|Photo © Br. Alfred Brousseau, Saint Mary's College|
for Hesperoyucca newberryi*
*A 2nd Hesperoyucca species commonly known as Newberry's yucca (H. newberryi (McKelvey) Clary) [23,54,108] is also recognized. Some sources suggest that Newberry's yucca is just a disjunct population of chaparral yucca occurring in Arizona [40,53,93,109,119]. In 1902, Trelease  indicated that there were several disjunct populations of chaparral yucca in Baja California, southern California, and Arizona and did not consider populations in Arizona a distinct species.
Various infrataxa of chaparral yucca are identified in the literature (see Synonyms) and are generally distinguished by differing growth forms. Chaparral yucca can grow as a solitary monocarpic rosette, as a caespitose form of several monocarpic rosettes, as a caespitose perennial form with polycarpic rosettes, and as a rhizomatous form with polycarpic rosettes. While these forms have been distinguished as unique subspecies and varieties in the literature, offspring from one form may or may not exhibit the parent form suggesting that branching characteristics are not taxonomically reliable . For details, see Plant growth.
Chaparral yucca infrataxa are no longer recognized [23,104]. Subspecies and varieties distinguished in older literature are presented below along with their identified growth forms.SYNONYMS:
for Hesperoyucca newberryi (McKelvey) Clary:
Yucca newberryi McKelvey, solitary monocarpic rosette 
|Chaparral yucca.||Newberry's yucca.|
|Maps courtesy of the Flora of North America Association. 2012, 17 February.|
Chaparral yucca and Newberry's yucca are native to North America and occur in many disjunct populations in northern Baja California, southern California, and/or northwestern Arizona . The Flora of North America  indicates that chaparral yucca occurs in southern California, and Newberry's yucca occurs in western Arizona.
Some studies have identified distributions for certain chaparral yucca growth forms [31,32,45,91]. Although several forms likely exist in any region, there are areas where particular growth forms dominate . In southern California, chaparral yucca populations south of the San Gabriel Mountains were dominated by solitary monocarpic rosettes, and populations north of the San Gabriel Mountains were primarily caespitose . Haines [31,32] found that rhizomatous forms occurred in the northern portion of chaparral yucca's California range, which included the San Rafael, Santa Ynez, and Santa Lucia mountains. Keeley and Keeley  reported in a review that chaparral yucca was primarily rhizomatous in the Central Coast Ranges. Webber  suggested that rhizomatous forms were common in central California where sites receive moderate rainfall. Caespitose forms dominated populations at the western margin of Mojave Desert from the San Bernardino Mountains to the Tehachapi Pass and the Piute and Greenhorn mountains. Solitary monocarpic forms dominated the southernmost part of chaparral yucca's California range in San Diego, Riverside, Orange, San Bernardino, and Los Angeles counties [31,32].
Chaparral yucca populations occur as far south as Baja California . In southern California, many chaparral yucca populations are widely separated. There are gaps between populations in the Santa Ana and Santa Monica mountains, the Santa Monica and Santa Ynez mountains, the San Rafael and Santa Lucia mountains, the Walker Pass region, and western Sequoia National Park. The density of chaparral yucca within any population is variable. Small isolated colonies are common in the Santa Lucia and Balkan mountains. Dense stands occur in the San Rafael Mountains and on slopes bordering the Mojave Desert [31,32]. Packrat midden records from low-elevation sites indicate that the distribution of the chaparral yucca was much wider and more continuous as recently as the last glacial (Wisconsinan) period of the Pleistocene .
Newberry's yucca is restricted to the vicinity of the Colorado River in the lower Grand Canyon in Mohave County, Arizona [23,113]. Arizona populations were initially discovered by Newberry in 1858 and were confirmed again in 1934 , but they were not consistently recognized as distinct species until 2009 . Newberry's yucca grows strictly as a solitary monocarpic rosette [23,54].
States (as of 2012 ):
United States: Chaparral yucca: CA; Newberry's yucca: AZ
Mexico: Chaparral yucca: BCN
Newberry's yucca is restricted to regions below the canyon rim on the south side of the Colorado River in Arizona .
Climate: Chaparral yucca is closely associated with coastal sage scrub and chaparral vegetation. These ecosystems occupy sites with mediterranean climates, where winters are cool and wet, and summers are hot and dry . The chaparral yucca-chamise (Adenostoma fasciculatum) association occurs in the mesomediterranean bioclimatic belt in semiarid regions of southern California . Evaluation of climate and soil moisture for 3 years on a south-facing coastal sagebrush site in southern California showed that the most extreme temperatures for the period included an average January temperature of slightly less than 50 °F (10 °C) and a July average of a little less than 95 °F (35 °C). Average soil temperatures at 6 to 40 inches (15-100 cm) deep ranged from about 50 °F (10 °C) in January to about 77 °F (25 °C) in July and August. Evaporation was least in February and March and greatest in July and August . Annual rainfall in chaparral vegetation types ranges from 8 to 39 inches (200-1,000 mm). Winter rain is episodic, and prolonged droughts are common (review ). Another source suggests a narrower average annual rainfall range of 14 to 25 inches (360-630 mm) in southern California chaparral .
Chaparral yucca occurs in parts of southern California where winter temperatures are very mild, winter rains are substantial, and summer temperatures are milder than those in Death Valley . Native garden guidelines indicate that chaparral yucca tolerates excess water and grows where temperatures reach as low as 10 °F (-12 °C) . Greenhouse experiments suggest that chaparral yucca may produce more roots in arid conditions. After 1.5-year-old chaparral yucca plants were watered at different frequencies for 7 months, the root surface:leaf area ratio was about 7 for rarely watered plants and 3.5 for frequently watered plants .
Newberry's yucca occurs along the Colorado River in northwestern Arizona, where winters are milder and rainfall is greater than in the surrounding areas .
Elevation: In California, chaparral yucca is most common from sea level to 4,500 feet (1,400 m) [83,117] but occasionally occurs at elevations up to 8,200 feet (2,500 m) [22,32,42,83,117]. In the Coast Ranges of California, chaparral yucca was much more abundant at low-elevation (400-1,500 feet (120-460 m)) than high-elevation sites (2,000-2,450 feet (610-750 m)) . Newberry's yucca is restricted to areas below the Grand Canyon rim (5,000 feet (1,500 m))  but may be most common at elevations of 1,000 to 2,500 feet (300-760 m) [54,113].
Soil: Although chaparral yucca tolerates a variety of soil types, it may be most abundant on rocky substrates. Throughout its range, chaparral yucca occupies porous, shallow soils and rocky outcrops  but can also occur in red clay soils . Chaparral yucca can be abundant in chamise chaparral with loose, rocky substrates . In California's Coast Ranges, it has been reported on sites with serpentine soils , but when the vegetation and environmental conditions of 67 coastal sage scrub sites were evaluated in southern California and northern Baja California, chaparral yucca was not found on serpentine or limestone substrates. In coastal sage scrub, chaparral yucca was especially common on sandstone substrates and sites with only moderate litter accumulations. It was absent from the most mesic sites and had low cover on sites with high exchangeable ammonium concentrations in the soil .
Plant communities: In California, chaparral yucca is most common in coastal sage scrub and chaparral [42,83] and may also occur, although much less commonly, in desert grasslands, creosote bush (Larrea tridentata) and other desert shrublands, and desert, oak (Quercus spp.), juniper (Juniperus spp.), and pine (Pinus spp.) woodlands [22,31,117]. In Arizona, Newberry's yucca occurred on schists in the Colorado River canyon with catclaw acacia (Acacia greggii) and mesquite (Prosopis spp.) . Additional studies of habitat and plant community relationships of Newberry's yucca were lacking.
Chaparral yucca's plant community relationships have been evaluated throughout southern California. Webber  reported that chaparral yucca was most common in chaparral and desert vegetation from sea level to 4,500 feet (1,400 m) and occasional in montane forests up to 7,500 feet (2,300 m). Haines  found chaparral yucca in the Upper Sonoran and Transition Life Zones with coastal sage scrub communities, chaparral vegetation, pine woodlands, and desert woodlands. Species common to the desert woodlands included Joshua tree (Yucca brevifolia) and California juniper (J. californica). Chaparral yucca was most common in coastal sage scrub and chamise chaparral communities. Rhizomatous forms were associated with coastal sage scrub and chaparral communities at elevations from 200 to 2,000 feet (60-600 m). Polycarpic caespitose forms were often found in desert woodland communities at 2,000 to 4,000 feet (600-1,200 m). Monocarpic caespitose forms occurred in coastal sage scrub and chaparral communities from sea level to 2,000 feet (600 m). Solitary monocarpic rosette forms were predominant in chaparral but were also found in coastal sage scrub, desert woodlands, and ponderosa pine (Pinus ponderosa) forests from 1,000 to 8,000 feet (300-2,400 m) . In the Liebre Mountains, chaparral yucca was considered widespread and occurred in grassland, scrub, and woodland vegetation .
Shrublands: Chaparral yucca is common in shrubland types in the mediterranean and desert regions in California. It occurred in dry coastal sage scrub, chaparral, and creosote bush communities in southern California at elevations from 1,000 to 7,900 feet (300-2,400 m) . In the San Gabriel Mountains, chaparral yucca occurred in pioneer and mature alluvial scrub vegetation on alluvial fans and floodplains  and in coastal sage scrub at elevations below 1,500 feet (460 m) . In Baja California, chaparral yucca was described in ocotillo-pachycereus-elephant tree (Fouquieria-Pachycereus-Pachycormus spp.) habitats , but additional information about chaparral yucca habitats in Baja California was lacking as of 2012.
Coastal sage scrub: In general, coastal sage scrub habitats are drier and have more open canopies than chaparral habitats. Coastal sage scrub communities also have a persistent herbaceous species component, while these herbs lack persistence beyond the first few postfire years in chaparral communities . Commonly, coastal sage scrub is dominated by California sagebrush (Artemisia californica) and sage (Salvia spp.) . In southern California, chaparral yucca is characteristic of coastal sage scrub [20,34,87]. When adjacent stands of coastal sage scrub and bigpod ceanothus (Ceanothus megacarpus) chaparral were compared in the Santa Monica Mountains, chaparral yucca density averaged 0.01/m² in chaparral and 0.07/m² in coastal sage scrub communities. The coastal sage scrub community was dominated by California sagebrush and San Luis purple sage (S. leucophylla). Aboveground biomass of dead wood in coastal sage scrub was only about 50% of that in chaparral. Productivity of the coastal sage scrub was rapid, seasonal, and closely associated with rainfall [26,27].
In southern California, chaparral yucca is characteristic of coastal sage scrub, which is sometimes referred to as soft chaparral [20,34,87]. Chaparral yucca sometimes codominates coastal sagebrush communities, especially in Santa Barbara County . On 120 Californian coastal sage scrub sites in southern California, frequency of chaparral yucca was greatest in the California broomsage-thickleaf yerba santa (Lepidospartum squamatum-Eriodictyon crassifolium)-chaparral yucca and the black sage-laurel sumac (S. mellifera-Malosma laurina) associations. Density of chaparral yucca, however, was never great . Along the coastal bases of the Transverse and Peninsular ranges in southern California, chaparral yucca is characteristic of Riversidean sage scrub, which occupies xeric sites that may be steep, severely drained, and/or have clay soils with very slow moisture release. Riversidean sage scrub vegetation is generally open and dominated by California sagebrush, eastern Mojave buckwheat (Eriogonum fasciculatum), and red brome (Bromus rubens). On the ocean side of the Santa Lucia Mountains at elevations below 2,000 feet (600 m), chaparral yucca occurs in central Lucian coastal scrub communities on exposed south slopes with shallow rocky soils. Shrubs are generally dense .
Chaparral: Various types of chaparral vegetation are habitat for chaparral yucca in California and Baja California. Chaparral types include chamise chaparral, which occurs throughout much of California; bigpod ceanothus chaparral, which occupies xeric slopes with rocky, poorly differentiated soils between 600 and 3,000 feet (200-900 m) in coastal southern California; southern mixed chaparral, which occurs in the coastal foothills below 3,000 feet (900 m) in southern California and northern Baja California; and semidesert chaparral, which is found at elevations of 2,000 to 5,000 feet (600-1,500 m) in the inner South Coast ranges .
Chaparral yucca is common in California's chamise chaparral but rarely contributes more than 10% cover to the community. Chamise chaparral generally occurs on hot, dry, south- and west-facing slopes with sandy, rocky soils that lack nutrients and horizon development . Chaparral yucca may be most abundant in chamise chaparral on ridgetops and steep south-facing slopes where overall plant cover is sparse [70,115]. A chaparral yucca-chamise association is recognized in the semiarid region of southern California .
In the Transverse and Coast ranges of southern California, chaparral yucca occurs with desert chaparral vegetation dominated by a mixture of chamise, red shank (Adenostoma sparsifolium), manzanita (Arctostaphylos spp.), and/or ceanothus (Ceanothus spp.) [8,64]. Chaparral yucca was dominant in desert chaparral in the inner South Coast Ranges and on the lower slopes of San Gabriel, San Bernardino, San Jacinto, Santa Rosa, and Laguna mountains. Desert chaparral is more open than other chaparral types, with overall plant cover of about 50%. Dominant species in the desert chaparral type include species from coastal chaparral and desert shrub communities such as chamise, bigberry manzanita (A. glauca), antelope bitterbrush (Purshia tridentata), California juniper, desert ceanothus (C. greggii), and birchleaf mountain-mahogany (Cercocarpus montanus var. glaber) .
Woodlands and forests: Occasionally, chaparral yucca occurs in woodlands and forests of California. It has been described in California juniper-blue oak cismontane woodlands on rocky outcrops , gray pine (Pinus sabiniana)-blue oak (Q. douglasii) woodlands on well-drained mediterranean sites in the Central Valley , and singleleaf pinyon (P. monophylla)-California juniper woodlands in the desert foothills of the San Gabriel Mountains . Chaparral yucca may also occur with bigcone Douglas-fir (Pseudotsuga macrocarpa) at elevations of 900 to 3,500 feet (270-1,100 m) in southern coastal California  and on steep south-facing slopes in the Santa Ana Mountains .See the Fire Regime Table for a list of plant communities in which chaparral yucca and Newberry's yucca may occur and information on the fire regimes associated with those communities.
|Photo © Keir Morse 2009.
Photo taken 9 May 2008 at Pinnacles National Monument.
Note the clustered rosette form.
Growth forms: Chaparral yucca is a perennial, acaulescent plant that grows from a caudex. Growth form varies from a solitary rosette to densely clumped rosettes connected by rhizomes or very short basal stems [42,99,123]. While any chaparral yucca population is usually a mixture of 2 or 3 growth forms , some studies report dominance of particular forms related to distribution and habitat; for more information, see General Distribution and Plant communities. In his 1893 study of chaparral yucca, Trelease  indicated that a caespitose form of about 8 to 10 crowns clustered around a single root system was most typical.
Chaparral yucca plants can be monocarpic or polycarpic. Newberry's yucca grows only as a monocarpic, solitary rosette. Both species are typically forbs [23,54]. When mature, this form produces a single unbranched flower stalk, and the entire plant dies after flowering [15,22,31,83]. Caespitose forms of chaparral yucca may produce individual rosettes that are monocarpic or polycarpic. Caespitose forms produce a small caudex (2-3.5 inches (5-9 cm) in diameter). The caudex usually protrudes 2 to 4 inches (5-10 cm) above the soil surface, so that the base is only slightly covered by soil, but crowded axillary rosettes cover the rest of the caudex. Caespitose and rhizomatous forms can support 3 to 200 rosettes, and rhizomes can connect rosettes 3 to 10 feet (1-3 m) apart . Rhizomatous forms often occur as large, rather open clumps of rosettes, while caespitose forms generally occur as dense clumps of numerous, tightly clustered rosettes. Rhizomes can measure 0.8 to 1.5 inches (2-4 cm) in diameter and 16 to 37 inches (40-95 cm) in length . A single rhizomatous clone may occupy an entire hillside [15,22,31,83]. Rhizomes are covered with a thick, bark-like layer. There are also chaparral yucca forms with both rhizomatous and caespitose characteristics .
Rhizomatous and caespitose forms live much longer than their monocarpic counterparts. Monocarpic forms complete their life span in as little as 4 to 6 years , while caespitose and rhizomatous forms may flower for decades (review ).
Vegetative and reproductive characteristics: Rosettes of chaparral yucca and Newberry's yucca are clusters of dense, narrow leaves 12 to 39 inches (30-100 cm) long. Leaves are flat, rigid, gray-green in color, and have minutely serrate margins and sharp terminal spines about 0.4 to 0.8 inch (1-2 cm) long [15,42,83]. When the vegetative characteristics of thousands of widely distributed chaparral yucca plants and forms were measured in southern California, the population averages for leaf length ranged from 16 to 43 inches (41-109 cm) . Chaparral yucca and Newberry's yucca produce large inflorescences on towering stalks. Inflorescences are compact panicles that measure 8 to 158 inches (20-400 cm) long and have hundreds to thousands of individual bell-shaped flowers. Flowering stalks are 5 to 13 feet (1.5-4 m) tall and about 12 inches (30 cm) thick [3,15,42,83,123]. In southern California, the population averages for flowering stalk height ranged from 5.9 to 17.4 feet (1.7-5.3 m), and panicle length ranged from 2.5 to 9.3 feet (0.8-2.8 m) . In Harbison Canyon, San Diego County, the smallest chaparral yucca in flower had a rosette that was 10 inches (25 cm) wide and a flowering stalk that was 46 inches (117 cm) tall, and the largest plant had a rosette that was 39 inches (100 cm) wide and a flowering stalk that was 13 feet (4 m) tall . Chaparral yucca and Newberry's yucca produce dehiscent capsule fruits. Fruits are about 1.2 to 1.6 inches (3-4 cm long), and seeds are thin, compressed, and about 7 mm long [54,83,123]. Fruits contain 6 columns of seeds . Seeds are winged .
Belowground characteristics: Chaparral yucca produces a spreading fibrous root system. Researchers excavated the root systems of 2 chaparral yucca plants from an open chamise chaparral site at about 4,500 feet (1,400 m) in the San Gabriel Mountains. The plants averaged 1.9 feet (0.6 m) tall, and the greatest concentration of roots occurred in the top 1 foot (0.3 m) of soil, which was about 6 inches (15 cm) of loose sand over about 1 foot (0.3 m) of broken, slightly weathered grandiorite rock. The number of lateral roots was similar in all directions, but roots growing uphill were shorter (6 feet (1.8 m)) than those growing downhill (11 feet (3.4 m)). Roots were found at a maximum depth of 2.5 feet (0.8 m) and spread a maximum of 11 feet (3.4 m). The longest root was 13 feet (4 m). Root tips averaged 0.2 inch (0.5 cm) in diameter, and older root portions were about half that .
The rhizomatous form of chaparral yucca has mature rhizomes that measure 2 to 6 feet (0.6-1.8 m) long and 1 inch (2.5 cm) in diameter .Raunkiaer  life form:
Flowers at the bottom of the inflorescence open first, and flowering progresses upward. Individual flowers are open for 3 to 6 days [3,91], and the flowering period for a plant ranges from 14 to 75 days. The length of the flowering period is negatively correlated with the date at which the first flower opens ; growth and development of inflorescences are more rapid at later blooming dates .
Chaparral yucca seed capsules mature after about 140 days, in late spring and summer [18,91]. They typically split in mid-August, and seeds are dispersed in gusty winds . Seed capsules remain attached to the stalk, and passive seed dispersal may occur for a year or more after dehiscence [3,24,91].REGENERATION PROCESSES:
Pollination by yucca moths: Researchers observed pollination of chaparral yucca by yucca moths in California's Riverside and San Diego counties. Adult moths emerged from the soil once chaparral yucca began flowering. Female yucca moths gathered pollen before or after mating with male moths in open flowers . Female moths dispersed before ovipositing, so the likelihood of cross pollination is maximized, but as the females collect pollen, tentacles may contact the stigmas and could result in some pollination within the same flower or inflorescence. Moths deposited their eggs before pollinating chaparral yucca flowers. When there were few flowers available, moths sometimes oviposited in developing seed capsules .
Field observations and experiments indicate that successful pollination of chaparral yucca flowers is unlikely without yucca moths . In California, small flies, bees, and beetles were observed gathering nectar from the base of chaparral yucca flowers, but none of these insects touched anthers or stigmas . In later observations made in southern California, many species of beetles, flies, bees, and butterflies occurred in and around chaparral yucca flowers, but there was no chaparral yucca pollen on any of them .
As the flowering season ends, chaparral yucca aborts unpollinated flowers and many small, immature fruits. Abortion rates for fruits may relate to capsule maturity and abundance of yucca moth eggs. The total number of flowers/inflorescence reaching anthesis is positively correlated with rosette size (P<0.05), but strength of this correlation may be greatest when flowering is preceded by little rain . Survivorship of capsules on the lowest third of an inflorescence was significantly greater (P<0.001) than those on the uppermost third of the inflorescence. When flowers at the top of the stalk were opening, capsules near the bottom of stalk were almost full size . Experimental evidence suggests that plants preferentially abort fruits with relatively high numbers of eggs. When researchers counted the number eggs in 26 pairs of aborted and retained fruits, aborted fruits contained more eggs (P<0.001) .
Cross vs. self-pollination: Fruit production has been observed in the absence of yucca moths. In California, small chaparral yucca fruits developed in an area where neither yucca moths nor oviposition sites were observed . In a nursery in Riverside County, California, a chaparral yucca plant produced an abundance of fruits and seeds, and the researcher found no signs of yucca moths or larvae .
Artificial pollination experiments indicate that self-pollinated flowers may produce fruits; however, production is lower for self-pollinated than cross-pollinated flowers. In experiments that included 4 chaparral yucca populations, capsule development by self-pollinated flowers ranged from 0% to 16% and by cross-pollinated flowers ranged from 8% to 22% (Wimber 1958 cited in ). In other artificial-pollination experiments, self-pollinated flowers produced 0% to 35.8% mature capsules, while cross-pollinated fruits produced 24% to 70% mature capsules . An experiment with strictly monocarpic chaparral yucca forms at the Sonoran Desert margin in Riverside County found no fruit production from 618 bagged flowers, while 886 unbagged flowers produced 75 fruits .
Seed produced by self-pollinated flowers is less likely to result in successful reproduction than seed produced by cross-pollinated flowers. Nearly 30% of self-pollinated and 52% of cross-pollinated flowers on monocarpic chaparral yucca plants in Orange County produced fruits. Self-pollinated flowers produced fewer seeds/capsule than cross-pollinated flowers, and the retention rates for fruits produced through self-pollination were 32% of those for fruits produced through cross-pollination. When self-pollination and cross-pollination were compared at later reproductive stages, both germination and seedling establishment were lower for seeds produced through self-pollination. Across all reproductive stages, reproduction by self-pollination was 8% to 32% less than reproduction by cross-pollination .
Seed production: Chaparral yucca and Newberry's yucca produce hundreds to thousands of seeds [49,51,54]. Seed production can vary by plant size, growth form, site and climatic conditions, and pollination method. Predation by yucca moth larvae, other insects, and mule deer can reduce seed production.
Studies and observations suggest that chaparral yucca does not produce seed until plants are 4 years or older. Webber  reported that monocarpic forms generally completed their lifecycle in 4 to 6 years, and after 40 years of observations, Horton  indicated that chaparral yucca first bloomed at 5 to 20 years old. In a horticultural study, 1-year-old chaparral yucca seedlings from seed collected in the Santa Ana Mountains were transplanted into a common garden. Just 7 of 77 plants produced flowers after 7 years. The researcher suggested that after a minimum of 7 years, a fraction of a chaparral yucca cohort reached reproductive maturity .
Several studies suggest that chaparral yucca plant size is correlated with flower, fruit, and seed production. Udovic  found that usually less than 10% of chaparral yucca flowers set fruit. Average flower production and flowering stalk height were significantly positively correlated with the average number of fruits produced (r²=0.68, P<0.001; r²=0.69, P<0.001, respectively). Pollinator abundance was not correlated with fruit set; on sites with an excess of pollinators, fruit set remained low . In San Bernardino County, monocarpic chaparral yucca forms produced fewer seeds when their flower stalks were shorter than the average inflorescence height of 9.5 feet (2.9 m). Plants with shorter than average inflorescences produced 5,500 seeds, and those with taller than average inflorescences produced 12,300 seeds. The number of weevil scars (see Predation) was greater on short than tall flowering stalks . Inflorescence height was also associated with fire damage. For more information, see Reproduction response to fire.
At the Sonoran Desert margin in Riverside County, abundance of fruits initiated was much less but was proportional to the abundance of flowers produced by monocarpic chaparral yucca forms. The number of flower buds reaching anthesis was directly proportional to rosette size, but the number of seeds produced per capsule was not correlated with plant size. Mature fruit set on undamaged plants, measured as the proportion of flowers that produced mature fruits, ranged from 1.4% to 18.6% in a year of below-average precipitation and 2.5% to 17.6% in a year of above-average precipitation. Maximum fruit set of 29.1% was recorded for a plant outside the study area. Reproductive characteristics for monocarpic plants monitored for 3 years in Riverside County are summarized below .
|Reproductive characteristics of monocarpic chaparral yucca for 3 years at the margin of the Sonoran Desert in southern California. Values are averages [2,3].|
|Reproductive attribute evaluated||Site 1 (1978)||Site 1 (1979)||Site 2 (1979)||Site 2 (1980)|
|Basal area of rosette (m²)||2.01||1.91||1.56||1.54|
|Maximum inflorescence height (m)||3.15||3.24||3.13||3.28|
|Number of open flowers||2,130||2,045||1,744||1,709|
|Number of fruits initiated/plant||not recorded||447||305||490|
|Number of fruits matured/plant||177||194||162||178|
|Mature fruit set (%)/plant||not recorded||9.0||9.2||10.0|
|Number of ovules/capsule||not recorded||207.4||205.0||not recorded|
Growth forms: In southwestern San Bernardino County, monocarpic chaparral yucca forms produced about 5 times the number of viable seeds of caespitose forms. Inflorescences were taller and flower and fruit production were greater for monocarpic than caespitose forms. Seed viability decreased as inflorescence height increased for monocarpic but not for caespitose plants. Number of viable seeds produced per plant increased with leaf surface area for monocarpic plants, and viable seed production was 3 times greater with a doubling of leaf surface area. Number of viable seeds produced per plant increased with an increase in the number of rosettes for caespitose plants, and viable seed production was 2.4 times greater when rosette number increased from 1 to 4 . See Germination for additional differences related to growth form.
|Average values for vegetative and reproductive characteristics of monocarpic and caespitose forms of chaparral yucca in San Bernardino County |
|Attribute evaluated||Monocarpic form||Caespitose form|
|Leaf surface area (m²)*||11.0||9.9|
|Inflorescence size (m)||3.7||3.0|
|Number of flowers produced/plant||1,600||981|
|Number of fruits produced/plant||150||55|
|Viable seeds (%)||82.5||62.7|
|Total viable seeds/plant||15,000||290|
|*For caespitose form, value is mean for all attached rosettes.|
Site and climatic characteristics: When reproductive characteristics were evaluated on a per fruit basis for various chaparral yucca forms, differences were less correlated with growth form than with site characteristics. Mature capsules were collected from throughout chaparral yucca's California range. Monocarpic forms produced the largest capsules with the most seeds, but when mature fruits from all growth forms were combined, the number of seeds/locule was positively correlated with elevation, distance from the coast, and total annual precipitation (P<0.05). The number of moth larvae/capsule ranged from 0 to 14 and averaged less than 3, regardless of growth form. The number of larvae/capsule was negatively correlated with elevation, distance from the coast, total annual precipitation, and average temperature (P<0.05). The number of capsules free of larvae was substantially greater for monocarpic than caespitose or rhizomatous forms .
|Average fruit, seed, and moth predation characteristics by different chaparral yucca populations and reproductive types |
|Growth form||Seed weight (mg)||Capsule length (mm)||Seeds/locule||Larvae/capsule||Seeds destroyed*
|Monocarpic (Population 1)||17.6||30.2||31.6||1.16||2.5-8.7|
|Monocarpic (Population 2)||21.3||30.8||28.8||0.94||2.4-7.1|
|Caespitose with monocarpic rosettes||18.1||26.8||24.9||0.99||4.2-11.6|
|Caespitose with polycarpic rosettes||18.0||25.9||24.9||1.83||6.8-11.9|
When chaparral yucca fruit set was compared in 18 southern California populations, average fruit production was least for populations in xeric desert scrub, intermediate in coastal sage scrub, and greatest in relatively mesic chaparral communities. Researchers suggested that fruit set differences were related to differences in moisture availability . Others have also suggested a link between moisture and seed production. Cox  reported that there was little or no chaparral yucca flowering in extremely dry years. In cismontane southern California, few inflorescences developed in dry years but many developed in years with concentrated spring rains. However, this relationship was not observed on the western margin of the Mojave Desert, where an abundance of flowers was generally produced regardless of rainfall. In this area, caespitose chaparral yucca forms dominated .
Pollination method: Less seed is produced by self-pollinated than cross-pollinated flowers. For monocarpic chaparral yucca plants in Orange County, self-pollinated flowers produced 88% of the seeds/capsule that cross-pollinated flowers did, and the retention rates for fruits produced by self-pollinated flowers was 32% of that for fruits produced by cross-pollinated flowers .
Predation: Yucca moths, other insects, and mule deer may reduce chaparral yucca flower and seed production. Yucca moth larvae generally consume 6 to 14 seeds within a fruit [3,18], yet a "fair percentage of the seeds are allowed to come to maturity" . In populations from a large portion of chaparral yucca's California range, generally less than 10% of the seeds were consumed by larvae, regardless of growth form. However, northern and coastal rhizomatous populations lost up to 25% of their seeds to larvae, and the number of capsules without larvae was substantially greater for monocarpic than caespitose or rhizomatous forms . Female sap beetles oviposit in chaparral yucca flower buds, and the emerging larvae feed on the pollen and ovaries, which causes flower bud and flower abortion. In coastal sage scrub northeast of Escondido, an estimated 25% or more of flower losses were due to the beetles . On the San Bernardino National Forest, weevils (Scyphophorus yuccae) and nonpollinating moths (Prodoxus aenescents and P. cinereus) significantly (P<0.05) reduced the percentage of flowers produced on at least one-third of an inflorescence in 1 of 2 populations . In Riverside County, mule deer consumed the inflorescences of a small number of marked chaparral yucca plants before any flowers opened .
Seed dispersal: Chaparral yucca produces flattened winged seeds , which are dispersed by wind when capsules split . When capsules were collected throughout chaparral yucca's California range, Keeley  found that yucca moth larvae were chiefly found near the base of the capsule. Larvae formed a chamber with their silk while feeding on seeds. The chamber could block dispersal of seeds beneath it, but because the larvae typically occur at the base of the capsule, dispersal potential of the uneaten seeds was unaffected by the larvae .
In a wet season in southern California, some chaparral yucca seeds germinated while attached to the pods of monocarpic plants. In January and February, germinated seeds were found in fruits attached to the inflorescence and in fruits lying in the litter beneath parent plants .
Seed banking: Longevity of chaparral yucca and Newberry's yucca seeds under field conditions was not reported in the reviewed literature (2012). After 3 years of storage at room temperature, 85% of chaparral yucca seeds germinated. After 3 years of storage at 40 °F (4 °C), 75 % of seeds germinated .
Germination: Viability of chaparral yucca seeds is generally high, but germination can be highly variable. Tetrazolium chloride tests indicated that the viability of chaparral yucca seed collected from native habitats was 50% to 100%. The percentage of seeds from these lots that developed into seedlings in the greenhouse ranged from 0% to 100% . After 3 years of storage at room temperature and cooler (40 °F (4 °C)), germination of chaparral yucca seed ranged from 74% to 85%. Seeds germinated 9 days after sowing and required no pretreatments .
Germination can be lower for seeds from caespitose than monocarpic forms, self-pollinated than cross-pollinated flowers, and insect-attacked than unattacked plants. Seeds collected from monocarpic chaparral yucca plants in southwestern San Bernardino County germinated faster and at a high percentage than seeds collected from caespitose plants. Seeds from monocarpic plants germinated after 5 days, and those from caespitose plants germinated after 8 days. Seeds from monocarpic plants germinated at a rate of 5%/day, and the rate for seeds from caespitose plants was 3.1%/day. Germination of seeds from monocarpic plants was 80% and for seeds from caespitose plants was 56%. See Growth forms for more about vegetative and reproductive characteristics for monocarpic and caespitose chaparral yucca forms . In Orange County, germination was lower for seeds from self-pollinated than from cross-pollinated flowers of monocarpic chaparral yucca plants. Germination of seeds produced from self-pollinated flowers was 67% of that for cross-pollinated seeds . More information about differences in the reproductive potential as related to self-pollinated and cross-pollinated seeds is presented in following discussions: Cross vs. self-pollination, Pollination method, and Seedling establishment. On the San Bernardino National Forest, 2 nonpollinating moths (Prodoxus spp.) significantly (P<0.05) reduced the germination of seeds collected from at least a third of the inflorescence in both chaparral yucca populations monitored .
In a wet season in southern California, some chaparral yucca seeds germinated while attached to the capsules of monocarpic plants. In January and February, germinated seeds were found in fruits attached to the inflorescence and fruits lying in the litter beneath parent plants. Researchers found 34 germinated seeds in 23 fruits attached to the inflorescence and 16 germinated seeds in 16 unattached fruits in leaf litter. Of the germinated seeds, 5 had shoots greater than 3.2 inches (8 cm) long and roots greater than 2.8 inches (7 cm) long, but average root length was 0.5 inch (1.4 cm). In the greenhouse, 3 of the germinated seeds produced shoots, but only 1 survived 30 days. When germinated seeds were planted while still attached to the fruits, none grew. Researchers also collect ungerminated seeds, but none developed into seedlings. It was unclear whether germination within fruits would result in any advantage to establishment in the field, beyond a potential decrease in predation .
Seedling establishment: Although chaparral yucca seedlings were often observed in the field, studies aimed at determining the conditions most suitable for seedling establishment were lacking as of 2012. In 4 years of field work, Webber  reported observing an "unlimited" number of chaparral yucca seedlings. In the survey of chaparral yucca populations throughout California, Hoover  found many fewer seedlings in chaparral yucca populations dominated by rhizomatous and caespitose forms than in populations dominated by monocarpic forms. After 40 years of observations in southern California, Horton  indicated that chaparral yucca seedlings were typically found in open areas around dead parent plants. For a description of chaparral yucca seedlings, see Arnott .
Seed produced by self-pollinated flowers is less likely to germinate and establish than seed produced by cross-pollinated flowers. Reduced fruit production, fruit retention, seed production, and germination resulted from self-pollination of monocarpic chaparral yucca plants in Orange County. Survival of seedlings from self-pollinated seeds was 71% of that from cross-pollinated seeds. Across all reproductive stages, reproduction by self-pollination was 8% to 32% less than reproduction by cross-pollination .
In arid conditions, chaparral yucca seedlings may allocate more to root development than what is allocated in moister conditions. When 1.5-year-old chaparral yucca seedlings were planted in redwood boxes and watered frequently or very little, the root surface:leaf area ratio after about 7 months was about 3.5 in the frequently watered boxes and 7 in the little-watered boxes .
Plant growth: Growth rates were not reported for chaparral yucca or Newberry's yucca as of 2012. Various chaparral yucca growth forms were produced from seed collected from a single rhizomatous form growing in the Refugio Canyon of Santa Barbara County. After 2 to 4 years in the greenhouse, most seedlings had no lateral branching, and 10% of 3-year-old and 30% of 4-year-old seedlings produced lateral shoots. The types of lateral shoots varied from true rhizomes 0.4 to 10 inches (1-25 cm) long and up to 1 inch (2.5 cm) thick to true rosette shoots attached directly to the parent stem. Some seedlings developed both rhizomes and rosette shoots. These findings suggested that branching characteristics were not taxonomically reliable . There is additional information available about the various chaparral yucca growth forms and their distribution, plant communities, seed production, and germination relationships and characteristics.
Vegetative regeneration: Caespitose and rhizomatous forms of chaparral yucca spread and regenerate vegetatively (, review by ). Top-kill or damage is not required for vegetative growth, but a high density of chaparral yucca sprouts have been reported after fire [106,107]. For more on vegetative regeneration following fire, see Fire adaptations and Plant response to fire.SUCCESSIONAL STATUS:
Shade relationships: Chaparral yucca is often more abundant in open than shaded conditions. In the San Jacinto Mountains, chaparral yucca occurred primarily in the openings of chamise chaparral vegetation but was also found within chamise clumps . Generally, the canopy is more open in coastal sage scrub than chaparral communities . When adjacent stands of coastal sage scrub and bigpod ceanothus chaparral were compared in the Santa Monica Mountains, chaparral yucca cover and density were greater in coastal sage scrub [26,27]. When the composition and environmental conditions of 67 coastal sage scrub sites were evaluated in southern California and northern Baja California, chaparral yucca was most common in relatively open stands with only moderate litter accumulations and light grazing disturbance .
Primary and secondary succession: Several sources report that within chaparral vegetation in southern California, chaparral yucca is a pioneer species in primary succession, which occurs on broken rock surfaces in the mountains and alluvial fans and washes in the valleys [16,35,36].
No consistent changes in the relative abundance of chaparral yucca were reported for early, intermediate, or late postfire succession in California chaparral vegetation. Changes in species compositions, species life forms, and species diversity, however, are not common in postfire succession in chaparral vegetation. Often the species present in the first postfire year are also present in mature communities [25,33,36]. In midelevation sites in cismontane southern California, researchers found no major shifts in life form or species composition in chaparral burned 0 to 8 times from 1910 to 2001 .
Cover of chaparral yucca was not very different in chaparral sites burned 2 to 96 years prior. Eighty-one sites on the coastal and desert sides of the San Gabriel and San Bernardino mountains were surveyed. On the coastal side, chaparral yucca cover averaged 0.4% on sites burned 2 to 8 years prior and 0.6% on sites burned 41 to 96 years prior. On the desert side, cover averaged 1.3% on sites burned 2 to 8 years prior and 1.1% on sites burned 41 to 96 years prior .
Importance of chaparral yucca increased from pioneer to intermediate and from intermediate to mature stages of succession in 2 of 3 alluvial scrub sites in coastal southern California. However, developmental stage of the communities was determined by identification of species representative of each developmental stage rather than time since fire or flooding .Browsing: Chaparral yucca appears sensitive to browsing. At site near Caliente, California, researchers reported that chaparral yucca populations occurred as small islands restricted to the steepest slopes in a "sea of overgrazed alien grasses". Even on shallow slopes, cattle took developing inflorescences of many chaparral yucca plants . When 67 coastal sage scrub sites were evaluated in southern California and northern Baja California, chaparral yucca was most common in relatively open stands with only moderate litter accumulations and light livestock grazing .
Survival and sprouting: Sprouting of chaparral yucca is considered common after fires that are not severe [12,15,118]. On 1-year-old burned sites on the Angeles National Forest, chaparral yucca mortality averaged 50%. Mortality was greater in decadent (>83 years old) than young chaparral stands . On 1- to 4 year-old burned sites in San Diego County, chaparral yucca failed to sprout when aboveground plant parts were entirely consumed by fire. The researchers suspected that survival and sprouting would be more likely in openings where fuel build up was low . Eighteen months after moderate to severe surface fires in blue oak woodlands in the foothills of Sequoia National Park, the density of chaparral yucca was 5 plants/ha. There were no dead chaparral yucca plants. The fire occurred on 29 June, when the air temperature was 86 °F (30 °C), relative humidity was 17%, and fine fuel moisture content averaged 3.5%. Flame lengths were 3.3 to 15 feet (1.0-4.6 m) .
Seedling establishment: While seedlings may occur in the 1st postfire year , sometimes establishment is delayed until 2 or more years after fire . Postfire seedling establishment was absent or delayed on severely burned sites . At San Diego State University's Oaks Biological Field Station, the average density of chaparral yucca seedlings was 0.4/m² in the first postfire growing season after a January prescribed fire on south-facing slopes dominated by 57-year-old chaparral vegetation. In the following year, brush was added to an adjacent site and burned in a prescribed fire in February. Chaparral yucca seedlings were not present in the 1st year after this fire, which was likely more severe .
Plant response to fire: Chaparral yucca sprouts following top-kill by fire. Although studies distinguishing the origin of sprouts from various chaparral yucca forms are lacking, sprouts may develop from the caudex, short basal stems, and/or rhizomes. Although the likelihood of sprouting or seedling establishment may be reduced when fires are severe, large increases or decreases in chaparral yucca abundance following fire are rare.
When the origin of chaparral yucca regeneration on burned sites was determined in the Santa Monica Mountains, sprouts were more common than seedlings, and when both seedlings and sprouts occurred on a burned site, abundance of sprouts was generally greatest. Both seedlings and sprouts occurred 2 years after a large October wildfire in coastal sage scrub. The average cover of chaparral yucca seedlings ranged from less than 0.05% to 0.1%, and average cover of sprouts ranged from 4.3% to 12.1%. No seedlings or sprouts were found on north-facing slopes . On sites burned 3 years and 8 months earlier in chaparral in the Santa Monica Mountains, the number of chaparral yucca sprouts and seedlings along 330-foot (100 m) transects averaged 8 and 2, respectively .
In early postfire succession (1-2 years after fire), chaparral yucca sprouts were often reported without seedlings, but not all plants survived fire. On 1-year-old burned sites on the Angeles National Forest, chaparral yucca mortality averaged 50%. Mortality was not consistently associated with vegetation type, precipitation levels, or fire season; however, stands with greater mortality were described as decadent before the fire .
|Density of sprouting and nonsprouting chaparral yucca plants 1 to 2 years after fire on the Angeles National Forest |
|Month of fire||Vegetation type||Annual precipitation (inches)||Density of sprouting plants||Density of nonsprouting plants|
|July||chamise-scrub oak (Quercus spp.)||<11||6||18|
One year after a mid-July fire in the chaparral-desert ecotone of the San Ysidro Mountains, an estimated 75% of chaparral yucca plants survived and had sprouted. Survival was estimated by comparing the number of sprouts to the number of skeletal remains on burned sites. Therefore, mortality may have been overestimated if some plants had flowered and died prior to the fire. There were no sprouts 2 months after fire, but density of sprouts increased from 4 to 10 months after fire. The number of sprouts per plant was high, and cover of chaparral yucca averaged 0.36% on burned and 0.15% on adjacent unburned plots [106,107].
|Density of chaparral yucca plants and sprouts as time since fire increased in the San Ysidro Mountains |
|Time since fire (months)||2||4||7||10|
|Density of sprouting plants (number/ha)||0||10||15||20|
|Density of sprouts (number/ha)||0||800||262||1,690|
Researchers estimated that about 80% of chaparral yucca plants sprouted by the first postfire spring following fall fires in coastal and interior chaparral in southern California. Survival was estimated by comparing the number of sprouts to the number of skeletal remains on burned sites. Estimated chaparral yucca survival was 81% in coastal and 86% in interior chaparral. There were no chaparral yucca seedlings in the first postfire growing season .
Reproduction response to fire: Increased chaparral yucca flowering has been reported after fire. Unburned tall flowering stalks that remain standing after surface fires may be an important postfire seed source, although insect damage can reduce the probability that flowering stalks remain standing on burned sites. “Profuse” chaparral yucca flowering can occur within 1 to 2 years of fire . Abundant chaparral yucca flowers were observed within weeks of an August wildfire in chaparral vegetation on the Los Padres National Forest. No flowers were observed in the prefire spring flowering season. Severity of the summer fire was not described . Fire-stimulated flowering may only happen for mature plants. Chaparral yucca seedlings that established in 1927 or 1932 after a summer fire in chamise chaparral in Barranca Canyon, southern California, had not flowered by 1949 . In San Bernardino County, flower stalks with weevil damage were shorter than those without damage, and short flowering stalks were often removed by or fell during fire. Seed production from short flowering stalks was reduced 72% to 77% by a summer fire, whereas 9% or less of the seed crop was removed by fire when flowering stalks were tall .
Abundance response to fire: Chaparral yucca abundance can be greater on burned than unburned sites [106,107,120], but chaparral yucca has also been described as a "retreater" on burned sites . Chaparral yucca abundance may be less after high-severity than low-severity fires. Often the differences in chaparral yucca abundance are not great between pre- and postfire or burned and unburned sites or in early seral and late-seral burned areas.
Cover of chaparral yucca was greater 2 years after than before a fall backfire in coastal sage scrub on the western slope of the Santa Monica Mountains in Los Angeles County. Prior to the fire, chaparral yucca cover averaged 6.8% on the site that had not burned in 22 years. Two years after the fire, chaparral yucca cover averaged 8.5% .
Severe fires may limit early postfire abundance of chaparral yucca. In the northwestern San Jacinto Mountains, chaparral yucca sprouts occurred in the 1st and 2nd postfire growing seasons after a fall wildfire consumed all litter, most shrub stems, and left charcoal, ash, or bare mineral soil between charred root crowns in manzanita chaparral. Density of chaparral yucca was lower in the 2nd postfire growing season on the burned site (2 plants/acre) than on a similar unburned site (6 plants/acre) . In the Santa Ynez Mountains near Santa Barbara, density of chaparral yucca was much greater on burned sites where fire burned in a fuelbreak where vegetation was cleared before burning (70 plants/900 m²) than in burned chaparral with standing vegetation at the time of fire (20 plants/900 m²). Burned areas were evaluated 9 years after a "moderate-intensity" prescribed fire .
In some studies, chaparral yucca was rare or absent from burned sites. In southern San Diego County, density of chaparral yucca was 128 plants/acre on unburned plots, but plants were absent from adjacent burned plots 5 months after a "very hot" fire in a Tecate cypress (Cupressus forbesii) grove. The fire burned in late October, was pushed by Santa Ana winds, and occurred when air temperatures were high and humidity levels were low . Chaparral yucca was described as a "retreater" when burned and unburned plots were compared 2 to 12 years after fire in a Joshua tree-singleleaf pinyon-California juniper ecotone at the southwestern edge of the Mojave Desert in San Bernardino County. Chaparral yucca occurred on unburned transects but was rare on burned transects .
Abundance of chaparral yucca was often similar on recently burned and long unburned sites, regardless of time since fire. In San Diego County, the cover of chaparral yucca did not change much in the first 4 years following fall wildfires. On north, south, and west slopes, cover ranged from 0% to 2% in the first 4 postfire years. On east slopes, however, chaparral yucca was less than 1% in the 1st and 2nd postfire years, 2% in the 3rd postfire year, and 7% in the 4th postfire year . Differences were also small when chaparral yucca abundance was compared in chaparral vegetation that burned 2 to 8 years and 41 to 96 years prior on the coastal and desert sides of the San Gabriel and San Bernardino mountains. Chaparral yucca seedlings and sprouts were common in the early postfire years. On the coastal side, chaparral yucca cover averaged 0.4% on sites burned 2 to 8 years earlier and 0.6% on sites burned 41 to 96 years earlier. On the desert side, cover averaged 1.3% on sites burned 2 to 8 years earlier and 1.1% on sites burned 41 to 96 years earlier .
See this Fire Study in FEIS for further information on responses of chaparral yucca and other chaparral and woodland species to fire:
Damage and recovery in southern Sierra Nevada foothill oak woodland after a severe ground fire (blue oak-interior live oak/annual grass community)FUELS AND FIRE REGIMES:
Fire regimes: Recurring fires are common in the coastal sage scrub and chaparral communities where chaparral yucca commonly grows. Fire intervals can be as short as 20 years for both types. Some studies suggest that increases in human ignitions, introduction and spread of nonnative annual grasses, and changes in fire behavior related to fire exclusion from coastal sage scrub and chaparral have resulted in more frequent fires than existed historically.
A review reports that fire behaves differently in coastal sage scrub and chaparral communities. Because fuels may be less abundant and more widely spaced in coastal sage scrub than in chaparral, fires may spread more rapidly and fire severity may be lower in coastal sage scrub than in chaparral (review by ). However, local conditions are variable, and open- to closed-canopy conditions are reported for coastal sage scrub . A review reports that the "normal" fire-return interval in coastal sage scrub is 30 years . The various types of southern California chaparral are considered especially flammable at 30 to 50 years old, although this depends on climate and local fuel accumulation rates (review by ).
Fire frequency did not vary between chaparral and coastal sage scrub from 1930 to 1978 in the western Santa Monica Mountains. The average fire-return interval was 20 years for both chamise chaparral and coastal sage scrub communities (Radtke unpublished information cited in ). A review indicates that fire-return intervals in chaparral vegetation range from approximately 10 to 100 years . However, in chaparral vegetation near the California-Baja California border, fires were rare in young stands and generally restricted to stands 50 years or older [79,80]. Although chaparral shrubs are fully mature at 12 to 18 years old, the absence of herbaceous vegetation in young stands may limit fire spread through the spaces between shrubs . In the Sierra San Pedro Mártir, Baja California, the fire-return interval in chaparral is usually 40 to 50 years. As stands reach 40 years or more, flammability increases with dead fuel accumulations, and fuel continuity increases in live and dead biomass . The average fire-return interval for chaparral vegetation in San Diego County, California, was 70 years from 1920 to 1971. In the 1,025,000-acre (414,800 ha) study area, 163,800 acres (66,300 ha) burned twice. Nearly 30 fires were 5,680 acres (2,300 ha) or larger, and the largest (Laguna Fire in 1970) was 145,800 acres (59,000 ha). Fires typically burned in late summer or fall, and most resulted from anthropogenic ignitions .
Several studies indicate that fire frequencies have increased from estimated historical frequencies in southern California's chaparral. Researchers propose several factors that may have contributed to the increase in fire frequency, including increases in human ignitions, introductions and spread of nonnative annual grasses, and changes in fire behavior related to fire exclusion. Chaparral communities generally burn in high-intensity, stand-replacing crown fires that range from thousands to tens of thousands of acres. A small percentage of fires has exceeded 100,000 acres (40,500 ha) in size. Chamise-dominated chaparral is described as "flashier" (in flammability) than manzanita-dominated chaparral . Analyses of sediment cores from the Santa Barbara Channel in southern California suggest that large fires occurred every 40 to 60 years in the 1700s and 1800s (Byrne and others 1977 cited in ). In the 1900s, the number of human ignitions increased, and the average fire-return interval was shorter, 30 to 35 years .
In the southeastern desert bioregion of California, researchers identified hotspots with relatively high fire frequencies and proportional area burned. They attributed increased fire occurrence to increased human ignitions and high fine fuel loads from nonnative annual grasses. Relative humidity is low in the summer (10%-30% in July), when fire spread is likely in the southeastern desert bioregion. From 1985 to 2001, there were 12 to 32 lightning strikes/100 km²/year; most lightning strikes occurred from July through September. Fire records from 1980 to 2000 indicate that the proportion of area burned was low (high was 0.3%/year) for the bioregion. However, at elevations below 4,200 feet (1,280 m) in the Mojave, Colorado, and Sonoran deserts, there were sites with a high number of human ignitions and an abundance of nonnative annual grasses; these sites burned 3 times in 15 years .
When fire regimes were compared in annual grassland, coastal sage scrub, and chaparral vegetation types in southern California and northern Baja California, total area burned was similar from 1972 to 1980 in annual grassland, coastal sage scrub, and chaparral vegetation for the 2 areas. In chaparral vegetation, however, the size of individual fires was much greater in southern California than in northern Baja California. In annual grasslands and coastal sage scrub, fire probability was strongly correlated with high precipitation levels the previous winter. Chaparral vegetation burned after wet and dry years. Most chaparral fires were small in northern Baja California but ranged from small to very large in southern California. The researcher suggested that fire suppression in southern California had decreased the number of chaparral fires but failed to reduce the total area burned, and therefore, fire size increased. Between 1911 and 1980, there were 41 chaparral fires in National Forests of southern California that exceeded the largest chaparral fire in Baja California . Fires in San Diego County were larger but less frequent than those in Baja California. Comparisons of past and current fire behavior and fire frequency in southern California and northern Baja California led Minnich  to suggest that active fire suppression has resulted in more fires burning outside of the historic late summer-fall fire season, escaping early control efforts, and spreading at high intensities during severe weather, namely Santa Ana winds and summer heat waves. Prior to active fire suppression in southern California, fires in chaparral exhibited a smolder and run behavior that could last for months in the dry season. This fire behavior supported irregular fire intensities that changed with fuel and weather conditions and created patchiness in stand structure. Major fire runs produced narrow strips of burned area and left many unburned islands .
See the Fire Regime Table for additional information on the prevailing fire regimes in vegetation where chaparral yucca and Newberry's yucca may occur. Find further fire regime information for the plant communities in which these species may occur by entering the species' names in the FEIS home page under "Find Fire Regimes".FIRE MANAGEMENT CONSIDERATIONS:
Palatability and nutritional value: No information was available on these topics.VALUE FOR REHABILITATION OF DISTURBED SITES:
The following tables provide fire regime information that may be relevant to chaparral yucca and Newberry's yucca habitats. Find further fire regime information for the plant communities in which these species may occur by entering the species' names in the FEIS home page under "Find Fire Regimes".
Fire regime information on vegetation communities in which chaparral yucca may occur. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models , which were developed by local experts using available literature, local data, and/or expert opinion. This table summarizes fire regime characteristics for each plant community listed. The PDF file linked from each plant community name describes the model and synthesizes the knowledge available on vegetation composition, structure, and dynamics in that community. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|Coastal sage scrub||Replacement||100%||50||20||150|
|Coastal sage scrub-coastal prairie||Replacement||8%||40||8||900|
|Surface or low||62%||5||1||6|
|California oak woodlands||Replacement||8%||120|
|Surface or low||91%||10|
|Surface or low||78%||13|
|Mixed evergreen-bigcone Douglas-fir (southern coastal)||Replacement||29%||250|
|Great Basin Shrubland|
|Creosotebush shrublands with grasses||Replacement||57%||588||300||>1,000|
|Interior Arizona chaparral||Replacement||88%||46||25||100|
|Great Basin Woodland|
|Juniper and pinyon-juniper steppe woodland||Replacement||20%||333||100||>1,000|
|Surface or low||49%||135||100|
Fire regime information on vegetation communities in which Newberry's yucca may occur. The majority of communities listed are based on information collected while researching both chaparral yucca and Newberry's yucca and likely represent a greater number of communities than are truly inhabited by Newberry's yucca.
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|Surface or low||15%||67|
|Desert grassland with shrubs and trees||Replacement||85%||12|
|Desert shrubland without grass||Replacement||52%||150|
|Southwestern shrub steppe||Replacement||72%||14||8||15|
|Surface or low||15%||69||60||100|
|Southwestern shrub steppe with trees||Replacement||52%||17||10||25|
|Surface or low||25%||35||25||100|
|Interior Arizona chaparral||Replacement||100%||125||60||150|
Replacement: Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed: Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.
Surface or low: Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [39,68].
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