Mafic Fens and Seeps
These saturated wetlands are similar to Calcareous Fens and Seeps but occur on soils weathered from mafic or ultramafic igneous and metamorphic rocks. In Virginia, these small-patch communities are known primarily from The Glades and Buffalo Mountain areas of the southern Blue Ridge plateau (Grayson and Floyd Counties, respectively), with an outlying site at Big Meadows in the northern Blue Ridge (Madison County). Habitats are hillside or foot-slope spring seeps and groundwater-saturated small stream bottoms. Microtopography is typically irregular, with hummock-and-hollow development, braided streams, areas of coarse gravel and cobble deposition, muck-filled depressions, and superficial to substantial peat accumulations. Soils, derived from underlying amphibolite, actinolite schist, gabbro, metabasalt, or similar rocks, are strongly to slightly acidic, with high magnesium and iron levels and moderately low calcium-to-magnesium ratios. Communities in this group that lack significant organic soils are technically "seeps," although the term "fen" is widely applied to various mafic or calcareous seepage wetlands in the southeastern United States (see Weakley and Schafale 1994 for further discussion).
The vegetation of these wetlands ranges from open woodlands and tall shrublands to wholly herbaceous, but is often a patch-mosaic of woody growth and herbaceous openings. In the woodland variants, the overstory is dominated by combinations of red maple (Acer rubrum), eastern white pine (Pinus strobus), pitch pine (Pinus rigida), and tulip-tree (Liriodendron tulipifera); the lower layers contain a peculiar mix of typical seepage swamp species, mafic indicators, and species commonly associated with acidic wetlands. Smooth alder (Alnus serrulata), meadowsweets (Spiraea latifolia and Spiraea tomentosa), stiff dogwood (Cornus racemosa), common ninebark (Physocarpus opulifolius var. opulifolius), maleberry (Lyonia ligustrina var. ligustrina), spicebush (Lindera benzoin var. benzoin), and alder-leaved buckthorn (Rhamnus alnifolia) are characteristic shrubs in both woodland understories and open fens.
The herbaceous flora of mafic fens contains many state-rare and unusual species, including several Coastal Plain-mountain disjuncts. Herbs that are particularly diagnostic of or abundant in these habitats include Canada burnet (Sanguisorba canadensis), large-leaved grass-of-parnassus (Parnassia grandifolia), bluejoint (Calamagrostis canadensis), short-leaf sneezeweed (Helenium brevifolium), bog goldenrod (Solidago uliginosa var. uliginosa), sterile sedge (Carex sterilis), field sedge (Carex conoidea), hairy fimbry (Fimbristylis puberula var. puberula), northern white beaksedge (Rhynchospora alba), marsh muhly (Muhlenbergia glomerata), royal fern (Osmunda spectabilis), cinnamon fern (Osmundastrum cinnamomeum var. cinnamomeum), woodland bulrush (Scirpus expansus), rough-leaved goldenrod (Solidago patula var. patula), sticky false asphodel (Triantha glutinosa), and twig-rush (Cladium mariscoides).
The processes that maintain these systems in open condition are poorly understood. All of the documented occurrences are small and have been disturbed to some degree by logging and/or grazing. Ditching and other hydrologic alterations, grazing, non-native weeds, woody succession and, in more open fens, perhaps fire exclusion are continuing threats to these naturally rare wetlands, most of which remain unprotected.Reference: Ogle (1982).
Click on the images below to open a larger image in a separate window.A dense sward of bluejoint (Calamagrostis canadensis) fills an herbaceous opening in a Northern Blue Ridge Mafic Fen at Big Meadows, Madison County (Shenandoah National Park). Stiff dogwood (Cornus racemosa) forms shrub thickets in the background. Photo: Gary P. Fleming / © DCR Natural Heritage.
Low herbaceous fen dominated by graminoids at The Glades, Grayson County. Photo: Irvine Wilson / © DCR Natural Heritage.
Mafic shrub fen at The Glades, Grayson County. Dominant species include smooth alder (Alnus serrulata), Canada burnet (Sanguisorba canadensis), and bluejoint (Calamagrostis canadensis). Photo: Irvine Wilson / © DCR Natural Heritage.
Herbaceous opening in a Northern Blue Ridge Mafic Fen; prevalent species include buckbean (Menyanthes trifoliata , foreground) and several sedges (Carex buxbaumii , C. exhinata ssp. echinata , and C. scoparia). Big Meadows, Madison County (Shenandoah National Park). Photo: Gary P. Fleming / © DCR Natural Heritage.