Fire Effects on Shortleaf Pine

Adaptations

Shortleaf pine is generally recognized as being a fire adapted species (Garren 1943, Masters et al. 2003).  It has also been termed fire resistant (Lawson 1990) or having characteristics of a fire resilient species (McCune 1988).   But at least one author erroneously considered it fire-intolerant (Givnish 1981).  Shortleaf is not as well adapted to fire as longleaf but more so than either loblolly or slash pine.  As with most other southern pines the thick platy bark gives considerable resistance from fire injury to the cambium (Figure 15). Shortleaf develops thick bark very early in the seedling and sapling stage and is an excellent self-pruner (Guldin 1986).  These attributes are thought to be adaptations to frequent low-intensity surface fire (Schwilk and Ackerly 2001). It is also one of the few pines that will sprout from the base following top-kill (Garren 1943, Fowells 1965)(Figure 16).  This trait has been observed in trees up to 8 years of age (Mattoon 1915) and about 2.4 m (8 ft) tall in the south (Garren 1943).  However in southern New Jersey trees up to 30 years of age have been noted to sprout (Little and Somes 1956).  The sprouting trait is often cited as being an adaptation to fire (Mattoon 1915, Keeley and Zedler 1998) but then again hardwoods in general aggressively sprout when top killed, many of which are considered fire intolerant.  Some evidence suggests that summer prescribed burns will prevent sprouting of shortleaf (Cain and Shelton 2000). 

Figure 15

Figure 16

Figure 17

By definition serotinous cones refer to cone persistence (Harlow and Harrar 1969) or the retention of viable seed in closed cones over a period of time (Pyne et al. 1996).   However, by usage, cone serotiny has come to mean those with a waxy or resinous coating that open only related to heat from an intense fire (see Givnish 1981, Keeley and Zedler 1998, Schwilk and Ackerly 2001).  Shortleaf cones are somewhat persistent (Harlow and Harrar 1969) but not considered serotinous in the current usage of this term.

Bud configuration and location enables shortleaf to rapidly recover from fire (Little and Somes 1956, Little and Mergen 1966).  In the first 1-2 years of seedling development in open-grown conditions, a basal double crook exhibiting lateral growth at the root collar develops below ground line with a number of attendant dormant buds (Little and Mergen 1966)(Figure 17).  These dormant buds are well protected underneath the soil surface and survive even high intensity fires.  When competition is severe or in shady situations, it may take individual stems up to 9 years to develop this crook (Little and Somes 1956).  Most seedlings will develop this crook but those that are open grown develop it in a shorter period of time (Little and Mergen 1966). However the propensity of shortleaf to sprout is quite variable.  Sprouting ability is related to presence of a well-developed crook, a well developed root collar and overall seedling vigor and less as a result of age or height of the seedling (Little and Somes 1956). 

Dormant buds are also found along the main stem and main branches.  Where these latent buds receive some protection, the plant will sprout from either location even if completely defoliated by fire (Mattoon 1915, Little and Somes 1956).  Trees developing from sprouts normally develop good form and produce a commercial product (Mattoon 1915, Little and Somes 1956).  Mattoon (1915) also noted occasional occurrence of double-stemmed trees, which presumably resulted from stem development from two sprouts expressing equal dominance.  Masters et al. (2005) observed abundant sprouting in densely stocked 4 year-old seedlings that developed as a result of seed tree and selective cuts, when subjected to a late growing season burn in an Oklahoma study.  However in the same area Nickles et al. (1981) were able to successfully top-kill competing shortleaf in an early regeneration setting using prescribed fire and herbicide.  Stands of shortleaf have been noted to develop of coppice origin.  For example a young stand of planted shortleaf pine west of Waldron, Arkansas was top-killed by arson fires in the late 1980s, resprouted and was subsequently thinned precommercially, and now is approaching merchantable size (L. Hedrick, U.S. Forest Service, personal communication).

Mature shortleaf are notably resistant to mortality from crown scorch (Komarek 1981) and will survive if terminal buds are not killed even if the complete crown is scorched (Wade and Johansen 1986).  However Cain and Shelton (2003a) noted that diameter and thus volume growth were reduced by half in mixed stands of loblolly and shortleaf when crown scorch was about 75%.  Also the needles of shortleaf apparently do not burn as readily as other southern pines (Komarek 1981).  Apparently some combination of needle configuration and lower flammability provides protection for the terminal buds.

Effects of Fire Frequency
Frequent low-intensity fires are well-tolerated by shortleaf pine (Masters et al. 2005).  Hermann (1995) found no decrease in survivorship of mixed shortleaf-loblolly dominated stands after 35 years of annual dormant season burning.  Other studies found little or no mortality to mature overstory shortleaf with a 3-year burning interval (e.g., Sparks et al. 1999).  However, short-term (8 years) diameter growth and volume yield of shortleaf were increased in a New Jersey study with fire every 2-3 years while thinning without fire has little to no effect on shortleaf growth (Somes and Moorehead 1954).  In a review of the effects of fire on shortleaf growth rates Huebschmann (2000) found mixed results with some studies reporting declines, some increases and some with no change to shortleaf growth.

Effects of Fire Season
Although normally very fire resistant, mortality following low intensity fires in October was noted in mature shortleaf and loblolly in an area long excluded from fire and which had heavy domelike litter accumulations from sloughed bark and needle cast at the base (Ferguson et al. 1961).  In this case fuel moisture was low (6%) and evidently residence time of the fire was long.  Mortality may have been a combination of basal cambium injury and injury to fine rootlets that had developed in the organic layer.  A key to the mortality here was that fire had been long excluded from the site, fuel accumulations were high and fuel moistures were very low.  Other studies that examined both late-growing and late-dormant season fire did not report mortality to overstory shortleaf under a range of fireline intensities although other parameters were the focus of their studies (Sparks et al. 1999, 2002).

Effects of Fire Intensity
As suggested above, shortleaf is resistant to even relatively high intensity fire that may completely scorch the crown of a given mature tree (Komarek 1981).  Trees 60 years old showed no loss of growth when subjected to light to moderate intensity summer burns (Yocum 1972).   However young trees may suffer as much as a 75% growth loss following a prescribed burn (Garren 1943).  Only in the rare occasion of low fuel moisture and high fuel loads have low intensity fires resulted in overstory mortality to shortleaf (Ferguson et al. 1961).