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NORTH AMERICAN SERPENTINE FLORA
Serpentine soil habitats occur in areas that have been geologically active, particularly where plate tectonics has occurred and uplifting of altered peridotites has exposed these formations (Brooks 1987; Coleman and Jove 1992). These formations are known throughout the world and often form linear zones that indicate the margins of old continents (Coleman and Jove 1992). These linear zones are very apparent in North America where two major zones occur, one on the western side of the continent from mid California, U.S.A., north to British Columbia, Canada, and another on the eastern part of the continent from the state of Alabama, U.S.A., north-east to Quebec, Canada.
In western North America (CPD Site NA16e), serpentine soil habitats occur along fault zones in the Central and North Coast and Cascade ranges near the subduction contact of the Pacific plate with the North American plate. Western Sierra Nevadan serpentine habitats are associated with the uplifting of the Sierran batholith and regions of vulcanism. The geographic area of serpentine habitats in the western U.S.A. includes California (2860 kmē), Oregon (1170 kmē) and Washington (520 kmē), and British Columbia in western Canada (Map 5). The serpentine habitats in western North America occur from sea-level to an elevation of 2900 m. Precipitation in western North American areas falls primarily between October and March; mean annual rainfall increases from 600 mm in the south to 2000 mm in the north-west. Some of the precipitation falls in the form of snow.
In eastern North America (CPD Site NA25), serpentine habitats occur in the U.S.A. in the states of Alabama, Georgia, South and North Carolina, Virginia, Maryland, New Jersey, Pennsylvania, New York, Massachusetts and Vermont (Reed 1986; Brooks 1987), and in Quebec (Brooks 1987) and Newfoundland (3000 kmē) (Roberts 1992) in Canada (Map 5). The serpentine areas of eastern North America occur at elevations from near sea-level to 1500 m. Precipitation occurs primarily from October to April with substantial amounts in the form of snow in the northern parts. In the southern areas, such as Alabama and Georgia, summer monsoons (July through September) may contribute to the annual precipitation.
Serpentine soil habitats are distinct due to a variety of influencing factors not controlled by local climate or elevational conditions. Specifically, the soil chemical and physical characteristics make these habitats very poor in terms of nutrients, sometimes toxic due to the presence of heavy metals, and potentially having lower soil moisture availability. The basic nutrient limitations of low nitrogen and phosphorus exist in serpentine soils. In addition, there is an imbalance between calcium and magnesium, where calcium is relatively low compared to what are often high concentrations of magnesium (Kruckeberg 1984; Brooks 1987). The heavy metals of chromium and nickel can occur in concentrations high enough to invoke toxic conditions for most plant species (Brooks 1987). In addition, the mineralogical conditions and equilibrium conditions of them has made serpentine rocks highly erodible under normal atmospheric conditions. The montmorillonite clays formed by the minerals adsorb more water than many other clay surfaces, thus reducing the available water to plants. All these factors reduce the ability of plants to adapt to serpentine soil habitats and has led to a highly specialized flora.
Vegetation types occurring in serpentine soil habitats include grasslands, chaparral, woodlands and forest. Also, in some areas there are extensive serpentine habitats referred to as serpentine barrens which are sparsely vegetated by annual and perennial herbaceous plant species.
In the western U.S.A. serpentine grasslands are relatively uncommon and mostly occur near the Pacific coast such as in the San Francisco Bay region of California (McCarten 1986, 1987). There are approximately 5000 ha of serpentine grassland in the western U.S.A. In California, grassland dominants include the native perennial bunchgrasses; Melica imperfecta, M. torreyana, Poa scabrella, Sitanion hystrix, and Stipa pulchra. In disturbed grassland areas, especially where extensive cattle grazing occurs, non-native grasses such as Lolium multiflorum, Bromus mollis, and B. rubens can be dominants. Locally, the grassland habitats can have a dominance of non-grass herbaceous plant species including the genera Allium, Gilia, Layia, Lasthenia, Microceris, Phacelia, Plantago and Zygadenus (Kruckeberg 1984; McCarten 1986, 1988a).
Serpentine chaparral vegetation is also restricted to the western U.S.A., primarily in California. This Mediterranean vegetation type is the more common serpentine vegetation in California and collectively covers over 1000 kmē. The dominant plant species include Quercus durata and Arctostaphylos viscida. These two species often occur in nearly pure stands; however, mixtures of these and other species occur in habitat gradients where other Arctostaphylos species may occur with Garrya congdonii, Rhamnus californica and several species of Ceanothus (McCarten and Rogers 1991).
Serpentine woodland vegetation occurs in a variety of types both in western and eastern North America. In western North America, serpentine woodlands include evergreen oak woodlands, often dominated by Quercus chrysolepis, and pine woodlands that are generally dominated by Pinus sabiniana. In eastern North America pines are also a dominant in woodland vegetation.
Forests on serpentine soils are extremely uncommon due to the low nutrient levels in the soil. However, some areas do have a higher density vegetation particularly montane areas with higher rainfall such as the Cascade Mountains in northern California and southern Oregon. In those areas Pinus jeffreyi and P. sabiniana form patchy forested areas. These forests are often interrupted by open areas of serpentine barrens, or as a function of slope the steeper areas may support chaparral or woodlands.
The serpentine barrens are recognized as a major habitat in serpentine areas throughout the world (Kruckeberg 1984; Brooks 1987). The areas are often exposed areas with dramatic changes in topography such as steep, high erodible slopes. The serpentine barrens are often areas where high concentrations of toxic heavy metals occur in the serpentine minerals. Extensive studies have been conducted on plants adapted to these serpentine barrens where heavy metals occur (Kruckeberg 1984; Brooks 1987; Baker, Proctor and Reeves 1992).
In general, the vegetation types on serpentine soils are representative of the vegetation one might expect based on climate and elevation. Two notable differences are that for some forest and woodland species their elevational zonation may be several hundred metres lower than in non-serpentine areas having the same climate and exposure (Kruckeberg 1984); the other difference is the lack of deciduous species which are predominately only found in very moist sites such as along creeks and watercourses.
The serpentine flora is recognized throughout the world for its high level of diversity (Brooks 1987; Baker, Proctor and Reeves 1992). In North America mostly local floristic studies have been conducted (McCarten 1986, 1988a), with the exception of Reed (1986) who has developed a detailed flora for serpentine areas in the eastern U.S.A. Reed's flora (1986) includes over 1600 taxa of plants that occur in serpentine areas of the eastern U.S.A. From that flora it is not clear how many may be endemic to serpentines and what proportion of the native endemic flora of the regions or states covered is represented. Reed's finding of a very high number of taxa of plants growing in serpentine habitats is consistent with other serpentine floras, including western North America where the flora may exceed 2000 taxa (Kruckeberg 1984; Brooks 1987; McCarten 1986, 1988a; McCarten and Rogers 1991).
The most significant aspect of the serpentine floras of North America, and elsewhere in the world, is the diversity which is primarily due to the high percentage of serpentine endemics. In California, Kruckeberg (1984) estimated that the serpentine endemic plant species represents 10% of the 2300 species of plants that are endemic to the California Floristic Province. Other estimates indicate that the serpentine endemics may be as high as 15% of the endemic flora (McCarten 1987). In Newfoundland, Canada, Roberts (1992) identified 190 plant taxa as endemic to the serpentine flora of that area.
Many of these serpentine endemics are recognized as rare and endangered plant species (Kruckeberg 1984; McCarten 1986, 1987, 1988a; McCarten and Rogers 1991; Callizo 1992). Over 25% of the 1300 plant taxa considered to be rare in California are represented by taxa that occur on serpentine soils (McCarten 1988a, 1995). Important genera that are broadly recognized as having speciated into and with the serpentine soil habitats in western North America include Allium, Brodiea, Calochortus, Calycadenia, Cordylanthus, Delphinium, Eriogonum, Hesperolinon, Phacelia, and Streptanthus to name a few (Kruckeberg 1984; McCarten 1988a).
In relation to the plant species diversity that occurs in serpentine soil habitats the low number of non-native plants, such as introduced weeds, is important. Due to the extreme habitat conditions, non-native species only occur in areas where the serpentine soils have been disturbed or altered such as through the addition of fertilizer. While many vegetation types have been invaded by non-native species, particularly in the Mediterranean climate areas of western North America, the serpentine areas have been resistant to this invasion. In undisturbed areas the local flora may be represented by over 95% native plant species. This is significant since many other areas of non-serpentine grassland and chaparral have as much as 50% or more of the flora non-native species.
Two examples of useful plants are the genus Hesperolinon (Linaceae) which is related to the flax of commerce (Linum usitatissimum), and Helianthus exilis, which is related to sunflower (Helianthus annuus). Helianthus exilis represents a novel genetic resource of potential agronomic value in which some of the characters of interest are its high linoleic acid content, cold tolerance and genetic variation in germinability (Jain, Kesseli and Olivieri 1992).
The significance of serpentine habitats and the vegetation that occur in them include all the reasons for recognizing values for plant conservation in general. Recognized social and environmental values include: (1) economic value of plants as resources for human uses, (2) role of plants in maintaining environmental stability, (3) the scientific value as a tool for understanding ecological processes, and (4) cultural and inspirational values (Given 1994).
The social values of specific serpentine plants is reflected in the few species that were mentioned as being useful plants, especially for their potential use in agriculture (Jain, Kesseli and Olivieri 1992). Other examples involve the use of serpentine adapted plants that have been identified as hyperaccumulators of heavy metals, such as nickel, that have been used in bioremediation of mine tailings (Kruckeberg 1984; Brooks 1987). The specificity of particular plant groups and species as endemics on serpentine soils having high concentrations of nickel has been suggested to be a means of identifying areas that produce these economically important minerals (Reeves 1992).
The environmental values of North American serpentine habitats follow closely from the high degree of plant species diversity and endemism in these areas. Serpentine endemics often occur in very localized areas and contribute significantly to local floral diversity. A significant number of serpentine endemics, or species that predominately grow on serpentine soils are either neo-endemics or palaeo-endemics (Raven and Axelrod 1978; Kruckeberg 1984). Representation of the primitive flora and the new rapidly speciating flora is critical to studies on plant evolutionary processes. Further, the serpentine floras in relatively undisturbed areas are generally represented by a higher percentage of native species (McCarten 1986, 1988a). The low nutrient conditions of the serpentine soils often limits the ability of non-native weedy plants to invade these habitats. Therefore, serpentine habitats are one of the few remaining areas where a significant level of invasion by non-native weedy plants has not occurred. Serpentine native bunchgrass areas are considered to potentially represent examples of non-serpentine native grassland plant communities prior to the invasion of non-native Mediterranean grasses.
Threats have been from industrial, urban, recreational, and occasionally agricultural development and other activities. Historically, mining for heavy metals including chromium, nickel, and mercury, have had a significant negative impact on serpentine habitats (Kruckeberg 1984; Brooks 1987; McCarten 1987, 1988a). Mining for heavy metals has been reduced. More recently, however, gold mining has begun to have an impact by processing large quantities of rock that are often associated with serpentine habitats. Gold mining activities often have both indirect and direct effects on serpentine areas by physically altering the surface and subsurface areas to gain access to the gold-bearing rocks. Indirect, but possibly more destructive with regard to the vegetation, is alteration of surface and ground water hydrology by changing the direction of natural precipitation run-off and adding water to some areas. Attempts to restore mine tailings have not always recognized the unique ecological conditions of serpentine habitats and fertilizers have been used in an attempt to grow plants not adapted to serpentine soils. In those cases the restoration experiments have failed.
Urbanization from housing developments and the development of infrastructural systems, such as major highways and roads, have often utilized the apparent poorly vegetated areas of serpentine habitat. These urban forms of development have directly reduced the amount of serpentine area particularly in the San Francisco Bay region of California (McCarten 1986, 1987). Indirect impacts from urban development have been the alteration of surface and ground water hydrology that altered the vegetation of any area, sometimes affecting endangered plant species populations (McCarten 1987). In addition, use of water, fertilizer, and pesticides has reduced the native endemic plant species ability to grow in some areas and allowed non-native weedy plants to invade serpentine habitats.
Recreational activities, particularly by off-road vehicles, have damaged often sensitive serpentine habitats. Specifically, areas that are perceived as unvegetated barrens, which often support endangered herbaceous plant species, are used for off-road vehicle use. These activities directly damage the plant species, increase the erosion rates of the soils, and otherwise physically damage the rare or unique conditions that occur in these areas.
Agricultural activities rarely affect serpentine areas due to the low nutrient levels, occurrence of toxic levels of heavy metals, and overall poor soil conditions. However, through liming and the use of fertilizers some lands have been converted to low quality arable lands. The more destructive agriculturally-related practise has been forestry. Even though serpentine areas are generally low in forest productivity these lands have been cut for timber resulting in accelerated erosion of the shallow soil. Also, related activities, such as road building and development of heavy equipment staging areas, has also directly damaged the serpentine soil habitat. Natural recolonization of disturbed serpentine soils is generally slow often taking decades for vegetation to become established.
Relatively little of the serpentine habitats in North America are preserved despite the numerous unique features and rare and endangered plant species (Kruckeberg 1984, 1992; McCarten 1987, 1995). Some preserves are managed by U.S. federal and state agencies, as well as some private conservation organizations.
Conservation areas in California: the U.S. Department of Agriculture's National Forest Service maintains some research natural areas (RNAs) including the Frenzel Creek RNA in the Mendocino National Forest, Colusa County. The Frenzel Creek RNA (270 ha) includes seven rare plant species and the plant communities of serpentine barrens, extensive chaparral, and a corridor of riparian vegetation including MacNab cypress (Cupressus macnabiana). Other RNAs may exist that include serpentine habitats, but the preserves are not specifically designed to protect these habitats. The Forest Service also maintains areas referred to as Botanical Areas (BAs) that have been identified as important for botanical resources such as the presence of rare and endangered plant species. The Butterfly Valley BA, in the Plumas National Forest, Plumas County, includes a wetland area known as a fen which flows over serpentine formations. The serpentine fen supports a range of wetland plants including the insectivorous cobra lily (Darlingtonia californica).
The U.S. Bureau of Land Management (BLM) maintains an Area of Critical Environmental Concern (ACEC) that includes serpentine habitat (McCarten 1988a). The Cedars ACEC, Lake and Napa counties, supports several plant communities including MacNab cypress woodland and extensive chaparral.
The California State agency, Department of Fish and Game, manages a serpentine area known as the Harrison Grade Ecological Reserve, in Sonoma County. This reserve includes Sargent cypress (Cupressus sargentii) woodland, serpentine chaparral, and serpentine seeps supporting a distinct wetland vegetation (McCarten 1988b). In addition, this reserve is habitat for a rare plant, Pennel's birds-beak (Cordylanthes tenuis subsp. capillaris).
The California Nature Conservancy manages the Ring Mountain Preserve in Marin County. This preserve, covering approximately 25 ha of serpentine habitat, is the only reserve protecting native serpentine bunchgrass habitat (McCarten 1986). In addition, four rare plants are known from the preserve including two endangered species, Tiburon Mariposa lily (Calochortus tiburonensis) and the Marin western flax (Hesperolinon congestum).
Numerous other sites have been identified in the North Coast ranges of California as areas needing preservation due to their unique floras and habitats associated with serpentine (McCarten 1988a, 1995). Currently, a serpentine ecosystem study of California is being conducted to identify areas needing preservation in the state (McCarten 1995).
In eastern North America, several sites of serpentine habitat are being preserved and managed by The Nature Conservancy. Details of the location, size, and flora were not available.
Overall, conservation efforts for protecting serpentine habitats and the associated flora have been poor even though these areas are widely recognized for their ecological value and floristic diversity. Regional efforts are needed to identify plant communities and floristic associations that should be protected.
Map 5. Serpentine Areas of North America, U.S.A. and Canada (CPD Site NA16e and NA25)
Baker, A., Proctor, J. and Reeves, R. (eds) (1992). Vegetation of ultramafic (serpentine) soils: proceedings of the first international conference on serpentine ecology. Intercept Limited, U.K. 509 pp.
Brooks, R. (1987). Serpentine and its vegetation. Dioscorides Press, Portland. 454 pp.
Callizo, J. (1992). Serpentine habitats for the rare plants of Lake, Napa, and Yolo counties, California. In Baker, A., Proctor, J. and Reeves, R. (eds), The vegetation of ultramafic (serpentine) soils: proceedings of the first international conference on serpentine ecology. Intercept Limited, U.K. Pp. 35-53.
Coleman, R. and Jove, C. (1992). Geologic origin of serpentinites. In Baker, A., Proctor, J. and Reeves, R. (eds), The vegetation of ultramafic (serpentine) soils: proceedings of the first international conference on serpentine ecology. Intercept Limited, U.K. Pp. 1-18.
Given, D. (1994). Principles and practice of plant conservation. Chapman and Hall, London. 292 pp.
Jain, S., Kesseli, R. and Olivieri, A. (1992). Biosystematic status of the serpentine sunflower, Helianthus exilis Gray. In Baker, A., Proctor, J. and Reeves, R. (eds), The vegetation of ultramafic (serpentine) soils: proceedings of the first international conference on serpentine ecology. Intercept Limited, U.K. Pp. 391-408.
Kruckeberg, A. (1984). California serpentines: flora, vegetation, geology, soils, and management problems. University of California Press, Berkeley. 180 pp.
Kruckeberg, A. (1992). Plant life of western North American ultramafics. In Roberts, B. and Proctor, J. (eds), The ecology of areas with serpentinized rocks: a world view. Kluwer, Dordrecht. Pp. 31-74.
McCarten, N. (1986). Serpentine flora of the San Francisco Bay region. California Endangered Plant Program, California Department of Fish and Game, Sacramento. 93 pp.
McCarten, N. (1987). Serpentine plant communities of the San Francisco Bay region. In Elias, T. (ed.), Conservation and management of rare and endangered plants: proceedings from a conference of the California Native Plant Society. California Native Plant Society, Sacramento. Pp. 335-340.
McCarten, N. (1988a). Rare and endemic plants of Lake County serpentine soil habitats. California Endangered Plant Program, California Department of Fish and Game, Sacramento. 137 pp.
McCarten, N. (1988b). Habitat management plan for the Harrison Grade Ecological Reserve. Technical report for the California Department of Fish and Game, Sacramento. 35 pp.
McCarten, N. (1995). Conservation strategies for an ecosystem level approach to preserving plant biodiversity in serpentine soil habitats. Conserv. Biol. (in review).
McCarten, N. and Rogers, C. (1991). Habitat management study of rare plants and communities associated with serpentine soil habitats in the Mendocino National Forest. U.S. Department of Agriculture, Mendocino National Forest, Colusa. 70 pp.
Raven, P. and Axelrod, D. (1978). Origin and relationships of the California flora. Univ. of California Publ. in Botany 72: 1-134.
Reed, C. (1986). Floras of the serpentinite formations of eastern North America. Contributions to the Reed Herbarium No. 30, Baltimore. 858 pp.
Reeves, R. (1992). The hyperaccumulation of nickel by serpentine plants. In Baker, A., Proctor, J. and Reeves, R. (eds), The vegetation of ultramafic (serpentine) soils: proceedings of the first international conference on serpentine ecology. Intercept Limited, U.K. Pp. 253-278.
Roberts, B. (1992). The serpentinized areas of Newfoundland, Canada: a brief review of their soils and vegetation. In Baker, A., Proctor, J. and Reeves, R. (eds), The vegetation of ultramafic (serpentine) soils: proceedings of the first international conference on serpentine ecology. Intercept Limited, U.K. Pp. 53-66.
This Data Sheet was written by Dr Niall F. McCarten (Jones and Stokes Associates, 2600
V Street, Suite 100, Sacramento, California 95818-1914, U.S.A. and University of
California Herbarium, University of California, Berkeley, California 94720, U.S.A.).
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