Fall-line Magnolia Bogs of the Mid-Atlantic Region
Magnolia bogs have long been regarded as one of the most interesting natural features in the Washington, D.C. area. W.L. McAtee, a Washington area naturalist who first defined these bogs in 1918, termed them "Magnolia Bogs" for the unique assemblage of sweetbay magnolia (Magnolia virginiana), sphagnum moss, and other bog flora. Occasionally they are referred to as "McAteean Bogs," after McAtee, or "Seepage Bogs." These bogs usually form on hillsides or slopes where a spring or seep flows from an upland gravel and sand aquifer over a thick, impervious layer of underlying clay which prevents the downward infiltration of water. This seepage flow and the highly acidic, gravelly soils create optimal conditions for the formation of bogs.
The term "bog" as applied here, although technically a misnomer, has traditionally been used by people in general, including botanists, to describe acidic, sphagnous wetlands that strongly resemble bogs. Magnolia bogs are actually acidic, fen-like seeps uniquely associated with high elevation gravel terraces of the inner coastal plain near the fall line, which divides the coastal plain and piedmont physiographic provinces in the Mid-Atlantic region. 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 George's County, Maryland, to their southern extent near Fredericksburg, Virginia.
Throughout their range, magnolia bogs were never common or very large, usually occupying an area an acre or less in size. Nevertheless, they are vitally important resources both for the pure, naturally filtered waters which flow continuously from them - even in drought periods - and the relic populations of ancient northward and westward migrations of often rare coastal plain flora, which have persisted in small communities well inland and fairly close to the piedmont. Included in these relic communities are plants such as bog clubmoss, twisted spikerush, slender beaksedge, bunched beaksedge, hairy umbrella-sedge, darkgreen sedge, bog yelloweyed grass, ten-angled pipewort, smooth winterberry, red milkweed, zigzag bladderwort, and Elliott's goldenrod. Other well-known bogs in Anne Arundel County, Maryland, that are more eastward of the fall line-such as the extirpated Glen Burnie Bog and the Magothy Bogs - are not characteristic magnolia bogs, despite some floristic similarities, because of different geological conditions and plant assemblages.
Peatlands, pocosins, fens, and bogs throughout the coastal plain are now extremely rare as a result of habitat disturbance, fire suppression, and fragmentation. Magnolia bogs are also increasingly rare, and surviving ones are degraded throughout their range because of extensive development of the gravel terraces that surround the bogs-destroying or severely depleting their water supply. Most of the famous ones surveyed by the Smithsonian Institution and W.L. McAtee nearly a century ago, like the Holmead Swamp, Terra Cotta Bog, and Powder Mill bogs, have long been destroyed (although we recently uncovered a small remnant of the latter, along with a small population of ten-angled pipewort). Some, like the Suitland Bog and Oxon Run bogs, have survived, although the Suitland Bog is greatly disturbed with the addition of a boardwalk, numerous out-plantings of non-native (to the site) carnivorous pitcher plants that rob valuable habitat for native species, a sewer line, and encroaching housing developments. Urbanization, storm water runoff, siltation, off-road vehicles, and invasive exotic plants have degraded most of the few remaining magnolia bogs and greatly threaten their future survival.
For the past five years, as part of a research project mainly for conservation purposes, we have been conducting an exhaustive search for any remaining magnolia bogs in the region. All available information regarding the historic magnolia bogs - going back to the Civil War - was also researched and documented. We have been aided in these surveys by other botanists with the Maryland Native Plant Society (MNPS), and the preservation of surviving magnolia bogs has become a major campaign of MNPS. Although most of the historic bogs have been destroyed, some new sites have been discovered - the mostly pristine but threatened Araby Bog is a stellar example.
A dozen magnolia bogs are known to exist today in Maryland, D.C., and Virginia, three of which are in the path of the proposed InterCounty Connector, and several small remnants of historic bogs like the Ammendale and Powder Mill Bogs have been discovered. While most of the rare orchids and lilies have largely disappeared, several very rare plants that had not been seen for many decades - halberd-leaved greenbrier, low rough aster, and Long's rush, for example - have been rediscovered. Several previously unreported plants for Maryland - including featherbristle beak sedge (Rhynchospora oligantha) - have also turned up.
Rod Simmons is a field ecologist with the City of Alexandria; Mark Strong a botanist with the Smithsonian Institution. They expect to publish their research on coastal plain magnolia bogs early next year.