.3 (1.0)||45.5 (1.0)||10.6 (2.2)||6.7 (0.5)|
Soaptree yucca provides cover for small mammals and birds. The white-throated woodrat frequently nests at the base of stems; when these dens are abandoned by woodrats they are often used by desert cottontails . Soaptree yucca is the only perch for birds in much of its range .
VALUE FOR REHABILITATION OF DISTURBED SITES:
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 . Soaptree yucca helps stabilize sand in dune areas . The stems often produce adventitious roots that increase sand stability .
Soaptree yucca is difficult to transplant. Campbell and Keller  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 .
OTHER USES AND VALUES:
Native Americans traditionally consumed young flowerstalks and the lowest part of the stem [4,9,24], and ground interior portions of the trunk into flour [4,9]. Historically, they also used yucca fibers (including that of soaptree yucca) for clothing and rope. Yucca fibers were economically important during World War I, when New Mexico and Texas "produced 80 million pounds of bagging and burlap" . Crushed soaptree yucca roots and stems can be used to make soap or shampoo [24,35]. Whole plants can be ground to pulp, much as trees are prepared for paper pulp, to provide a valuable emergency livestock feed [24,27,35,52]. Complete population recovery following extensive soaptree yucca harvest has not been observed .
OTHER MANAGEMENT CONSIDERATIONS:
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 . 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 . 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 .
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 . 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 .
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  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 .
GENERAL BOTANICAL CHARACTERISTICS:
Soaptree yucca is a tree or shrub. It has woody, succulent stems often 9-12 feet (3-4 m) high (occasionally up to 30 feet (9 m) . Utah yucca is smaller, with stems up to approximately 4.5 feet (1.3 m) . Growth form of soaptree yucca varies from a single, erect, trunklike stem to several stems in clumps of 1.6 to 8.2 feet (0.5-2.5 m) in diameter . Like all yucca species with dehiscent fruits, soaptree yucca is rhizomatous. The species is unique in that the rhizome develops downward and later begins lateral root extensions. The "vertical rhizome" as described by Webber  commonly grows to 3-5 feet (1-1.5 m) deep, and 3-6 inches (8-15 cm) in diameter. Lateral roots are 6-10 inches (15-20 cm) long and 1-3 inches (2.5-8 cm) in diameter . Leaves are slender, sharply pointed, 2-3 feet (0.6-1 m) in length, and grow in variably loose to densely crowded clumps . Stems are often clumped because they are derived from common rhizome systems . Leaves grow in a tuft at the top of the stem . The stem can be either upright or procumbent . The inflorescence is an open, branching panicle , producing a capsular, fleshy, and dehiscent fruit. The fruit commonly contains about 150 viable ovoid seeds .
Soaptree yucca produces from seed and by sprouting. Soaptree yucca has an obligate mutualistic relation to its pollinator the yucca moth (Tegeticula yuccsella) . 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 . 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].
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 . 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 . In the Chisos Mountains of the Rio Grande area of Texas, soaptree yucca "reached best development" on eroded rocky slopes .
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) .
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 .
Soaptree yucca occurs in both early and later succession. Early descriptions of honey mesquite (Prosopis glandulosa) dunes list soaptree yucca as a secondary colonizer . Honey mesquite was dominant on unprotected areas with frequent wind erosion or sand aggradation, while soaptree yucca established after several years of favorable conditions on sites where topography or honey mesquite had stabilized soils . A more recent source cites soaptree yucca as a major component of dune vegetation in the Mesilla Basin of New Mexico. The author reported that mounds of sand formed around soaptree yucca, implying that it did not require other shrubs to stabilize sand and advance succession .
In Arizona, New Mexico, and Texas soaptree yucca flowers between May 15 and July 15; fruit ripens between August 1 and October 1; and seed is dispersed in September and October . On the Jornada Experimental Range soaptree yucca initiated new leaf growth in May and June and continued producing new leaves throughout the summer . Leaf elongation occurred until early fall. The period of most rapid growth was at the end of July, when average length of new leaves doubled in 1 week . During their 1st year leaves grow parallel to the stem. In subsequent years they are oriented outward, providing some protection for the apical meristem . Leaves remain green for 3-5 years. Dry leaves remain on the stem .
FIRE ECOLOGY OR ADAPTATIONS:
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 . Thomas and Goodson  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||< 35 to < 100|
|desert grasslands||Bouteloua eriopoda and/or Pleuraphis mutica||5-100|
|plains grasslands||Bouteloua spp.||< 35|
|grama-galleta steppe||B. gracilis-P. jamesii||< 35 to < 100|
|blue grama-tobosa prairie||B. g.-P. mutica||< 35 to < 100|
|paloverde-cactus shrub||Cercidium microphyllum/Opuntia spp.||< 35 to < 100|
|creosotebush||Larrea tridentata||< 35 to < 100|
|Ceniza shrub||L. t.-Leucophyllum frutescens-Prosopis glandulosa||< 35|
|galleta-threeawn shrubsteppe||Pleuraphis jamesii-Aristida purpurea||< 35 to < 100|
|mesquite||Prosopis glandulosa||< 35 to < 100|
|mesquite-buffalo grass||P. g.-Buchloe dactyloides||< 35|
|Texas savanna||P. g. var. glandulosa||< 10 |
IMMEDIATE FIRE EFFECT ON PLANT:
Fire generally top-kills soaptree yucca, but it can be fire tolerant depending on the intensity and frequency of fire [40,43]. Two studies, both low-severity spring fires, one in the Sierrita Mountains near Tucson, Arizona, and one in the Whetstone Mountains of southeastern Arizona, found 25% and 27% mortality of soaptree yucca after several months, respectively [20,43]. In both cases, damage to the apical meristem was a frequent cause of mortality. Leaves remaining on the stem, whether dead or living, insulate conductive tissue and reduce fire damage to the stem and vascular tissue . Meristem protection also increases fire tolerance of soaptree yucca: apical meristems are protected by the dense terminal rosette and belowground meristems along rhizomes are protected by soil [38,42,43].
PLANT RESPONSE TO FIRE:
Soaptree yucca sprouts from rhizomes, the root crown, and undamaged stems after fire . Frequent production of belowground sprouts makes soaptree yucca unique among leaf succulents [38,42]. In the Whetstone Mountains of Arizona, 51% of soaptree yucca survived fire only by producing sprouts from belowground meristems , 33% of surviving soaptree yucca regrew from the apical meristem , 7% regenerated from the apical meristem and produced belowground sprouts, and 2% grew from seed. Few (7%) soaptree yucca were unburned, showing that the species is a fire tolerator rather than evader . The authors concluded that in most cases, regeneration of soaptree yucca occurs via vegetative reproduction rather than from seed trees surviving on unburned patches . Seedling establishment following fire is not thought to be a reliable means of recovery for soaptree yucca in most years, but it may be important during wet years following fire [42,43]. The estimated time for yucca species to recover to prefire density is 2-5 years .
DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
An extensive body of research has been published on fire effects in semidesert grassland, oak savanna, and Madrean oak woodlands of southeastern Arizona, including the response of soaptree yucca to fire. See the Research Project Summary of this work for more information on soaptree yucca and more than 100 additional species of herbaceous and woody plant species, birds, small mammals, and grasshoppers.
FIRE MANAGEMENT CONSIDERATIONS:
It is reasonable to expect 25% mortality of soaptree yucca following fire [20,43]. This may be problematic where fire frequently occurs : 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 .
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 .
1. Addicott, John F. 1986. Variation in the costs and benefits of mutualism: the interaction between yuccas and yucca moths. Oecologia. 70: 486-494. 
2. Alexander, Robert R.; Pond, Floyd W. 1974. Yucca (L.) Yucca. In: Schopmeyer, C. S., ed. Seeds of woody plants in the United States. Agric. Handb. 450. Washington, DC: U.S. Department of Agriculture, Forest Service: 857-858. 
3. Barbour, M. G.; MacMahon, J. A.; Bamberg, S. A.; Ludwig, J. A. 1977. The structure and distribution of Larrea communities. In: Mabry, T. J.; Hunziker, J. H.; DiFeo, D. R., Jr., eds. Creosote bush: Biology and chemistry of Larrea in New World deserts. U.S./IBP Synthesis Series 6. Stroudsburg, PA: Dowden, Hutchinson & Ross, Inc.: 227-251. 
4. Bell, Willis H.; Castetter, Edward F. 1941. Ethnobiological studies in the American Southwest. IV. The utilization of yucca, sotol, and beargrass by the aborigines in the American Southwest. University of New Mexico Bulletin. 5(5): 1-74. 
5. Bernard, Stephen R.; Brown, Kenneth F. 1977. Distribution of mammals, reptiles, and amphibians by BLM physiographic regions and A.W. Kuchler's associations for the eleven western states. Tech. Note 301. Denver, CO: U.S. Department of the Interior, Bureau of Land Management. 169 p. 
6. Brown, David E. 1982. Chihuahuan desertscrub. In: Brown, David E., ed. Biotic communities of the American Southwest--United States and Mexico. Desert Plants. 4(1-4): 169-179. 
7. Campbell, R. S. 1929. Vegetative succession in the Prosopis sand dunes of southern New Mexico. Ecology. 10(4): 392-398. 
8. Campbell, R. S.; Keller, J. G. 1932. Growth and reproduction of Yucca elata. Ecology. 13(4): 364-374. 
9. Castetter, Edward F.; Opler, M. E. 1936. Ethnobiological studies in the American Southwest. III. The ethnobiology of the Chiricahua and Mescalero Apache. University of New Mexico Bulletin. 4(5): 1-63. 
10. Chew, Robert M.; Chew, Alice Eastlake. 1965. The primary productivity of a desert-shrub (Larrea tridentata) community. Ecological Monographs. 35: 355-375. 
11. Chew, Robert M.; Chew, Alice Eastlake. 1970. Energy relationships of the mammals of a desert shrub (Larrea tridentata) community. Ecological Monographs. 40(1): 1-21. 
12. Eyre, F. H., ed. 1980. Forest cover types of the United States and Canada. Washington, DC: Society of American Foresters. 148 p. 
13. Fowler, Harold G. 1983. The bee Xylocopa californica arizonensis (Hymenoptera: Anthophoridae) and Yucca elata (Agaveceae): nests, populations, behavior and importance for nutrient cycling in the Chihuahua Desert. Folia Entomologica Mexicana. 56: 75-83. 
14. Gardner, J. L. 1951. Vegetation of the creosotebush area of the Rio Grande Valley in New Mexico. Ecological Monographs. 21: 379-403. 
15. Garrison, George A.; Bjugstad, Ardell J.; Duncan, Don A.; [and others]. 1977. Vegetation and environmental features of forest and range ecosystems. Agric. Handb. 475. Washington, DC: U.S. Department of Agriculture, Forest Service. 68 p. 
16. Hakkila, Mark D.; Holechek, Jerry L.; Wallace, Joe D.; [and others]. 1987. Diet and forage intake of cattle on desert grassland range. Journal of Range Management. 40(4): 339-342. 
17. Henrickson, James; Johnston, Marshall C. 1986. Vegetation and community types of the Chihuahuan Desert. In: Barlow, J. C.; [and others], eds. Chihuahuan Desert--U.S. and Mexico, II. Alpine, TX: Sul Ross State University: 20-39. 
18. Herbel, Carlton H.; Nelson, Arnold B. 1966. Species preference of Hereford and Santa Gertrudis cattle on a southern New Mexico range. Journal of Range Management. 19: 177-181. 
19. Holechek, Jerry L.; Tembo, Ackim; Daniel, Alipayou; [and others]. 1994. Long-term grazing influences on Chihauhuan Desert rangeland. The Southwestern Naturalist. 39(4): 342-349. 
20. Humphrey, R. R. 1949. Fire as a means of controlling velvet mesquite, burroweed, and cholla on southern Arizona ranges. Journal of Range Management. 2: 175-182. 
21. James, Craig D.; Hoffman, M. Timm; Lightfoot, David C.; [and others]. 1993. Pollination ecology of Yucca elata: An experimental study of a mutualistic association. Oecologia. 93(4): 512-517. 
22. James, Craig D.; Hoffman, M. Timm; Lightfoot, David C.; [and others]. 1994. Fruit abortion in Yucca elata and its implications for the mutualistic association with yucca moths. Oikos. 69(2): 207-216. 
23. Kartesz, John T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. Volume I--checklist. 2nd ed. Portland, OR: Timber Press. 622 p. 
24. Kearney, Thomas H.; Peebles, Robert H.; Howell, John Thomas; McClintock, Elizabeth. 1960. Arizona flora. 2d ed. Berkeley, CA: University of California Press. 1085 p. 
25. Kerley, G. I. H.; Tiver, F.; Whitford, W. G. 1993. Herbivory of clonal populations: cattle browsing afftects reproduction and population structure of Yucca elata. Oecologia. 93: 12-17. 
26. Krausman, Paul R.; Kuenzi, Amy J.; Etchberger, Richard C.; [and others]. 1997. Diets of mule deer. Journal of Range Management. 50(5): 513-522. 
27. Krochmal, A.; Paur, S.; Duisberg, P. 1954. Useful native plants in the American southwestern deserts. Economic Botany. 8: 3-20. 
28. Kuchler, A. W. 1964. United States [Potential natural vegetation of the conterminous United States]. Special Publication No. 36. New York: American Geographical Society. 1:3,168,000; colored. 
29. Langford, R. P. 2000. Nabkha (coppice dune) fields of south-central New Mexico, U.S.A. Journal of Arid Environments. 46(1): 25-41. 
30. Muller, Cornelius H. 1940. Plant succession in the Larrea-Flourensia climax. Ecology. 21: 206-212. 
31. Nelson, A. B.; Herbel, H. M.; Jackson, H. M. 1970. Chemical composition of forage species grazed by cattle on an arid New Mexico range. Bulletin 561. Las Cruces, NM: New Mexico State University, Agricultural Experiment Station. 33 p. 
32. Nelson, Enoch W. 1934. The influence of precipitation and grazing upon black grama grass range. Technical Bulletin No. 409. Washington, DC: U.S. Department of Agriculture. 32 p. 
33. Parmenter, Robert R.; Van Devender, Thomas R. 1995. Diversity, spatial variability, and functional roles of vertebrates in the desert grassland. In: McClaran, Mitchel P.; Van Devender, Thomas R., eds. The desert grassland. Tucson, AZ: The University of Arizona Press: 196-229. 
34. Paysen, Timothy E.; Ansley, R. James; Brown, James K.; [and others]. 2000. Fire in western shrubland, woodland, and grassland ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-volume 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 121-159. 
35. Powell, A. Michael. 1988. Trees & shrubs of Trans-Pecos Texas including Big Bend and Guadalupe Mountains National Parks. Big Bend National Park, TX: Big Bend Natural History Association. 536 p. 
36. Raitt, Ralph J.; Pimm, Stuart L. 1977. Temporal changes in northern Chihuahuan Desert bird communities. In: Wauer, Roland H.; Riskind, David H., eds. Transactions of the symposium on the biological resources of the Chihuahuan Desert Region: United States and Mexico; 1974 October 17-18; Alpine, TX. Transactions and Proceedings Series Number 3. Washington, DC: U.S. Department of the Interior, National Park Service: 579-589. 
37. Raunkiaer, C. 1934. The life forms of plants and statistical plant geography. Oxford: Clarendon Press. 632 p. 
38. Reay, Frances; Reay, Brian. 1987. Survival of succulents after fire in South Australia. British Cactus and Succulent Journal. 5(1): 23-26. 
39. Shiflet, Thomas N., ed. 1994. Rangeland cover types of the United States. Denver, CO: Society for Range Management. 152 p. 
40. Smith, Gretchen; Holechek, Jerry L.; Cardenas, Maual. 1996. Wildlife numbers on excellent and good condition Chihuahuan Desert rangelands: an observation. Journal of Range Management. 49(6): 489-493. 
41. Stickney, Peter F. 1989. Seral origin of species originating in northern Rocky Mountain forests. Unpublished draft on file at: U.S. Department of Agriculture, Forest Service, Intermountain Research Station, Fire Sciences Laboratory, Missoula, MT; RWU 4403 files. 10 p. 
42. Thomas, P. A. 1991. Response of succulents to fire: a review. International Journal of Wildland Fire. 1(1): 11-22. 
43. Thomas, P. A.; Goodson, P. 1992. Conservation of succulents in desert grasslands managed by fire. Biological Conservation. 60(2): 91-100. 
44. U.S. Department of Agriculture, Soil Conservation Service. 1994. Plants of the U.S.--alphabetical listing. Washington, DC: U.S. Department of Agriculture, Soil Conservation Service. 954 p. 
45. Wallmo, O. C. 1955. Vegetation of the Huachuca Mountains, Arizona. The American Midland Naturalist. 54: 466-480. 
46. Wansi, Tchouassi; Pieper, Rex D.; Beck, Reldon F.; Murray, Leigh W. 1992. Botanical content of black-tailed jackrabbit diets on semidesert rangeland. The Great Basin Naturalist. 52(4): 300-308. 
47. Webber, John M. 1960. Hybridization and instability of Yucca. Madrono. 15: 187-192. 
48. Webber, John Milton. 1953. Yuccas of the Southwest. Agriculture Monograph No. 17. Washington, DC: U.S. Department of Agriculture, Forest Service. 97 p. 
49. Welsh, Stanley L.; Atwood, N. Duane; Goodrich, Sherel; Higgins, Larry C., eds. 1987. A Utah flora. The Great Basin Naturalist Memoir No. 9. Provo, UT: Brigham Young University. 894 p. 
50. Winder, John A.; Walker, Derek A.; Bailey, Calvin C. 1995. Genetic aspects of diet selection in the Chihuahuan Desert. Journal of Range Management. 48(6): 549-553. 
51. Wood, John E. 1969. Rodent populations and their impact on desert rangelands. Bulletin 555. Las Cruces, NM: New Mexico State University, Agricultural Experiment Station. 17 p. 
52. Wood, M. Karl; Mayeux, Herman S., Jr.; Garcia, E. L. 1990. Early use of soaptree yucca as emergency feed. Rangelands. 12(4): 213-216.