Photo by Michael Charters, Southern California Wildflowers
AUTHORSHIP AND CITATION:
Meyer, Rachelle. 2011. Ceanothus leucodermis. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ .
NRCS PLANT CODE :
chaparral white thorn
chaparral whitethorn ceanothus
The scientific name of chaparral whitethorn is Ceanothus leucodermis Greene (Rhamnaceae) [54,62,96].
At the ecotone of forest and chaparral communities on the west slope of Sierra Corona, chaparral whitethorn may
hybridize with Palmer ceanothus (Ceanothus palmeri) or whitethorn ceanothus (Ceanothus cordulatus) .
|North American distribution of chaparral whitethorn as of 2011. Map courtesy of USDA, NRCS. 2011. The PLANTS Database. National Plant Data Team, Greensboro, NC. (2011, December 17).|
Caparral whitethorn occurs in the following US and Mexican states as of 2011 :
United States: CA
Mexico: Baja California [4,32,54,55,92]
Chaparral whitethorn occurs from northern Baja California, Mexico [4,32,54,55,92] northward through the mountains of southern California [23,55,87,96,121] and along the Coast Ranges [23,54,55,60,87,121], the Transverse Ranges [18,54,55], and the Sierra Nevada foothills [54,55,87,96,121,134], with a few occurrences as far north as Shasta [55,121], Siskiyou [23,74], and Humboldt counties .
SITE CHARACTERISTICS AND PLANT COMMUNITIES:
Site characteristics: Chaparral whitethorn is most common at elevations below 6,000 feet (1,800 m) [21,23,54,96], on sites with rocky [54,96,121] or sandy soils [102,107,115]. It occurs on dry [23,55,66,121] to mesic sites [55,66]. Chaparral whitethorn is more common on coastal sites than desert sites [13,47,88]. It occurs in areas with a mediterranean climate [38,118,120].
Elevation and aspect: Chaparral whitethorn distribution extends into higher elevations in southern California compared to central California. In central California, chaparral whitethorn has been reported at elevations ranging from 660 feet (200 m) in Madera County  to 3,730 feet (1,137 m) in the Kings River Ranger District of Sequoia-Kings Canyon National Park . In Yosemite National Park, chaparral whitethorn was listed as a major component of the chaparral-oak woodland community that was most prevalent below 4,000 feet (1,200 m). In areas that have burned severely, this community may occur at higher elevations, although it is unclear whether chaparral whitethorn occurs there . In southern California, chaparral whitethorn has been reported on sites from about 1,000 feet (300 m) [58,85,107] to over 7,000 feet (2,100 m) , and is most common from about 3,500 (1,070 m) [58,88] to 6,000 feet (1,800 m) [34,55,58,85].
In southern California, abundance of chaparral whitethorn is influenced by elevation and aspect. On southern exposures, chaparral whitethorn reaches its greatest frequency and cover at higher elevations than on northern exposures in the San Bernardino Mountains  and coastal drainages . In the San Bernardino Mountains, chaparral whitethorn reached its highest cover on southern exposures over 5,000 feet (1,500 m; 22%) and on northern exposures over 4,000 feet (1,200 m; 18%) . In coastal drainages of southern California, chaparral whitethorn cover was greatest on northern exposures from 3,000 to 5,000 feet (900-1,500 m), and on southern exposures from 5,000 to 8,000 feet (1,500-2,400 m) . Cover of chaparral whitethorn was positively associated with elevation and northern aspects on burned and unburned sites in San Diego county . On desert exposures in the San Gabriel and San Bernardino mountains, chaparral whitethorn occurs at higher elevations than on coastal exposures .
Chaparral whitethorn is more common on coastal aspects than desert aspects [13,47,88]. In southern California, the maximum cover of chaparral whitethorn in desert drainages was 7% from 3,000 to 4,000 feet (900-1,200 m) on northern exposures. Maximum cover in coastal sites was 24.2% from 5,000 to 6,000 feet (1,500-1,800 m) on southern exposures . Although not statistically significant, chaparral whitethorn was more common and had greater cover on sites with coastal exposures than on sites with desert exposures in the San Gabriel and San Bernardino mountains .
In Baja California, Mexico, chaparral whitethorn was reported in a chaparral and pinyon-juniper woodland site at 4,000 feet (1,220 m)  and in mixed chaparral over 3,900 feet (1,190 m) on the western flanks of the Sierra San Pedro Mártir .
Soil: Chaparral whitethorn typically occurs on rocky [54,96,121] and coarse-textured soils [30,52,54,102,107,115], with reported sand content ranging from 39.5%  to 78% . In the southern Sierra Nevada foothills, chaparral whitethorn occurred on sites with sand contents ranging from 39.5% to 64%. Its highest cover was on the site with 64% sand, although soil texture was one of several factors that likely contributed to its abundance .
Reported pH at sites with chaparral whitethorn ranged from 5.8 to 7.4 [98,102,136]. The pH of 7.4 occurred in an ash layer on a site in the San Jacinto Mountains burned 1 year previously . The highest reported pH value of soil was 6.71 and occurred on a site on the west slope of the southern Sierra Nevada .
Percent soil organic matter on sites in the southern Sierra Nevada foothills with chaparral whitethorn ranged from 2.3% to 3.1% .
Moisture availability: Chaparral whitethorn occurs on sites that range from dry [23,55,66,121] to mesic [55,66]. Chaparral whitethorn is a characteristic species in communities ranging from huckleberry oak (Quercus vacciniifolia) chaparral, which occurs on dry ridges and xeric exposures, to interior live oak (Q. wislizenii) and north slope chaparral, which are comparatively mesic chaparral communities .
Chaparral whitethorn is a characteristic species of the California chaparral province, which has a mediterranean climate, with moist winters and dry summers [118,120]. Annual precipitation in the California chaparral province ranges from 12 inches (305 mm) at San Juan de Dios in Baja California to 28 inches (702 mm) at San Gabriel Dam north of Glendora, California . Only 10% to 25% of annual precipitation occurs in the warmest months . Summer is typically a period of intense drought with occasional thunderstorms in July and August . Fog from the coast may provide some moisture on coastal sites .
|Table 1: Average annual precipitation in areas with chaparral whitethorn|
|Location||Annual precipitation (mm)|
|About 10 miles northeast of Escondido, California||406 |
|Cleveland National Forest||460 |
|Idyllwild, California||646 |
|San Gabriel Mountains||1,220 |
Chaparral whitethorn is less tolerant of drought than many chaparral species [107,108]. It was less resistant to cavitation than 8 Rhamnaceae species  and 3 nonsprouting chaparral species, and similar in resistance to 3 of the 4 other sprouting species tested. . In a common garden experiment, chaparral whitethorn irrigated to imitate wet winter months had 0% mortality, while chaparral whitethorn exposed to ambient precipitation had 5.4% mortality in 25% full sunlight and 54.5% mortality in full sunlight. Ambient precipitation consisted of no to trace rainfall from 3 March to 21 October followed by greater than average rainfall .
Temperature: The mediterranean climate has cool winters and hot summers [118,120]. In the Cleveland National Forest, chaparral whitethorn occurred in an area with average maximum and minimum air temperatures of 82.9 °F (28.3 °C) and 46.4 °F (8.0 °C) in summer and 56.5 °F (13.6 °C) and 31.5 °F (-0.3 °C) in winter . Depending on latitude and elevation, the frost-free period in much of California ranges from 160 to 200 days. In an area along the southern California coast, the frost-free period is 300 days .
Plant communities: Chaparral whitethorn is a common chaparral species. It is less common in oak woodland-chaparral transition, oak woodland, conifer forest-chaparral transition, and conifer-dominated communities. Chaparral whitethorn is listed as a dominant, codominant or indicator species in communities described in the following publications:
Chaparral whitethorn is characteristic of several chaparral communities [13,18,23,46,77,86,91,96] but rarely occurs as a dominant species (, Sawyer and Keeler-Wolf 1995 cited in ). It is common in mixed chaparral [15,36,88] and scrub oak (Quercus spp.) chaparral [15,36,49,59,104,138], including huckleberry oak chaparral and interior live oak chaparral . Chaparral whitethorn may codominate with oaks such as interior live oak and scrub oak [18,88]. The frequency of chaparral whitethorn in National Forests along the southern and central coast of California was 29% in mixed chaparral plots and 30% in scrub oak chaparral plots. Outside of National Forest boundaries, chaparral whitethorn frequency was 9% in mixed chaparral plots and 5% in scrub oak plots . Chaparral whitethorn can be common in chamise (Adenostoma fasciculatum) chaparral along the southern Coast Ranges and interior of southern California [15,36,59] but rarely codominates with chamise . Chaparral whitethorn codominated with chamise and birchleaf mountain-mahogany (Cercocarpus montanus var. glaber) on sites in southern California (Holl and other 1980 cited in ). Chaparral whitethorn was present in 26% of chamise plots within National Forests on the southern and central coasts of California and 1% of plots outside National Forests in this region . When chaparral whitethorn occurs in Ceanothus chaparral it may be a component , a codominant , or the dominant species (Sawyer and Keeler-Wolf 1995 cited in ). Chaparral whitethorn occurs in Ceanothus-manzanita (Arctostaphylos spp.) chaparral [20,52], and may be a codominant in this community . It also occurs in chamise-manzanita chaparral , mesic north slope chaparral , mountain-mahogany (Cercocarpus spp.) chaparral , desert transition chaparral [91,130], and coastal transition chaparral, comprised of both hard and soft chaparral species. On the southern and central California coast, chaparral whitethorn occurred on 3% of coastal transition chaparral plots in and outside of National Forests . In the San Bernardino Mountains, it had 1% cover in desert transition chaparral .
Chaparral whitethorn occurs in the oak woodland-chaparral transition [8,17,59] and oak woodlands [4,14,31,96]. Chaparral whitethorn occurred on steep south-facing slopes in low elevations of the woodland-chaparral zone in the San Bernardino Mountains  and in chaparral-oak woodland in Yosemite National Park . Chaparral whitethorn was a component of oak woodland savanna dominated by gray pine (Pinus sabiniana), blue oak (Quercus douglasii), and interior live oak [4,14,31]. It occurred in the understory of a Jeffrey pine-California black oak (P. jeffreyi-Q. kelloggii) woodland at Cuyamaca Rancho State Park . The frequency of chaparral whitethorn in hardwood woodland plots outside of National Forests in interior California was 6% .
Chaparral whitethorn is a component of chaparral-conifer forest ecotones [1,93] and occurs in several conifer-dominated communities [4,15,59,85,91,135]. Frequency of chaparral whitethorn was 20% in conifer forest plots on lands outside of National Forests in southern California . In Baja California, chaparral whitethorn occurs near the chaparral/mixed-conifer forest ecotone . On Cuyamaca Peak, thick chaparral with chaparral whitethorn was adjacent to a Arizona cypress (Cupressus arizonica) stand . Chaparral whitethorn occurs in the shrub layer of knobcone pine (Pinus attenuata) forest , bigcone Douglas-fir (Pseudotsuga macrocarpa) forest , Coulter pine (Pinus coulteri) forest [4,91,135], Coulter pine-California black oak stands , and pinyon-juniper (Pinus spp.-Juniperus spp.) communities . Chaparral whitethorn had 3% cover in Coulter pine-chaparral communities and 2% cover in Coulter pine-black oak communities . It occurred in 3% of pinyon-juniper plots in interior California on lands outside of National Forests . It may have been an associate of brush that occurred to varying degrees under ponderosa pine forest in a study area on the west side of the Sierra Nevada in Madera County .
Chaparral whitethorn sometimes occurs in riparian habitats [6,32,48]. On the Los Padres National Forest, it was present in a community where red willow (Salix laevigata) and mule-fat (Baccharis salicifolia) were characteristic species . Chaparral whitethorn occurred just upland of the fan palm (Erythea armata) zone in an oasis in Baja California . It was a component of riparian coastal scrub in the San Gabriel Mountains .
Form and architecture: Chaparral whitethorn is an erect shrub 5 to 12 feet (2-4 m) tall [23,54,96,107,121] with branches that are rigid, divaricate, and spiny [23,96,121].
Photo by Michael Charters, Southern California Wildflowers
Leaves: Chaparral whitethorn is evergreen [23,54,96,121] with alternate leaves [23,54,121], 0.2 to 1.5 inches (0.5-3.8 cm) long [23,54,96,121] and 0.2 to 0.5 inch (0.5-1.3 cm) wide [23,121]. Chaparral whitethorn leaves are generally elliptical-oblong [54,96,121] to ovate [96,121] with entire to minutely serrate margins [23,54,96]. Leaves are 3-veined [23,54,121], waxy, and generally smooth [23,54,96], although they may be covered in short, fine hairs [54,96]. Leaves of most evergreen shrubs live for 2 or more years .
Reproductive structures: Flowers occur in simple, dense clusters [96,121] 1 to 4 inches (2.5-11 cm) long [54,96,121]. Chaparral whitethorn fruit is a capsule [23,96] less than 0.25 inch (0.6 cm) wide that is sticky [23,54,96,121]. The average mass of chaparral whitethorn seed was 7.12 mg on a wedgeleaf ceanothus-chaparral whitethorn site .
Roots: In southern California, chaparral whitethorn individuals had prominent tap roots more than 12 feet (3.7 m) deep, and additional major roots grew downwards. Of 4 individuals with an average height of 4.8 feet (1.5 m), the maximum root length was 21 feet (6.4 m). The maximum outward spread of roots was 10 feet (3.0 m) and the average spread was 8.4 feet (2.6 m). Given that the average spread of tops was 3.6 feet (1.1 m), root spread was about 50% greater than crown spread .
On a site in southern California, average live basal burl area of chaparral whitethorn was 160 cm² on a site burned about 2 years previously and 151 cm² on an unburned area .
Chaparral whitethorn has root nodules [29,30,52,53] formed by the nitrogen-fixing bacteria Frankia [97,98]. In a 2-year-old burned chaparral community near Lakeside, California, 63% of excavated chaparral whitethorn individuals were nodulated, and a rough estimate of nitrogen fixation by chaparral whitethorn seedlings was 0.87 kg N/ha/year . In the San Gabriel Mountains, 2- and 3-year-old chaparral whitethorn sprouts were often nodulated. Nitrogen fixation was low during the dry period in summer and fall and during winter cold. Preliminary data indicated that juvenile stages fixed more nitrogen than mature stages .
Longevity: Chaparral whitethorn seedlings establish in the first few postfire years, so stand age generally reflects the age of the community. Chaparral whitethorn is most common in stands less than 40 years old [47,91,138]. Vogl and Schorr  considered Ceanothus short lived, because of the low densities of living Ceanothus and occurrence of dead Ceanothus in mature (≥42 years) manzanita chaparral . However, Horton  considered chaparral whitethorn a long-lived shrub, and Syphard and Franklin  state that chaparral whitethorn generally lives about 60 years. In the San Gabriel and San Bernardino Mountains, chaparral whitethorn apparently lived longer on coastal exposures than desert exposures . Chaparral whitethorn was a characteristic species in chaparral-oak woodlands in Yosemite National Park that were described as "becoming decadent after 30 to 40 years" . For information on occurrence of chaparral whitethorn in stands of various ages, see Successional Status. See Plant response to fire for information on how time since burning affects chaparral whitethorn abundance.
Chaparral whitethorn typically flowers from April [23,24,96] to June [23,96,121], although flowering as early as February as been reported . Fruit ripens in July and August  and seeds are dispersed in late July. In an oak woodland savanna in the Sierra Nevada, about 95% of chaparral whitethorn seed was dispersed in a 2-week period in late July with low levels of seed dispersal continuing into August . Seeds mature in September and October . Chaparral whitethorn seedlings emerge in spring, but germination rates are low except during early postfire years. Following prescribed burns in the western slope of the Sierra Nevada, 66% of all chaparral whitethorn seedlings emerged in the week of 14 to 21 March. The following year, peak emergence was before 13 March on south-facing slopes and before 3 April on north-facing slopes .
Chaparral whitethorn regenerates via seed and can sprout after fire in much of its range. In some years chaparral whitethorn produces abundant seed. Seeds are dispersed short distances by dehiscing capsules. Persistence of chaparral whitethorn seeds in the soil seed bank is unknown. Germination rates and seedling establishment are generally low, except after fire. Seedling establishment is high after fire, although many seedlings die in the first few postfire years.
Pollination and breeding system: Perfect flowers are characteristic of the Ceanothus genus . Insects pollinate Ceanothus flowers [51,83].
Seed production: Seed production in Ceanothus, including chaparral whitethorn, varies extensively from year to year [21,64]. On an unburned site dominated by Ceanothus in southern San Diego County, chaparral whitethorn fruit production ranged from 4 to 2,454 fruits/m² of aerial cover over a 3-year study. Based on this and estimates of aerial cover per hectare, chaparral whitethorn produced from 47,600 to 29,200,000 seeds/ha annually. Viability of seeds was 53% in the 1st year and 44% in the 2nd year, indicating production of 23,000 to 12,800,000 viable seeds/ha annually. Precipitation is suggested as a major source of annual variation in seed production, with high production occurring the year following high-precipitation years .
Age of first seed production has not been reported for chaparral whitethorn. Generally, Ceanothus begin producing seeds at 4- to 10-years-old, and Ceanothus sprouts produce at least some seeds at 3- to 6-years-old .
Seed dispersal: Chaparral whitethorn seed is initially dispersed when capsules mature and dehisce explosively [31,65]. In an oak woodland savanna in the central Sierra Nevada, 36% of chaparral whitethorn seeds occurred beneath the canopy of the parent plant, 42% occurred at the edge of the parent plant crown, and the remainder occurred from the edge up to 30 feet (9 m) from the parent plant. Only 1.9% occurred 30 feet away. Most seeds were cast southeast from the parent plant .
Ceanothus seeds may also be dispersed by birds and rodents . Fruit of chaparral whitethorn is eaten by small mammals, birds, and insects , but whether this represents seed predation or seed dispersal is unclear. From 90% to 99% of annual seed production of chaparral species may be eaten by rodents .
Seed banking: On a Ceanothus-dominated site in southern San Diego County, estimated chaparral whitethorn seed bank density in summer was 835,000 viable seeds/ha. Seed production was much higher than this in a high-production year, suggesting that seed banks fluctuate with inputs and outputs such as animal dispersal, seed predation, erosion, and decay .
Although persistence of chaparral whitethorn in the seed bank is unknown, Ceanothus seeds are generally long lived . Desert ceanothus (Ceanothus greggii) [41,103,105] and redstem ceanothus (Ceanothus sanguineus) [37,76] seeds may remain viable in the soil for decades, deer brush (Ceanothus integerrimus) seeds may remain viable for over 100 years , and seeds of snowbrush ceanothus (Ceanothus velutinus) can germinate after "several centuries" in the soil [5,22,35,42,63].
Germination: Heat stimulates germination of chaparral whitethorn seeds [65,67,77], and seedlings often establish after fire (See Fire adaptations). Germination rates of chaparral whitethorn seeds exposed to 212 °F to 248 °F (100-120 °C) for 5 minutes were 48% to 68%, while those not exposed to heat had germination rates of 7% or less . In another laboratory study, chaparral whitethorn seeds exposed to temperatures ranging from 160 °F to 280 °F (71-140 °C) exhibited germination rates of 24% to 79%, while unheated seeds had a maximum germination rate of 15% . Germination rates of chaparral whitethorn seeds that have not been heated range from less than 5% [65,67] to 15% . Light does not significantly influence chaparral whitethorn germination . Details of these studies are discussed in the Fire adaptations section.
Gibbens and Shultz  suggest that chaparral whitethorn requires heat or some other mechanism to break the seed coat, then a cool stratification period, followed by warmth.
Seedling establishment and plant growth: Seedling establishment is common following fire, with generally high mortality in the first few years after establishment. General observations of seedling establishment are discussed in the Fire adaptations section, and seedling mortality is discussed in the Time since fire section.
Chaparral whitethorn seedling establishment on unburned sites has been observed in the Sierra Nevada foothills [40,112]. Chaparral whitethorn seedlings were frequent in a unburned stand comprised of chamise, wedgeleaf ceanothus, and chaparral whitethorn with a closed canopy and infrequent canopy gaps on the western slope of the southern Sierra Nevada foothills . Chaparral whitethorn seedlings occurred at densities <2,000/acre following brush mashing and no burning treatment on the west side of the Sierra Nevada in Madera County .
Chaparral whitethorn grows quickly, with sprouts growing faster than seedlings . In desert-exposed sites in the San Gabriel and San Bernardino mountains, several chaparral species such as scrub oak, Mojave ceanothus (Ceanothus greggii var. vestitus) and true mountain-mahogany (Cercocarpus montanus) "approached" the height of chaparral whitethorn 20 to 40 years after fire . In the western Sierra Nevada, chaparral whitethorn grew faster than wedgeleaf ceanothus . After a November wildfire in a chaparral community in southern California, chaparral whitethorn sprouts grew up to 7 feet (2 m) in 10 years . In the year following a prescribed fire, chaparral whitethorn seedlings growing away from other plants were frequently 2 to 3 inches (5-8 cm) tall within a month of emergence, 7 to 8 inches (18-20 cm) in 3 months, 12 to 13 inches (30-33 cm) in 5 months, and 18 to 20 inches (46-51 cm) in 9 months. Chaparral whitethorn seedling root lengths were up to 5 or 6 inches (13-15 cm) 2 weeks after germination, 14 to 15 inches (36-38 cm) after a month, and 3 to 4.5 feet (1-1.4 m) after 3 months. Chaparral whitethorn seedling roots averaged about 3.4 inches (8.6 cm) in depth by 15 days after emergence, and doubled their depth in 2 weeks. Roots reached depths of 22.5 to 32.25 inches (57-82 cm) in about 3 months .
Vegetative regeneration: Chaparral whitethorn sprouts following damage from fire or cutting [10,23,46,47,52,57,101,107,108,120] and was classified as a facultative sprouter by Pratt and others . Sprouts are described as originating from stumps [10,23,57], root crowns [52,82], and basal burls [4,46,61,73]. See Fire adaptations for more information on postfire sprouting.
Chaparral whitethorn populations in central California do not sprout to the same extent as those in southern and coastal California [11,121,122,133]. Many of the references that categorize chaparral whitethorn as a sprouting species are focused on southern California [25,34,47,66,124]. Chaparral whitethorn in more northern populations, including the Sierra Nevada foothills, apparently sprout less frequently or not at all [11,122,133]. A description of California range plants notes that chaparral whitethorn sprouts readily after fire in southern California, while in Tulare county and northward it does not sprout . However, chaparral whitethorn was noted to sprout to some extent after bursh mashing and burning in Madera county, although seedlings were more common than sprouts [39,40]. Seedlings were also more common than sprouts on some low elevation  and desert exposed  sites in southern California
Although chaparral whitethorn has been observed in chaparral communities up to about 60 years old [47,102,138], it is most common in communities less than 40 years old [47,91,138]. Chaparral whitethorn seedling establishment is common in the first few years after fire [46,66,108,136] (see Fire adaptations and Time since fire). Although mortality of these seedlings may be high [40,57,72], cover of chaparral whitethorn often increases as survivors grow . In the San Gabriel and San Bernardino mountains, peak cover of 7% to 8% occurred in the 9- to 21-year-old age class on both coastal and desert exposures. Average cover in the 22- to 40-year-old age class was 4.2% on coastal sites and 1.8% on desert sites. In 41- to 96-year-old stands, chaparral whitethorn cover averaged less than 0.5% on both coastal and desert sites. However, the relationship with time since fire was not significant . As communities mature, Ceanothus generally decline in abundance  and occurrence of standing dead Ceanothus individuals increases [89,136].
In mixed chaparral comprised of varying amounts of chamise, manzanitas, Ceanothus, and scrub oaks, chaparral whitethorn is typically more common in stands of less than 30 years old than in older stands. Ceanothus, including chaparral whitethorn, may be codominant in scrub oak mixed chaparral stands that are 10 to 40 years old and then begins to decline . Chapparal whitethorn codominated with bigberry manzanita (Arctostaphylos glauca) in chaparral that had not burned in 25 years  and was common in 25-year-old mixed chaparral . It occurred in a mixed chaparral community in the Cleveland National Forest that had not burned in 34 years . In the San Jacinto Mountains, chaparral whitethorn was present in manzanita chaparral that had not been burned in at least 42 years . It occurred with 4% to 22% cover in chamise or mixed chaparral surveyed from 1929 to 1934 but was not found when the same sites were surveyed 60 to 65 years later . In contrast, chaparral whitethorn in the southern Sierra Nevada foothills had 3.3% cover in a 55-year-old chamise stand, which was higher than in 4-, 14-, and 35-year-old chamise stands .
Without disturbance some sites with chaparral whitethorn succeed to other community types, such as oak or pine woodland [89,139]. In some communities, conversion may take many decades . In bigcone Douglas-fir stands in the east Transverse Ranges, chaparral whitethorn occurs in the first few years after fire. By about 50 years after fire, canyon live oak (Quercus chrysolepis) is typically dominant and few Ceanothus shrubs remain due to senescence. In Coulter pine stands, chaparral whitethorn usually dies out after about 25 to 40 years . In a review, Minnich  reports 6.1% chaparral whitethorn cover in burned bigcone Douglas-fir stands 4 years old, 0% in 10- to 12-year-old stands, 3.8% in 18 to 19-year-old stands, and 0.7% in 21- to 25-year-old stands. Chaparral whitethorn did not occur in 38-, 56-, or 64-year-old stands. Horton  notes that bigcone Douglas-fir invades south-facing slopes with chaparral comprised of interior live oak, chaparral whitethorn, bigberry manzanita, and Eastwood manzanita (Arctostaphylos glandulosa) "very slowly". Lack of senescence of chaparral whitethorn in a chaparral-mixed forest ecotone at the James San Jacinto Mountains Reserve 25 years after an August wildfire led the authors to speculate that recovery of the mixed forest may take over 100 years . Vigorous chaparral communities more than 100 years old suggest that on some sites disturbance is not required for chaparral to persist indefinitely . Chaparral whitethorn is not likely to occur in these old chaparral stands, but some Ceanothus individuals persist in mature stands despite generally high mortality .
Shade tolerance: Chaparral whitethorn is at least moderately shace tolerant. Irrigated chaparral whitethorn in 25% full sunlight had 0% mortality, and chaparral whitethorn experiencing ambient precipitation had lower mortality in 25% full sunlight than those in full sunlight  (for details of this study see Moisture availability). Light did not significantly influence chaparral whitethorn germination rates. See Fire adaptations for details of germination studies.
Immediate fire effect on plant: Although fire may kill chaparral whitethorn [64,73], topkill is more common [9,64,66,73]. According to Keeley , fire-caused morality of chaparral whitethorn may reach 50%. Less than 20% of chaparral whitethorn burl area was killed by fire in a southern California Ceanothus stand, based on area of dead and living chaparral whitethorn burls in burned and unburned areas. Chaparral whitethorn was classified as having moderate adult mortality from fire . July and August burns to clear land for grazing in Madera and Lake counties resulted in top-kill rates ranging from 65% to 95%. Total survival or mortality was not reported .
Postfire regeneration strategy :
Tall shrub, adventitious buds and/or a sprouting root crown
Ground residual colonizer (on site, initial community)
Fire adaptations and plant response to fire:
Sprouting: Chaparral whitethorn typically sprouts after top-kill from fire or cutting [10,23,101,120], although vegetative regeneration seems more prevalent in the southern portion of its range (see Vegetative regeneration). Chaparral whitethorn sprouts were observed 1 and 2 years after September and October wildfires in manzanita chaparral in the San Jacinto Mountains of southern California . Chaparral whitethorn sprouting was observed 1.5 years after an early December prescribed fire in a dense chaparral understory on a Jeffrey pine-California black oak woodland at Cuyamaca Rancho State Park .
Chaparral whitethorn sprouts grow quickly, and sprouting is often described as vigorous [47,52,57]. In the mountains north of San Bernardino, sprouts grew faster than seedlings [47,57]. Chaparral whitethorn sprouts had grown to a height of 7 feet (2 m) 10 years after the November 1938 Fern Canyon wildfire in the San Dimas Experimental Forest  (see Table 2).
|Table 2: Average height (inches) of chaparral whitethorn sprouts and seedlings 1 to 15 years following a November wildfire in chamise chaparral |
Germination and seedling establishment: Germination of chaparral whitethorn seedlings is common following fire, although germinants experience high mortality (see Time since fire). Chaparral whitethorn is commonly grouped with species that establish as seedlings on burned sites [46,66,108]. Seedlings may (e.g., [40,112], see Seedling establishment) or may not occur on sites that have not burned recently [73,136]. Chaparral whitethorn seedlings were observed 1 and 2 years after September and October wildfires in manzanita chaparral in the San Jacinto Mountains . Numerous chaparral whitethorn seedlings were observed in the first years following the November 1938 Fern Canyon wildfire in chamise chaparral  (see Table 2 and Table 5). Chaparral whitethorn occurred mainly as seedlings during the first decade after fire in chaparral on north-facing slopes exposed to arid conditions of the high Mojave Desert plateau in the San Gabriel and San Bernardino mountains . It established from seed banks following fires in bigcone Douglas-fir communities [4,89].
Heating stimulates germination of chaparral whitethorn seeds; other aspects of fire (char and smoke) apparently do not affect germination . Laboratory studies indicate that germination is stimulated by temperatures up to about 270 °F (132 °C) sustained for 5 minutes . Seeds exposed in the laboratory to temperatures of 280 to 300 °F (138-149 °C) for 5 minutes did not germinate [65,67,120] (see Table 3 and Table 4). The soil surface may reach much higher temperatures during chaparral fires . For example, maximum temperatures reached in chaparral litter during fire on sites in common manzanita (A. manzanita) chaparral in Humboldt county ranged from 555 to 960 °F (290-515 °C) at 0.5 inch (1 cm) depth, and temperatures at this depth remained over 150 °F (66 °C) for 26 to 34 minutes. Maximum temperatures generally decrease with increasing depth in the litter, duff, and soil . Given these temperatures, mortality of seed in the litter is likely, and germinants emerge from beneath a layer of duff or soil that protects them from lethal temperatures but is heated sufficiently to stimulate germination .
|Table 3: Mean percent germination of chaparral whitethorn seeds subjected to laboratory treatments |
|Heat treatment||No heat||160 °F
|No heat||160 °F
|*Temperature treatment means with the same letter
were not significantly different (P>0.05); light and dark experiments considered
**0.25 g of char added to paper.
|Table 4: Percent germination of chaparral whitethorn seeds exposed to various temperatures |
Treatments: Temperatures (°F) exposed to for 5 minutes
|Sampling date||Control (no heat)||140-160||160-180||180-200||200-220||220-240||240-260||260-280||280-300|
Plant response to fire: Chaparral whitethorn typically establishes and often reaches peak abundance within the first few years after fire. For instance, in the east Transverse Ranges in southern California, chaparral whitethorn occurs early in succession after fire in bigcone Douglas-fir stands and Coulter pine stands . Postfire response of chaparral whitethorn is influenced by time since fire, site characteristics (such as elevation and aspect, postfire precipitation, plant community composition), and fire characteristics (including severity, frequency, and season). Other, interacting factors that may influence the postfire response of chaparral species in general include stand and seed bank composition [10,110], stand age, fuel characteristics (such as the amount of dead wood), and recreation and urbanization .
It has been suggested that increased nitrogen in the soil following fire could inhibit postfire nitrogen fixation. On a site in southern California, increases in ammonium and phosphate in the first 2 years following fire were limited primarily to the first 4 inches (10 cm) of soil. Because chaparral whitethorn forms nodules at deeper soil layers, where nitrogen levels were relatively unaffected by fire, chaparral whitethorn nitrogen fixation was not impacted .
Time since fire: Chaparral whitethorn typically sprouts and/or establishes from seed within the first few postfire years [57,81,136]. Mortality of recently established seedlings is often high [40,57,72,136], but cover typically increases as sprouts grow and seedlings mature [16,57,71]. The only study to test the association of chaparral whitethorn abundance with time since fire found that cover was greatest (7% to 8%) in the 9- to 21-year-old age class and lower in older stands. However, chaparral whitethorn cover differences among stands of different ages were not significant . See Successional status for more details of that study and a discussion of chaparral whitethorn in older stands.
Typically, chaparral whitethorn sprouts [64,81,136] and seedlings [30,39,40,73,136] establish in the first 2 years following fire; after this period, both forms of regeneration decline. However, chaparral whitethorn was listed as one of 4 species that did not occur within 2 years of burning on plots surveyed in central and southern coastal California, despite being present before fire . Sprouts were observed 1 year after fire in manzanita chaparral in the San Jacinto Mountains , 1.5 years after fire in a dense chaparral understory on a Jeffrey pine-California black oak woodland at Cuyamaca Rancho State Park , and 2 years after fire in a Ceanothus-dominated site in southern San Diego County . In mixed chaparral in Madera County, new seedlings were observed as soon as late spring after brush mashing follwed by early spring burning, and as late as the third growing season after early spring or fall burning . No seedlings were recorded 1.5 years after an early December prescribed fire in a dense chaparral understory on a Jeffrey pine-California black oak woodland at Cuyamaca Rancho State Park . For more details of this study see the Research Project Summary.
Following a peak in seedling establishment 1 or 2 years after fire, chaparral whitethorn seedlings throughout their range commonly experience high levels of mortality. Density of seedlings declined during the 15 years after fires north of San Bernardino  (see Table 5, below). In the San Jacinto Mountains in southern California, density on a 2-year-old burn site was 886 seedlings/acre less than on a separate 1-year-old burn site. This indicated an estimated 57% seedling mortality rate in the 2nd postfire year , assuming site differences had negligible influence. Following a fall wildfire in southern California chaparral, 98% of chaparral whitethorn seedlings recorded in March had died by June . Although chaparral whitethorn seedlings experienced high mortality in the first 2 years after brush mashing followed by early spring burning in mixed chaparral in Madera County, density of chaparral whitethorn was more than 4,000 seedlings/acre in the 4th and 5th postfire years . High mortality in these last 2 studies may have been related to competition with grasses  or herbs  (see Site conditions). Following an initial peak in seedling establishment in the first 2 years after brush mashing follwed by fall burning, chaparral whitethorn seedling density was relatively stable at about 6,000 seedlings/acre through the 5th postfire growing season on a site in Madera county . Chaparral whitethorn sprout density in chamise chaparral north of San Bernardino , and in manzanita chaparral in the San Jacinto Mountains , was comparatively stable in the first few postfire years.
|Table 5: Chaparral whitethorn seedling density (average number/milacre) on 3 sites following wildfires |
Time since fire in years
Within 5 years of fire, seedling establishment and production of sprouts apparently slow while established plants grow and cover increases. Seedling establishment peaked the second year after a brush mashing and burning treatment in Madera county and declined the following summer . By the 4th year after a November wildfire in a chaparral community in southern California, the shrub layer was dense and included chaparral whitethorn sprouts. Chaparral whitethorn seedlings were also abundant and comprised a substantial portion of the cover . Four years following an October 2003 high-severity crown fire in the Cuyamaca Mountains in San Diego county, chaparral whitethorn cover averaged 38.7% in a chaparral community, <3% in the other communities, and 4.77% across all burned areas . In the San Jacinto Mountains, chaparral whitethorn cover was greater than 5% by the 5th postfire year ; cover of chaparral whitethorn sprouts and seedlings were <0.2% each in the first 2 postfire years on this site . The pattern of increasing cover with succession is not universal. In mixed chaparral comprised predominantly of chamise, scrub oak, Eastwood manzanita, and chaparral whitethorn, chaparral whitethorn cover was 27% in the 3rd postfire year and 12% in the 4th postfire year on west-facing slopes, and 3% in both the 3rd and 4th postfire year on north-facing slopes .
Chaparral whitethorn may increase in cover and/or frequency in 5- to 15-year-old stands. In a burned fuelbreak in the Santa Ynez Mountains of the central coast of California, shrub recovery was slow in the first 5 years but increased following maturation of chaparral whitethorn seedlings that had established following fire. Total shrub cover, of which chaparral whitethorn was a substantial component, increased from <10% at 5 years after fire to about 25% 7 years after fire. By 9 years after fire shrub cover was nearly 40% . Twelve years after a fire in the mountains north of San Bernardino, chaparral whitethorn cover had increased on 77% of quadrats compared to 3 years before the fire .
In the southern foothills of Sequoia National Park, chaparral whitethorn nutrient levels varied with time since fire. Leaf concentrations of nitrogen and potassium peaked from about 5 to 15 years after fire. Leaf phosphorus, calcium, and lignin concentrations were highest in the oldest age class sampled, >60 years. Magnesium concentrations peaked in intermediate stand ages, and cellulose concentrations varied little with stand age. The association with lower levels of key nutrients in later stages led to the hypothesis that lack of nutrients influences senescence of chamise-dominated stands .
Site conditions: Topography likely influences chaparral whitethorn's postfire response, with available information suggesting good postfire establishment of chaparral whitethorn at intermediate elevations (4,300-5,000 feet (1,310-1,520 m)) [33,57] on west-facing slopes [33,71]. In chamise chaparral in southern California, chaparral whitethorn had the strongest postfire response on the highest elevation site, at 5,000 feet (1,520 m), with sprouts growing quickly and seedlings comprising a substantial portion of the cover . Chaparral whitethorn occurred at low levels (<3% cover) in several community types following a high-severity October wildfire in Cuyamaca Rancho State Park, and it was the dominant species (38.7%) in a mixed chaparral community that occurred at around 4,300 feet (1,310 m) on steep, west-facing slopes with low basal area of conifers . In the 4 years following a fire in mixed chaparral, chaparral whitethorn either did not occur or occurred at <1% cover on south- and east-facing slopes. On north-facing slopes, chaparral whitethorn cover reached 4% in the 2nd postfire year and was 3% in the 3rd and 4th postfire years. On west-facing slopes chaparral whitethorn cover was 1% in the 1st postfire year, 3% in the 2nd postfire year, 27% in the 3rd postfire year, and 12% in the 4th postfire year . On a southern California chaparral site Ceanothus seedlings, including chaparral whitethorn, were abundant and in some instances occurred exclusively on flat areas and gentle slopes where a deep ash layer accumulated and persisted .
Data from 2 southern California study areas suggest that sprouting of chaparral whitethorn may be more prevalent at higher elevations. In a chamise chaparral community, sprouts grew quickly on the highest elevation site, at 5,000 feet (1,520 m). At 2 sites below 3,000 feet (910 m), chaparral whitethorn sprouts did not occur . The proportion of chaparral whitethorn populations comprised of sprouts on sites burned in a November wildfire increased with increasing elevation from a site at 3,300 feet (1,000 m) dominated by chamise to a site at 5,100 feet (1,550 m) codominated by hairy yerba santa (Eriodictyon trichocalyx) and chaparral whitethorn. See Table 6 for density and cover of chaparral whitethorn seedlings and sprouts at 3 elevations .
|Table 6: Density and cover of chaparral whitethorn sprouts and seedlings 7 months after a November wildfire at 3 elevations |
Postfire precipitation is generally considered a major factor in the response of vegetation to fire in chaparral communities  and likely influences chaparral whitethorn seedling establishment. On the west side of the Sierra Nevada, variability in the number of chaparral whitethorn seedlings establishing in the few months after spring burns was attributed to postfire rainfall, with increased rainfall associated with greater establishment . In southern California, dry years caused mortality of a large percentage of chaparral whitethorn seedlings that had established following fire .
Presence of grasses, forbs, and fast growing shrubs may reduce chaparral whitethorn seedling establishment following fire due to competition for resources such as water and sunlight. Postfire seeding with grasses reduced survival of chaparral seedlings including chaparral whitethorn, likely because soil moisture was limited . Competition with grasses was suggested as a cause of higher mortality rates in chaparral whitethorn seedlings that established 2 years after brush amshing and burning compared to those that established in the 1st postfire year. Competition with shrubs also led to higher mortality rates in chaparral whitethorn in this study. Postfire mortality of wedgeleaf ceanothus and chaparral whitethorn combined was higher on plots with Yerba santa (Eriodictyon californicum), which grows more quickly and overtops seedlings of both species, than on plots where Yerba santa was absent . High rates of chaparral whitethorn seedling mortality after fire in southern California chamise chaparral may have been due to competition with herbs .
Fire characteristics: Fire severity influences the response of vegetation to fire in chaparral communities , although severity effects may be shortlived . Chaparral whitethorn likely benefits from fires of any severity. Following wildfires in southern California chaparral in 2003, chaparral whitethorn seedling recruitment was positively correlated (P=0.032, r=0.4) with fire severity, and chaparral whitethorn sprouting was negatively associated with fire severity (P=0.044, r=-0.45) . In the southern Sierra Nevada foothills, chaparral whitethorn frequency was similar on quadrats burned at low (12%) and high (13%) severity . Chaparral whitethorn was widespread  and comprised a substantial component of dense shrub thickets  following high-severity fire in conifer forests of southern California.
Long fire-return intervals (about 50 years or more) and short fire-return intervals (about 15 years or less) may be detrimental to chaparral whitethorn. Long fire-return intervals would reduce exposure to heat that stimulates germination (see Fire adaptations) and result in increased stand age. Chaparral whitethorn is most common in young communities (see Successional status and Time since fire). However, fire-return intervals less than about 10 to 20 years would likely result in chaparral whitethorn decline , especially in areas where chaparral whitethorn does not sprout [8,39]. Direct mortality would likely be greater, since a fire that only top-kills a sprouting individual would kill a nonsprouting individual . Also, chaparral whitethorn seedlings are less likely to reach reproductive age if the fire-return interval is short (see Seed production). A seed source is critical to recovery of nonsprouting Ceanothus, and frequent fires in these communities may eliminate them from a site [8,39,141]. A single fire may result in decline of nonsprouting Ceanothus when or where seed production is low . Keeley and Zedler  suggest that sprouting chaparral species generally perform well on sites that burn frequently, and obligate seeders perform better on sites with return intervals of ≥100 years. Given the lower rates or absence of sprouting, frequent fire may reduce chaparral whitethorn populations in the northern portion of its range [11,121,122,133], and perhaps at low elevations  or on desert exposures  in the southern part of its range.
Fall fires may result in greater spring seedling establishment than fires in other seasons [8,40]. For instance, seedling establishment after brush mashing followed by fall burns was much higher than after early spring burns on the west side of the Sierra Nevada in mixed chaparral . Burning in late spring following mashing resulted in <2,000 seedlings/acre. Early spring burning after mashing resulted in over 6,000 seedlings/acre later that spring, and about 20,000 seedlings/acre the following spring. Many of those seedlings died the following summer, and about 4,000 seedlings/acre remained by the 4th and 5th growing seasons. When mashing was followed by fall burning, about 35,000 seedlings/acre established the following spring, and about 6,000/acre remained by the 4th and 5th growing seasons after burning. Increased germination after fall burning is likely because the conditions necessary for germination occur in the right order and at a time of year when moisture is less limiting. Because chaparral whitethorn sprouting was limited in this area, fall burning was recommended to maximize seedling establishment of chaparral whitethorn and nonsprouting species . In woodland-grass chaparral in the western Sierra Nevada, a community where chaparral whitethorn is an important component, brush species sprouted and seedlings established in the spring after fall and winter burning. Brush species sprouted in the late spring and summer after late spring fires, but seedlings did not establish until the following spring .
Generally, sprouts of chaparral species have greater survival following late summer or fall fires than spring or early summer fires, because rapid growth during the spring and early summer deplete carbohydrate reserves .
FUELS AND FIRE REGIMES:
Fuels: Chaparral stands are typically quite flammable due to growth form, proportion of dead wood in fuel loads, and high content of resin, oil, wax and other volatile products in chaparral shrubs [84,117]. Chaparral shrubs tend form expansive acreages of dense shrub growth with interlocking crowns . These factors contribute to the large, fast-moving fires typical of chaparral .
Measured fuel characteristics of chaparral whitethorn describe leaf area, leaf energy content, and proportion of dead wood. Maximum total shrub dry biomass among 9 chaparral whitethorn shrubs measured was 50 pounds (22.7 kg), and dry biomass averaged 22% dead wood . Chaparral whitethorn leaf area in a California chaparral community was 1.4 cm² and leaf density was 0.012 g/cm² (Fischbeck and Kummerow 1977 cited in ). The specific leaf area of chaparral whitethorn was 8.22 mm²/mg on a wedgeleaf ceanothus-chaparral whitethorn site . In a test of 8 California chaparral species, chaparral whitethorn leaf energy content was 20.16 kJ/g, the 3rd lowest value .
Litter accumulation of chaparral whitethorn has been addressed in 2 studies [75,95]. Annual litter accumulation, measured in a chaparral whitethorn community from postfire years 10 through 14, averaged 340 kg/acre and ranged from 280 to 570 kg/acre. Since the stand changed in size and density as it aged, the author warns that the values should not be extrapolated or generalized . Chaparral whitethorn in California chaparral had generally low litter fall over a 4-year period, with a peak of 150 g/m² and the next highest value less than 100 g/m². Peak litter fall occurred in late summer and fall (Mooney et al 1977b cited in ).
On the San Dimas Experimental Forest, dry weight and depth of organic surface layers in chaparral types dominated or codominated by chaparral whitethorn 11 years after a stand-replacing wildfire were less than those in other chaparral types in the same area. Mean dry weight of surface organic layers (litter and duff combined) ranged from 200 kg/acre in chaparral whitethorn-eastwood manzanita stands to 2,300 kg/acre in chamise-chaparral whitethorn stands. Surface organic layers in pure stands of chaparral whitethorn had a mean dry weight of 1,100 kg/acre. Depth of organic surface layers in these communities ranged from 0.1 inch (0.3 cm) where chaparral whitethorn was dominant to 0.2 inch (0.5 cm) where it was codominant. Cover of surface organic layers on the site 11 years after fire was 85% and depth was 66% of prefire levels . Organic matter accumulates as stands age. In a canyon live oak-bigberry manzanita-chaparral whitethorn community in southern California that had not been burned in 55 years, dry weight of litter was 15,060 kg/acres and the average litter depth was 0.6 inch (1.5 cm) (Kittredge 1939 cited in ). More details of recovery of the forest floor in several types of chaparral communities, including those dominated by chaparral whitethorn, are described by Kittredge .
Dead wood may comprise a substantial proportion of fuel loads in chaparral communities, especially in older stands. For example, as chamise-Ceanothus stands age, total fuels decline and dead fuels increase due to Ceanothus senescence . In chamise chaparral in the San Dimas Experimental Forest that had not burned in 18 years, there was 379.9 g/m² of standing dead stems, comprised mainly of chaparral whitethorn . Vogl  suggested that the abundance of senescent chaparral whitethorn in a Coulter pine stand may result in severe fires. Due to typically low dead fuel accumulations in an approximately 35-year-old mixed chaparral community in southern California consisting primarily of scrub oak (Quercus dumosa), chamise, desert ceanothus, and chaparral whitethorn, prescribed fires had low flame heights and poor rates of spread despite relatively hot prescription conditions (see Fire Management Considerations) . For general estimates of live and dead fuel loads and surface-to-volume ratios of fuels in chaparral vegetation, see Rundel .
In areas burned 3 times in 10 years in a woodland-grass chaparral community where chaparral whitethorn was important, sites that were heavily grazed before the 2nd and 3rd fires had less fuel, which resulted in unburned areas .
Fire regimes: Chaparral whitethorn tolerates a range of fire regime characteristics reflected by its occurrence in a variety of communities such as chaparral, oak woodland, conifer forest, and riparian (see Plant Communities). Chaparral whitethorn may be most abundant when fire-return intervals are about 20 to 30 years  (see Successional status and Time since fire). If fire-return intervals are shorter than the time it takes to reach reproductive maturity, chaparral whitethorn may be negatively impacted [8,39,120], especially in chaparral whitethorn populations with limited or no sprouting ability [8,39]. Depending on the site and plant community characteristics, fire-intervals of more than about 40 years may result in chaparral whitethorn senescence and eventual elimination from a stand . On some sites extended periods without fire would likely result in conversion from chaparral whitethorn communities to communities such as such as oak or pine woodland [89,139] (see Successional Status and Fire characteristics). Conversely, Vogl  suggests that the abundant senescent chaparral whitethorn in Coulter pine stands may result in severe fires that could convert these communities into shrublands . Despite senescence or even absence from older stands, chaparral whitethorn may establish from the soil seed bank when these areas burn (see Germination and seedling establishment).
Fire regimes in communities where chaparral whitethorn occurs range from stand-replacement fires [66,133,139] in chaparral communities to low-severity surface fires in oak woodlands and conifer communities, such as Jeffrey pine forests (see the Fire Regime Table). Fires in chaparral occur in the summer and fall [36,66,133] and are largest in fall because Santa Ana winds increase fire spread . Lightning was the primary source of historical ignitions in chaparral , with human-caused ignitions becoming more common over the past several decades [46,66,70,118]. Estimates of fire-return intervals for surface fires in oak savanna and conifer forests range from 10 to 30 years (see the Fire Regime Table), while fire-return intervals in chaparral generally range from about 30 to 100 years [36,70,118,133]. The average fire-return interval across chaparral types and ownerships in central and southern coastal California was 50 years and ranged up to 100 years . In chaparral communities from San Diego to Ventura counties, fire-return intervals since 1950 ranged from 29 to 38 years . The majority of chaparral in the Sierra Nevada bioregion was grouped into a 40- to 60-year fire-return interval category . Rundel  stressed the variability of fire-return intervals, noting that chaparral vegetation can carry a fire within a few postfire years and may also remain unburned for 100 years. For example, in National Forests of southern and central coastal California, coastal transition communities have average fire-return intervals of 39 years, while fire-return intervals in scrub oak chaparral outside of National Forests in this region average 83 years. Chaparral whitethorn occurred on an average of 3% to 5% of plots in these communities . See the Fire Regime Table for further information on fire regimes in vegetation communities in which chaparral whitethorn may occur.
FIRE MANAGEMENT CONSIDERATIONS:
Most chaparral whitethorn populations would likely benefit from fire of any severity [68,112] at intervals of about 20 to 40 years [8,39,47,89]. Although sprouting and seedling establishment occur after spring, summer, or fall fires, seedling establishment may be greatest after fall fires  (see Fire characteristics). Chaparral whitethorn recovery time (see Time since fire) and fire-return intervals that allow for chaparral whitethorn persistence (see Fire Regimes) likely vary with plant community , topography [33,57,71,72] (see Site conditions), and proportion of the population that sprouts [8,39]. In areas where chaparral whitethorn does not sprout following fire, short fire-return intervals would likely result in declines in chaparral whitethorn. Long fire-return intervals (greater than about 40-60 years) are also likely to result in the loss of chaparral whitethorn from the shrub layer, since germination and sprouting are stimulated by fire, and chaparral whitethorn senesces after about 40 years (see Successional Status).
Fire prescription information is available for various chaparral communities. A prescription for manzanita-Ceanothus chaparral based on the 1978 National Fire Danger Rating System recommended air temperatures from 30 to 75 °F (–1-24 °C) and relative humidity from 30% to 65% for spring headfires, air temperatures from 40 to 90 °F (4-32 °C) and relative humidity from 20 to 40% for fall backing fires, and windspeeds 10 mph or less for both . Summer and fall prescribed fires in a chaparral community comprised of scrub oak, chamise, desert ceanothus, and chaparral whitethorn were conducted when live fuel moisture in scrub oak ranged from 78% to 130%, northeast-southeast winds were less than 13 km/h, relative humidity ranged from 21% to 42% and air temperature ranged from to 54 to 86 °F (12-30 °C) . In a Jeffrey pine-California black oak woodland with a dense chaparral understory, a prescribed fire intended to reintroduce fire into Cuyamaca Rancho State Park and determine impacts of prescribed burning on the local vegetation was conducted in December when air temperatures ranged from 64 to 75 °F (18-24 °C), relative humidity from 18% to 25%, fuel moistures from 6% to 8%, and wind speeds were less than 6.4 km/h . Green  provides recommendations for burning in chaparral including those regarding topography, season, time of day, live and dead fuel moistures, fuel volume and continuity, chemical composition of fuel, and weather conditions for prescriptions including wind, relative humidity, and air temperature. Information regarding ignition methods and smoke management for prescribed burning in chaparral are also included .
Although it has been suggested that fairly frequent fires are needed in chaparral to reduce the risk of large, high-severity wildfire that result from fuel accumulation [2,10,17,87,90,102], there is evidence that flammability and fire severity do not increase with stand age [46,66,69,70,118] and that fire has not been successfully excluded from chaparral [46,66,70]. Based on differences in fire regime and fire characteristics between northern Mexico and southern California, Minnich  concluded that fire exclusion was resulting in larger, more severe fires. Stand age was a significant predictor of fire severity based on data from several southern California chaparral sites burned in 2003 . In contrast, fuel accumulation and resulting fire severity were not related to the age of chaparral in other studies of California chaparral [46,66,69], and chaparral stands can carry fire within a few years of fire [46,118]. In southern and central California, chaparral stands 11 to 20 years old represented significantly (P<0.05) more of the area burned (38%) than older age classes . In addition, some data suggest fire suppression efforts have not resulted in longer fire-return intervals or less frequent fires that would result in increased fuel accumulation [46,66,70]. For instance, since 1910 the number of fires per decade has increased (P<0.01) in southern and central California, and in 8 of 9 counties fire-return intervals since 1950 are shorter than those before 1950 .
Firebreaks have been constructed in chaparral communities to decrease fuel uniformity  and increase the possibility of successful suppression [43,94], especially in the wildland-urban interface . The effectiveness of fuelbreaks likely depends on many factors including fuelbreak design, consistent maintenance, surrounding fuel loads, and fire characteristics . It is generally accepted that even large fuelbreaks cannot contain chaparral fires during severe weather conditions [43,140], specifically those that cause long-range spotting [43,94].
Erosion is a major concern following fires in chaparral communities. Information is available on general effects of fire on soil  and soil erosion [10,26,27,124] in chaparral communities. Suppression activities such as road building and creation of fuelbreaks may cause or exacerbate erosion . Seeding with grasses to prevent postfire erosion may reduce seedling survival of chaparral whitethorn  and chaparral shrubs in general .
FEDERAL LEGAL STATUS:
Information on state- and province-level protection status of plants in the United States and Canada is available at NatureServe.
IMPORTANCE TO WILDLIFE AND LIVESTOCK:
Chaparral whitethorn is not often used by livestock but may be important to deer and bighorn sheep, especially young plants and new shoots. For the wildlife value of California chaparral-mountain shrub communities that may include chaparral whitethorn, see Garrison and others . Some chaparral whitethorn communities provide cover for bighorn sheep (Holl and others 1980 cited in ).
Palatability and nutritional value: Chaparral whitethorn provides better quality browse for domestic sheep, domestic goats, deer, and bighorn sheep than for cattle. In a guide to California range plants, chaparral whitethorn was ranked as excellent to good quality forage for deer, good to fair for domestic sheep and goats, poor to useless for cattle, and useless for horses . In the western Sierra Nevada, deer browsed chaparral whitethorn extensively [39,40], with utilization ranging from 10% to 60% . Conrad  notes that deer and bighorn sheep prefer chaparral whitethorn in open stands. In southern California, goats showed intermediate preference for chaparral whitethorn on sites burned 1 or 5 years previously . In conifer plantations in the Tahoe National Forest, chaparral whitethorn was used to some extent by sheep but was considered a less desirable browse species than deer brush . Small mammals, birds, and insects eat chaparral whitethorn fruit .
Young growth of chaparral whitethorn is preferred. Sprouts are more palatable than previous years' growth . In conifer plantations on the Tahoe National Forest, sheep had "fair utilization" of chaparral whitethorn early in the growing season, but utilization declined to almost none late in the season . In the western Sierra Nevada, chaparral whitethorn seedlings were highly preferred by deer .
Young leaves have higher crude protein levels than mature leaves. Crude protein levels in chaparral whitethorn leaves were 21.2% of dry weight in April and 13.2% in August . On the San Joaquin Experimental Range, crude protein levels were 16% in young leaves and 11% in fully developed leaves. The calcium to phosphorus ratio was about 11:1 in fully developed chaparral whitethorn leaves, and crude fiber levels ranged from 6.5% to 9.5% throughout the year .
In the western Sierra Nevada, chaparral whitethorn tolerated heavy deer browsing over multiple seasons. Chaparral whitethorn was able to grow slowly but steadily under continuous browsing. Despite heavy use, chaparral whitethorn reached mature size. However, continuous heavy browsing for more than 2 to 3 years may cause extensive mortality . Chaparral whitethorn browse yield on a site burned in early spring was 360 pounds/acre after the 5th growing season .
Cover value: Chaparral whitethorn communities provided cover for bighorn sheep on some sites in southern California. About 70% of bighorn sheep observations occurred in chaparral whitethorn-mountain mahogany-chamise stands on 50% to 90% slopes with south and southeast aspects (Holl and others 1980 cited in ).
VALUE FOR REHABILITATION OF DISTURBED SITES:
Chaparral whitethorn has been mentioned as having some potential for erosion control. Horton  noted that it may be established by direct seeding on road slopes and disturbed areas above 4,000 feet (1,200 m). However, it had not been fully tested as an erosion control species , and, although deep, its roots are less branched than those of some other chaparral species . Kittredge  states the shallow litter layer in chaparral whitethorn communities would provide less protection from rain and more losses from evapotranspiration than the more developed litter layers of other plant communities.
Preliminary research suggests that chaparral whitethorn has some potential as a species that could be harvested and burned to produce energy. Chaparral whitethorn exhibited rapid growth in the first growing season following harvesting in June . In a study of the biomass potential of several species, chaparral whitethorn had the highest potential photosynthesis rate (7.6 mg carbon dioxide/gram dry weight/hour) of the 8 California chaparral species tested .
The fruits of chaparral whitethorn and blossoms of some Ceanothus species will form lather when rubbed in water .
OTHER MANAGEMENT CONSIDERATIONS:
Control of chaparral whitethorn on rangelands has been attempted using herbicides  and burning followed by seeding of preferred grasses . Foliar-applied herbicide was especially effective for killing sprouts. See Vallentine  for details. Chaparral shrub seedling survival, including that of chaparral whitethorn, was reduced by seeding with grasses after burning, but only when seeding was done within a year of burning or clearing . The ability of the target chaparral whitethorn population to sprout influences the options available for control . Fire is most effective as a control method for chaparral shrubs that do not sprout. Repeated burning is necessary to kill seedlings  (see Fire management considerations).
|Fire regime information on vegetation communities in which chaparral whitethorn 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
|California oak woodlands||Replacement||8%||120|
|Surface or low||91%||10|
|Surface or low||78%||13|
|Surface or low||74%||30|
|Mixed evergreen-bigcone Douglas-fir (southern coastal)||Replacement||29%||250|
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 [50,79].
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