|
A Magnolia Bog
|
| The Araby Bog is a diverse, 6.5 acre Magnolia Bog in Charles County, Maryland with a large, open section that gives rise to a pristine perennial stream that flows into nearby Mattawoman Creek. Magnolia Bogs are acidic, fen-like seeps (Shetler 2000, Thomas 2000) associated with gravel terraces of the inner Coastal Plain near the fall line that are named for the unique assemblage of Sweetbay Magnolia (Magnolia virginiand), Sphagnum moss, and other bog flora (McAtee 1918, Shetler 1970). Occasionally they are referred to as "McAteean Bogs", after W.L. McAtee who first defined them in 1918 (Shetler 2000), or "Seepage Bogs" (Fleming et al. 2001). These rare wetlands differ geologically and hydrologically from the famous, rain-fed (ombrotrophic) peat bogs of New England and northern Europe because they occur in terrace gravel deposits and are spring-fed. Their distribution generally follows the fall line in a narrow east-west band from the Laurel area, at the northern extent of their range, in Prince Georges County, Maryland, to their southern extent, near Fredericksburg in Caroline County, Virginia, and are perhaps best developed in the Washington, D.C. vicinity (Simmons and Strong 2001). Throughout their range they were never common or very large, usually occupying an area an acre or so in size (McAtee 1918). Other well known bogs near Washington that are more eastward of the fall line, like the Glen Bumie and Magothy Bogs, are not characteristic Magnolia Bogs, despite some floristic similarities, because of different geological conditions and plant assemblages (Shreve et al. 1910, Sipple 1999, Severn River Association 2000). | |
|
Terrace gravel soil profile
Terrace gravel soil profile
showing thick clay lens 
Exposed gravel-sand substrate at Araby Bog
Bog Clubmoss (Lycopodiella appressa) and spring-flushed gravel-sand substrate
Sphagnum moss and Sweet Magnolia leaves
Open Section of Araby Bog
Twisted Spikerush (Eleocharis tortilas)
Twisted Yellow-eyed Grass (Xyris torta)
Virginia Meadow Beauty (Rhexia virginica)
|
Magnolia Bogs are enlarged springs or seeps that usually form on a slope where a perched water table intersects the ground surface above an impervious clay lens or aquiclude. Such seepage areas are particularly associated with terrace gravel formations which hold large amounts of rainwater in the porous sand and gravel lenses. The soils of high elevation gravel terraces in the vicinity of the fall line are composed of beds of cobble (gravel), sand, silt, and clay that were deposited by the Potomac River during glacial melting millions of years ago (Weems 1995). They are weathered, very acidic (pH 4.0), and generally infertile as a result of calcium ions and other nutrients associated with rich soils leaching into stream valleys below over a long period of time (Simmons 1995). The soils of Magnolia Bogs are also very acidic, sandy, and gravelly because they are derived from terrace gravel deposits. As a result, they are not characterized by accumulated peat or organic soils as true peat bogs are, except in some super-saturated areas. Nor are they characterized by the deep, mucky soils of more commonly encountered wetlands in the region, though occasionally these soils occur in small pockets within the Magnolia Bog complex.
Extensive studies of Magnolia Bogs throughout their range reveal most, if not all, to include dense, shaded thickets of ferns, shrubs, and large magnolias, as well as open, sunny areas dominated by graminoids (grasses and grass-like plants), a diversity of herbaceous plants, and scattered shrubs (Hitchcock and Standley 1919, Simmons and Strong 2001). Sphagnum moss is a dominant groundcover in both settings because of the permanently saturated, acidic conditions throughout the bog (pH 4.2-5.0) (Simmons 1995, Long 1999). The gravel and sand substrate is frequently exposed as well, "flushed...to the surface or slightly overflowed...by a constant flow of clear, cool spring water' (McAtee 1918). A characteristic "suite" of plant species, many of which are rare in Maryland and Virginia, comprises the flora of Magnolia Bogs and in addition to the aforementioned factors makes them unique (McAtee 1918, Hitchcock and Standley 1919, Shetler 1970, Simmons and Strong 2001, Fleming et al. 2001). Several rare dragonflies and damselflies (Fleming et al.. 2001) and a rare copepod (Thomas 1991) are also associated with Magnolia Bogs. Growing on forested slopes slightly above the bogs, in the permanently moist soil (capillary fringe) that surrounds the feeder-seeps, are often lush carpets of Ground Pine {Lycopodium obscurum\ ferns, scattered ericads (shrubs in the Heath Family), and other characteristic plants of acidic seeps. These areas are usually extensive and are typically associated with Magnolia Bogs (Simmons and Strong 2001). Constant groundwater outflow that passes through these seeps and accumulates in the bogs then continues on as small perennial streams (Hitchcock and Standley 1919, Simmons and Strong 2001), sometimes forming acidic Seepage Swamps (Fleming et al. 2001). Such a condition exists at the Araby site. Magnolia Bogs are therefore important components of the intricate hydrological cycle of terrace gravel communities, which also include upland Oak-Chestnut-Heath Forest, acidic seeps, perennial streams, and Seepage Swamps. Magnolia Bogs have become increasingly rare and surviving ones degraded throughout their range because of extensive development of the gravel terraces that surround the bogs, which destroys or severely depletes their water supply (Rosenstock 2001). Most of the famous ones near Washington surveyed by the Smithsonian Institution and others nearly a century ago, like the Powder Mill Bogs, Holmead Swamp, and Terra Cotta Bog, have been destroyed. Some, like the Suitland Bog and the Oxon Run Bogs, have survived, although the Suitland Bog is greatly disturbed and faces further damage by encroaching development. The Little Paint Branch Bogs (small remnants of the once-extensive Powder Mill Bogs) exist under power lines and are therefore degraded by invasive exotic plants and utility maintenance, especially herbicides and utility vehicles. Siltation has damaged the bogs at the Beltsville Agricultural Research Center and channelized stormwater runoff from a housing development placed too near has mostly destroyed the Franconia Bog in Springfield, Virginia, Fairfax County's largest and best remaining Magnolia Bog. Urbanization, stormwater runoff, siltation, utility maintenance, and invasive exotic plants have degraded most of the remaining bogs. In contrast, the Araby Bog is unique among known Magnolia Bogs because of its pristine, undisturbed condition. Unfortunately, it is currently threatened by the proposed Falcon Ridge and Hunters Brooke development projects because of the extensive clearing, re-grading, and construction planned for the land surrounding the bog, especially the groundwater recharge areas. Peatlands, pocosins, fens, and bogs throughout the Coastal Plain are extremely rare as a result of habitat disturbance, fire suppression, and fragmentation (Fleming et al. 2001). Similarly, Magnolia Bogs of the mid-Atlantic region are globally-rare natural communities (Morse 2000, Shetler 2000, Thomas 2000, Simmons and Strong 2001) with dwindling prospects for future survival. Unless adequate protection is uniformly given to these sites, most of them will disappear in the decades to come. To ensure some degree of conservation and stewardship, as well as recognition of their being highly-rare natural communities, it is strongly recommended that all Magnolia Bogs, regardless of rare species content, be designated a "Wetlands of Special State Concern" or similar status. |