Serpentine soil

300px|thumb|[[Solidago multiradiata, Erigeron aureus, and Adiantum aleuticum in rocky serpentine soil]]

Serpentine soil is an uncommon soil type produced by weathered ultramafic rock such as peridotite and its metamorphic derivatives such as serpentinite. More precisely, serpentine soil contains minerals of the serpentine subgroup, especially antigorite, lizardite, and chrysotile or white asbestos, all of which are commonly found in ultramafic rocks. The term "serpentine" is commonly used to refer to both the soil type and the mineral group which forms its parent materials.

Serpentine soils exhibit distinct chemical and physical properties and are generally regarded as poor soils for agriculture. The soil is often reddish, brown, or gray in color due to its high iron and low organic content. Geologically, areas with serpentine bedrock are characteristically steep, rocky, and vulnerable to erosion, which causes many serpentine soils to be rather shallow. The shallow soils and sparse vegetation lead to elevated soil temperatures and dry conditions. Together, these factors create serious ecological challenges for plants living in serpentine soils.

Parent rock

Serpentinite is a meta-igneous rock formed by the metamorphic reaction of olivine-rich rock, peridotite, with water. Serpentinite has a mottled, greenish-gray, or bluish-gray color and is often waxy to the touch. The rock often contains white streaks of chrysotile running through it, which are a type of naturally occurring asbestos. Asbestos is linked to an array of human health conditions such as mesothelioma from long-time exposure of breathing in the dust particles. Caution should be taken when working in serpentine soils or when working with crushed serpentine rocks.

Serpentinite most often forms in oceanic crust near the surface of the earth, particularly where water circulates in cooling rock near mid-ocean ridges: masses of the resulting ultramafic rock are found in ophiolites incorporated in continental crust near present and past tectonic plate boundaries.

Serpentine soils are derived from ultramafic rocks. Ultramafic rocks are igneous or metamorphic rocks that contain more than 70% iron or magnesium minerals.

Distribution

thumb|Serpentine outcrop high in the [[Siskiyou Wilderness of northwest California—here Jeffrey pine dominates the landscape.]]

Serpentine soils are widely distributed on Earth, in part mirroring the distribution of ophiolites. There are outcroppings of serpentine soils in the Balkan Peninsula, Turkey, Newfoundland, the island of Cyprus, the Alps, Cuba, and New Caledonia.

Botany

thumb|[[Soldiers Delight Natural Environment Area in Maryland]]

Ecologically, serpentine soils have three main traits: poor plant productivity, high rates of endemism, and vegetation types that are distinct from neighboring areas.

Serpentine plant communities range from moist bogs and fens to rocky barrens and must be able to tolerate the harsh environmental conditions of such poor soil. As a result, they are often drastically different from non-serpentine soil areas bordering the serpentine soils. In California, shrubs such as leather oak (Quercus durata) and coast whiteleaf manzanita (Arctostaphylos viscida ssp. pulchella) are typical of serpentine soils. An adaptive mechanism to high magnesium soils allocates more resources to deep-growing roots. Heavy metals stunt growth, induce iron deficiency, cause chlorosis, and restrict root development. Serpentine-tolerant plants are evolutionarily younger than non-serpentine plants. The heterogeneity of serpentine communities coupled with their patchy distribution limits gene flow but promotes speciation and diversification.

Serpentine barrens

thumb|right|Unlike most ecosystems, in serpentine barrens, there is less plant growth closer to a stream, due to toxic minerals in the water.

Serpentine barrens are a unique ecoregion found in parts of the United States in small but widely distributed areas of the Appalachian Mountains and the Coast Ranges of California, Oregon, and Washington. Barrens are especially likely in ultramafic areas which never had substantial deposits of non-ultramafic, more nutrient-rich material as carried by water (including glaciers) or wind (which delivers loess, sand or volcanic ash). In California, 10% of the state's plants are serpentine endemics. The barrens occur on outcrops of altered ultramafic ophiolites.

They are named for minerals of the serpentine group, resulting in serpentine soils, with unusually high concentrations of iron, chromium, nickel, and cobalt. Serpentine barrens, as at Grass Valley, California, often consist of grassland or savannas in areas where the climate would normally lead to the growth of forests.

Serpentine soils can be amended to support crops and pasture land for cattle grazing. This can be done by adding soluble calcium carriers such as gypsum to the soil. By adding gypsum or other calcium compounds a more favourable calcium-to-magnesium ratio can be developed, creating a better balance of plant nutrients. This, however, poses a possible implication to grazing cattle. An article from the Journal of Trace Elements in Medicine and Biology discerned that 20% of the grazing animals had toxic levels of nickel in their kidneys, and 32% had toxic levels of copper in their liver. Further study is needed to see if this will potentially have a negative effect on human health as it pertains to beef consumption.

Examples

Soldiers Delight Natural Environment Area in Baltimore County, Maryland, covers 1,900 acres of serpentine barren. The area has over 38 rare, threatened, and endangered plant species; as well as rare insects, rocks, and minerals.

thumb|right|Serpentine Barrens in Rock Springs Nature Preserve, Lancaster County, Pennsylvania

Rock Springs Nature Preserve in Lancaster County, Pennsylvania is a property conserved by the Lancaster County Conservancy that is a prime example of a serpentine barren. It was originally a grassland, but wildfire suppression led to the conversion of the area to forest. This barren contains the rare serpentine aster (Symphyotrichum depauperatum), as well as a number of rare species of moths and skippers.

In Chester County, Pennsylvania, the Nottingham Park, aka Serpentine Barrens, was recommended by UMCES as deserving of National Natural Landmark designation, on numerous grounds. They included supporting a number of rare and endemic species, an intact population of pitch pine, and also the site having historic significance.

Buck Creek Serpentine Barrens in Nantahala National Forest in Clay County, North Carolina, is another example. Dominant rock types are serpentinized dunite and olivine, with variable soil depths ranging from and rock outcrops representing 5–10% of the local landscape. The U.S. National Vegetation Classification for this community is "Southern Blue Ridge Ultramafic Outcrop Barren" and believed to be unique to the Buck Creek area. In 1995, the United States Forest Service began active conservation management of the site, primarily with prescribed controlled burns, which, along with some manual cover removal, has been successful in regenerating populations of previously sparse species. In addition to over 20 conservationally listed plant species, Rhiannon's aster (Symphyotrichum rhiannon) was described in 2004 and is endemic to these barrens.