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Wildlife, Animals, and Plants |
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VALUE AND USE
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].
PALATABILITY: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].
NUTRITIONAL VALUE:Soaptree yucca foliage, particularly actively growing apices, is low in acid-detergent lignin [25]. 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 [26]:
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 [25].
COVER VALUE: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 [8]. Soaptree yucca is the only perch for birds in much of its range [25].
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 [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].
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" [27]. 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 [48].
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 [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].
Related categories for SPECIES: Yucca elata | Soaptree Yucca |
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