Natural Communities - Serpentine Grasslands
Tens of thousands of acres of grassland dotted with Blackjack and Post Oaks once stretched across northern Maryland and nearby Pennsylvania. Prior to European settlement, most of Baltimore and Harford Counties and adjacent counties in Pennsylvania were covered by this prairie-like grassland. English settlers seeing this virtually treeless expanse referred to it as "The Barrens."
For thousands of years, Native American fire-hunting kept the grasslands relatively free of woody vegetation and created patches of bare ground for herbs to colonize. When European settlement eliminated large-scale frequent fires, the grassland areas were replaced by woodlands. Prairie-like vegetation persisted on serpentine, a dry and nutrient-poor soil. Unfortunately, most of the prairie-like serpentine grasslands have been destroyed in the last century by mining and development, and invading pines and junipers threaten to take what remains.
Four major remnants of the globally rare serpentine grassland still exist in Maryland, occupying less than five percent of the original grassland area but harboring at least 34 rare and endangered plant species. These rare plants at Soldier' Delight, Robert E. Lee, and two privately owned natural areas are increasingly threatened by the encroachment of pines and junipers.
Serpentine, the Snake-colored Soil
Serpentine, or serpentinite, is a mineral producing dry, nutrient-poor soil deadly to plants not specially adapted to its unusual chemistry. In folklore, the name "serpentine" is attributed to the soil's resemblance to a mottled greenish-brown snake dwelling on similar soils in northern Italy. The greenish soil color comes from fragments of the underlying bedrock containing magnesium silicate. Toxic to plants, as much as one-third of the bedrock may be made of magnesium. The soil can be very dark in color, depending on its iron, chromite, and magnesium content.
High levels of magnesium in the soil block a plant's ability to take in soil nutrients, especially calcium. Because they are shallow and low in organic material and clay, serpentine soils also cannot hold water or nutrients well.
Serpentine soils often have pockets of naturally occurring heavy metals toxic to plants, such as chromium, cobalt, and nickel. Also, these soils are normally acidic near the surface, but less so in deeper layers. As wind and water erode the soil, non-acidic layers are exposed, creating varied habitat for plants.
In the eastern U.S., scattered serpentine lands occur in a band stretching from Alabama to Canada, with the largest and most floristically distinctive part located in southeastern Pennsylvania and northern Maryland. One of the four remaining serpentine areas in Maryland, the Soldiers' Delight Natural Area near Baltimore, is the largest in eastern North America, encompassing 2000 acres, and is among the most species-rich in the world.
To live in dry, magnesium-rich, nutrient-poor, and, in some cases, toxic soils, most serpentine plants have developed special adaptations in form or internal chemistry. Some plants are extra-efficient at absorbing the crucial nutrient calcium, which offsets the negative effects of magnesium. Some herbaceous species on serpentine soils avoid the heat by being very hairy or having specialized leaf types. For example, the Serpentine Chickweed, which occurs only on serpentine soil, produces a thick covering of hair on its leaves and stems. This dense layer of hair serves to reflect excessive sunlight which could cause leaves to overheat. In addition, it serves as a humidity trap, preventing the leaves and stems from wilting during hot summer droughts.
Another adaptation is expressed by the dominant plant of the serpentine grassland, Little Bluestem grass. During droughts as well as during the midday hours of the summer, this species prevents excessive water loss by rolling its long, narrow leaves inward. In addition, extensive root systems allow many plants to tap more moisture from the dry, shallow soils.
Streams or open seeps crossing the grasslands allow moisture-loving species to survive in the otherwise hot, dry grasslands. Streams also expose layers of soil with greatly different acidity, allowing plant species which prefer either acidic or basic soils to exist side by side. The harsh conditions imposed by serpentine soils limit woody plant growth, so trees do not reach the same size they would in a richer habitat.
Rare Species of the Serpentine Grasslands
Visitors to the serpentine grassland are greeted with a fascinating array of true prairie grasses, such as Little Bluestem, Indian Grass, and Purplish Three-awn. Brightly colored blazing stars, asters, goldenrods, and other wildflowers sparkle against the subtle hues of the swaying grasses. Serpentine grasslands are not as rich in species or as densely vegetated as many other Piedmont habitats. However, serpentine outcrops harbor an unusually high percentage of rare or isolated species.
Uncommon plants are able to colonize serpentine grasslands for many different reasons. Plants that cannot compete in richer habitats may survive in the sparsely vegetated serpentine grassland, thriving as they adapt to its harsh conditions. For plants originally from another locale, the grassland may resemble their original habitat in temperature, moisture availability, or soil characteristics.
Some plants more common to the south or west of Maryland have become established in the hot, dry grassland. These plant populations are said to occur at the northern or eastern limit of their range. Plant populations such as these are important to protect because, in adapting to the new habitat, they may become markedly different from the rest of the species. Preserving these outlying populations ensures that the genetic diversity of the species will be maintained.
Of the rare plants on Maryland's serpentine outcrops, the Serpentine Aster (Aster depauperatus) is the only species restricted to this habitat throughout its range. Rather than migrating from a similar habitat type as many other plants have, the Serpentine Aster actually evolved on serpentine soils. A low, branching plant with small, daisy-like white flowers, this fall-blooming aster occurs at only four sites in Maryland and is a federal candidate for listing as an Endangered or Threatened species.
The Sandplain Gerardia (Agalinis acuta) has numerous funnel-shaped pink blossoms that are open from late summer through early fall. Once more widespread in acidic, nutrient-poor Coastal Plain soils, this federally Endangered species has been largely eradicated by development and now occurs at only one site in Maryland, a serpentine grassland in Baltimore County. State biologists have been conducting research to determine what role disturbances, such as breaking up the soil surface, play in the survival of this plant.
Currently listed as an Endangered species in Maryland, the rare and beautiful Fringed Gentian (Gentianopsis crinita) once flourished in at least five locations in the state. Scattered along streams and beside open seeps, it now thrives at only two serpentine areas in Maryland, its silky, purplish-blue flowers radiant in full sunlight.
The small, pink blossoms of the state-rare Fameflower (Talinum teretifolium) are usually open for only a few short hours on sunny days. This species is confined almost solely to serpentine outcrops and is listed as a Threatened species in Maryland. Bees are attracted to the bright patches of star-shaped flowers, which contrast sharply with the otherwise stony soil.
Biologists are now exploring the likely existence of rare insects associated with serpentine plants. Insect populations from prairies or other barren habitats are drawn to certain plants of the serpentine grassland used by larvae as food or by adults as nectar sources. Rare insects are much more likely to be associated with serpentine grasslands than are rare birds or mammals. Grasses and other flowering plants grow sparsely on serpentine soils and may not provide sufficient cover or food for resident populations of grassland birds or mammals.
Edward's Hairstreak (Satyrium edwardsii), a butterfly rarely seen south of Pennsylvania, inhabits the edge of oak thickets at one of Maryland''s serpentine areas. It is listed as an Endangered species in Maryland. Its larvae feed on Scrub Oak and Post Oak, while adults, emerging in mid-summer, seek the nectar of the grassland's milkweeds and goldenrods. The hairstreak's larvae are often surrounded by armies of Allegheny Mound-builder Ants, protecting them from raids by parasitic wasps. In return, the ants feed harmlessly upon a fluid produced by the larvae. Both species benefit from this unusual relationship.
Biologists at Soldiers' Delight Natural Area have sporadically seen a brightly colored leafhopper species so new to science that it hasn't yet been given a name. Even in serpentine grasslands, this leafhopper is scarce, making the study of its food and habitat requirements difficult. Although most of its close relatives are desert-dwellers to the south and west of Maryland, a rare black pear-shaped beetle (Polypleurus perforatus) makes its home under dead wood in serpentine grasslands and other barren habitats.
Some rare insects that occur in serpentine areas may once have been more common in Maryland's glades or barrens or in prairie habitats of the Midwest. In the last three centuries, these habitats have been greatly reduced by development and disruption of the natural disturbances that once kept them open.
History and Management of the Grasslands
Today, visitors to serpentine areas will find a sparsely vegetated mosaic of stunted oak and pine forests surrounding small grassy openings. These habitat islands are the last remaining fragments of what was once an extensive prairie that covered most of the eastern United States when the climate was hotter and drier, 25,000 years ago.
As the climate cooled and the prairies receded, the inhospitable serpentine soils remained relatively open and grassy for thousands of years before European settlement, thanks to lightning fires and the fire-hunting habits of Native Americans. Native Americans often set extensive fires as a hunting method to entrap deer, driving them toward open areas where they could be killed easily. The grasslands created by these fires also provided important habitat for deer and other game species.
William Bose Marye (1886-1979), a renowned Maryland historian, devoted his life to studying the history of the local grassland ecosystem. His writing vividly describes the vast and inhospitable "barren," traversed by birds on their migratory flights. "One of the outstanding sights of the Barrens must have been great flocks of birds, such as crows, blackbirds, and pigeons, winging their way across the waste," writes Marye. According to an early settler, "The Flocks of these Birds of Passage are so amazingly great, Sometimes, that they darken the Sky; nor is it uncommon for them to light in such Numbers on the Larger Limbs of Mulberry-Trees and Oaks as to break them down."
After the decimation of Indian tribes by smallpox in the late 17th century, fire no longer played a significant role in the ecology of serpentine grasslands, and woody vegetation began to invade. Since the inhospitable serpentine soils were often the last areas to be forested, the serpentine grasslands were valued by settlers for livestock forage. Grazing and trampling by livestock slowed the encroachment of woody plants, but were not as effective as fire in maintaining the prairie.
With the removal of livestock from serpentine areas, probably in the 1930s, the loss of grassland greatly accelerated. Mining of serpentine areas in the early twentieth century for chromite, talc, asbestos, magnesite, soapstone, and other minerals also took its toll. In recent decades, the rapid pace of residential development has laid claim to most of the remaining serpentine grasslands.
In serpentine areas today, patches of prairie-like grasslands persist mainly on the drier, south-facing slopes, surrounded by an ever-encroaching forest of Virginia Pine and Eastern Red Cedar.
An ecological management program has been initiated to halt the spread of these conifers and to restore some of the largest historical grasslands now occupied by pine woodlands. With the aid of community volunteers, pines and cedars are cut and transported off site for disposal. Native oaks are not cut since they were historically a part of this natural community. When conditions are right, these areas are then burned, using state-of-the-art fire management techniques. State personnel involved are professionally trained in planned burns as well as fire suppression and were among the first in the country to be called upon to assist in fire control during the Yellowstone National Park fire of 1988. Whenever possible, the public is invited to witness and participate in restoration efforts, including planned or prescribed burns.
For more information please contact:
Maryland Department of Natural Resources
Wildlife and Heritage Service
Tawes State Office Building, E-1,
Annapolis, MD 21401
Phone (410) 260-8540
Toll-free in Maryland: 1-877-620-8DNR, Ext. 8540
Text by Ann Prince and other staff of the
Wildlife and Heritage Service.
Illustration and layout by Josephine Thoms,
DNR Land Planning Services.
Map by John Prince,
National Office of The Nature Conservancy.
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