Introduction

Ultramafic, or colloquially “serpentine,” rocks and soils have dramatic effects on the vegetation that grows on them. Many plants cannot grow in serpentine soils, leaving distinctive suites of plants to occupy serpentine habitats. Plants that do grow on serpentine soils may be stunted, and plant distributions are commonly sparse relative to other soils in an area. Plant communities on serpentine soils are usually distinctive, even if one does not recognize the plant species. Because of these distinctive features, ultramafic rocks and serpentine soils are of special interest to all observers of landscapes. Geology underlies both conceptually and literally the distinctive vegetation on serpentine soils. The occurrence of special floras on particular substrates within particular regions makes rocks and soils of key significance to plant evolution and biogeography. Sophisticated interpretations of these interrelationships require a combined knowledge of geology, soils, and botany that few people possess. Even highly specialized professionals generally lack the requisite expertise in all three disciplines. The science of ecology, which in principle concerns interactions among all aspects of the environment, seldom incorporates a deep understanding of rocks and soils. Some scientists have attempted to bridge this gap through creating a discipline known as geoecology (Troll 1971, Huggett 1995), which forms the basis for our interdisciplinary exploration of serpentine rocks and soils in western North America. The term “serpentine” is applied in a general sense to all ultramafic rocks, soils developed from them, and plants growing on them. Ultramafic rocks are those with very high magnesium and iron concentrations. The word serpentine is derived from the Latin word serpentinus, meaning “resembling a serpent, or a serpent’s skin,” because many serpentine rocks have smooth surfaces mottled in shades of green to black. The distinctive chemistry of ultramafic rocks and serpentine soils restricts the growth of many plants and makes them refuges for plants that thrive in serpentine habitats, including serpentine endemics (species that are restricted to these soils) and other species that have evolved means of tolerating these habitats. Often the means of tolerance include visible adaptations such as slow growth and relatively thick, spiny foliage.