Plant life of western North American ultramafics

Abstract

In Western North America, ultramafics occur with decreasing abundance from California, Oregon, Washington, to British Columbia. All the occurrences are now considered parts of ophiolite suites, and are associated with the north-south trending cordilleras and their plate tectonics. The greatest concentrations of ultramafics, mostly as serpentinized peridotite, are in northwestern California and southwestern Oregon.

Soils weathered from ultramafic rocks are either devoid of vegetation (barrens) or support sparse but often distinctive floras. Cation exchange capacities range from 5.2 to 43 m . equivs 100 g^-1 dry soil; pH values are around neutral (6.0 to 8.8); Mg/Ca quotients are invariably greater than 1.0; deficiencies of nitrogen and phosphorus are common and can be corrected by the addition of these elements, only in the presence of adequate calcium. Tissue analysis of serpentine plants often reveals high concentrations of magnesium and nickel.

Vegetation on ultramafic soils takes the form of distinctive variants of conifer or mixed conifer-hardwood forest, chaparral, or grassland. Often the serpentine (S) vegetation is sharply delimited from adjacent nonserpentine (NS) types, both by physiognomy (e.g., chaparral on serpentine, forest on nearby nonserpentine), and by species composition. The most striking contrasts in vegetation (S vs. NS) are in California and Oregon. Contrasts in S-NS vegetation are lessened in the Pacific Northwest, possibly because of increased precipitation, or the short post-Pleistocene history of the region, or both. Proctor (Chapter VI) notes a similar lessening of contrasts in S-NS vegetation in the United Kingdom.

Floras on ultramafic soils can be strikingly unusual. Three types of floristic elements can be found: (1) serpentine endemics, (2) local or regional indicator species, and (3) bodenvag species, taxa widespread on S and NS habitats. Also many NS taxa may be excluded from adjoining S soils.

The greatest concentration of species endemic to serpentine is in the Klamath-Siskiyou mountain complex of northwestern California and southwestern Oregon, with secondary concentrations in the North Coast and South Coast ranges and the Sierra Nevada of California. Endemics occur in all life-forms: trees and shrubs (e.g., Cupressus sargentii, Quercus durata, Ceanothus jepsonii), herbaceous perennials (e.g., Calochortus tiburonensis, Fritillaria liliacea, Lilium bolanderi); and annuals (e.g., Streptanthus batrachopus, Layia discoidea, Clarkia franciscana). Endemics belong to genera abundantly represented in the regional flora.

Widespread species that appear as local or regional indicators of serpentine include trees like Calocedrus decurrens and Pinus jeffreyi, shrubs (e.g., Heteromeles arbutifolia, Adenostoma fasciculation, Ceanothus cuneatus) and herbs (e.g., Streptanthus glandulosus, Darlingtonia californica, Aspidotis densa, Xerophyllum tenax).

Indifferent or bodenvag species are often racially differentiated into tolerant and intolerant biotypes.

The fauna on western North American serpentines has received but scant attention, and merits closer study. Butterfly species are known to be closely tied to serpentine plants as food sources; one instance of plant mimicry of butterfly eggs is cited.

The evolution of a serpentine flora may involve a variety of speciational routes. The most probable sequence for diploid taxa could involve (1) genetic preadaptation to serpentine within a NS species; (2) racial fixation of the preadapted genotype; (3) further morphological and physiological divergence yielding an infraspecific variant; (4) attaining species status by further genetic and ecological isolation. This sequence is illustrated by Streptanthus, a genus of western North American crucifers, with varying degrees of fidelity and narrow endemism to California and Oregon serpentines. A more rapid mode of speciation on serpentine, saltational speciation by catastrophic selection, has been proposed.

Adaptation to ultramafic soils is likely to involve both physiological and morphological modifications. Xerophy-tism, nanism, glaucescence, plagiotropism and color changes (anthocyanic, chlorotic) are frequent attributes of serpentine species. A few species possess the ability to accumulate over 1000 μgg ^-1of nickel in their foliar dry matter (hyperaccumulators).

Western North American serpentines have been exploited for minerals, timber, grazing and agriculture, with consequent effects on their floras. Mining for mercury, nickel and chromium, as well as geothermal power develop-ments, have created the greatest disturbances to them. Only modest efforts have been made to preserve samples of serpentine vegetation. Some state and federal wilderness areas include serpentine vegetation; other serpentine areas are ‘protected’ either by neglect or because they are valued as watershed areas. A very few natural areas specifically for serpentine vegetation have been established in the three Pacific Coast states. None are known for British Columbia.

The red-rock forest may seem hellish to us, but it is a refuge to its flora…. it is the obdurate physical adversity of things such as peridotite bedrock which often drives life to its most surprising transformations. David Rains Wallace, The Klamath Knot (1983).