High Allegheny Wetland

This system occurs along the high plateau of the Allegheny Mountains, immediately west of the Allegheny Front at elevations between 730 and 1430 m. Wetlands in this system are drained by low-gradient, meandering, intermittent to small headwater streams. Drainage is impounded in high, flat-lying basins by natural dams or "knickpoints" of resistant sandstone. In addition to poor moisture drainage, cold air drains from the surrounding uplands to pool in the flat basins, which function as frost pockets. Rainfall is plentiful, averaging about 1300 mm/year. Communities in this system may have substrates of shallow to deep peat or, less commonly, mineral soil. Soils are acidic to circumneutral. These high Allegheny wetlands form complex mosaics ranging in size from a few hectares to 6000 hectares. Forested swamps occupy the less disturbed margins or slightly higher "islands." This system has a distinctly northern character in its resemblance to bogs and swamps of New England. However, the striking absence of Chamaedaphne calyculata and Picea mariana, two abundant and common species of northern bogs and swamps, as well as the presence of species characteristic of the Southern Appalachians, such as Hypericum densiflorum, Vaccinium erythrocarpum, and Rhododendron maximum, distinguishes this system from its northern counterpart. Ombrotrophic bogs are rare but occur in undisturbed portions of a few of the larger wetlands. The more central, flood- or beaver-influenced portions contain shrub swamps, sedge fens, wet meadows, and open marshes. Forested swamps are dominated by Picea rubens, with varying cover by Acer rubrum, Tsuga canadensis, and Betula alleghaniensis var. alleghaniensis. It is likely that the role of Pinus strobus played a greater role in the structure and function of this system historically than it does today (Maryland Geological Survey and Curran 1902). Residual white pines in remote areas of this system in Cranesville Swamp in Maryland also suggest this possibility. Where limestone or calcareous shale influences seepage water, Abies balsamea and Fraxinus nigra are typical canopy dominants. Common shrub species are Viburnum nudum var. cassinoides, Rhododendron maximum, Vaccinium myrtilloides, Alnus incana ssp. rugosa, Hypericum densiflorum, Ilex verticillata, and Aronia melanocarpa. Herbaceous species frequently include Rubus hispidus, Solidago uliginosa, Juncus effusus, Eriophorum virginicum, Osmunda cinnamomea var. cinnamomea, Polygonum sagittatum, Carex folliculata, Carex gynandra, Leersia oryzoides, Galium tinctorium, Solidago rugosa, Symplocarpus foetidus, Lycopus uniflorus var. uniflorus, Scirpus cyperinus, Carex scoparia var. scoparia, and Carex trisperma var. trisperma. Sphagnum spp. and Polytrichum spp. dominate the bryophyte layer. This system is maintained by a spatially complex mix of seepage, low-energy flooding, beaver activity, and rainfall. Undisturbed examples exist (e.g., Cranberry Glades), where old-growth swamp buffers the central peatlands, which have been dated to 10,000 years. In presettlement time, some wetland mosaics in this system had significant forested components (e.g., Canaan Valley, Cranesville Swamp in West Virginia; Finzel Swamp, Hammel Glades in Maryland), while others (e.g., Cranberry Glades, Big Run Bog) were largely open peatlands with forested swamp only on the margins.

These High Allegheny wetlands form complex mosaics of small-patch communities. Forested swamps occupy the less disturbed margins or slightly higher "islands" in the wetland mosaic. Ombrotrophic bogs are rare but occur in undisturbed portions of a few of the larger wetlands. The more central, flood- or beaver-influenced sections contain shrub swamps, sedge fens, wet meadows, and open marshes. A number of species have northern affiliations, including some that are disjunct (e.g., Abies balsamea, Larix laricina, and Andromeda polifolia var. glaucophylla). The shrub strata include characteristic central Appalachian species (e.g., Rhododendron maximum), Appalachian endemic species (e.g., Ilex collina), and species with a more southern affiliation (e.g., Vaccinium erythrocarpum). Forested swamps are dominated by Picea rubens, with varying cover by Acer rubrum, Tsuga canadensis, and Betula alleghaniensis var. alleghaniensis. Where limestone or calcareous shale influences seepage water, Abies balsamea and Fraxinus nigra are typical canopy dominants. Common shrub species are Viburnum nudum var. cassinoides, Rhododendron maximum, Vaccinium myrtilloides, Alnus incana ssp. rugosa, Hypericum densiflorum, Ilex verticillata, Aronia melanocarpa (= Photinia melanocarpa), Viburnum recognitum, and Kalmia latifolia. Herbaceous species frequently include Rubus hispidus, Solidago uliginosa, Juncus effusus, Eriophorum virginicum, Osmunda cinnamomea var. cinnamomea, Polygonum sagittatum, Carex folliculata, Carex gynandra, Leersia oryzoides, Galium tinctorium, Solidago rugosa, Symplocarpus foetidus, Lycopus uniflorus var. uniflorus, Scirpus cyperinus, Carex scoparia var. scoparia, Carex trisperma var. trisperma, Carex stipata, and Calamagrostis canadensis var. canadensis. Sphagnum spp. and Polytrichum spp. dominate the bryophyte layer.

This system occurs along the high plateau of the Allegheny Mountains, immediately west of the Allegheny Front at elevations between 610 and 1430 m. Wetlands in this system are drained by low-gradient, meandering, intermittent to small streams that form the headwaters of larger (often high-gradient) mountain rivers. The system is underlain by gently folded sedimentary rocks of Carboniferous and Devonian age. Drainage is impounded in high, flat-lying basins by natural dams or "knickpoints" of resistant sandstone (Pottsville and Price formations). These sandstone layers come to the surface along the gently dipping axes of breached anticlines or synclines, or occasionally on the gently dipping limb of a fold. Cold air drains from the surrounding uplands to pool in the flat basins, which function as frost pockets. Rainfall is plentiful, averaging about 1300 mm/year. Communities in this system may have substrates of shallow to deep peat (a few centimeters to up to 3 m depth) or, less commonly, mineral soil. Soils are acidic to circumneutral, with pH ranging from 3.1 to 6.5. High values for soil organic matter, total exchange capacity, exchangeable nitrogen, soluble sulphur, and phosphorus are typical. Most soils are low in boron, copper, potassium, and manganese.

This system is maintained by a spatially complex mix of seepage, low-energy flooding, beaver activity, and rainfall. Drainage in the flat headwater basins is partly impounded by resistant sandstone at the basin outlet. Low-gradient, meandering headwater streams provide regular low-energy inundation. Seepage from surrounding forests provides nutrients at the margins of the wetland mosaic, and where limestone or calcareous shale is present, circumneutral wetlands are maintained. Beaver activity encourages the cycling of early- to mid-successional types. In the rare ombrotrophic bogs, rainfall is the only source of moisture. Many of the forested swamps in this system were logged during 1880-1920, and some were subsequently burned and/or heavily grazed. Flat headwater basins function as frost pockets, catchment areas for cold air draining from surrounding uplands. Cool temperatures (mean annual temperature 6.7-9.4 degrees C) and high rainfall (1220-1680 mm/year) are characteristic. Floristic diversity is controlled by underlying sedimentary rocks that weather to form high diversity of nutrient, acidity, and drainage conditions (Byers et al. 2007).

The logging boom of 1880-1920 resulted in profound alteration of the landscape, with 99% of forest harvested or burned. Denuded slopes resulted in sediment transport into the wetlands, and railroad beds were placed along most streambeds resulting in channelization and barriers. Upland forests have recovered to some degree, providing buffer to wetlands. Current threats include mining activities and home developments, grazing, fragmentation due to road construction and logging, excessive deer herbivory, and invasive species. Frangula alnus can be a major invasive species, particularly in the Allegheny National Forest. Additional threats here include clearcutting in the watershed, as well as activities associated with shale gas development, such as roads and pipelines. In Maryland and Virginia, communities in this system have been greatly impacted by ditching and draining for agriculture and silviculture. Climate change, natural gas development, and wind turbines also pose threats to this system.

The system occurs in a southwest/northeast-trending band about 40 km wide and 200 km long along the high, flat plateau of the Allegheny Mountains. The eastern boundary is the Allegheny Front, and the western boundary is the heavily dissected, lower elevation Allegheny Plateau. Minimum elevations range from 730 m in the north to 940 m in the south (Droop Mountain, West Virginia). The maximum elevation is 1422 m on Mount Porte Crayon, West Virginia.