The Magnolia Bog--Must this ancient habitat vanish?
The mid-Atlantic region was once sprinkled with a type of delicate wetland called variously a Fall-line Bog, Gravel-seep Bog, Coastal Plain Swamp, or Magnolia Bog. As these names suggest, a Magnolia Bog springs from a distinctive confluence of soils, topography, hydrology, and plant distribution. Restricted to coastal plain formations, where perpetual seeps and springs are fed by rainwater stored in sands and gravels atop impervious clay layers, Magnolia Bogs are found only in a band along the fall line from northern Virginia to New Jersey, and nowhere else. Today, most of these ancient habitats have been lost. So few remain that they have been placed on the global watchlist as vulnerable to extinction.
Magnolia Bogs tend to form where gravel terraces begin sloping to form nascent streams. Here, seeps, or springs, may sustain perpetually saturated conditions. Rainwater, arrested by the deep leaf litter covering a forested terrace, infiltrates the gravelly soils until meeting a layer of clay, where it is stored as a perched water table. This water table supplies seeps where the clay layer is exposed by a sloping topography. With a gentle slope, the reluctant flow of water sustains the requisite soggy conditions.
Over millennia, percolating water leaches nutrients from the gravelly soils while the terrace forests confer acidic conditions. The stage is set: scarce nutrients, acidic soils, and water-saturated conditions. Add plants that are tolerant to these challenges and also adapted to our climate, and we have the makings of the Magnolia Bog.
It is the Sweetbay Magnolia that lends its name to these wetlands. With shiny, elongated leaves and showy cream blooms supported tens of feet high, the sweetbay can be the first clue one is approaching an enchanting experience embedded within the surrounding forest. Spongy hummocks of sphagnum moss are also characteristic and reminescent of true peat bogs, source of the gardeners peat-moss. In times of high rainfall, the sphagnum hummocks can be expected to moderate flooding, and thus serve as a second line of defense, after the surrounding forest itself, to prevent destructive scouring of streams.
Other inhabitants often include elephantine-leaved Skunk Cabbage and shoulder-height Cinnamon Fern. Other plants to be found include Netted Chain Fern, Royal Fern, and the to-be-avoided Poison Sumac, as well as a wide variety of additional wetland associates, such Swamp Azalea, Swamphaw, Native wisteria, Swamp Sweetbells, Highbush Blueberry, Winterberry, and various grasses and sedges.
As water wanders through a Magnolia Bog, it gradually gathers to form a perpetual stream, never mind the intermittent one often indicated by USGS topo-maps. The acidity levels decline and utterly clear water begins to support the aquatic web of life, from microbes to minnows, amphibians to brook lamprey, and much else.
The bog not only tempers flooding during storms but, between rainfalls, supplies purified high quality baseflow sustaining downstream integrity. Thus the impairment of a Magnolia Bog to the general encroachment and changes in hydrology wreaked by housing developments is a loss even greater than that of a special and now-rare habitat. Even denizens as remotely based as the Atlantic Ocean may suffer, for migratory fish spawn in waters receiving the gifts of the bog.
A case in point is Mattawoman Creek, in Charles County, Maryland. The headwaters of some tributaries to Mattawoman originate even today in Magnolia Bogs. And Mattawoman is the most productive known spawning and nursery creek for migratory river herring in the Chesapeake Bay system. Perhaps it is telling that one of its tributaries, the second most popular small stream to spawning herring in the Potomac River drainage, itself originates in a Magnolia Bog. Today these few remaining bogs are highly threatened by county policies that allow for their impairment by sprawling developments.
At present, at least two Mattawoman bogs are known to be within sites proposed for housing developments. There may be more, because assessments are spotty and the Mattawoman watershed, coinciding as it does with a gravity-fed sewer district, has been designated a "development district", one larger than the District of Columbia, by proclamation of the county's Comprehensive Plan. One of the threatened bogs is especially notable for its exceptional size and its location adjacent to a state Wildlands and to a historic plantation. Its jeopardy is particularly disappointing because it occurs within a "deferred" portion of the development district, that is only supposed to be considered for development two decades hence, if ever, according to the Comprehensive Plan. However, the Charles County Government has come up with a justification for ignoring its own promise, and has given preliminary approval to dense development that would severely degrade the bog. The Araby Bog, named for the adjacent historic plantation, deserves all of our efforts for preservation.
-------------
MORE ABOUT BOGS
From web site: http://www.state.va.us/~dcr/dnh/environ.htm, the following description of one of the "Conservation Priorities of the Future" includes Magnolia bogs:
Coastal Plain Seepage Swamps and Bogs
Seepage swamps are groundwater-fed wetlands which remain saturated year-round. They are characterized by sweetbay magnolia, titi, poison sumac, sedges, and rare plants such as white-fringed orchis, purple pitcher-plant, New Jersey rush, golden colicroot, and swamp pink. Unusual animals also reside here, such as the sphagnum sprite and seepage dancer damselflies. Non-forested seepage areas containing an abundance of sphagnum moss are often referred to as bogs. Ecologists speculate that many of the so-called bog plants were more common in Virginia when frequent fire kept the competing woody growth in check. Presently, most of the remaining bogs occur in the clearings maintained along powerlines, railroads, or roadsides.