Appalachian Hemlock and Northern Hardwood Forest

This forested system of the eastern U.S. ranges from central New England west to Lake Erie and south to the higher elevations of Virginia and West Virginia. It is one of the matrix forest types in the northern part of the Central Interior and Appalachian Division. Northern hardwoods such as Acer saccharum, Betula alleghaniensis, and Fagus grandifolia are characteristic, either forming a deciduous canopy or mixed with Tsuga canadensis (or in some cases Pinus strobus). Other common and sometimes dominant trees include Quercus spp. (most commonly Quercus rubra), Liriodendron tulipifera, Prunus serotina, Acer rubrum, and Betula lenta. It is of more limited extent and more ecologically constrained in the southern part of its range in northern parts of Virginia and West Virginia.

The canopy is characterized and often usually dominated by northern hardwoods (e.g., Fagus grandifolia and Acer saccharum), often with Tsuga canadensis, but may also contain large amounts of Pinus strobus and Quercus spp. Tsuga canadensis can dominate the canopy on cool/moist sites at higher elevations and in shaded coves, valley bottoms and riparian areas. Bottomlands and toeslopes may also contain Fraxinus americana, as well as Platanus occidentalis (Whitney 1990). Other common associates may include Acer rubrum, Prunus serotina, Betula lenta, Tilia americana, Pinus strobus, Liriodendron tulipifera, Quercus spp., and Magnolia acuminata. The subcanopy and shrub layers are usually well-developed and may include Viburnum lantanoides (= Viburnum alnifolium), Viburnum acerifolium, Hamamelis virginiana, and Cornus alternifolia. A dense, low to high shrub layer of Rhododendron maximum and sometimes Kalmia latifolia is sometimes present. Common herbaceous species include Maianthemum canadense, Onoclea sensibilis, Huperzia lucidula (= Lycopodium lucidulum), Dryopteris carthusiana (= Dryopteris spinulosa), Oxalis montana, and Mitchella repens (Lutz 1930, Braun 1950).

This system occurs predominantly on mesic sites over a broad range of topographic conditions, such as protected low and midslopes and valley bottoms, at elevations from 305 to 1360 m. Soils are usually acidic and retain some moisture except during severe droughts. They are moderately well-drained to well-drained loamy or silty soils, and are rocky and usually deep in depressions among boulders. Forests in this system are also associated with high-elevation periglacial boulderfields. In the Central Appalachian center of its range, its ecological amplitude is somewhat broader, and it becomes the matrix forest in some areas of Pennsylvania, Maryland, West Virginia. At Shenandoah National Park, this system spans a broad range of environmental settings from steep west-facing slopes to south-facing gentle slopes.

In general, this system is dominated by long-lived, mesophytic species that form multi-layered uneven-aged forests. Canopy dynamics are dominated by single and multiple disturbances encouraging gap phase regeneration (Abrams and Orwig 1996). Larger disturbances include windthrow, insect attack and icestorms. Although stand-replacing wind events are rare, small to medium blowdown events are more common and occur at greater frequency on the plateau and exposed sideslopes (Ruffner and Abrams 2003).This system is currently being devastated in large parts of its range by the hemlock woolly adelgid (Adelges tsugae). This sucking insect is continuing to cause close to 100% mortality in some areas as it spreads from the north into the southern United States. The insect will most likely cause canopy hemlocks to be replaced by other canopy trees. Historically, this system was probably only subject to occasional fires. Fires that did occur may have been catastrophic and may have led to even-aged stands of pine and hemlock. Fire suppression appears to have increased the extent of this system at the expense of oak-pine systems.

Fire Regime Description (from Landfire 2007a): Historically, this system was probably only subject to occasional fires. Fires that did occur may have been catastrophic and may have led to even-aged stands of pine and hemlock. Due to the predominance of cool, moist site conditions, surface and replacement fires are extremely rare, occurring at 700- to 1000-year intervals. Most protected sites are essentially fire-free. The principal cause of fuel formation leading to fire in northern hardwood ecosystems is broad-scale, storm-driven windthrow of catastrophic proportions (Hough 1936, Runkle 1982). The importance of red maple, sweet birch, northern red oak, and especially black cherry in contemporary Central Appalachian examples of this community group reflects secondary succession following catastrophic logging and fire disturbances in the early part of the twentieth century. Sugar maple and beech, both abundant in understory layers and locally codominant in the overstory, appear positioned to assume dominance as current secondary stands mature. However, beech bark disease and excessive deer browsing are serious threats to the future viability of the largest stands on Allegheny Mountain (VDNH 2007).

This system is currently being impacted in large parts of its range by losses or declines of several dominant tree species. Tsuga canadensis is heavily impacted by hemlock woolly adelgid (Adelges tsugae), a sucking insect that is continuing to cause close to 100% mortality in hemlocks; Orwig and Foster (1998) documented high mortality or high foliar loss by Tsuga canadensis; changes in forest composition resulted in rapid understory responses to canopy openings such as prolific establishment of Betula lenta, as well as invasion by Ailanthus altissima, Microstegium vimineum, and others). Other important trees in this system impacted by insect damage include Fraxinus americana by the emerald ash borer (Agrilus planipennis) and Fagus grandifolia by the insect Cryptococcus fagisuga, which causes fungal infections known as beech bark disease. A number of tree species of this system are also damaged by Asian long-horned beetle (Anoplophora glabripennis). Past logging in many areas has altered this system by creating a predominantly even-aged structure that contains a much higher proportion of shade-intolerant species. In many areas, this change in species composition is further aggravated by decades of overbrowsing by deer (Runkle 1982, Abrams and Orwig 1996), which has significantly reduced the hemlock component and reduced species and structural diversity in many areas. Loss of foundation tree species changes the local environment and associated ecosystem processes; those forests dominated by a few foundation species are dependent on a small number of strong interactions that may be susceptible to fluctuation among unstable states (Ellison et al. 2005). Additional threats include logging, development, overbrowsing, and road construction, which fragments large patches (Fike 1999, NYNHP 2013b). Climate change also threatens to allow expansion of woolly adelgid beyond its current restrictions in cooler climates (Paradis et al. 2007). Another threat is acid deposition due to high precipitation in the mountains downwind from coal power plants. Combined with repeated logging cycles which remove carbon and calcium sinks, this may cause soil cations to be leached and depleted and toxic elements to accumulate (Connolly et al. 2007).

This system is found from southern New Hampshire south to Virginia and West Virginia, and possibly in adjacent Kentucky.