Fire Ecology of Loblolly Pine Forests

Species Adaptations to Fire

The bark of southern pines is thicker and has better insulating qualities than most hardwood species in the region. Of the southern pines, only longleaf and slash pine have bark with a higher resistance to fire damage than mature loblolly (Hare 1965). However, young loblolly pines have relatively thin bark and are susceptible to girdling by fire until they are approximately two inches in diameter (Wade 1993). An experimental study that utilized a propane-fired backfire simulator similarly found that most trees with a ground diameter greater than 5 cm were resistant to girdling by flames within the normal range of backfire intensities (Greene and Shilling 1987).

Crown scorch can also cause fire-induced mortality. The percentage of crown that is scorched depends on both tree size and flame height (McNab 1977, Waldrop and Van Lear 1984). Prescribed winter burns in a mixed loblolly-shortleaf stand in southeastern Arkansas caused an average of 95% crown scorch for seedlings < 0.9 m (3 feet) in height, 90% crown scorch for seedlings >= 0.9 m (3 feet) in height, and 80% for saplings greater than 1.3 cm (0.5 inches) dbh (Cain and Shelton 2002). However, even severe crown scorch may have little effect on the survival and growth of larger trees. In a 17-year old pine plantation in South Carolina, co-dominant trees that were completely scorched suffered only 20% mortality, whereas intermediate trees suffered 20-30% mortality (Waldrop and Van Lear 1984). No dominant trees died as a result of crown scorch. In a 19-year old naturally regenerated loblolly pine stand in southeastern Louisiana, incidence of severe crown scorch following a winter burn was greatest in dense, lightly thinned plots that had a large number of small trees (Lilieholm and Hu 1987). The only trees that experienced significant fire-induced mortality were in the suppressed crown class.

Unlike most hardwoods and some southern pine species such as shortleaf and longleaf pine, loblolly pine does not resprout from the root collar when it is topkilled by fire. However, widespread establishment of both loblolly and shortleaf pine seedlings often occurs following fires. The combustion of the litter layer facilitates seed germination, and fire-induced overstory mortality leads to higher light levels that favor the rapid growth of young loblolly pine seedlings. Loblolly pine regeneration is greatly enhanced when fire coincides with heavy seed production (Cain and Shelton 2002). Although loblolly pine produces some seeds every year, bumper seed crops of more than 200,000 seeds/ha (81,000 seeds/acre) generally occur every 3 to 6 years (Baker and Langdon 1990). Effective dispersal distances range from 61 to 91 m (200 to 300 feet) downwind from the source and 23 to 30 m (75 to 100 feet) in other directions

The resistance of hardwoods to topkill from fire also increases with tree size (Waldrop et al. 1987). Larger trees have thicker bark that is less susceptible to cambial damage, and foliage that is higher above the forest floor and less susceptible to crown scorch. Hardwood responses to fire can vary greatly among species. For example, many mature oaks and hickories have relatively thick bark (Harmon 1984). In contrast, species such as red maple, sweetgum, and American beech have thinner bark and are presumably more likely to be girdled by fires. In addition to differences in bark thickness, hardwood species also exhibit differences in the insulating qualities of their bark. In an experiment that controlled for bark thickness, sweetgum, American holly, and black cherry required the least time to achieve lethal cambial temperatures when exposed to flame (Hare 1965). Red maple, water oak, dogwood, tupelo, and birch took longer to reach lethal temperature, and southern magnolia and sweetbay were the most resistant to cambial damage. However, these differences may not translate directly into inter-specific variation in tree mortality under field conditions (Harmon 1984).

Hickories and oaks also tend to invest more energy in root development than species such as red maple and yellow poplar, and are therefore able to resprout repeatedly after being topkilled. In some cases, vigorous resprouting from rootstocks can actually increase the density of hardwood stems following a burn (BROKEN-LINK Brose and Van Lear 1998). If fire does not occur for several years, well-development root systems will send up vigorous sprouts that rapidly reach a height and bark thickness where they can survive most low-intensity surface fires (Van Lear 1991). However, the spouting ability of most hardwoods declines with tree size, so that larger trees may not resprout if they are killed by a high-intensity fire.

See also: Fire Autoecology of Plants.

Autogenic Succession

In the absence of fire, loblolly pine forests exhibit predictable successional trajectories following land clearing, timber harvesting, or other stand-replacing disturbances. Loblolly and shortleaf pines are typically the first trees species to colonize abandoned agricultural lands in the South. The association between loblolly pine and old-field vegetation is so strong that it has been nicknamed “old-field” pine (Baker and Langdon 1990). Loblolly and shortleaf pines are effective at colonizing these open sites because of their prolific seed production, long-distance dispersal, and high drought tolerance once seedlings are established (Bormann 1953). If abandoned fields are very large, however, pine establishment will be dispersal limited and colonization will proceed much more slowly (Golley et al. 1994, Pinder et al. 1995). The rapid growth of loblolly pine in full sunlight allows it to quickly dominate abandoned fields once it is established (Baker and Langdon 1990). With an adequate seed source nearby, an even-aged cohort of loblolly pine is usually established within 3 to 10 years. (McQuilkin 1940, Oosting 1942).

Pine recruitment typically ceases after approximately 25 years and an understory of hardwoods begins to establish (Oosting 1942, Monette and Ware 1983). Establishment of hardwoods is facilitated by the gradual opening of the pine canopy as self-thinning occurs, and by the accumulation of pine litter that increase soil moisture retention and protects hardwood seeds from desiccation. As overstory pines die as a result of lightning strikes, insect attacks, disease, and senescence, they are gradually replaced by understory hardwoods. Species richness of trees, shrubs, and other understory plants all increase over the first 70 years following field abandonment (Nicholson and Monk 1974). In the absence of major disturbances, shade-tolerant hardwoods will begin to supplant the pines and form mixed pine-hardwood stands within 50-100 years (Monette and Ware 1983, Christensen and Peet 1984). Although hardwoods account for the majority of stems as succession proceeds, the basal area of stands 100-150 years old can still be dominated by a few large, old loblolly pines (Pederson et al. 1997, Abrams and Black 2000). In the absence of major disturbances, the forest will eventually succeed into an oak-hickory dominated climax forest over most of the loblolly range (Quarterman and Keever 1962, Jones 1988, White and Lloyd 1998), or a beech-magnolia climax forest in some portions of the Gulf Coastal Plain (Delcourt and Delcourt 1974, 1977, Glitzenstein et al. 1986).

Forest succession following clear-cutting can differ in several ways from old-field succession. Old-field succession usually results in an initial cohort of pure pine that can persist for several decades before hardwoods establish. Although some mixed pine-hardwood stands in the Southeast represent this middle stage of old-field succession, many others are the result of clear-cutting or selective logging of pine (Ware et al. 1993). After timber harvesting, the forest floor remains largely intact and many tree species can rapidly reestablish from rootstocks or buried seed. Therefore, oaks and other hardwood species typically establish along with pines in the initial post-disturbance cohort and are present throughout succession (Abrams and Black 2000, Harcombe et al. 2002). Nonmerchantable species, such as blackgum, were often left on cutover lands in the early 20^th century (Abrams and Black 2000). In other cases, partial cuts left a mixture of smaller pines and hardwoods (Glitzenstein et al. 1986). These remnant trees can influence post-cutting succession by providing a local seed source, and may still remain a century of more after cutting. In addition, native understory flora can establish relatively rapidly after clear-cutting, instead of requiring decades to reinvade abandoned fields (Hedman et al. 2000).

Fire Effects on Vegetation

The effects of individual fires can be highly idiosyncratic, and are sensitive to weather and fuel conditions at the time of the burn. Both mortality of overstory pines and recruitment of oaks and hickories increased after a surface fire in a 35-year old stand of old-field pine (Oosting 1944, Oosting and Livingston 1964). Thus, a single low-severity fire can increase the rate of succession from pine to hardwood dominance. In contrast, crown fire in another portion of the same stand killed most overstory trees, leading to the regeneration and rapid growth of a new pine cohort. In this case, fire acted to retard succession and maintain a pine-dominated forest. The application of two prescribed burns in a hurricane-impacted stand in east Texas similarly resulted in a shift toward a pine-dominated savannah vegetation type (Liu et al. 1997). However, in this case, the amount of compositional change was slight because the prescribed fires were low-intensity and patchy. Results from another study in East Texas also indicate that the impacts of a single fire on understory vegetation may be minimal if high fuel moisture, low wind speeds, and cool temperatures result in a patchy, low-intensity burn (Rideout and Oswald 2002)

When loblolly pine forests are subjected to repeated fires, the cumulative effects of these fires depend on frequency of burning. For example, uneven-aged forests of loblolly and shortleaf pine in southeastern Arkansas were subjected to winter burning at three, six, and nine-year return intervals (Cain et al. 1998). Areas burned at three-year intervals had lower cover of understory woody plants than unburned controls and areas burned at six- or nine-year intervals. Only the areas burned at three-year intervals exhibited reduced importance of small hardwood stems and a compositional shift toward more fire-tolerant species relative to the unburned areas. All of the burning treatments resulted in higher cover of graminoids and composites than in the unburned controls. Understory species composition of loblolly pine forests in the coastal plain of South Carolina varied similarly along a fire frequency gradient (White et al. 1990). Only the annually burned plots supported high abundances of early-successional grasses, composites, and legumes. Periodically burned plots contained many of these species as well, although some of the poorer competitors were not present. Unburned controls completely lacked this suite of opportunistic “fire followers”, but contained other shade-tolerant, fire-sensitive species that were absent in the burned plots.

The season of burning also influences the cumulative effects of multiple fires. In general, summer burns topkill more woody stems and affect larger-size stems than winter burns (Brender and Copper 1968). In the Upper Coastal Plain of Alabama, 20-30 year old loblolly and shortleaf pine forests were subjected to both winter and summer burns at a three-year interval (Hodgkins 1958). Plots burned during the winter had lower canopy cover than control plots, and plots burned during the summer had lower canopy cover than those burned during the winter. These differences in cover mainly resulted from different densities of sapling-size trees. Burning during both seasons also resulted in higher shrub and vine cover than in the control plots, but shrub and vine cover was higher in plots burned during the winter than in those burned during the summer.

Experimental plots in the Santee Experimental Forest, located in the coastal plain of South Carolina, were burned at different frequencies (annual, biennial, and periodic) in two different seasons (winter versus summer) from 1946 through 1989 (Waldrop et al. 1992). Periodic burning in both seasons and annual winter burning increased the density of small < 2.5 cm (1 inch) diameter hardwood stems, decreased the density of 2.5 to 12.5 cm (1 to 5 inches) diameter hardwood stems, and had little effect on larger hardwood stems. Densities of small stems were higher than 40,000 stems/ha (16,200 stems/acre) under annual winter burning, compared with less than 20,000 stems/ha (8,100 stems/acre) under period burning, mainly because of increased sprouting of sweetgum (Waldrop et al. 1987). In contrast, annual summer burning greatly reduced hardwood stem densities for all sizes less than 12.5 cm (5 inches). Only the repeated summer fires were able to deplete the carbohydrate reserves enough to kill some of the hardwood rootstocks. Density of shrub stems was lower under annual burning than under periodic burning, and lower under annual summer burning than under annual winter burning. Understory cover was dominated by woody plants under periodic burns. Cover of grasses and forbs was higher under annual winter burns, and comprised the majority of understory cover under annual summer burns.

Although summer burns generally have higher intensities and cause higher mortality than winter burns, this generalization may not hold in some vegetation types. In a shortleaf pine dominated forest in west-central Arkansas, winter burns actually had higher intensities and consumed more fuel than summer burns, even though live and dead fuel moistures were both higher in the winter (Sparks et al. 2002). This difference was attributed to a change in fuel quality between seasons. In the summer, fuel loads were lower and were dominated by live herbaceous fuels. In the winter, fuels were dominated by litter from hardwood leaf fall and dormant herbaceous plants.