Fire Effects on Vegetation

Fire Effects

Vegetation

Most research addressing the role of fire in Table Mountain pine stands has been limited to post-wildfire studies, which suggest that high-intensity prescribed fires are needed to remove the forest canopy and expose mineral soil for successful regeneration. Zobel (1969) found that serotinous cones opened in lightly burned areas, but that seedlings survived only where fires killed overstory trees and erosion exposed mineral soil. Likewise, Sanders (1992) observed the greatest proportion of Table Mountain pine seedlings in high- and moderate-intensity burn areas, where the canopy was open and mineral soil exposed. Williams and Johnson (1992) found that seeds were abundant on the ground in lightly disturbed stands where no fire occurred. However, seedlings were successful only on microsites with thin litter layers (<4 cm) and where the canopy was more open than in surrounding stands. Such microsites were usually created by ice storms (Williams 1998). Also, Williams et al. (1990) found that hardwood litter creates barriers to pine seedling establishment.

Stand-replacement prescribed burning has been studied at 3 separate burn units in the southern Appalachian Mountains including the Grandfather Ranger District, Pisgah National Forest (Welch et al. 2000); Tallulah Ranger District, Chattahoochee National Forest (Waldrop and Brose 1999); and a burn unit managed by both the Andrew Pickens Ranger District, Sumter National Forest and the Buzzard’s Roost Preserve of the South Carolina Heritage Trust Program (Waldrop et al. 2002). These burn units will be referred to as the Grandfather, Tallulah, and Buzzard’s Roost burns, respectively. The burns conducted for all 3 studies varied in their effects on opening the forest canopy and removing litter and duff. Impacts on vegetation were largely a function of fire intensity. The prescriptions applied in these studies produced 4 fire intensities defined by Waldrop and Brose (1999): low, medium-low, medium-high, and high. Briefly, these categories were described as subcanopy ground fires (low), subcanopy ground fires with hot spots where jackpot fuels occurred (medium low), flames reaching into overstory tree crowns (medium high), and flames equal to or exceeding tree height (high). All intensities were observed in the Tallulah burn and all but high intensity was observed in the Buzzard’s Roost burn. Only the medium-low intensity was observed at the Grandfather burn.

High- and medium-high intensity fires were the only ones of sufficient intensity to kill enough of the overstory to achieve conditions of stand replacement. High-intensity fires in the Tallulah burn killed nearly all overstory trees, leaving only 1.0 m² of basal area per ha (Table 1). Medium-high intensity fires at Tallulah and Buzzard’s Roost were also effective at killing overstory trees, leaving only 1.6 and 7.6 m² per ha of basal area, respectively. Mortality was high across all diameter size classes following both high- and medium-high-intensity fires. Sunlight reaching the forest floor may have been adequate for seedling survival following fires of both intensities.

Table 1. Characteristics of Table Mountain pine stands during the year following stand-replacement prescribed burning.

¹Waldrop and Brose (1999)
²Waldrop et al. (2002)
³Welch et al. (2000)

Medium-low- and low-intensity fires reduced canopy cover (Table 1), but residual basal area may be too high in all 3 studies to allow sufficient pine regeneration. Medium-low-intensity fires reduced basal area to 11.1 m² per ha at the Tallulah burn and 10.8 m² per ha at the Buzzard’s Roost burn, but left 25.9 m² per ha at the Grandfather burn. Low-intensity fires had little effect on basal area, leaving 22.7m² per ha at the Tallulah burn and 19.2 m² at the Buzzard’s Roost burn. Mortality was greatest in lower d.b.h. classes (< 15 cm d.b.h.) following fires of medium-low and low-intensity. Shade from surviving trees after low- and medium-low intensity fires may prevent pine seedling survival.

Post-burn counts of Table Mountain pine seedlings in the Tallulah and Grandfather burns suggest that fires were of sufficient intensity to open serotinous cones throughout the burn units, even in areas of low-intensity. Post-burn pine densities ranged from 3,448 to more than 22,500 stems per ha (Table 1) in these two units. An unexpected result was that the lowest pine densities in the Tallulah burn were in areas burned at the highest intensity. This suggests that cones were consumed or seeds killed by intense heat, or that the seedbed became less suitable by excessive exposure to sunlight and evaporation.

Although plots in high-intensity burn areas had fewer seedlings, if they are well dispersed, the 3,448 seedlings per ha present in those areas should create pine-dominated stands. However, Table Mountain pine seedlings were found at only 51 % of the sampling points, indicating that portions of burned areas had no pine regeneration. Hardwoods will likely dominate such areas. Plots in areas burned at medium-high intensity also had low pine stocking (64 percent) and may become dominated by hardwood sprouts. Areas burned at medium low and low intensity should have sufficient numbers of seedlings to create pine-dominated stands if they receive sufficient sunlight.

Prolific hardwood sprouting was observed following fires of all intensities (Table 1). Under all fire intensities there were over 20,000 rapidly growing stems per ha one year after burning. Competition from these sprouts may eliminate any pine regeneration after a fire of any intensity. This result suggests that multiple, low-intensity fires may be necessary to reduce hardwood abundance while maintaining a seed source among large pines. Continued measurement of the stands created in these studies is needed to provide management recommendations for post-burn cultural treatments.

Soil and Water

No studies have been completed that directly address the impacts of fire in Table Mountain pine stands on soil and water quality. Studies by Zobel (1969), Williams and Johnson (1992), and Williams et al. (1990) document that wildfires exposed mineral soil and caused erosion in some areas. However, other areas had a duff layer thick enough to interfere with Table Mountain pine regeneration. Waldrop and Brose (1999) examined differences in forest floor characteristics by fire intensity after a prescribed fire. Post-burn duff depth was not related to fire intensity and averaged about 5 cm throughout the burn unit. Soil exposure was minimal. Rainfall the day after burning probably prevented smoldering of duff and prevented erosion.

The need for mycorrhizae is generally accepted for southern pine seedlings grown in nurseries, but it has not been studied for nontimber species such as Table Mountain pine. Ellis et al. (2002) examined the relationship of fire intensity to mycorrhizal development on Table Mountain pine roots. Pisolithus tinctorius, Suillus granulatus, and Cenococcum spp. were the predominant symbionts that formed mycorrhizal root tips in Table Mountain pine stands. Two years after burning, seedlings growing in areas burned at medium-low and medium-high fire intensities had twice as many mycorrhizal root tips (40 %) as did seedlings from sites burned at high intensities (22 %), indicating a lasting negative impact of high-intensity prescribed fires. Laboratory results were similar, showing that mycorrhizal roots tips are less common after fungi have been exposed to temperatures over 50^oC and almost absent after exposure to temperatures up to 80^oC. These results suggest that poor pine regeneration could result from poor formation of mycorrhizal root tips after high intensity fires. Frequent low-intensity burning would be one means of avoiding loss of mycorrhizal fungi.