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Whitethorn ceanothus is not an indicator species for any specific habitat types, but is listed as a "moderate site indicator" in forested communities of the upper montane in the southern and central Sierra Nevada [79].
Other overstory species commonly associated with whitethorn ceanothus are California black oak (Quercus kelloggii), bitter cherry (Prunus emarginata), and Brewer oak (Q. garryana var. breweri).
Common understory associates of whitethorn ceanothus include: huckleberry oak (Q. vacciniifolia), bush chinquapin (Chrysolepis sempervirens), pinemat manzanita (Arctostaphylos nevadensis), greenleaf manzanita (A. patula), creeping snowberry (Symphoricarpos mollis), redflower currant (Ribes sanguineum), Sierra gooseberry (R. roezlii), California hazel (Corylus cornuta var. californica), deerbrush (Ceanothus integerrimus), prostrate ceanothus (C. prostratus), snowbrush ceanothus (C. velutinus), Sierra mountain misery (Chamaebatia foliolosa), salal (Gaultheria shallon), Pacific rhododendron (Rhododendron macrophyllum), and Saskatoon serviceberry (Amelanchier alnifolia) [21,24,52,60,63,69,98].
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Whitethorn ceanothus is a native, evergreen shrub. It has a low-lying, flat-topped growth habit, especially at higher elevations, and generally reaches heights between 2 and 5 feet (0.6-1.5 m). Whitethorn ceanothus can form a continuous ground cover up to 12 feet (3.7 m) in diameter. The many short, rigid, intricately-branched stalks end in sharp spines. The numerous leaves are alternate and distinctly 3-ribbed from the base. They are small, elliptic to egg-shaped, and blunt at the tip. The leaf margins are entire or finely toothed. Flowers are in dense clusters borne on a panicle-like inflorescence. The fruit is a capsule that is slightly crested or horned and somewhat sticky at maturity. It is small, tipped by a threadlike 3-forked appendage, and 3-lobed. Each lobe has a ridge on its back and is 3-celled with each cell bearing 1 hard, round seed or nutlet. [13,25,26,47,73,74, 78,98]
Whitethorn ceanothus has high nitrogen concentrations in its foliage and soil beneath the plant due
to the nitrogen-fixing bacteria Frankia spp., suggesting that it may enhance nitrogen availability
for the surrounding area [75,76].
RAUNKIAER [83] LIFE FORM:
Phanerophyte
REGENERATION PROCESSES:
Whitethorn ceanothus regenerates by seed and vegetative means. Germination from seeds stored in
the soil is the primary mode of reproduction for whitethorn ceanothus and generally takes place
after fire [40,54]. Prolific sprouting occurs from a lignotuber
when damage is done to the top of the plant [40,50]. Whitethorn ceanothus is an obligate sprouter
in moderate fuel-consumption burns and a facultative seeder following high fuel-consumption burns [53].
Pollination: Whitethorn ceanothus is an insect-pollinated species [67].
Breeding system: Outcrossing is the most common breeding system in Ceanothus spp., with limited autogamy [67].
Seed production: Ceanothus spp. are prolific seed producers. Thousands of viable seeds can be produced per hectare and remain dormant in the soil and duff layer until disturbance stimulates germination [38,48]. At approximately 4 years of age, whitethorn ceanothus is capable of seed production and reaches a maximum seed load of approximately 4,500 seeds per plant by the age of 20 to 25. Seed production remains high until the plant is senescent, around 40 years of age, or where the plants are inhibited by shade [18].
Seed dispersal: Whitethorn ceanothus seeds are autochorus, meaning that the main mode of seed dispersal is by the plant itself [55]. The seeds are small and tend to stay where they fall, unless they are carried off by birds, rodents, or ants [22,67]. Ceanothus seeds can also be forcibly ejected from the seed pods when they ripen [22,55].
Seed banking: Because of the large number of seeds produced by whitethorn ceanothus, substantial seed reservoirs exist in the soil and duff layer. The number of seeds in the soil can be expected to vary from year to year depending on the amount of seeds produced and seeds utilized by consumers [22]. Keeley [55] notes a relationship between the seed bank size and stand age where whitethorn ceanothus is found; the older the stand the larger the seed bank will be. Seed quantities of whitethorn ceanothus combined with littleleaf ceanothus (Ceanothus parvifolius) in over-mature mixed coniferous forests can number up to 1.9 million/acre [68].
Germination: Whitethorn ceanothus is heavily dependent upon fire for germination [11]. The seeds of whitethorn ceanothus are refractory: they require a heat stimulus alone or in conjunction with other conditions such as cold stratification [55]. Fire, or similar heat treatment, is necessary to crack the seed coat to allow for absorption of water and subsequent germination. Maximum germination occurs with the sequence of fire and a natural stratification period of 10 to 16 weeks [10,17,38,55,80,82]. Whitethorn ceanothus was observed to be germinating profusely on logged areas, perhaps because of heat from insolation [86].
Fire aids in the germination of seeds but can also destroy them. High intensity or high frequency fires can be detrimental to the seed bank and can result in lower rates of germination [22,68,80,82]. Overall, fire appears to be beneficial in promoting seed numbers, germination, and seedling population [52,68].
The initiation of germination depends on the following: how long the seeds had been dormant; length of time since the last soil disturbance; heat of previous fires; amount of moisture in the soil; persistence of the winter snow pack; soil temperatures; sun exposure; and how deeply the seed was buried [18].
Seedling establishment/growth: Gratkowski [37] notes that whitethorn ceanothus seedling establishment is much higher in burned areas than in nonburned areas, where it is almost nonexistent. Biswell [11] states that seedlings are seldom seen except where there has been fire. Nevertheless, the mortality of Ceanothus spp. seedlings is high during the first 2 postfire years [22]. Environmental factors that affect the growth and survival of these seedlings include insects and other animal browsing, damping-off fungus, soil moisture, exposure, and soil temperature. The first 2 weeks after emergence are the most critical for survival [18].
Asexual regeneration:
Layering is common in whitethorn ceanothus at higher elevations
where the weight of snow forces branches and stems down to the ground [61]. Stump-sprouting from a
lignotuber is also common after the top of the shrub is damaged
[40,50].
SITE CHARACTERISTICS:
Whitethorn ceanothus grows best in open situations [10,17]. Dry, open flats, pine forests, rocky ridges, and
washes that have well-drained soils are common sites where whitethorn ceanothus is found [23,47,52,74,98]. Areas
that have been burned or logged are very favorable for populations of whitethorn ceanothus [2,27,71]. It can also
survive and thrives in the shade of coniferous timber [98].
Whitethorn ceanothus is known to occur between 2,100 to 11,000 feet (650-3,350 m) in elevation [7,14,23,47,52,74,
102]
SUCCESSIONAL STATUS:
Whitethorn ceanothus can be classified as both an early- and late-successional species. It is considered
a pioneer species most commonly associated with early successional stages because of its ability to germinate
from seed and sprout after disturbance [15,81]. Since whitethorn ceanothus is one of the 1st plants to become
established on denuded soils, it has immediate value in soil protection and later serves as a nurse crop for
coniferous species [98]. It is possible that whitethorn ceanothus plays an important role in succession by
providing a more favorable microclimate of nutrient rich microsites in otherwise harsh growing conditions [76].
In normal seral conditions whitethorn ceanothus is overtopped and killed by the conifer species that become established in their shade. The brushfields can, however, significantly hinder conifer regeneration and slow the rate of forest succession. If there is repeated fire, whitethorn ceanothus brushfields can become semipermanent communities [31]. Where fire is lacking, it does not establish and populations decrease, and in some cases are eliminated, in the shade of dense tree canopies [10].
A study by Fernau and others [30] along the west face of the central Sierra Nevada classified whitethorn
ceanothus as a late-successional species, with populations peaking 26-32 years after disturbance. Findings
published by Minnich [70] also provide evidence that it is capable of existing in later successional stages.
He states that chronosequences of postsuccession show the establishment of seedlings of whitethorn ceanothus
without fire, apparently from seed caches.
SEASONAL DEVELOPMENT:
Seasonal development of whitethorn ceanothus begins with emergence and extends through a growing season
ranging from 75 to 95 days depending on emergence date and elevation at which it is found. Germination
of seedlings begins in late May and June, and they are usually well established by 4 weeks of age. The
seedlings were observed to grow at a rate of 0.2 inches (0.5 cm) per week until late August when they began
to go dormant. After the 1st growing season whitethorn ceanothus grow very rapidly. In mature whitethorn
ceanothus new growth and flower bud swelling occur at the same time, ranging from late May to mid-June
depending on elevation. Blossoming is expected about 10 days later and continues for approximately 23 days
[18].
Whitethorn ceanothus flowers between May and August dependent on where location [23,52,74,78]. Seed maturity is reached in the late summer, generally in August or September.
Fire regimes: Whitethorn ceanothus occurs in a mixed fire regime. The presettlement fire regime for communities where whitethorn ceanothus occurs consisted of frequent, low-severity burns and larger stand-replacing fires. Postsettlement, this trend has changed and produced more lethal fires with higher severities and longer intervals [16]. The fire return intervals in the montane chaparral, where whitethorn ceanothus most often occurs, are "probably quite variable" due to the influence of poor growing conditions [88]. Fire in the subalpine forest habitats where whitethorn ceanothus occurs is infrequent due to limited productivity and fuel accumulation in association with short growing season and heavy snow cover [70].
The following table provides fire return intervals for plant communities and ecosystems where whitethorn ceanothus is important. Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".
| Community or ecosystem | Dominant species | Fire return interval range (years) |
| California montane chaparral | Ceanothus and/or Arctostaphylos spp. | 50-100 |
| pinyon-juniper | Pinus-Juniperus spp. | <35 [77] |
| Sierra lodgepole pine* | Pinus contorta var. murrayana | 35-200 |
| Jeffrey pine | Pinus jeffreyi | 5-30 |
| Pacific ponderosa pine* | Pinus ponderosa var. ponderosa | 1-47 [4] |
| interior ponderosa pine* | Pinus ponderosa var. scopulorum | 2-30 [4,5,64] |
| coastal Douglas-fir* | Pseudotsuga menziesii var. menziesii | 40-240 [4,72,84] |
| California mixed evergreen | Pseudotsuga menziesii var. menziesii-Lithocarpus densiflorus-Arbutus menziesii | <35 |
| canyon live oak | Quercus chrysolepis | <35 to 200 [4] |
| California black oak | Quercus kelloggii | 5-30 [77] |
Fire removes most of the above ground whitethorn ceanothus biomass, leaving nitrogen-rich, bare mineral soil. Following fire, patches of whitethorn ceanothus maintain higher amounts of available nitrogen for up to 6 months on both low- and high-intensity burn plots. If these areas continue to have more available nitrogen than other patch types, this long-term effect may enhance tree seedling growth [76].
The Research Project Summary Plant response to prescribed burning with varying season, weather, and fuel moisture in mixed-conifer forests of California provides information on prescribed fire and postfire response of many plant community species including whitethorn ceanothus.
FIRE MANAGEMENT CONSIDERATIONS:The evolution of whitethorn ceanothus and fire together suggests that the use of prescribed burning is a viable management option in areas to promote whitethorn ceanothus. The use of fire to promote whitethorn ceanothus is suitable because of the fire-stimulated germination of seeds and stump-sprouting in moderate fuel consumption burns. The species adds nitrogen to the soil, possibly enhancing conifer regeneration.
Conversely, overstocked or well-established stands of whitethorn ceanothus add significantly to fuel loading. Extensive stands of whitethorn ceanothus and montane chaparral patches exhibited the greatest mean fire spread/minute and the greatest flame lengths compared to forested types [101]. Snyder [91] notes that during succession, brush cover is still significant even as the basal area of trees doubles. If these populations persist as noted they could act as ladder fuels, increasing the possibility of a stand replacing fire.
Fuel loading becomes an important factor to recognize when using prescribed fire as a land management tool. The changing fire regime in communities where whitethorn ceanothus becomes established, to infrequent, high-severity fires [16], suggests that using fire to manage whitethorn ceanothus should be done with caution.
Palatability/nutritional value: Whitethorn ceanothus is an important browse species for deer because of its high crude protein content and palatability [9,22,25,49]. It does not, however, provide sufficient nutrients for optimum growth and development in mule deer and is more valuable when consumed along with other species [58]. It is considered as a poor to fair browse species for livestock such as cattle, domestic sheep, and goats [22,25,73].
Cover value:
No information is available on this topic.
VALUE FOR REHABILITATION OF DISTURBED SITES:
Whitethorn ceanothus has considerable value when used for site amelioration. It is used for
erosion control on slopes, terraces, and steep banks [78,89,90]. Whitethorn ceanothus is very
important for rehabilitation because of nitrogen-fixing and other soil-building attributes. It
is tolerant of severe sites and can play a soil building role on these sites [22,49]. It has
successfully been used for these purposes in the Lake Tahoe Basin and Lost Canyon, California
[15,19,89,90].
A list of available/potential seed sources can be acquired in the USDA, NRCS Commercial
Sources of Conservation Plant Material, available online [99].
OTHER USES:
No information is available on this topic.
OTHER MANAGEMENT CONSIDERATIONS:
There a few main points to consider when managing whitethorn ceanothus. It increases with
disturbance and forms very dense, difficult to penetrate brushfields. It is regarded as a
very strong competitor that can hinder tree seedling growth and subsequent conifer regeneration
[22,57,94]. Whitethorn ceanothus may compete with tree seedlings, but it also provides ongoing
nitrogen that may improve the site for conifer establishment and growth after establishment.
Soil nitrogen, however, probably becomes most important for tree seedlings after they have
cleared the initial hurdle of establishment [49,76]. In the past, herbicide treatments and
allowing livestock grazing in whitethorn ceanothus brushfields were used as management options
to open the dense shrub canopy. This increased the amount of light reaching the young conifers in
the understory and decreased brush competition for soil moisture and nutrients. Initial applications
of herbicide resulted in 100% top kill and 20% whitethorn ceanothus mortality with plant mortality
reaching 90% after the 3rd application [35,39,40,43,92,103]. A study done by Hobbs and Radosevich
[48] concluded that the control of evergreen woody species such as whitethorn ceanothus in young
conifer plantations improved tree seedling growth and in some cases survival.
Conversely, whitethorn ceanothus is valuable as a browse species and if populations decrease, the amount of available forage for wildlife and livestock is also decreased [57]. Prescribed burns are often used in land management to encourage the establishment of whitethorn ceanothus because of its importance as a browse species [27]. It is also beneficial as a nurse plant for conifer and Sierra gooseberry seedlings, providing shade and available nitrogen [81].
Tappeiner [95] reported the average size of open-grown whitethorn ceanothus plants on ungrazed, 9-year-old tree plantations in the Sierra Nevada and Klamath Mountains. Average crown diameter was 5.7 feet (1.7 m) and average height was 3.2 feet (.98 m). This information may be useful in predicting shrub development.
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