Ecotypic Response to Ultramafic Soils by Some Plant Species of Northwestern United States

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Abstract
Soils high in magnesium derived from ultramafic rocks (serpentine, peridotite, and dunite) in northwestern United States support endemic as well as wide-ranging but edaphically indifferent(bodenvag) species. The latter occur widely on diverse rock formations of the region. Severalbodenvag species are shown to respond ecotypically to ultramafic soils. Of 18 species tested, all but three are differentiated into strains either tolerant or intolerant of ultramafic soils. Tests for edaphic preferences were conducted with seedlings and mature transplants on ultramafic soils. Growth performances were determined in greenhouse pot tests, outdoor soil bins, and by transplants in the wild. Herbaceous perennials (e.g.,Achillea millefolium, Fragaria virginiana, Prunella vulgaris, Rumex acetosella) gave the clearest ecotypic differences. Woody species either showed only slight ecotypic response(Spiraea douglasii var.menziesii andGaultheria shallon) or delayed the expression of their genotypic adaptability(Pinus contorta). Where ultramafic abut non-ultramafic soils, those populations ofbodenvag species that grow in non-ultramafic habitats can have a significant proportion of individuals tolerant to ferromagnesian soils (e.g.,Achillea millefolium). This suggests gene flow between populations of contrasting edaphic sites and possibly preadaptedness for the ultramafic habitat. Strains of two introduced weeds(Prunella vulgaris andRumex acetosella) have become ecotypically tolerant to ultramafic soils, probably within the last 75 years.

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