Northern hardwoods are high elevation forests on moist sites with northerly aspects in the Southern Appalachians. They provide a diverse habitat for various birds and other wildlife including federally endangered species. Wind and ice storms are the most frequent disturbances, while natural fires are very rare in this fire-sensitive community. Within the past few centuries, northern hardwoods and other communities in the Southern Appalachians have undergone a range of fire regimes related to Native American burning, early European land clearing, heavy logging and subsequent slash fires, and finally fire suppression. There are very few stands of old growth and late-successional northern hardwoods left in the Southern Appalachians, since almost all stands were harvested during the last century. While fire suppression had negative effects on fire-adapted communities like oak and pine forests, it generally had a neutral to positive effect on fire-sensitive northern hardwood forests. Due to the sensitivity of the northern hardwood forest type to fire, prescribed fire programs do not play a large role in their management. For more information about the northern hardwood community type, please refer to the following sections:

• Ecosystem description: where to find northern hardwoods, community subtypes, important plant and animal species.
• Historic fire regime: natural fire regime, fire frequency in comparison to other community types, anthropogenic fires.
• Effects of fire on vegetation and wildlife: fire resistance and regeneration methods of tree species, birds favored by post-fire conditions, community shifts.
• Fire suppression: effects of fire suppression on plant community and wildlife.

In the Southern Appalachians, northern hardwood forests occur at elevations between 4000 and 5500 ft (1200 – 1650 m), mostly in high elevation coves but also on upper slopes with relatively high precipitation. North of the Asheville Basin, northern hardwoods are also likely to occur on high base rock at higher elevations (Simon et al. 2004). Due to low temperatures and increased rainfall at high elevations, conditions on these sites are usually mesic. Soils comprise various upland series including umbric dystrochrepts and typic haplumbrepts (Schafale & Weakley 1990).

Mesophytic tree species dominate the canopy, particularly American beech (Fagus grandifolia), yellow birch (Betula alleghaniensis) and yellow buckeye (Aesculus flava).  Depending on geographic region and subtype, associated canopy species may include basswood (Tilia americana var. heterophylla), sugar maple (Acer saccharum), white ash (Fraxinus americana), black cherry (Prunus serotina), eastern hemlock (Tsuga canadensis), red spruce (Picea rubra) or northern red oak (Quercus rubra) (Schafale & Weakley 1990; Ulrey 1999; Pearson 2004). Yellow birch-spruce forests and high elevation red oak forests belong in the broader ecological group of high elevation hardwood forests. However, their fire ecology is different from the northern hardwoods.

Towards higher elevations, northern hardwoods grade into spruce-fir forests, grass or heath balds or high elevation rocky summits. As elevation decreases, they become rich cove forests on mesic sites or chestnut oak forests on drier sites. On drier, more exposed sites of equivalent elevation, northern hardwoods may grade into high elevation red oak forest (Schafale & Weakley 1990).

Typical northern hardwood forests and their subtypes (boulderfield forests and beech gaps/slopes) cover about 197,000 acres (= 3.5 %) in the Southern Appalachians (Simon et al. 2004). In addition to seedlings of canopy species like yellow birch, beech, buckeye and maple, understory species may include mountain ash (Sorbus americana), Allegheny serviceberry (Amelanchier laevis), striped maple (Acer pensylvanicum), and hophornbeam (Ostrya virginiana). The most common or abundant species in the usually dense and diverse herb layer are Pennsylvania sedge (Carex pensylvanica), wood nettle (Laportea canadensis), Roan snakeroot (Ageratina altissima var. roanensis) and wild leeks, also known as ramps (Allium tricoccum) (Schafale & Weakley 1990; Simon et al. 2004).

Boulderfields are relicts of periglacial activity in the Pleistocene and offer a variety of wet and dry microsites. They are dominated by yellow birch with gooseberry (Ribes glandulosum) as a characteristic shrub species. Beech gaps/slopes can occur in south-facing gaps or on ridge tops within the spruce-fir zone where spruce and fir are actually lacking. In well-developed beech gaps, beech is the typical canopy species over dense beds of Pennsylvania sedge (Schafale & Weakley 1990).

Northern hardwoods provide important habitat for various bird species like the Appalachian yellow-bellied sapsucker (Sphyrapicus varius appalachiensis), ruffed grouse (Bonasa umbellus), black-throated blue warbler (Dendroica caerulescens), veery (Catharus fuscescens), rose-breasted grosbeak (Pheucticus ludovicianus) and blue-headed vireo (Vireo solitarius). The sapsucker, a subspecies strictly endemic to the Southern Blue Ridge, prefers rather open high-elevation forests with dead trees for cavities, while ruffed grouse, warbler and vireo like dense brush and shrub understory. The grosbeak favors edge habitats with a mixture of dense shrubs and trees (Hunter et al. 1999; Chris Kelly, personal communication).

Other animals using the northern hardwood habitat are southern rock vole (Microtus chrotorrhinus), red squirrel (Tamiasciurus hudsonicus) and the Carolina northern flying squirrel (Glaucomys sabrinus coloratus). The flying squirrel is a federally endangered species that occurs in mature stands within the spruce-fir / northern hardwood ecotone. Long-tailed shrew (Sorex dispar) might inhabit rock outcrops within the northern hardwood zone and southern water shrew (Sorex palustris punctulatus) can be found along mountain streams above 3200 ft (975 m) (Chris Kelly, personal communication).

Lightning fires seem to be rather infrequent in the Southern Appalachians and most often occur on mid-elevation ridges and upper slopes with south, southwestern or western aspect (Barden & Woods 1974; Harmon 1982; Runkle 1985; Bratton & Meier 1998). Northern hardwoods, however, are typically found on higher elevation north-facing slopes rendering natural fires even more unlikely. Due to the mesic conditions in northern hardwood forests fires are not easily ignited. The thick and moist litter layer usually causes fires to smolder and move slowly (Parshall & Foster 2002; Frelich & Lorimer 1991). These circumstances lead to fire return intervals between 200 and 400 years (Leenhouts 1998) and even longer for stand replacement fire regimes (300 to 500 years) (Stanturf et al. 2002). These values, especially the fire return interval of 200 to 400 years, are valid for the conterminous United States and include numbers of Northeastern northern hardwood forests. Due to the rough topography of the Southern Appalachians and the fact that Appalachian northern hardwood forests are interspersed with other, possibly fire prone communities, the actual fire return intervals might be shorter. Unfortunately, only little is known about actual fire regimes in Southern Appalachian high elevation forests.

In comparison to other forest types, northern hardwoods take intermediate positions regarding the influence of fire. Fahey & Reiners (1981) compared modern fire records of different forest types in Maine and New Hampshire. Their results show that northern hardwoods burn less often than pine forests but more often than spruce-fir forests.  The percentage of area burned is smallest in spruce-fir forests followed by northern hardwoods, while it is largest in pine and birch-aspen forests. Charcoal records indicate that this is not only a phenomenon of modern times: fire was very uncommon in northern hardwoods at least during the past 1000 years (Parshall & Foster 2002). Since there are interactions between climate, vegetation, and fire frequency, charcoal data cannot ultimately tell us whether a change in fire frequency induces vegetation change or if altering vegetation (and therefore fuel load) causes changes in fire frequency.  However, charcoal records show that fire frequency declined with the expansion of hardwood taxa and that there were no fires during the time of hardwood dominance (Clark et al. 1996).

Nevertheless, fires do occur in northern hardwood forests. Especially in the Southern Appalachians, an area with diverse plant communities across short distances, fires burning in a neighboring community can creep into northern hardwoods. Northeastern northern hardwoods also show evidence of fire, but those fires have been linked to human activities like land-clearing operations or logging (Lorimer 1977; Frelich & Lorimer 1991). In New Hampshire the highest burning percentages in northern hardwood forests were closely associated with highest population densities (Fahey & Reiners 1981) and in Canada charcoal data suggest that even in prehistoric times fire occurrence and Indian occupation coincided (Clark & Royall 1995; see also A History of People and Fire).

Northern hardwood forests are a fire-sensitive community-type.  Fire resistance of trees depends mainly on thickness and insulating abilities of their bark as well as their diameter and how the bark thickness tapers along the bole (Harmon 1984). Thin bark makes practically all the major tree species in northern hardwood forests very susceptible to fire injury. The cambium of black cherry, for example, reaches lethal temperatures faster than in any other eastern hardwood species, and the cambium of sugar maple may be injured even without external damage (Uchytil 1991; Coladonato 1991).  Fire can easily damage the shallow roots of basswoods and usually top-kills American beech (Sullivan 1994a; Coladonato 1991), so even occasional or light fires can have dramatic impacts.

Beech, black cherry and basswood regenerate by root suckering or stump sprouting (Coladonato 1991; Sullivan 1994a, Uchytil 1991), but yellow birch and sugar maple are poor sprouters after fire and depend on wind-dispersed seeds. However, yellow birch is likely to be a part of the post-fire regeneration in northern hardwoods, since fire seems to enhance its seed germination and seedling establishment (Sullivan 1994b; Timenstein 1991). While light surface fires may favor sugar maple seedlings over basswood, hotter fires destroy sugar maple reproduction providing the necessary light gaps for basswood (Sullivan 1994a).

Since even light fires may damage thin-barked northern hardwoods, fire usually opens up the canopy. Trees not instantly killed but having suffered fire damage are more susceptible to decaying fungi that can cause delayed mortality (Sullivan 1994a; Colodonato 1991). Such open woodlands with sugar maple, beech and birches provide ideal habitat for the yellow-bellied sapsucker. Ruffed grouse, vireo and black-throated blue warbler, however, benefit from prolific resprouting that provides a dense shrubby understory (Chris Kelly, personal communication).

Repeated or intense fires can change moisture conditions and convert a mesic site into a more xeric one, thus promoting oak rather than northern hardwood species (Van Lear & Watt 1993). In fact, widespread, centuries-long Native American burning practices most likely influenced the establishment and stabilization of oak-pine communities in the Appalachians even on mesic sites (Brose et al. 2001). Charcoal studies suggest that fires set by Late Archaic and Woodland peoples increased populations of fire-tolerant oaks, chestnut and pines in Kentucky and support the theory of Indian burning playing a role in the transition from northern hardwoods to white pine-oak forests in Ontario, Canada (Delcourt et al. 1998; Clark & Royall 1995).

While the era of fire suppression in the 20^th century had negative effects on fire-adapted and fire-dependent communities like oak and pine forests, it has generally had a neutral to positive effect on fire-sensitive northern hardwood forests.  Many former oak forests have, in the absence of fire, been replaced by other hardwood species like red maple (Acer rubrum), yellow poplar (Liriodendron tulipifera) and hickories (Carya spp.). However, northern hardwoods had a chance to recover from high-intensity slash fires after large-scale logging in the late-1800s and early 1900s. Presently, about 68% of Southern Appalachian high elevation hardwoods are in a mid-successional stage, 18% have reached sapling or pole size, about 13% are late successional and 1% is early successional (Hunter et al. 1999).

Fire suppression and management towards old-growth forest in northern hardwoods has resulted in a lack of large openings, which in turn seems to have caused the present rarity of Appalachian yellow-bellied sapsuckers in the Great Smoky Mountains National Park.  Although sapsuckers need mature trees for their cavities, they depend on young trees for harvesting sap and insects to feed their young ones. Other bird species also profit from some kind of disturbance causing a more complex vegetation structure: black-throated blue warbler and veery demand a dense understory; ruffed grouse needs more early-successional habitat (Hunter et al. 1999).   Prescribed fire programs, however, have never focused on fire-sensitive northern hardwood communities, since other management techniques like thinning or shelterwood cutting can create a more complex vegetation pattern providing the necessary habitat for those bird species.