Prescribed Fire in Loblolly Pine Forests

Fuel/Fire Hazard Reduction

Fuel loading in loblolly pine forests increases with time since burning, reflecting litter accumulation and growth of understory vegetation following a fire (Wade et al. 2000). Prescribed fire is widely used as a fuel reduction treatment throughout the loblolly pine region, mainly because of its relatively low cost and effectiveness at reducing fine fuel accumulations. Winter burns in the upper Coastal Plain of South Carolina resulted in a mean 52% reduction of litter depth, along with a 28% reduction in 1-hour fuel loading, a 15% decrease in 100-hour fuel loading, and a 3% decrease in 1000-hour fuel loading (Scholl and Waldrop 1999). The mean loading of 10-hour fuels actually increased by 22%, probably as a result of canopy scorch and pruning of lower branches in the forest canopy. The treatments also caused a mean 43% reduction in total live fuel loading, and a 61% reduction in live understory shrub fuels.

In an experimental study of alternate fuel treatments in the Piedmont of South Carolina, prescribed burning alone resulted in greater fuel reductions than either thinning alone or thinning followed by burning (Waldrop et al. 2004). Treatments that included fire were particularly effective at reducing the loading of litter fuels. In contrast, treatments that included thinning increased the loading of fine woody fuels in the first post-treatment year. When the first year post-treatment fuel loadings were used as inputs, the prescribed burning treatment produced the lowest flame height and spread rate, whereas the thinning treatments produced higher fire intensities (Mohr et al. 2004).

The effectiveness of prescribed fire at reducing fuels will depend on the weather conditions preceding and during the fire; the amount, types, and spatial distribution of fuels; and the methods used for burning. When prescribed fires are implemented at too low of an intensity, their impact on fuels loadings may be negligible (Rideout and Oswald 2002). The effects of prescribed burning may also be sensitive to other vegetation management activities. Fuel consumption was found to be greater in slash that had been felled in the spring, versus the winter, because continuing transpiration of cut vegetation in the spring resulted in lower fuel moisture and greater flammability (Waldrop 1997).

See also: Using Prescribed Fire for Hazardous Fuel Reduction.

Timber Management

Prescribed fire is frequently used in southern pine management for site preparation prior to seeding or planting (Schultz 1997). Burning is effective both at reducing the vigor of competing hardwoods, and is favored because of its low cost. However, burning alone is rarely sufficient to eliminate hardwood competition or to meet other management objectives, and is therefore usually combined with other mechanical or chemical treatments. Burning immediately before harvesting is often the most effective method of hardwood control, because shaded understory hardwoods have lower vigor than those growing in the sunlight of recent clear-cuts. Alternately, burning late in the growing season and planting or seeding immediately thereafter provides pine seedlings with the greatest competitive advantage over hardwoods. Low-severity winter burning may actually stimulate the sprouting of hardwoods in the following growing season. In addition to suppressing hardwood competition, prescribed burning also reduces the litter layer and creates suitable conditions for natural regeneration (Cain and Shelton 2002).

Once a cohort of loblolly pines is established, fire can be applied as a tool for pre-commercial thinning. Prescribed burning was used to reduce the density of a young loblolly pine stand in the lower Piedmont of Georgia from more than 24,700 trees/ha (10,000 trees/acre) to 8,600 trees/ha (3,500 trees/acre) (McNab 1977). Tree diameters in the pre-fire stand ranged from 1.3 cm (0.5 inches) to 15.2 cm (6 inches), and no trees greater than 10.1 cm (4 inches) in diameter were killed by the fire. This method is only recommended in situations where trees sizes span a relatively wide range of diameters.

Prescribed fire can also be applied during the rotation to reduce competition from woody vegetation. Numerous studies have demonstrated that fire can reduce the dominance of understory trees and shrubs. The effects of fire are usually greater under more frequent burning regimes, particularly when annual growing season burns are applied (White et al. 1990). However, these reductions in understory vegetation do not always translate into more rapid growth of crop trees. Under controlled conditions, growth of loblolly pine seedlings was enhanced by long term, annual winter burning (McKevlin and McKee 1986). In contrast a comparison of growth rates of overstory loblolly pines over a 41-year period showed negligible differences in the rate of basal area growth per hectare between an unburned control and plots subjected to different seasons and frequencies of fire (Waldrop et al. 1987). This lack of response was attributed to the fact that the stands were already 40 years old at the onset of the experiment.

Several other factors may also limit the potential for increased tree growth in response to understory vegetation control with prescribed fire. Although mature trees are seldom killed by the crown scorch from controlled surface fires, severe crown scorch may result in a temporary decrease in growth. Following a single prescribed burn in a 17-year old loblolly pine plantation in South Carolina, height growth of crop trees was reduced compared to an unburned control and diameter growth was unaffected (Waldrop and Van Lear 1984). Diameter growth in a 19-year old naturally regenerated loblolly pine stand in southeastern Louisiana decreased following a prescribed fire (Lilieholm and Hu 1987). Smaller trees and trees with more crown scorch had the largest growth reductions, and the growth of most trees recovered within three years after the fire. Burning also releases N and other essential nutrients to the atmosphere as gasses and particulates (Carter and Foster 2004). When frequent prescribed burning is combined with short-rotation timber harvesting and broadcast burning for site preparation, nutrient losses may translate into reduced site productivity and lower timber yields.

See also:  Using Prescribed Fire in the Silviculture of Loblolly Pine.

Wildlife Management

Upland game species such as cottontail rabbit (Sylvilagus floridanus), fox squirrel (Sciurus niger), gray squirrel (Sciurus carolinensis), white-tailed deer (Odocoileus virginianus), bobwhite quail (Colinus virginianus), mourning dove (Zenaida macroura) and wild turkey (Meleagris gallopavo)are all found in loblolly pine or loblolly-hardwood stands (Mobley and Balmer 1981). Many non-game species and some threatened and endangered species also utilize these forest habitats. Most species native to the region evolved in the diverse habitats of former pre-settlement forests and are not adapted to conditions in pure pine forests. For example, white-tailed deer prefer different habitats and food sources at different points in their life cycle. Species such as the red-cockaded woodpecker (Picoides borealis), gopher tortoise (Gopherus polyphemus), and indigo snake (Drymarchon corais) require open savannah habitats that are not replicated in most closed-canopy pine forests. However, by maintaining diversity in stand structure, age and timber types, with some mast-producing hardwoods where feasible, habitat for many can be provided with little economic impact on timber production (Johnson 1987). Prescribed fire can be used to alter both the structure of forest stands and the plant species available for wildlife nutrition, and is thus an important tool for managing wildlife habitat in loblolly pine forests.

Regenerating pine stands often provide a large quantity of woody vegetation, herbaceous plants, and fruits beneficial to some mammals and birds (Dickson and Huntley 1987). This early-successional habitat generally deteriorates within 7–10 years as canopy closure shades out non-pine vegetation. A few species such as the brown-headed nuthatch (Sitta pusilla) are strongly associated with pure pine stands. However, the species richness and abundance of bird communities generally increases over the course of forest succession as young pine stands mature and develop a hardwood component (Meyers and Johnson 1978).

Most bird species in southern pine forests are more sensitive to understory vegetation than to overstory structure (Johnston and Odum 1956). Grassy understories favor species such as bobwhite quail and Bachman’s Sparrow (Aimophila aestivalis). Shrub dominated understories favor other species such as the Carolina Wren (Thryothorus ludovicianus), the great crested flycatcher (Myiarchus crinitus) and the Summer Tanager (Piranga rubra), the Yellow-throated Vireo (Vireo flavifrons), the Eastern Wood Peewee (Contopus virens), and the Hooded Warbler (Wilsonia citrine) (Meyers and Johnson 1978). Thus, to the extent that prescribed fire influences the composition and structure of understory vegetation (Waldrop et al. 1987, Waldrop et al. 1992, Cain et al. 1998), it will also affect bird habitat.

Winter burns are often more effective than summer burns at promoting the development of wildlife foods, because summer fires may temporarily eliminate critical browse plants needed in the fall and early winter. Loblolly pine stands in South Carolina burned every winter for 20 years showed an increase in food plants valued by wild turkey, such as winged sumac (Rhus copallina), beggartick (Bidens spp.) and partridge pea (Cassia spp.), whereas adjacent plots burned annually in the summer had lower levels of these species (Degraff et al. 1991). On the Coastal Plain of South Carolina annual winter burns produced high quantities of forbs producing superior habitat for quail (Lewis and Harshbarger 1976).

However, not all species are best served by annual winter burning. Frequent fires can weaken browse plants to the point where they seldom produce much foliage and fruit, limiting their habitat value. In pine-hardwood forests, understory burning is recommended at 5-10 year intervals to provide ample berries and mast for black bears (Hamilton 1981). White-tailed deer benefit from a more frequent regime of spring burns (Halls 1978). A 3 to 5 year burn regime keeps browse within reach without having the detrimental effect on fruit and mast production and spring burning promotes resprouting of vegetation. For southeastern forests in general, prescribed winter burning over a 3 to 6 year interval is recommended to increases the quality, quantity, palatability and accessibility of a variety of grasses, forbs and shrubs that provide food for many game and non-game species (Schultz 1997).

Habitat for several threatened and endangered species such as the Gopher tortoise, indigo snake and red-cockaded woodpecker are improved by burning (Wade and Lunsford 1988). Red-cockaded woodpeckers (RCW) prefer old growth, open, pine dominated stands for nesting and foraging. RCWs form nesting and roosting cavities by excavating holes infected with the red heart fungi (Phellinus pini) in large pines. These fungi decay the heartwood of the tree while leaving the exteriors intact (Conner and Locke 1979) and mature trees are more likely to be affected. The average loblolly pine cavity tree is more than 75 years old and greater than 23 cm (9 inches) dbh (Schultz 1997). A single family may require more than 50 ha but may thrive in as little as 28 ha in high quality habitat (Conner et al. 2001). Stands adequate for RCW contain at least 1.7 m²/ha (7.4 ft²/acre) of pine basal area/ha. The presence of hardwoods may reduce the quality of RCW habitat by attracting other woodpecker species and increasing competition for pine resources. Prescribed fire is useful for reducing hardwood component but old cavity trees may be very susceptible to intense understory fires. For this reason colony sites should be protected by raking fuel at least 3 m (10 feet) away from cavity trees and burning the areas separately with cool backfires (Conner and Locke 1979).

Other species of birds, such as Bachman’s sparrow, brown-headed nuthatch and pine warbler do well on pine stands managed for RCW’s (Schultz 1997). Other birds associated with early successional pine stands, such as common yellowthroat (Geothlypis trichas), yellow-breasted chat, field sparrow (Icteria virens), grasshopper sparrow (Ammodramus savannarum) and indigo bunting (Passerina cyanea) often benefit from prescribed burns (Dickson 1981). Zebahazy et al. (2004) reported that bird abundance did not change because of fuel reduction treatments, including both thinning and burning, but species richness increased. Foliage-gleaning and canopy nesting species were more common in thinned sites than in burned. A drawback is that these treatments may increase risk of predation on birds.

Most lightning-caused fires occur in the summer when conditions are driest and most amphibian species are underground or close to water. Prescription burning often occurs in the spring and some amphibians may be migrating to water for reproduction or dispersing to breeding sites and may be more vulnerable to fire (Pilliod et al. 2003). However, Schurbon and Fauth (2003) suggest that a burn interval of approximately 5 years applied during the growing season is better for amphibian diversity in longleaf pine stands. Russell et al. (1999) suggests that returning prescribed fire to the southeastern coastal plain would likely benefit herpetofauna by restoring historical mosaics of successional stages, habitat structures and vegetative species composition. In the southeast firebreaks created around isolated wetlands may be harmful to species that do poorly in ponds where hardwood succession and canopy closure has resulted from years of fire suppression.

See also: Using Prescribed Fire to Improve Wildlife Habitat and Fire Effects on Fauna