Serpentine Soils

by

Nishanta Rajakaruna, Robert S. Boyd

• LAST REVIEWED: 05 January 2022
• LAST MODIFIED: 25 February 2014
• DOI: 10.1093/obo/9780199830060-0055

Introduction

Serpentine soils are weathered products of a range of ultramafic rocks composed of ferromagnesian silicates. Serpentine more accurately refers to a group of minerals, including antigorite, chrysotile, and lizardite, in hydrothermally altered ultramafic rocks. Common ultramafic rock types include peridotites (dunite, wehrlite, harzburgite, lherzolite) and the secondary alteration products formed by their hydration within the Earth’s crust, including serpentinite, the primary source of serpentine soil. Serpentine soils are generally deficient in plant essential nutrients such as nitrogen, phosphorus, potassium, and sulfur; have a calcium-to-magnesium (Ca:Mg) molar ratio of less than 1; and have elevated levels of heavy metals such as nickel, cobalt, and chromium. Although physical features of serpentine soils can vary considerably from site to site and within a site, serpentine soils are often found in open, steep landscapes with substrates that are generally shallow and rocky, often with a reduced capacity for moisture retention. Due to the intense selective pressure generated by such stressful edaphic conditions, serpentine soils promote speciation and the evolution of serpentine endemism, contributing to unique biotas worldwide, including floras with high rates of endemism and species with disjunct distributions. The biota of serpentine soils has contributed greatly to the development of ecological and evolutionary theory, as well as to the study of the genetics of adaptation and speciation. Plants growing on serpentine soils also provide genetic material for phytoremediation and phytomining operations. Habitats with serpentine soils are undergoing drastic changes due to ever-expanding development, deforestation, mining for heavy metals and asbestos, exotic-species invasions, climate change, and atmospheric deposition of previously limiting nutrients such as nitrogen. Such changes can have drastic impacts on serpentine floras and affect bacteria, fungi, and fauna associated with serpentine plants and soils. Habitats with serpentine soils provide ample opportunities for conservation- and restoration-oriented research directed at finding ways to better manage these biodiversity hotspots.

General Overviews

Serpentine-containing rocks, such as serpentinite, are an important cultural and historical material. Serpentinites can be easily worked and are used by many cultures for tool making, decorations, jewelry, ceremonial carvings, and amulets, as well as for magic, such as for protection from snake bites. Nickel, cobalt, chromium, and asbestos (e.g., chrysotile) are largely extracted from serpentine-containing rocks. Several significant treatments of serpentine soils, plants, and other biota, including serpentine as a model for ecological, evolutionary, and applied studies, have been published since 1975. Proctor and Woodell 1975 provides the first review of serpentine ecology, with a focus on factors that limit plant growth on serpentine soils. Brooks 1987 is a comprehensive treatment on serpentine floras from around the world, including individual chapters devoted to geology, soils, and nutritional and elemental stressors plants encounter when growing on serpentine soils. Roberts and Proctor 1992 complements Brooks 1987 in describing soil-plant relations of serpentine from sites in North America, Europe, Africa, Asia, and Australia. Jaffré 1980 provides a review of serpentine plants from endemic-rich New Caledonia, including tissue concentrations of heavy metals in plants found on serpentine soils. Brady, et al. 2005 is an extensive review of studies on the ecology and evolution of serpentine plants, with a particular focus on adaptation and speciation. Although restricted to western North America, Alexander, et al. 2007 is a thorough treatment of the geology, hydrology, and soils as well as the biodiversity (microbes, fungi, animals, and plants) of serpentine soils of California and western North America. The most recent treatment, Harrison and Rajakaruna 2011, is a collection of papers written by experts in their respective fields, asking what serpentine-associated studies have revealed about broader theoretical questions in geology, evolution, and ecology. The chapters on topics relating to earth history, evolution, ecology, and conservation confirm the value of serpentine as a model in multiple disciplines in the natural sciences.

• Alexander, Earl B., Robert G. Coleman, Todd Keeler-Wolf, and Susan P. Harrison. 2007. Serpentine geoecology of western North America: Geology, soils and vegetation. New York: Oxford Univ. Press.

  Although geographically restricted, this book explores the ecology of serpentine habitats, focusing on soils and plants but including information on other organisms (animals, fungi, microorganisms) where feasible.

• Brady, Kristy U., Arthur R. Kruckeberg, and Harvey D. Bradshaw Jr. 2005. Evolutionary ecology of plant adaptation to serpentine soils. Annual Review of Ecology, Evolution, and Systematics 36:243–266.

  DOI: 10.1146/annurev.ecolsys.35.021103.105730

  An excellent review of plant adaptation to serpentine soils, this paper covers the defining features of serpentine soils and the mechanisms proposed for serpentine tolerance. It also addresses the evolution and genetics of serpentine adaptation and how speciation may occur in this type of habitat.

• Brooks, Robert R. 1987. Serpentine and its vegetation: A multidisciplinary approach. Ecology, Phytogeography & Physiology 1. Portland, OR: Dioscorides.

  A classic overview of serpentine geology and ecology, this volume provides a summary of early work on this habitat type and includes information on soils, plants, animals, agriculture, and vegetation.

• Harrison, Susan P., and Nishanta Rajakaruna, eds. 2011. Serpentine: The evolution and ecology of a model system. Berkeley: Univ. of California Press.

  The nineteen chapters discuss how metal-enriched serpentine habitats have been used or can be used to address major questions in earth history, evolution, ecology, conservation, and restoration.

• Jaffré, Tanguy. 1980. Étude écologique du peuplement végétal des sol dérivés de roches ultrabasiques en Nouvelle Calédonie. Travaux et Documents de L’ORSTOM 124. Paris: ORSTOM.

  An early study of serpentine vegetation in a biological hotspot with very high endemism, this monograph contains information on the climate and soils, as well as the vegetation, of serpentine sites in New Caledonia. There is also extensive information about the elemental concentrations of plants growing on these soils, including hyperaccumulators.

• Proctor, John, and Stanley R. J. Woodell. 1975. The ecology of serpentine soils. Advances in Ecological Research 9:255–366.

  DOI: 10.1016/S0065-2504(08)60291-3

  An early review of serpentine plant ecology that focuses on factors that may determine the relative infertility of serpentine soils, including Ca:Mg ratio, Mg/Ni interactions, metal toxicity, and low levels of major plant nutrients.

• Roberts, Bruce A., and John Proctor, eds. 1992. The ecology of areas with serpentinized rocks: A world view. Geobotany 17. Dordrecht, The Netherlands: Kluwer Academic.

  DOI: 10.1007/978-94-011-3722-5

  An early summary of serpentine ecology in major global areas. After a brief geological review, thirteen contributed chapters cover serpentine ecology in areas within North America, Europe, Africa, Australia, and Asia. Coverage reflects the limited state of knowledge at the time, but the book is a good entry point into the pre-1990s literature.