Oaks and Fire

Although oak species are not dependent on fire for regeneration as are some species of pines, oaks are well adapted to an environment that includes periodic fire.  Oak species have several biological adaptations that enhance their ability to survive on sites exposed to frequent fire, including: high reprouting capacity, thick bark, resistance to rotting after scarring, and rapid acorn germination on fire-prepared seed beds. 

Perhaps the most notable of these adaptations is the generas high resprouting capacity following fire (Johnson 1993).  Oak species have a concentration of dormant buds near the root collar. These buds often remain an inch or more below the soil surface where they are protected from lethally high temperatures (Korstian 1927).  Also, sprouts arising from belowground buds tend to have fewer pathogens and therefore have higher survival than aboveground sprouts (Roth and Hepting 1943).  This tenacious ability of small oak rootstocks to resprout repeatedly following frequent top-kill enables oak to dominate the advanced regeneration pool in areas where fire occurs at frequent intervals.  For example, resprouting has been reported in many studies as a key mechanism maintaining survival of oak seedlings above that of maple seedlings following fire or heat stress (Curtis 1959, Grimm 1984, Hengst and Dawson 1994, Huddle and Pallardy 1996).  In another study, Waldrop and Lloyd (1991) reported that oak mortality rates after 26 years of biennial summer burning in mature pine stands in the Coastal Plain were still below 50%, whereas mortality rates of other woody species ranged from about 60 to 80%. This resprouting mechanism is especially critical to the success of northern red oak, as it has thinner bark, and therefore is less resistance to heat damage than other oaks (Crow 1988).

Frequent fires also give oaks several regeneration advantages over competing species, such as: preparing seedbeds and encouraging acorn caching; discouraging acorn and seedling predators; reducing fire-intolerant competitors; and xerifying sites by removing the forest floor and opening the canopy.  Fire, by removing excessive litter buildup from the forest floor, is important for the successful germination of acorns (Van Lear and Watt 1993).  Not only are areas of thin litter preferred by squirrels and bluejays for acorn burial (Galford and others 1988), but oak seedlings from freshly germinated acorns are more likely to emerge than from areas with heavy litter cover.  But, while removal of thick litter may expedite the germination process, it is important that some humus remain after fires. The humus layer keeps the surface of the soil porous, so that uncached acorns can more easily penetrate the soil, retains moisture, and provides support for the new seedling (Carvell and Tryon 1961).

Fire helps to control insect predators of acorns and new seedlings. Recent studies indicate that prescribed burning may reduce populations of oak insect pests when conducted under proper conditions and at the appropriate time in the insects life cycle (Galford and others 1988). Martin and Mitchell (1981) illustrated how insect populations can be reduced or eliminated directly or indirectly by fire.  Burning may also reduce rodent habitat, eliminating another source of acorn predation (Hannah 1987). A reduction in acorn predators would allow more acorns to be scattered and buried by jays and squirrels, thus enhancing the probability of successful germination.  Evidence suggests that jays collect and disperse only sound nuts (Darley-Hill and Johnson 1981, Deen and Hodges 1990), which implies that if these acorns escape predation they will result in well-established first-year seedlings. 

Effects of Fire Suppression on Oaks

Due to the competitive edge fires give oaks over fire-intolerant hardwoods, oaks are favored by frequent fires in the southern Appalachians, particularly on high-quality sites.  In the absence of fire, oaks often lose this competitive advantage to species that are intolerant of fire but that grow faster in forest openings.  Decades of fire suppression in the southern Appalachians have had several negative effects on oak regeneration. The absence of fire since the turn of the 20th century has allowed species that are intolerant of fire to become established and grow to a size where they—because of thicker bark associated with age—can now resist fire (Van Lear and Watt, 1993). At greater than 5 cm (2 in.) d.b.h., yellow-poplar becomes almost as fire resistant as oaks (Maslen 1989). Mockernut and pignut hickories, scarlet oak, red maple, and blackgum are examples of species that are often found on sites where fire has been long absent (Harmon 1984, Martin 1989).

Suppression of fire has also allowed shrubby understory species to occupy drier sites where fire was once frequent and oak was more dominant. In particular, rhododendron has dramatically increased its extent (Van Lear and Waldrop 1989, Martin 1989). Impenetrable thickets of ericaceous species, such as rhododendron, mountain laurel, and huckleberry, now often dominate midstories and understories of hardwood stands in the southern Appalachians and prevent desirable hardwood regeneration from becoming established (Beck 1989).