The oak-hickory forest type (Braun 1950, Barrett 1994) occurs primarily on average to dry upland sites throughout the central hardwood region. The dominants typically are white oak (Quercus alba), northern red oak (Quercus rubra), mockernut hickory (Carya tomentosa), pignut hickory (Carya glabra), black oak (Quercus velutina), yellow poplar (Liriodendron tulipifera) and occasionally, white pine (Pinus strobus) (Schafale and Weakley 1990). In the Southern Appalachians, oak-hickory forests are found at low elevations on sites less exposed than oak-pine forests and more exposed than cove forests. Depending on disturbance history, oak-hickory can also be found on moist upland sites.

Fire has been a vitally important disturbance agent influencing the structure and composition of this forest type. Oak-hickory forests historically had an understory fire regime (Van Lear and Waldrop 1989).  Native Americans burned these forests frequently to promote grasses and attract game, among other reasons. Although precise presettlement fire frequencies are not known, conservative estimates suggest fire return intervals of 2.8 years (Cutter and Guyette 1994) to 14 years (Buell and others 1954, Guyette and Day 1997).  These frequent Indian fires likely maintained oak-hickory forests as barrens, prairies and oak savannas before European contact. Oaks and hickories were favored by these frequent fire regimes, because they both have adaptations that make them resistant to fire, such as thick bark. As a result, these species dominated the canopy as old, large, fire-resistant trees. Shrubs, understory trees, and woody debris were likely rare in these fire-maintained forests. 

When Native American populations were decimated by new diseases after European contact, the frequency and extent of fire decreased substantially throughout the South and grasslands, savannas, and woodlands succeeded to closed forest. Subsequent settlement of the oak-hickory forests by Euro-Americans did increase fire frequencies to some extent, since these early settlers used fire for many of the same reasons as the Native Americans. However, fire suppression activities in the 1900s decreased fire frequency once more in oak-hickory forests, as they did throughout the South.  Presently, the fire regime of oak-hickory forests is characterized by very infrequent, low-intensity surface fires.

These altered fire regimes have profoundly changed the oak-hickory forest by allowing it to succeed to mixed mesophytic and northern hardwood species such as red maple, eastern white pine, sugar maple, and beech. On drier mountainous sites, fire exclusion allows ericaceous shrubs such as mountain laurel and rhododendron to move from riparian areas into upland forests (Elliott and others 1999). Although the forest floor rarely dries enough to support surface fire, this ericaceous shrub layer is flammable; and when it burns, it typically supports intense, stand-replacement fires that alter successional pathways, reduce site productivity, negatively impact streams, and threaten human life and property.

Until recently, land managers failed to appreciate the role of prescribed fire in restoring and maintaining open oak-hickory forests (Lorimer 1993). However, research indicates that fire can be used in hardwood stands to establish and release oak-hickory regeneration (Barnes and Van Lear 1998, Brose and Van Lear 1998, 1999, Christianson 1969).

Distribution

The upland oak-hickory forest type occupies nearly 114 million acres (46.1 million ha) of commercial forestland in the eastern United States-more than any other single forest type in the country, excluding types in Alaska (USDA Forest Service 1980). It is found along the prairie borders from Texas north to the Dakotas, eastward throughout the Central States and Appalachian region into southern New England, and from the northern hardwood transition zone southward into the Piedmont Plateau and Coastal Plain. The type is well developed in the Cumberland Plateau and portions of the Appalachian Mountains and most continuous in the Ozark and Ouachita Highlands (Fowells 1965).

Soil types

Within this vast area oak-hickory occurs on soils ranging from cool-moist Boralf and Orthod Spodosols to warm-dry Mollisols and Alfisols. These soils are derived from glacial material, residual sandstone, shales, and limestones and from gneisses, schists, and granites and range from clay to loamy. The type is found on all topographic positions from dry rocky ridges to deep coves to well-drained valley floors (Fowells 1965).

Climate

The climate over the range of the oak-hickory type is humid except in the southwestern fringe where it is moist subhumid to dry subhumid. Average annual precipitation is 22 inches (560 mm) in central Texas, from 30 inches (760 mm) to 40 inches (1015 mm) in the Central States, and about 80 inches (2030 mm) in the southern Appalachians. Generally, about one-half or more of the annual precipitation falls in the dormant season, and from 2 to 8 week periods with little or no rainfall are common late in the growing season, particularly in the western part of the range. Average annual snowfall is 100 inches (2540 mm) or more in the North, from 15 to 20 inches (380 to 510 mm) in the Central States, and a trace in the South. Mean annual temperature averages 40° F (4.4° C) in the North, 55° F (12.8° C) in the Central States, and 65° F (18.3° C) in the South. The frost-free period averages 100 days in the North, 180 days in the Central States, and 290 days in the South (Fowells 1965).

Major species

Within the broad oak-hickory type, the Society of American Foresters has defined nine forest cover types as oak types. Oaks and hickories are also listed as components of 31 other forest cover types (Eyre 1980). The great range of climate, soil, and topography results in widely different stand compositions. White oak (Quercus alba L.), northern red oak (Q. rubra L.), and black oak (Q. velutina Lam.) are found throughout the type. Other common oaks on drier sites are scarlet oak (Q. coccinea Muenchh.) and chestnut oak (Q. prinus L.) in the Appalachians and adjoining areas; northern pin oak (Q. ellipsoidalis E. J. Hill) and bur oak (Q. macrocarpa Michx.) in the northern and western parts of the range; and post oak (Q. stellata Wangenh.), blackjack oak (Q. marilandica Muenchh.), bluejack oak (Q. incana Bartr.), and southern red oak (Q. falcata Michx.) in the West and South. Pignut hickory (Carya glabra (Mill.) Sweet), mockernut hickory (C. tomentosa (Poir.) Nutt.), shagbark hickory (C. ovata (Mill.) K. Koch), and bitternut hickory (C. cordiformis (Wangenh.) K. Koch) are consistent but minor components of the type.

The mixture may also contain yellow-poplar (Liriodendron tulipifera L.), ashes (Fraxinus L. spp.), elms (Ulmus L. spp.), sugar maple (Acer saccharum L.), red maple (A. rubrum L.), blackgum (Nyssa sylvatica Marsh.), black locust (Robinia pseudoacacia L.), black cherry (Prunus serotina Ehrh.), black walnut (Juglans nigra L.), and many other hardwood species. On suitable sites, shortleaf pine (Pinus echinata Mill.), loblolly pine (P. taeda L.), pitch pine (P. rigida Mill.), Virginia pine (P. virginiana Mill.), eastern white pine (P. strobus L.), eastern hemlock (Tsuga canadensis (L.) Carr.), and eastern redcedar (Juniperus virginiana L.) may also be present. The oak-hickory type often grades into the oak-pine type in the Piedmont Plateau (Braun 1950).

Understory species

Many species of understory trees and shrubs occur in oak-hickory stands. Some of the most common ones are flowering dogwood (Cornus florida L.), gray dogwood (C. racemosa Lam.), sassafras (Sassafras albidum (Nutt.) Nees), sourwood (Oxydendrum arboreum (L.) DC.), eastern redbud (Cercis canadensis L.), downy serviceberry (Amelanchier arborea (Michx. f.) Fem.), blueberry (Vaccinium L. spp.), mountain laurel (Kalmia latifolia L.), witch-hazel (Hamamelis virginiana L.), beaked hazel (Corylus cornuta Marsh. var. cornuta), and bramble (Rubus L. spp.) (Braun 1950, Fowells 1965).

The fire regime of the oak-hickory forest has varied spatially and temporally because of changing cultural influences before, during, and after Euro-American settlement.  Before such influence, the fire regime for oak-hickory forests was dictated by the activity of Native Americans because they were the primary ignition source (Abrams 1992, Buckner 1983, Denevan 1992, Pyne 1997). Lightning fires are uncommon in many of these regions because thunderstorms occur primarily during the growing season, usually accompanied by rain (Barden and Woods 1974; Ruffner and Abrams 1998). 

Numerous authors (DeViro 1991, Patterson and Sassamen 1988, Stewart 1951, 1963, Van Lear and Waldrop 1989) have discussed the vast extent to which Native Americans used fire.  Native Americans in the Central Hardwoods Region (Delcourt and Delcourt 1997, 1998, Olson 1996), the Appalachians and Piedmont (Van Lear and Johnson 1983), and the Northeast (Buell and others 1954, Day 1953) commonly used fire for numerous reasons throughout the year. Surface fires predominated and burned over large areas; only natural barriers or unfavorable weather stopped them. Hough (1877) thought the “oak openings,” “barrens,” and prairies east of the Mississippi resulted from Native American use of fire to promote grass growth and attract game (see Oak Savannas). He stated, “Scarcely a year passes without the occurrence of fires of sufficient extent to attract public notice.”  One historian (Russell 1983) agreed that fire frequency was greater near camps and villages than would be expected by lightning, but found no strong evidence that Native Americans burned large areas in the Northeast.

While ecologists and historians generally agree that anthropogenic fires were common in oak-hickory forests, exact presettlement fire frequencies are not known. Delcourt and Delcourt (1997, 1998) believe they varied considerably depending on closeness to Native American habitation. Other references point out that Native Americans maintained an extensive trail system throughout the East that was kept open with fire. Euro-American explorers reported many areas treated with annual and biennial fires (Barden 1997, Buckner 1983, Day 1953). Dendrochronological studies, which give conservative estimates, suggest fire return intervals of 7 to 14 years in the mid-Atlantic and Ozark regions (Buell and others 1954, Guyette and Day 1997). Cutter and Guyette (1994) reported a fire-return- interval of 2.8 years during 1740 to 1850 on a ridgetop in the Mark Twain National Forest. Pre-settlement fires in southern New England generally occurred during the spring and summer (Bromley 1935, Christianson 1969). Brown (1960) believes the prevalence of oak in Rhode Island is the result of a long history of fire. The fire regime was probably more pronounced in Southern areas than in Northern areas due to more favorable climatic conditions for ignition and spread, greater populations of Native Americans, and vegetation more conducive to burning.

The frequency and extent of Indian burning decreased substantially after European contact, which introduced new diseases and decimated their population by 90% or more over the next 100 to 150 years (Denevan 1992, Dobyns 1983, MacCleery 1993). As a result grasslands, savannas, and woodlands succeeded to closed forest (Buckner 1983, Denevan 1992, Dobyns 1983, MacCleery 1993, 1995, Pyne 1997). Subsequent settlement of the oak-hickory forests by Euro-Americans, who used fire for many of the same reasons as the Native Americans, increased the frequency and extent of burning (Abrams 1992, Pyne 1997, Van Lear and Waldrop 1989). Fire return intervals were shortened to 2 to 10 years with many sites burning annually (Cutter and Guyette 1994, Guyette and Day 1997, Holmes 1911, Sutherland and others 1995, Sutherland 1997). For example, the barrens of Pennsylvania and Maryland were burned annually at least through 1731 (Tyndall 1992).

Presently, the fire regime of oak-hickory forests is characterized by infrequent, low-intensity surface fires that occur during the spring and fall. They are caused almost exclusively by humans, and burn small areas (Pyne and others 1996). Lightning is a minor ignition source (Barden and Woods 1974, Ruffner and Abrams 1998). Fire return intervals have lengthened from a few years to several millennia (Harmon 1982), the longest fire-free intervals in the history of the Central Hardwoods Region (Ladd 1991).

Frequent fires ignited by Native Americans maintained open oak-hickory forests with a groundcover of grasses and forbs. Oaks and hickories were favored by these frequent fire regimes because they both have several adaptations that make them resistant to fire, such as their thick bark (See: Oaks and fire). These species dominated the canopy as old, large, fire-resistant trees. Densities of dominant trees probably varied from 20 to 40 per acre. Shrubs, understory trees, and woody debris were likely rare (Barden 1997, Buckner 1983, Denevan 1992, Pyne 1997).  Hardwood regeneration comprised of seedling sprouts were dominated by oak and hickory because these species initially emphasize root development over stem growth and have the ability to sprout repeatedly (Barnes and Van Lear 1998, Brown 1960, Van Lear 1991). With fire excluded for a few years, the well-developed rootstocks sent up vigorous stems that often developed sufficient size and bark thickness to withstand future fires. Where windstorms blew down trees over large areas, the replacement stand was even aged. Consequently, the forest was uneven aged, consisting of even aged patches.

Effects of Fire Suppression in Oak-hickory Forests

Reduction of fire has profoundly changed the oak-hickory forest by allowing the forest to succeed to mixed mesophytic and northern hardwood species such as red maple, eastern white pine, sugar maple, and beech. In the absence of fire, these species become established in the understory, grow into the midstory, and eventually change the composition of the canopy. Stem densities are often hundreds per acre. During the growing season, the dense shade from these fire-sensitive species reduces the abundance and richness of forbs and grasses and inhibits development of oak and hickory regeneration. Consequently, when a dominant oak or hickory dies, its reproduction is not capable of sufficient growth to capture the canopy opening.  Instead, the growing space is filled by mesophytic and northern hardwood species (Abrams and Downs 1990, Crow 1988, Lorimer 1985, McGee 1984).

According to Olson (1996) the brushy character of many oak-hickory sites is the result of an interruption in the chronic fire regime that allows shrubs and hardwoods to capture the site. When the area again burns several years later, these stems are top killed producing a dense growth of sprouts that can dominate the site for decades, especially with occasional fire. On drier mountainous sites, fire exclusion allows ericaceous shrubs such as mountain laurel and rhododendron to move from riparian areas into upland forests (Elliott and others 1999). These shrubs are shade tolerant and evergreen, shading the forest floor throughout the year. Hardwoods cannot regenerate beneath them (Baker and Van Lear 1998), and without disturbance, these heath thickets are the climax plant community on some sites.  Although the forest floor rarely dries enough to support surface fire, this ericaceous shrub layer is flammable; and when it burns, it typically supports intense, stand-replacement fires that alter successional pathways, reduce site productivity, negatively impact streams, and threaten human life and property (such slopes are favored building sites).

Altered fire cycles have also impacted the “low elevation rocky summit” vegetation type where fire historically maintained the hardwood scrub savanna (Hallisey and Wood 1976). Fire exclusion over the past 50 years resulted in an increased hardwood overstory and a dramatic decline in herbs such as blazing star and some woody scrub species such as bear oak (Barden 2000).

Until recently, land managers failed to appreciate the role of prescribed fire in restoring and maintaining open oak-hickory forests (Lorimer 1993). However, research indicates that fire can be used in hardwood stands to establish and release oak-hickory regeneration (Barnes and Van Lear 1998, Brose and Van Lear 1998, 1999, Christianson 1969).

Until recently, foresters failed to appreciate the role of prescribed fire in maintaining open oak-hickory forests and in facilitating regeneration of these species (Lorimer 1993). Regeneration of oak was attempted only with timber harvesting and herbicides, which generally hastened the successional replacement of oaks by mixed mesophytic species (Abrams and Scott 1989). Research indicates that fire can be used in hardwood stands to establish and release oak-hickory regeneration (Barnes and Van Lear 1998, Brose and Van Lear 1998, 1999, Christianson 1969).

Understory burning of mature, uncut hardwood stands can help establish oak and hickory regeneration by preparing seedbeds (Barnes and Van Lear 1998). Acorns and hickory nuts are often buried by wildlife, particularly squirrels and blue jays, which prefer burned areas because of the thin root mat. The fires also top-kill or eliminate many of the shrubs and small trees that shade the forest floor. In a less shaded environment, the acorns and hickory nuts germinate and the new seedlings begin developing their root systems. Eventually, the regeneration replaces canopy trees.

In this approach, fires are initially applied at a frequent interval (annual or biennial) depending on season of burn and severity of the shade. Once oak seedlings are established, fire is withheld for a few years (Cottam 1949), and then periodically reapplied once or twice a decade. This minimizes mortality of the oak regeneration by allowing time for root systems to develop. This approach may take 15 to 20 years for results to be apparent.

If oak and hickory regeneration is present in the understory, a two-step shelterwood harvest combined with a prescribed fire can be used for release (Brose and others 1999a,b). This shelterwood-burn technique appears to be a reasonable mimic to the disturbance regime of oak-hickory forests before Euro-American influence. It has considerable value as a silvicultural method, a wildlife management tool, and a means for restoring habitats such as oak savannas and open woodlands.  This method, and other methods of using fire in combination with silvicultural treatments are explained in the section: Using Prescribed Fire for Oak Management.