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Those nice open slopes on Brothers, Navaho, and Earl are the main example of
Ultramafic geology in the Pacific Northwest-
Ultramafic geology of Pacific Northwest
The largest and most extensive occurrences of ultramafic rock are in northern California crossing somewhat into southern Oregon. Almost all of these areas are serpentinite. Central Oregon, British Columbia and Alaska have relatively small outcrops of ultramafics, typically in the forms of peridotite and dunite with localized serpentinization (Kruckeberg, 1992).
Washington has some extensive ultramafic areas. All occurrences are north of Snoqualmie Pass – which is a boundary between the southern volcanic part of the Cascades and the complex lithology of the north (Kruckeberg, 1992).
* The only ultramafics on the east side of the Cascades are just north of Snoqualmie Pass in the Wenatchee Mountain Range, centered around Mt. Stuart. Largely composed of peridotite, it is know as the Ingalls Complex.
* Further north, serpentinized outcrops of peridotite occur in the Stillaguamish watershed.
* The largest exposure of dunite in North America is located at Twin Sisters Mountain in Skagit County on the Nooksack River. It is mantle that was emplaced during the Cretaceous Period.
* Sumas Mountain is the most northerly Washington exposure and is composed of dunite.
* The San Juan Islands have many scattered low elevation ultramafics. It is part of an ophiolite complex of serpentinized and alpine peridotites and that were formed in the Jurassic Period.
The Ingalls Complex is an ophiolite suite that surrounds the Mt. Stuart Batholith of the Wenatchee Mountain Range. This complex has been described by Kruckeberg (1992) as follows:
These montane outcrops, confined to northern Kittitas and southern Chelan counties, cluster around the spectacular Stuart Range, a serrate massif of old granodiorite (the Mt. Stuart Batholith). Old altered volcanics (greenstones), sedimentary rocks, gneisses and schists and acid igneous granodiorite border or even interfinger with the ultramafics. The region is thus lithologically rich and complex.
The terrain is rugged, with steep slopes and high ridges that culminate in ultramafic peaks (e.g., Iron, Earl, Navaho and Ingalls peaks).
This, the largest Washington occurrence of ultramafics, covering over 300 km<sup>2 of rugged montane-alpine terrain, is a landscape of high treeless ridges, scantily forested slopes in a transitional montane climate from the cool, moist Pacific side of the Cascades to the drier, more continental east side.
Figure from top of Esmerelda Basin trail showing contrast between two south facing slopes and a northeast facing slope with clearcuts. The two south facing slopes have ultramafic soils.
Non-ultramafic rocks in the Ingalls Complex are metagabros and metadiabases that intrude into a ‘…metabasaltic cover of massive flows, pillow lavas and pillow breccias with interbeds of argillite, chert and coarse breccia’ (Miller, 1980). The Ingalls opiolite suite covers about 400 km<sup>2 and originated as a thrust zone in the late Jurassic, forming a boundary between oceanic lithosphere and metamorphic continental crust (Miller, 1980). Ultramafic rocks are mostly peridotite, pyroxenite and serpentinite. Serpentinite occurs mostly at the contact between massive peridotite and non-ultramafics (Kruckeberg, 1992).
Soils
The processes of soil formation on ultramafic rocks are generally different from those on acidic rock, largely due to high concentrations of Fe and Mg. Soils formed on serpentinite generally produce more exaggerated effects than other types of ultramafics. Soil profile development is generally slow and poor, with average pH between 6-7.5. Soil moisture holding capacity is generally low. Cation Exchange Capacity is typically high, as is the Mg/Ca ratio. Ultramafic soils are often lacking in nitrogen, phosphorous and sometimes molybdenum; they are moderate to high in Cobalt, chromium, iron and nickel. Because soil development is poor and slopes are steep, ultramafic soils are not stable. This affects the establishment of pioneering vegetation and, as a consequence, further slows soil development.
Crop and timber productivity is highly variable, but mostly very poor. Land use is mostly restricted to livestock grazing and wildlife values. Food crops and livestock are susceptible to picking up and even concentrating heavy metals – a factor to consider especially if grown for consumption. Timber can be harvested from sites that are not so severe that they exclude tree growth, but regeneration after harvest is often difficult and growth is poor. Ultramafic sites in forested regions are often classified as non-productive, especially as the amount of serpentine increases. Many ultramafic soils have been found to benefit from the addition of calcium.
Plant physiology of ultramafic soils
Around the world, ultramafic outcrops can be characterized by altered vegetation forms: endemism, indicator species, edaphic races. Why do ultramafic support such unique plant species communities? This question has been scientifically investigated since the beginning of the 20<sup>th century and physical, chemical and biological components have been isolated to determine a cause.
Much of the scientific work has gone into determining plant response to ultramafic chemistry including tissue analysis, nutrient cultures and field and greenhouse trails with fertilizers. There have also been several vegetation studies to determine patterns in plant communities (e.g., Cooke 1992). The synthesis of these varied works gives the impression that there is no single factor that describes the effect(s) of ultramafic on plants. Instead, there must be a host of factors, that may or may not be present on a given site, that culminate to create the so called, “serpentine effect” (Kruckeberg, 1992).
Ultramafic soils lack or a very low in several major nutrients, especially nitrogen and phosphorous. (Walker et al, 1955). However, this cannot be the single cause of the serpentine effect since plants that are intolerant to ultramafic conditions still grow poorly in many ultramafic soils that are fertilized to make up for mineral deficiencies. It is interesting to note that good growth can often, but not in all cases, be achieved by adding calcium (Turitzin, 1981).
Ultramafic soils can be very alkaline – but this is not likely a major contributor to infertility, since not all infertile serpentine soils are alkaline and some very alkaline soils are very fertile.
Some serpentine soils are deficient in molybdenum (Walker et al, 1955), but the role this plays in site productivity is not clear, and likely not a big player in the serpentine story (Kruckeberg, 1992). Molybdenum deficiencies become most apparent when there is adequate nitrogen and phosphorus.
Magnesium, nickel, cobalt, iron, mercury and chromium toxicity could be a contributor to infertility. These heavy metals are not decisively known to affect plant growth and pH levels are often too high for significant absorption. Toxicity of magnesium and nickel is increased with decreased calcium and high nickel concentrations may affect nutrient absorption by some plant species (Kruckeberg, 1992).
According to Kruckeberg (1992), calcium deficiency has been linked to poor growth and some ultramafic soil adapted plants seem better able to absorb calcium. But addition of Ca does not always promote soil productivity; so this cannot be a single cause either. In fact, calcium is only required in micro-quantities and soils with low pHs, which are also calcium limited, do not show unusual vegetation.
The cause for unusual biological expressions on serpentine soils should be viewed with an eye to the whole system, a complex set of conditions. Conditions from one ultramafic soil will differ, and what limits productivity on one site may not in another, but the overall impression from site-to-site is often quite similar.
Adaptations and tolerances
Especially on sites with extreme serpentine conditions, plants exhibit traits that are often absence in non-ultramafic surroundings. Tolerant races and species often develop specific to their native ultramafic soil and may not be found on other ultramafic soils. Following are some examples from Kruckeberg (1992):
* Magnesium restrictors. Some plants have been found to tolerate high magnesium conditions by restricting uptake – these plants require high soil concentrations before they can begin to absorb. When planted in non-ultramafic soils, these plants cannot absorb sufficient quantities of magnesium and they grow poorly. This is true for a grass species endemic to the Ingalls Complex of the Wenatchee Mountains – Poa curtifolia (Kruckeberg, 1992)
* Calcium extractors. Races of species native to serpentine soils are often better able to take up calcium than races of the same species native to calcium sufficient sites.
* Heavy metal resistors and hyperaccumulators. Most plants found on serpentine soils are able to exclude heavy metals from uptake, but some genera (Thlaspi and Alyssum) and families (Cruciferae and Caryophyllacea) are hyperaccumulators of heavy metals (esp. nickel). Neither mechanism is well understood. Hyper accumulators may be able to retain heavy metals in a form that is rendered harmless.
* Pathogen escapees. Some species have been shown to thrive on serpentine soils because pathogens that severely restrict the species elsewhere cannot tolerate the conditions in serpentine soils while the plant can.
Ecology
Serpentinized ultramafic outcrops occur around the world and are often associated with unusual, rare, endemic and different races of plants. Ultramafic landscapes are “unmistakable, especially in their more extreme forms. They display barren stony ground with the glistening mottled rock exposed everywhere, widely spaced, low-growing herbs as the only signs of plant life; and where woody vegetation is present it occurs as sparse open stands of shrubs and often with stunted conifers (Kruckeberg, 1992).”
California serpentines support rich endemic populations of plants that are in sharp contrast to neighboring non-serpentines. This serpentine effect is diminished in Washington, with just a few endemic taxa and often only a quantitative shift in forest vegetation from productive to unproductive conifer forest (Kruckeburg, 1992)
Ecology of ultramafic soils in the Wenatchee Mountains
While not as notable for the number of endemic species when compared to the serpentines of California, the ultramafics of the Wenatchee Mountains are still vegetatively interesting. Plant communities more often reflect those local species that can handle ultramafic conditions. Not only must a species be able to tolerate the chemical conditions of ultramafics, they must be highly drought-tolerant. A general pattern that occurs is that mesic species are restricted to non-ultramafic soil and xeric species are typically dominant in the ultramafic areas (Kruckeberg, 1992). Extreme ultramafic areas are typically treeless barrens, that are often more similar to alpine areas. Vegetation is mostly restricted to herbaceous perennials: 80% hemicriptophyes, 14% chamaephytes and 6% geophytes (Kruckeberg, 1969).
One typical expression of ultramafic conditions is an elevational shift or change in plant communities – tolerant species that may not be able to compete at a given elevation in the adjacent non-ultramafic light-limited communities, may be able occupy a position in the open ultramafic soils. In less severe areas, unusual mixes of conifers can occur. Subalpine tree species such as, whitebark pine (Pinus albicaulis), subalpine fir (Abies lasiocarpa) and common juniper (Juniperis comunis) are found at lower elevations whereas lower montane species, Ponderosa pine (Pinus ponderosa), lodgepole pine (Pinus contorta var latifolia), white pine (Pinus monticola), Douglas-fir (Psuedotsuga menziesii) and others, are found at higher elevations (Kruckeberg, 1992).
While is notable which species are able to tolerated varying degrees of ultramafic conditions, it is also important to recognize the species and genera that are avoiders. Kruckeberg (1969) and del Moral (1974) found that figwort, legume, rose and lily families and genera are often missing or rare on ultramafic soils and are common on the adjacent non-ultramafic soils. According to Kruckeberg (1969), the following species are clear-cut avoiders (no tolerance) of serpentine soils:
* Shrubs: snowbrush (Ceanothus velutinus), Douglas maple (Acer glabrum), mountain boxwood (Pachystima myrsinites).
* Ferns: lace fern (Cheilanthes gracillima), parsley fern (Cryptogramma crispa).
* Herbs: arrow-leaved balsamroot (Balsamorhiza saggitata), Sitanion jubatum and<span class="Normal--Char" style=" font-style: italic; ">Penstemon spp.
* Lupins: Lupinus. hypoleuca and L. nardosmia
* The crustose lichen (Rhizocarpon geographicum). This is such a strong avoider that it can be used to indicate small interfingers of non-ultramafics.
View east from the top of the Esmeralda Basin trail. Valley bottom is the likely contact between ultramafics (left side) and non-ultramafics.
Though there are not very many in the Wenatchee ultramafics, endemic species can be a major part of the ultramafic plant communities (Kruckeberg, 1992). Polystichum lemmonii is a small fern that can form extensive clumps in rocky serpentine outcrops. It is a strict serpentine plant and is the only endemic that is found in serpentine areas ranging from British Columbia to Northern California. Three plants, Poa curtifolia, a type of grass, Lomatium cuspidatum, from the carrot family, and Chaenactis thompsoni,i a species of pincushion, may be the only plants on the most barren sites.
Local indicator species of ultramafic soils are plants that exhibit a high fidelity to ultramafics. Examples of these species include: Newberry’s knotweed (Polygonum newberryi), Fendler’s pennycress (Thalspi fendleri) and a rare paintbrush Castilleja elmeri.
Another group of plants that can be considered local indicator species are species that grow in non-ultramafic substrates in the rest of Washington but are found predominantly on serpentine soils in the Wenatchee Mountains. Indian’s dream fern (Aspidotis densa) has been found to favor limestone and serpentine in Washington. Trapper’s tea (Ledum glandulosum), which is related to Labrador tea and is found mostly on the west side of the Cascades, crosses east of the crest where there are serpentine soils. Other species include, Salix brachycarpa (a type of willow), alpine buckwheat (Eriogonun pyrofolium), alpine sandwort (Arenaria obtusiloba), an ivesia (Ivesia tweedyi), and Drummond’s anemone (Anemone drummondii) (Kruckeberg, 1992).
Web Links
Geological Information:
http://www.minerals.net/mineral/silicate/phyllo/serpenti/serpenti.htm
Ecological Example (Maryland):
http://www.dnr.state.md.us/wildlife/serpentine.html
http://jrm.library.arizona.edu/data/1986/391/3rosi.pdf Effects of serpentine on nutrition of grazing sheep.
Ecological Example (Mojave Desert)
http://bio-ccb13.stanford.edu/whatsnew/views/genetic.pdf
References
* Brooks, R.R. 1987. Serpentine and its vegetation. Dioscorides Press. Great Britain.
* Cooke, S.S. 1994. The edaphic ecology of two western North American composite species. PhD thesis, University of Washington, Seattle.
* Cyberman, C. 1988. The effects of serpentine soils on the morphology of two Wenatchee Mountain flowering plants. M.S. thesis, University of Washington, Seattle. Walker, R.B., H.M.
* Dann, K.T. 1988. Traces on the Appalachians: a natural history of serpentine in Eastern North America. Rutgers University Press. New Brunswick.
* del Moral, R. 1974. Species patterns in the upper North Fork Teanaway River drainage, Wenatchee Mountains, Washington. Syesis 7: 13-30.
* Diane, R.E., B. Goettle and D.H Wright. 1998. Draft recovery plan for serpentine soil species of the San Francisco Bay Area. U.S. Fish and Wildlife Service, Region 1.
* Grover, R. 1960. Some aspects of Ca-Mg Nutrition of plants with special reference to serpentine endemism. PhD thesis, University of Washington, Seattle.
* Kruckeberg, A.R. 1969. Pant life on serpentinite and other ferromagnesian rocks in northwestern North America. Syesis 2: 15-114.
* Kruckeberg, A.R. 1984. California Serpentines: flora, vegetation, geology, soils and management problems. University of California Press. Berkeley.
* Kruckeberg, A.R 1992. Plant life of western North American ultramafics. In:Roberts, B.A. and J Proctor (eds), The ecology of areas with serpentinized rocks. Kluwer Academic Publishers. Netherlands.
* Malpas, J. 1992. Serpentine and the geology of serpentinized rocks. In:Roberts, B.A. and J Proctor (eds), The ecology of areas with serpentinized rocks. Kluwer Academic Publishers. Netherlands.
* Miller, R.B. 1980. Structure, petrology and emplacement of the ophiolitic Ingalls Complex, North Central Cascades, Washington. PhD thesis, University of Washington, Seattle.
* Turitzin, S.N. 1981. Nutritient limitations to plant growth in a California serpentine grassland. Amer. Midland Naturalist. 107: 95-99.
* Walker, R.B. and P.R Ashworth. 1955. Calcium-magnesium nutrition with special reference to serpentine soils. Plant Physiol. 30
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