Historic Fire Regimes of Shortleaf Pine

Historic Fire Regime

Shortleaf pine, because of its wide distribution and site adaptability, occurs across the widest range of fire regimes of any southern pine (see Frost 1995, 1998; Schmidt et al. 2002).  The understory fire regime is 1-18 years based on fire chronology studies, historical accounts and the effects of long term fire frequency studies on shortleaf regeneration. However, depending on topographic position within a given physiographic region, a given stand may have historically been prone to burn at the more frequent or at the less frequent end of the range. The mixed fire regime cycle is likely 2-50 years and somewhat follows drought cycles across the range of shortleaf.  The stand replacement fire cycle is likely >100 years particularly where this species overlaps the range of and is associated with Virginia, pitch and Table Mountain pine.

Based on the classification of Schmidt et al. (2002), shortleaf pine occurs in three general fire regimes: Fire Regime I, characterized by low severity, nonlethal surface fire at intervals of 0 to 35 years; Fire Regime III, characterized by mixed severity fire at 35 to 100+ years; and Fire Regime IV, characterized by stand replacement severity fire at 35 to 100+ year intervals (Schmidt et al. 2002).  Jackson et al. (1999) considered it within the category of a low-intensity surface fire regime.  But Keeley and Zedler (1998) considered shortleaf within the group of pines characterized by a fire regime of predictable stand replacing fires.  However, these broad categorizations based on morphological and physiological groupings have limited utility in describing the specific historic fire frequency that perpetuates stands of shortleaf pine across the eastern landscape composed of widely ranging topographic and edaphic characteristics.   The topographic, edaphic and natural disturbance variation across a landscape creates highly variable eastern fire regimes.   Rapid plant community response in the southeastern U.S. also influences the fire regime that characterizes shortleaf stands.   This rapid response is a result of the number of growing days across the region, high rainfall and the response of dominant vegetation types to varying fire frequency.  Historically, frequent low-intensity fire maintained open mid-stories without ladder fuels. Various vegetation types, both overstory and understory, will assert dominance very rapidly based on fire frequency or lack of fire at much less than a 35-year interval.

Wade et al. (2000) reported that the fire regime for those Society of American Foresters (SAF) cover types that included shortleaf as one of the overstory dominants was from 2-15 years for Shortleaf 75, 3-10 years for Loblolly-shortleaf 80, and <10 years for Shortleaf pine-oak 76.  Stanturf et al. (2002) also list 2-15 years as the fire regime for shortleaf in the south.  Garren (1943) indicated that shortleaf along with loblolly thrives in areas of about a 10-year fire interval, but Chapman (1944) suggests that shortleaf could tolerate fire at more frequent intervals because of its ability to sprout.  However, in the New Jersey Pine Barrens the burn interval postulated was 16-26 years (Lutz 1934).  Frost (1995, 1998) gave general fire frequency regimes that were very nearly associated with physiographic regions, but did not specifically refer to a fire frequency for shortleaf cover types.  Associating the range of shortleaf with these general maps gives a range for presettlement fire frequency regimes of from 1 to 100 years.  However, throughout most of shortleaf pine’s distribution the range of fire frequency was reported as 1-25 years.

The most accurate information about fire regimes comes from fire chronology studies of tree rings.  Even so, fire chronologies are conservative because low intensity surface fires may not cause a fire scar to every tree in a given area (Masters et al. 1995, Jurney et al. 2004).  Also, trees in sheltered topographic situations may be protected.  As one goes back further and further into the tree-ring record, sample size declines and in some areas a representative sample of old trees may not be available.  Often logging, senescence and natural mortality and even natural disturbances such as fire may limit the sample size available for the oldest periods of time.  Relatively few studies have been conducted in shortleaf types and fewer still from east of the Mississippi River.

Masters et al. (1995) developed a fire chronology from tree-ring analysis in mixed shortleaf pine-hardwood stands in the Ouachita Highlands of southeast Oklahoma, and found the range of fire frequency to be at 1-12 year intervals, with an area mean fire frequency of 3.4 years on a  5,701 ha (14,088 ac) area.  The recurring theme in this fire chronology was periods of frequent fire followed by apparent periods of fire absence. This was much shorter than the 7.3 years mean fire interval from 1788-1817 for one shortleaf pine reported by Johnson and Schell (1985) in central Arkansas and the mean fire frequency of 32 years or less for the area of Hot Springs National Park prior to 1938.  They used predominantly shortleaf pine to date fires which is much less prone to fire scarring than the hardwood species used in the above study because shortleaf bark is quite resistant to fire (Splat and Reifsnyder 1962, Lowery 1968).  Therefore their results likely represent a considerable underestimate of fire-return intervals.  Other studies documenting fire chronology in shortleaf pine types include work in the Ozark Highlands of Missouri (Guyette and Dey 1997, Batek et al. 1999, Stambaugh et al. 2005).  Batek et al. (1999) reported that in the 1800s, as fire frequency increased, so did relative dominance of shortleaf.  They reported a range of fire frequencies from 2-50 years with most (86%) sample trees in the 2-18 year range. One fire chronology study from the Highland Rim in extreme southeast Tennessee on an oak barrens site reports periods of annual burning in the 1700s and 1800s (Guyette and Stambaugh 2005).  This area is within the region of historic major importance of shortleaf pine given by Haney (1962) therefore the fire chronology may be applicable only by inference.

Mixed severity fires may play a role in perpetuation of pine dominance over hardwoods in the New Jersey pine barrens (Little and Moore 1945).  Severe fires of 15- to 40-year intervals were reported from about 1800 to the mid-1940s and resulted in mixed stands of shortleaf and pitch pine (Little and Moore 1945).  Mixed severity fires also are noted to occur in the south where individuals within a stand succumb to even relatively low intensity fires under extremely dry conditions (Ferguson et al. 1961).  Likely mixed severity fires interacted with major wind disturbance events and the associated abundant fuels.

Keeley and Zedler (1998:240, 250) placed shortleaf within the group of pine species characterized by predictable stand-replacing fires, based on life-history characteristics and morphological traits.  However, this characterization is somewhat flawed because shortleaf historically occurred in sparse, open pine-bluestem stands over much of its range, and these open stands lacked the ladder fuels to carry a fire to the crowns of dominant trees (see Vogl 1972).  As well, shortleaf is unique in that it shares life history, physiological and morphological traits with a number of divergent pine groups some of which did not fit in the stand-replacing fire category (McCune 1988, see also Jackson et al. 1999, Keeley and Zedler 1998).  Not-withstanding, stand replacement fires do occasionally occur in this type but may be decidedly different than the sweeping crown fires that are typically associated with the designation of stand replacement fire for western forests.  Our best guess is that they may occur at >100 year intervals.  They may be either high intensity crown fires in young regenerating stands, in immature stands, and occasionally in mature stands.  A stand replacing fire event may occur as surface burns under protracted late summer drought, with the end result that overstory trees in a given stand are killed (similar to Ferguson et al. 1961).  Prescribed stand replacement fires have been used to restore mixed pine-hardwood stands in the southern Appalachians of North Carolina where the pine component was represented by Virginia, pitch and shortleaf pines (Vose et al. 1997). 

Recorded stand replacement fires since the 1960s in the Ouachita National Forest of Arkansas and Oklahoma include the Walker Mountain Fire, the Eagleton Fire and the Page Fire.  The Walker Mountain Fire occurred in September 1965 in Oklahoma, and burned 10,000 acres (G. Bukenhofer, U.S. Forest Service, personal communication).  The Page Fire occurred near Page, Oklahoma in April 1975 and burned approximately 4,000 acres (G. Bukenhofer, U.S. Forest Service, personal communication).  The Eagleton fire, in Arkansas, was a railroad ignited fire on October 30, 1963 and burned 13,673 acres (http://www.oldstatehouse.com/educational_programs/classroom/arkansas_news/detail.asp?id=448&issue_id=32&page=5).  Turner (1935) reported that crown fires in this region of the country had not been recorded in historical records and therefore attributed several old-growth even-aged stands as the result of tornados in southwest Arkansas.  However, the stands were not mapped as to configuration nor were the area of the stands given. 

Recent crown fires may, in part, be the result of fire exclusion in regenerating stands.  Where frequent fire is commonplace, stand structure develops such that open stand conditions prevail and there is an absence of ladder fuels (Guyette and Dey 1997, Masters et al. 2005).  The understory is dominated by fire tolerant grasses that are prone to burn with a low to moderate intensity surface fire (Vogl 1972, Masters 1991a, Masters and Engle 1994, Masters et al. 1995, Sparks et al. 2002).  Even under these open conditions,  burning under late summer drought conditions may result in some mortality to overstory shortleaf pine through heat damage at the root collar, crown scorch to the point where buds are killed, or other direct or indirect thermal effects.  These effects resemble a mixed fire regime, or in some cases, a stand replacement event, even though a fire never carries through the canopy.