Managing with Prescribed Fire

Ecosystem Maintenance and Restoration

Vegetation

After decades of fire suppression, ridgetop pine communities of the Southern Appalachians are entering later seral stages and disappearing.  They typically have Table Mountain pines (Pinus pungens) and pitch pines (P. rigida) in the overstory, which are being replaced by more shade-tolerant oaks.  Previous research suggests that high-intensity stand-replacement fires are needed to restore these communities because they will open the forest canopy and expose mineral soil.  However, this work was based on observations made after wildfires.

In a comparison of regeneration success after prescribed fires of varying intensity, high and medium-high intensity fires killed most overstory trees and provided adequate sunlight for pine seedlings (Waldrop and Brose 1999).  Medium-low and low intensity fires did not kill overstory trees and left too much shade on the forest floor.  Post-burn duff was deep and did not vary by fire intensity.  Sufficient seedling densities to restore pine-dominated stands (< 9,000 per ha) occurred after all but the highest intensity fires.  Many seedlings survived the first growing season as their roots penetrated duff up to 7.5 cm deep to reach mineral soil.  Hardwood rootstocks sprouted on sites treated with all fire intensities and may out-compete pine seedlings without repeated fires.

Poor regeneration after high-intensity fires was unexpected because these fires were suggested by previous research.  However, greenhouse and field studies of seedbed habitat showed that pine seedlings had better survival in the presence of low shade and thin duff than in full sunlight and with no duff (Mohr et al. 2002, Waldrop et al. 2002).  These results suggest that high intensity fires reduce seedbed habitat quality by drying the site.  Another study showed that high-intensity fires reduced mycorrhizal abundance and, therefore, limited moisture availability for germinants (Ellis et al. 2002).  A study of seed biology showed that poor regeneration after high-intensity fires was not likely caused by a poor seed source (Gray et al. 2002).  Rather, the fires may have consumed cones or killed seed.

Four studies provide evidence that ridgetop pine communities were historically created and maintained by multiple low-intensity fires rather than a single stand-replacement fire.  A dendrochronology study shows that these stands are uneven aged with trees ranging from 50 to 150 years old (Brose et al. 2002).  Pines regenerated frequently from approximately 1850 to 1950, probably due to open conditions maintained by low-intensity fires.  Mountain laurel (Kalmia latifolia) became more common after 1950, probably due to fire exclusion.  The seed biology study indicates that viable seed occur on trees as young as 5 years, suggesting an adaptation to frequent burning (Gray et al. 2002).  A study of multiple low-intensity fires shows that ridgetop sites have open understories and begin to support pine regeneration after three low-intensity prescribed fires (Randles et al. 2002).  

Pine regeneration can become established by single fires of relatively high intensity or multiple low-intensity fires, indicating that crown fires are too hot because they potentially damage the site.  Medium-high intensity fires, which reach into the lower crowns of pines, are safer and provide abundant regeneration.  Multiple low-intensity fires require a greater investment of time but better mimic historic burning regimes.  This knowledge will allow a wider burning window and increase worker safety because severe weather conditions are not required for low-intensity fires.  Because prescription guidelines developed by the above studies work are more conservative and safer, there will be a greater opportunity for prescribed burning to be accepted by the community and by land managers.  Regeneration of these stands by means other than prescribed burning is unlikely because most stands are in remote locations and inaccessible to equipment.  

Implementation

It is generally perceived that contemporary Table Mountain pine communities are legacies of the intense wildfire era of the early 20th century and are dependent on high-intensity crown fires for regeneration (Zobel 1969, Barden and Woods 1974, Sanders 1992, Williams 1998).  This perspective is supported by several facts.  Their almost exclusive occurrence on steep, dry, south- and west-facing ridges and upper slopes places them where fires moving uphill would reach their highest intensities (Zobel 1969, Sanders 1992, Williams 1998).  The species has silvical characteristics, such as black seeds, cone serotiny, need for a mineral soil seed bed, and shade intolerance, indicative of pines that evolved under such conditions (Della-Bianca 1990).  Some post-burn regeneration evaluations support this hypothesis as the most abundant and successful pine seedlings occurred where intense fires killed all the overstory and understory and removed the duff layer (Groeschl et al. 1992, Sanders 1992, Groeschl et al. 1993).  Conversely, low-intensity fires that did not open the canopy and reduce duff thickness failed to adequately regenerate pitch and Table Mountain pine (Williams et al. 1990, Groeschl et al. 1993, Elliott et al. 1999, Welch et al. 2000).      

However, evidence exists indicating that intense crown fires are not necessary and frequent, low-intensity surface fires may be the correct fire regime for this forest type.  Table Mountain pine has characteristics such as thick, flaky bark, self-pruning of lower branches, and cone production at young age, that suggests the species evolved in a frequent, low-intensity surface fire regime (Della-Bianca 1990, Little and Garrett 1990).  Cones of Table Mountain pine will open at temperatures as low as 30°C and the resins that seal the cones degrades after a few years (McIntyre 1929, Fraver 1992).  Williams and Johnson (1992) reported that Table Mountain pine released seeds throughout the year with peak dissemination occurring in the spring and summer.  The topographic location of Table Mountain pine stands would make them susceptible to lightning strikes and subsequent small, isolated fires (Barden and Woods 1974).  Waldrop and Brose (1999) studied a variable-intensity prescribed fire conducted in 1997.  They found that Table Mountain pine regenerated better in areas experiencing a moderate-intensity surface fire (partial canopy removal) than it did in the full sunlight created by a high-intensity crown fire.  Also, roots of the new pine seedlings were capable of penetrating duff up to 7.5 cm thick.  In a related study, Mohr et al. (2002) reported that Table Mountain pine seedlings survived better in partial shade on a 5 cm duff layer than they did in full sunlight on mineral soil.

Research Gaps

Several research projects have been completed to determine the type and intensity of prescribed fires that are needed to regenerate Table Mountain pine at the southern end of its range.  These projects suggest that either a single stand replacement fire or multiple low intensity fires can be successful though both methods have limitations when relied upon solely.  However, different prescribed burning techniques have not been tested in the central or northern portion of the species’ range.   Also, there are no guidelines for selecting stands for burning that have a high probability of producing successful regeneration.  Factors such as cone production, seed viability, stand successional stage, overstory density, and shrub density may determine whether or not a particular stand can be regenerated with prescribed fire.  This information would allow managers to prioritize stands for burning by identifying which are at the greatest risk.

Studies of prescribed fire impacts in Table Mountain pine communities have been limited to regeneration success.  Prescribed fires have dramatic impacts on numerous components of other ecosystems including wildlife, soils, insects, diseases, and nutrient cycling.  Impacts of prescribed fires should be studied for all ecosystem components in Table Mountain pine communities over a range of fire intensities.

Research to date has not has not been able to address management needs in the regenerated stands.  Competition from sprouts of hardwoods or shrubs may become a major concern.  Pines may need to be released from this competition by cutting, herbicide application, or understory burning.  The techniques or need for these practices have not been addressed.  Also, sites with Table Mountain pine regeneration are usually xeric and may not support rapid sprout growth.  Study sites where Table Mountain pine regeneration has been successful should be followed for several years to address these questions.