Serpentine Barrens

"Goat Hill Serpentine Barrens in SE Pennsylvania. Photo by Josh Klostermann"

Serpentine Barrens are unique ecoregions that can be found globally, in small patchy pockets of land with serpentine bedrock and soil. The term "serpentine barren" generally refers to a serpentine outcrop and its associated vegetation. These habitats are notable for their distinct flora with high rates of endemism. The difference in vegetative communities is the product of chemical, physical, and biotic edaphic factors but is primarily driven by the toxic chemical composition of the serpentine bedrock and soil. These systems are used to study edaphic endemism and plant speciation. [2] They are also used as an analog to brownfields in restoration ecology because of the high heavy metal content in the soil.

Rock formation:

"Soldiers Delight Serpentine Barren, MD. Photo by Josh Klostermann

Serpentine is not the name of a single mineral. Instead, it is a name used for a large group of minerals that fit this generalized formula: (X)2-3(Y)2O5(OH)4.

In this formula, X will be one of the following metals: magnesium, iron, nickel, aluminum, zinc, or manganese; and, Y will be silicon, aluminum, or iron. The appropriate generalized formula is therefore as follows: (Mg,Fe,Ni, Mn,Zn)2-3(Si,Al,Fe)2O5(OH)4.

Chrysotile, antigorite, and lizardite are three of the primary serpentine minerals. There are many other serpentine minerals, most of which are rare.

Serpentine minerals form where peridotite, dunite, and other ultramafic rocks undergo hydrothermal metamorphism. Ultramafic rocks are rare at Earth's surface but are abundant at the oceanic moho, the boundary between the base of the oceanic crust and the upper mantle.

They are metamorphosed at convergent plate boundaries where an oceanic plate is pushed down into the mantle. This is where they are subjected to hydrothermal metamorphism. The source of water for this process is seawater entrained in the rocks and sediments of the oceanic slab.

During hydrothermal metamorphism, olivine and pyroxene minerals are transformed into or are replaced by serpentine minerals. Some of the metamorphic rocks produced here are composed almost entirely of serpentine minerals. These serpentine-rich rocks are known as "serpentinites."

Extensive areas of Earth's surface are underlain by serpentinites. These areas occur near present or ancient convergent plate boundaries. They are locations where remnants of an oceanic plate are exposed at the surface. The remnant portion of the plate was either thrust up onto land, accreted onto the edge of a landmass, or exposed by uplift and deep weathering.

These areas of exposed oceanic plate are known as ophiolites. They are often the source of valuable minerals that might include magnetite, chromite, chrysoprase, jade, and serpentine.

Article by: Hobart M. King, PhD, RPG [6]

Evolution and soil edaphic endemism:

Serpentine soils are characterized by their harsh chemical composition. They contain metals such as iron, nickel, chromium, and cobalt which are toxic to most plants. They are also deficient in essential plant nutrients such as N, P, K. Their physical conditions also make it tough for plants to adapt. Outcrops are often found on steep rocky slopes, with high rates of erosion and therefore shallow topsoils. These physical conditions create an environment that holds little moisture and low nutrient levels. The biota found using these soils is sparse. Little plant life leads to further erosion and also increases soil temperatures. [2]

Together the chemical, physical, and biological factors that characterize serpentine soils culminate in the term coined by (our lord and savior) Hans Jenny in 1980, known as the "Serpentine Syndrome". This term is an expression used to indicate the cumulative effect that these 3 factors have on the development of vegetative communities growing in serpentine soil. [2] The "Serpentine syndrome" is one of the classic and most well-studied cases of edaphic plant endemism.

From a birds-eye view, serpentine areas look like islands with steep vegetative gradients as boundaries. They are like archipelagoes of specialized habitat for species with disjunct or endemic distributions. This makes them textbook examples for the study of plant speciation and island biogeography. Vegetative communities growing in serpentine barrens differ from surrounding areas by having a notably distinct physiognomy and community structure, high rates of endemism, low species richness, a dominance of specific taxa, a predominance of xerophytic species, low productivity, and the co-existence of high and low ph-tolerant species. [1]

Pennsylvannia and Maryland Serpentine Barrens:

The serpentine barrens of Eastern North America are found in thin, patchy, sinusoidal-bands, that work their way down the coast. Historically, the barrens of Pennsylvania and Maryland were maintained as grasslands for hunting grounds by the Susquehannock and other unknown Native American Tribes. Marye (1955) concluded that fire deliberately set by Native Americans was responsible for maintaining the grassland/savannah communities found in these serpentine habitats of the Northeast. [1] Post-European invasion and settlement, these areas were used for grazing due to their "barrenness". Barrens that were not grazed by livestock transitioned into a forest habitat. By 1930 almost all grazing in this area had ceased and conifer invasion and expansion began. Presently, afforestation has occurred in more than 90% of undeveloped serpentine barrens due to fire suppression. Current restoration efforts such as prescribed burns are being used to improve the health and quality of these globally important habitats. [7]

Flora:

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Below are compiled lists of the flora found at Soldiers Delight Serpentine Barrens. These species are typical of the Eastern Serpentine Region of SE Pennsylvania and Maryland.

of Wildflowers at Soldiers Delight Serpentine Barrens

of Woody Plants at Soldiers Delight Serpentine Barrens

of Ferns and Allies at Soldiers Delight Serpentine Barrens

of Lichens at Soldiers Delight Serpentine Barrens

Threats and Conservation

Major threats to the eastern serpentine barrens are afforestation, invasive species, and development. [7]

Citations:

1. Anderson, R. C., J. S. Fralish, and J. M. Baskin, editors. 1999. Savannas, barrens, and rock outcrop plant communities of North America. Cambridge University Press, Cambridge, UK ; New York, NY, USA.

2. Brady, K. U., A. R. Kruckeberg, and H. D. Bradshaw Jr. 2005. Evolutionary Ecology of Plant Adaptation to Serpentine Soils. Annual Review of Ecology, Evolution, and Systematics 36:243–266.

3. Jenny, H. 1980. The Soil Resource. Springer New York, New York, NY.

4. Nottingham_Evaluation_Review.pdf. (n.d.). .

5. Rajakaruna, N. 2004. The Edaphic Factor in the Origin of Plant Species. International Geology Review 46:471–478.

6.Serpentine: mineral, gem, ornamental stone, asbestos source. (n.d.). . https://geology.com/minerals/serpentine.shtml.

7. Latham, R. E. 1993. The Serpentine Barrens of Temperate Eastern North America: Critical Issues in the Management of Rare Species and Communities. Bartonia:61–74.