|Listed||April 1, 1998|
|Description||Upright, non-burl-forming shrub with rough gray or reddish bark; bristly twigs; and white flowers.|
|Habitat||East Bay Hills on north and east facing slopes where bare, siliceous, mesic soils with low fertility exist.|
|Threats||Compositional and structural changes due to fire suppression methods that result in shading and competition from native and alien plant species, disease, the ongoing effects of habitat fragmentation resulting from past urbanization.|
Pallid manzanita, Arctostaphylos pallida, is an upright, non-burl-forming shrub in the heath family (Ericaceae). The species grows from 6.5 to 13.0 ft (2-4 m) high with rough, gray, or reddish bark. The twigs are bristly. The ovate to triangular leaves are bristly, strongly overlapping, and clasping; they are 1.0-1.8 in (2.5-4.5 cm) long and 0.8-1.2 in (2-3 cm) wide. The dense, white flowers are urn-shaped and 0.2-0.3 in (0.5-0.8 cm) long. The flowering period is from December to March.
Pallid manzanita seems to prefer to grow in limited locations of the East Bay Hills on north and east facing slopes where bare, siliceous, mesic soils with low fertility exist. The species is found from 656-1,460 ft (200-445 m) elevation, primarily on thin soils composed of chert and shale. The plants are generally found in Arctostaphylos -dominated chaparral that is often surrounded by oak woodlands and coastal shrub. The two largest occurrences, occupying a total area of 29 acres (11.7 hectares), are found in maritime chaparral, a habitat with mesic environmental conditions due to a maritime influence. The smaller population at Sobrante Ridge has the least human impact of all known populations. It had an estimated 1,700-2,000 plants in the mid-1980s, and the status and vigor of the plants appeared good. The population remains in good shape and, although some management is needed, the potential for long-term viability is high. The Sobrante Ridge site has more open space than other occurrences and recruitment of pallid manzanita is taking place in areas with bare and exposed gravel.
Pallid manzanita is found only in the northern Diablo Range, a part of the inner South Coast Range of California. The Diablo Range extends in a northwest to southeast direction as a more or less continuous mountain chain, 20-30 mi (32-48 km) wide, for approximately 190 mi (306 km) from San Pablo Bay in central California to Polonio Pass in northeast San Luis Obispo County. The altitude of the Diablo Range varies from 2,000 to 4,200 ft (610 to 1,280m) and is broken by five east to west passes. These passes divide the Diablo Range into several distinct units: Contra Costa Hills, Mt. Diablo, Mt. Hamilton Range, Panoche Hills, San Carlos Range, and Estrella Hills. Pallid manzanita occurs in the Contra Costa Hills section of the Diablo Range.
The overall current range of pallid manzanita is similar to that known at the time the species was first described. The extant populations of this species are thought to be smaller, however, due to habitat destruction and fragmentation by urbanization. Although pallid manzanita occupies most of its historic range, local habitat destruction due to residential development has resulted in losses of up to 50% in some locations along Manzanita Way in the Oakland Hills. Only two large populations are known, one at Huckleberry Ridge, the presumed type locality in Alameda and Contra Costa Counties, and the other at Sobrante Ridge in Contra Costa County. The remaining occurrences, all located in Alameda or Contra Costa Counties, are all small, and most have fewer than 10 individuals. Of the 13 documented occurrences of pallid manzanita, six are considered to be declining, while the trend of the remaining seven is uncertain or unknown. One of the latter populations has fewer than 50 plants and was planted outside of its native habitat, where its long-term survival is not likely. Two other occurrences are considered to have been planted.
The largest known population of pallid manzanita occurs at Huckleberry Ridge, although an estimated 50% of the original habitat at this site has either been developed for housing or is privately owned. Development eliminated a large number of pallid manzanita plants and fragmented the remaining habitat at this site. An estimated 2,400-2,700 plants were present in this population during the mid 1980s. A fungal infection during the early 1980s resulted in branch and stem dieback in over 50% of the plants at Huckleberry Ridge, and the condition of the population was described as poor. Many of the smaller populations occur in coastal scrub. These occurrences of pallid manzanita are all small with few individuals and their long-term viability is questionable. The largest is estimated to have 65 individuals, some of which were planted. Several other occurrences were also planted, and many small populations are located along roadcuts where plants appear to have established naturally after the soil was disturbed. Some of these occurrences have only one or several individuals and are in poor condition. Many of these smaller populations are shaded by planted and naturalized Pinus radiata and Cupressus spp. (cypresses).
More than half of the remaining habitat for the species, including both large populations and numerous smaller populations, occur on lands owned by the East Bay Regional Park District. Other small populations occur on lands owned by the East Bay Municipal Utility District, the City of Oakland, Pacific Gas and Electric power line easements, or on other privately owned lands.
The main threats to all populations of pallid manzanita are compositional and structural changes due to fire suppression methods that result in shading and competition from native and alien plant species, disease, the ongoing effects of habitat fragmentation resulting from past urbanization, and chance events (due to the small size of the few remaining populations). Some populations of the species are threatened, to a lesser extent, by herbicide spraying and hybridization. Furthermore, the existing regulatory mechanisms do not provide adequate protection.
The long-term viability of pallid manzanita is in doubt because of past and present fire suppression policies and inactive or ineffective fire management plans. In the 1800s, before the expansion of urban areas into the East Bay Hills, major natural or human-caused fires driven by dry "Diablo Winds" periodically burned through manzanita habitat during the late summer and fall. Since these fires rarely threatened the lower lying communities of Berkeley and Oakland, they were allowed to rage unchecked. These occasional fires, though initially destructive, had positive long-term benefits for pallid manzanita, as described below. Fire management practices changed in the period from about 1900 to 1940 as human safety became an issue; eventually unrestricted burning was no longer allowed. This intense focus on fire suppression has helped protect people from fires as homes have climbed up to the crest of the East Bay Hills since the middle of the century, but it has harmed pallid manzanita populations.
Fire suppression in the East Bay Hills, in combination with increased browsing of tree and shrub seedlings and acorns by deer and livestock, has altered habitats within the range of pallid manzanita. Open-canopied oak woodlands maintained historically by frequent fire have been converted in its absence into closed-canopied woodland forests dominated by native trees like Umbellularia californica (California bay) or alien conifer and eucalyptus forests. These forests and woodlands with their denser canopies create a microclimate unsuitable for healthy pallid manzanita plants. A good example of this is the small population of pallid manzanita at upper East Ridge that persists in the understory of a closed-canopy forest of California bay and Arbutus menziesii (madrone). It is estimated that the site may not have burned in more than 100 years. Most of the 14 adult pallid manzanita in this population are unhealthy and show signs of fungal infections and bark striping. On sites shaded with excessive canopy, the ability of the shade intolerant pallid manzanita plants to maintain live tissue is thought to decline, resulting in the partial shutdown of growing cells and tissue sloughing that manifests as bark striping. Bark striping is therefore now believed to be a stress response by some species of manzanita to the absence of fire. At the Huckleberry Ridge population, pallid manzanita plants are generally wider than they are tall, a consequence of growing away from the overstory canopy to reach light, and all of the pallid manzanita plants displayed bark striping.
Fire suppression can also alter the reproductive dynamics of pallid manzanita stands. Based on differing survival responses of chaparral plants to fire, manzanitas can be divided into burl forming and non-burl forming. Burls lay at the base of the main stem of the plant and contain stored nutrients and shoot-forming embryonic tissues. The burl-forming types are capable of surviving fire by resprouting from these burls. The second group does not form burls. Instead, stand persistence is based on the establishment and maintenance of a seed bank in the soil. This seed bank may lay dormant within the soil for as much as 100 years or more. When a fire passes through an area, the seeds are scarified and thus become capable of germinating. However, fire is not the only way seeds can be scarified. Mechanical disturbances, such as crushing, can also crack the seed coat and enable the seeds to germinate. Both types of manzanita can also regenerate by layering, a method that does not require fire. Branches sprout roots at points at which they are covered by soil and leaf litter. This produces a clone of the original plant. Of the three methods of regeneration, only seed reproduction results in genetic recombination, crucial to the maintenance of genetic diversity.
Stand regeneration in pallid manzanita is based primarily on seed reproduction. At the Sobrante Ridge population, pallid manzanita is closely associated with open stands of canyon live oak and interior live oak and recruitment of both pallid manzanita and oaks is occurring on bare and exposed gravel. The effects of fire are evident at this site, occurring possibly 20-30 years ago. In contrast, the effects of fire are not evident at the Huckle-berry Ridge population and fire may have not occurred there for 70 years or longer. This pallid manzanita population is unhealthy due to the negative effects of a dense California bay-madrone canopy, and reproduction is poor. In a 1993 fuel management and habitat improvement experiment at the Huckleberry Ridge site, a small area overgrown with a dense stand of pallid manzanita, was cleared and the cut vegetation piled and burned. Seedlings of pallid manzanita were present the following year. Hand pulling of the invasive alien, French broom, was necessary during 1994 and 1995. During a site visit in March of 1997, 40-50 pallid manzanita were present. Most were 4-6 in (10-15 cm) tall, vigorous, and well-branched. The seedlings were found on the barer soil areas. In addition to continued invasion by French broom (Cytisus monspessulanus ), native coyote brush had begun to invade the site.
The exact degree of importance of fire in relation to this manzanita's reproductive strategy is not yet defined, since seed reproduction can also occur as a result of site soil disturbance. Evidence exists that mechanical scarification, such as crushing, stimulates germination in several manzanita species, including pallid manzanita. New seedlings of pallid manzanita have appeared in areas where mechanical scarification had recently taken place including exposed gravel clearings and fire breaks at the Sobrante Ridge, at several road cuts along Skyline Boulevard, and at Huckleberry Ridge where grading and removal of plants has occurred for residential development.
Fire is however thought to have been the primary historical process by which seed regeneration was initiated and it has other valuable effects beyond seed scarification. The accumulated leaf and bark litter, fallen fruits, and roots of Arctostaphylos species have a self-inhibitory effect on seed germination. Fire is believed to remove these toxic materials and promote germination of Arctostaphylos and other herbs and shrubs. Fire also recycles nutrients in the soil. The excessive accumulation of dead leaf and bark material also results in the retention of soil moisture. Higher soil moisture levels allow fires to conduct heat through the soil more effectively; this has the potential to destroy the existing pallid manzanita seed bank.
The genetic integrity of pallid manzanita is threatened by hybridization with other species of Arctostaphylos introduced into the vicinity of pallid manzanita populations. Urban development over the last 10 years has approached to within 100 ft (30.5 m) of the pallid manzanita population at Sobrante Ridge. At Huckleberry Ridge some homes along Manzanita Drive have pallid manzanita within their landscaping. These houses have helped to introduce exotic landscape and weedy plant species that compete with the remnant population. At least three other species of Arctostaphylos have been used for landscaping on private lands along Manzanita Way, a road that borders the Huckle-berry Ridge Preserve. Hybrids between a common associate of pallid manzanita, A. tomentosa ssp. crustacea (brittle leaf manzanita), are known to occur in two separate populations. Hybrids have also been observed between A. pallida and A. glauca (bigberry manzanita) in Oakland parks. Pallid manzanita closely resembles A. pajaroensis (Pajaro manzanita), a species native to the Pajaro River area. Hybrids may be occurring between these two species in areas where residents have planted A. pajaroensis along Huckleberry Ridge. Hybridization with any of these taxa could result in a hybrid manzanita swarm replacing pure A. pallida.
Herbicides have been used to eradicate eucalyptus associated with pallid manzanita in many areas in the Oakland Hills. The exact effect herbicide spraying has on pallid manzanita has not been studied, however, roadside spraying has had negative effects on regeneration of pallid manzanita along Skyline Boulevard.
Urban development in the East Bay Hills has fragmented the natural habitat of pallid manzanita. Splitting the habitat into smaller, more isolated units can alter the physical environment by changing the amount of incoming solar radiation, water, wind, and nutrients for the remnant vegetation. Small populations, in particular, are threatened by shading from planted eucalyptus, Pinus radiata, and cypresses, and by competition with other aggressive alien plant species including French broom, periwinkle (Vinca sp.), and German ivy (Senecio mikanoides ). In addition, a higher proportion of these fragmented natural areas are subject to the external factors of invasion of non-native plants, foot traffic, and increased erosion that disrupt natural ecosystem processes.
Over-utilization is not currently believed to be a threat to pallid manzanita, but unrestricted collecting for scientific or horticultural purposes, or excessive trampling of seedlings by individuals interested in seeing rare plants, could cause the numbers of plants to significantly decline. This species, though not known to be sought after by collectors, is commercially cultivated. Many members of this genus, including numerous San Francisco Bay area taxa, are collected for cultivation by local horticulturists for interior decoration and landscape plantings because this family of plants possesses attractive bark, leaves, and hard wood. The desirability and accessibility of pallid manzanita could therefore make the plants subject to collection if their precise location was publicized. Possible unauthorized cutting of pallid manzanita was evident at the Sobrante Ridge Regional Preserve population where public access trails and photographic displays of this species are established throughout manzanita habitat.
Disease remains a significant threat to this species. Approximately 50% of the Huckleberry Ridge population of pallid manzanita was affected in the 1980s by a fungal infection that attacked the roots of the plants, causing branch and stem dieback. The Huckleberry Ridge population remains in poor condition. If the cold and wet weather conditions that induced the fungal infection are repeated, another infection could occur, resulting in reduced vigor of the population.
Botryosphaeia fungal infections can cause changes in leaf pigmentation thus affecting the plant's photosynthetic capabilities, destroy branches, and lead to the eventual death of whole plants. Pale chlorotic leaves, possibly due to Botryosphaeia fungi, were evident at the East Ridge population, where 14 mature pallid manzanita plants grow under a canopy dominated by Umbellularia californica, Arbutus menziesii, and introduced Pinus radiata. In addition, urban expansion has resulted in the planting and subsequent spread of many exotic and native species of trees and shrubs. Many of these species grow faster than pallid manzanita and, in some locations, completely shade them. Excessive shade and overcrowding can cause a slow decline in the plant's overall health and vigor that can lead to the spread of Botryosphaeia fungi and an unknown root fungus.
Pallid manzanita is also at risk due to the inadequacy of existing protective regulations. Pallid manzanita is listed as an endangered species under the California Endangered Species Act. Although the direct taking of state-listed plants is prohibited, California law does not prohibit the indirect taking of such plants via habitat modification or land use changes by the owner. Other state laws provide for full disclosure of the potential environmental impacts of proposed projects. Even if projects are identified as having potential to do considerable environmental damage and to harm listed species, they may still be approved because of "over-riding considerations." The Alameda Manzanita Management Plan of 1987, whose mission was to improve the condition of the species and help in its recovery, has been only partly carried out due to limited fundings and conflicting fire management policies.
Conservation and Recovery
Pallid manzanita exists at two large and 11 small occurrences. The majority of its habitat is on East Bay Regional Park District property. The two largest occurrences of pallid manzanita are protected from further direct habitat destruction resulting from urbanization or land use conversion. Residential development is no longer considered a significant threat; although residential development eliminated a large number of pallid manzanita plants on Huckleberry Ridge, further direct habitat destruction is not anticipated. Most of the remaining population at Huckleberry Ridge, as well as the other large pallid manzanita population on Sobrante Ridge, is on lands now owned by the East Bay Regional Park District and is protected from further direct habitat destruction resulting from urbanization or land use conversion. The smaller pallid manzanita populations occur either on other park lands or on privately owned lands that have already been developed. Several large bay trees at upper East Ridge were cut at the base in 1985 to improve light conditions for some pallid manzanita. As a result, many pallid manzanita responded with new growth.
Current fire management focuses on controlling fires, but these practices can be modified in specific areas for listed species. On East Bay Regional Park District and East Bay Municipal Utility District lands, where the majority of pallid manzanita populations occur, the habitat has been managed by fire suppression and brush removal. Mechanical removal of exotic plants has been the primary method used to improve growing conditions mostly for isolated individual plants. Due to the continued expansion of urbanization adjacent to pallid manzanita habitat, and the catastrophic Oakland Hills fire of 1991, mechanical removal of highly flammable vegetation remains the predominant method used to reduce the fuel load in pallid manzanita chaparral habitat. East Bay Regional Park District has reduced the amount of flammable dead plant material in the Huckleberry Ridge population. The reduction in plant litter, and the pruning of some competing exotics, has helped to stimulate germination and growth of the species at Sobrante Ridge, Huckleberry Ridge, and two other lesser locations. A fire management plan that includes the possibility of prescribed burns to address the needs of pallid manzanita for germination and seedling establishment is currently being developed by the East Bay Regional Park District in cooperation with other state agencies.
U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232
Telephone: (503) 231-6121
Sacramento Fish and Wildlife Office
U.S. Fish and Wildlife Service
2800 Cottage Way, Room W-2605
Sacramento, California 95825-1846
Telephone: (916) 414-6600
Fax: (916) 460-4619
U. S. Fish and Wildlife Service. 22 April 1998. "Determination of Threatened Status for One Plant, Arctostaphylos pallida (Pallid Manzanita), from the Northern Diablo Range of California." Federal Register 63 (77): 19842-19850.