Fire Effects on Longleaf Pine Vegetation

The effect of fire on vegetation in high quality longleaf pine land is to maintain the existing plant community. In other words, the effect of a single burn in a habitat accustomed to, and maintained with, frequent fire is remarkably slight (Frost 1998). A few longleaf pine trees may be killed, primarily in the smaller size classes (Glitzenstein et al. 1995). Depending on fire season and intensity a certain percentage of tree-sized scrub oaks and hickories may be top-killed most of which will re-sprout (Glitzenstein et al. 1995). The above ground parts of ground layer plants will be mostly consumed by the fire, but the below ground parts will mostly survive to send up new shoots. Annual or biennial species present prior to the burn will germinate and grow to maturity following the fire. Within a few months post-fire the entire community will be reconstituted, to all appearances the same as before.

In contrast, effects of individual fires in long-unburned longleaf stands can promote meaningful changes, depending on time since fire, accumulated fuels, and burn conditions on the day of the burn (e.g. Plocher 2003). Prolonged periods of fire suppression can lead to invasions of hardwood trees, declines in grasses, and consequent reductions in community flammability (Streng and Harcombe 1982, Maliakal et al. 2000). Prescribed burning under these conditions may be difficult or ineffective, though Provencher et al. (2001a) found that even patchy, low intensity fires promoted increases in ground layer species richness in long unburned west Florida sandhills. On the other hand, accumulated woody fuels can lead to high intensity fires and substantial canopy mortality. For example, thirty years of fire exclusion in relict Virginia longleaf pine stands resulted in prescribed fires with flame heights up to 6 m and post-fire canopy mortality rates ranging from 19 to 46.7% (Plocher 1999).

Fires in long fire-suppressed sites may produce extreme results where the ensuing stand consists of a relatively few surviving longleaf pine trees along with hardwood sprouts and reduced populations of the typical perennial longleaf ground layer plants. In place of the latter, there may be a pronounced increase in “weedy” annuals and short-lived perennials germinating from buried seeds in the soil. These stands are often dominated by such species as pokeweed (Phytolacca americana), pinweed (Hypericum gentianoides), fireweed (Erechtites hieracifolia), Polypremum procumbens, and dogfennels (Eupatorium compositifolium, E. capillifolium). These ruderal (weedy) species are generally not encountered in well-maintained fire frequented sites except in very localized soil disturbances or hotspots (Maliakal 2000, Glitzenstein et al. 2003).

Because effects of individual fires are slight in properly maintained longleaf pine sites it is more productive to discuss effects of burn regimes, i.e. cumulative impacts of repeated fires occurring over long time periods. Most authors recognize three components to a fire regime: fire frequency, fire season, and fire intensity (DeBano et al. 1998).

Effects of Fire Frequency

Some effects of fire frequency on longleaf vegetation are relatively well established whereas others are still uncertain. In general, increases in fire frequency are correlated with increased herbaceous dominance, particularly of grasses, and decreased importance of woody plants (Waldrop et al. 1992, Beckage and Stout 2000, Glitzenstein et al. 2003). Fire frequency effects on vascular plant species richness are less well established. Three different relationships have been documented in the literature:

1. Two studies did not find a significant effect of fire frequency on species richness. Beckage and Stout (2000) did not find a fire frequency effect on species richness in a central Florida sandhill and Brockway and Lewis (1997) found no significant differences among three fire frequency treatments (annual, biennial, triennial burning) in a Georgia flatwoods. Beckage and Stout’s (2000) study was correlational rather than experimental and the lack of a significant effect may have been due to low statistical power. Although Brockway and Lewis’ (1997) reported no significant effect among annual, biennial, and triennial burning treatments, all three of these fire frequencies produced species richness levels (34.9 plant species) that were approximately twice that measured on long unburned (>40 years) forest plots (17.5 plant species).
2. Mehlman (1992) found that species richness in the long-term Stoddard Fire Plots at Tall Timbers Research Station increased as fire frequency increased, but tended to plateau at the shortest fire return intervals. Beckage and Stout (2000) further clarified this mathematical relationship in Mehlman’s (1992) data and termed it the “saturation effect”. The Stoddard plots are located in old-field loblolly stands, and may, historically, have been dominated by shortleaf pine (R. Masters, personal communication), so the relevance of these results to undisturbed longleaf sites may be debatable.
3. Data from long-term studies of longleaf pine flatwoods sites in Francis Marion NF, SC, and Osceola NF, FL indicated a linear relationship between fire frequency and species richness (Glitzenstein et al. 2003). This relationship was strongest at small scales and became marginally non-significant at the largest scale investigated (0.1 ha). If differing relationships between fire frequency and species richness are in fact real properties of different habitats or regions, and are not just artifacts of experimental design and analytical procedures, the implications for management would be significant. More studies are clearly needed.

In addition to the above studies of fire frequency effects on species composition, there is also a recently published study of the influence of fire frequency on population scale parameters of rare species. Gray et al. (2003) investigated effects of fire frequency on extinction, colonization and persistence of 32 rare longleaf ground layer plants at Fort Bragg and Camp Mackall military bases in southeastern North Carolina. They concluded that annual or biennial burning was needed to maintain or increase the numbers of populations of these rare species. Less frequent fires resulted in an unacceptably high rate of local extinctions.

Effects of Fire Season

Season-of-burn effects on longleaf pine vegetation were reviewed by Robbins and Myers (1992) and by Streng et al. (1993). Both reviews concluded that season of burn might be important, but that interpretation of many published studies was complicated by lack of replication, pseudo-replication, and potentially confounding effects of fire behavior and year of burn. Subsequent studies have also produced inconsistent results. The following conclusions may apply.

1. Compared to fires at other seasons, there appears to be a consistent positive effect of growing season fires on flowering and fruiting of dominant grass species and some composites (Biswell and Lemon 1943, Parrot 1967, Streng et al. 1993, Brewer and Platt 1994). Brewer and Platt (1994) also documented a positive effect of growing season fires on clonal proliferation in the dominant longleaf sandhill forb grass-leaved goldenaster (Pityopsis graminifolia).
2. In contrast, detailed studies of legumes have not detected an effect of season of fire on reproductive biology or nitrogen fixation rates (Hiers et al. 2000, 2003).
3. Growing season fires appear to enhance top-kill rates of oak trees and other tree-sized hardwoods in longleaf pine stands (Robbins and Myers 1992, Streng et al. 1993, Boyer 1993, Glitzenstein et al. 1995, Provencher et al. 2001a). Partly this is a function of fire behavior. That is, dry windy conditions promoting more intense fires are especially likely to occur during spring months. Fire behavior alone does not explain the entire effect of season of fire on oak topkill, so there is probably a residual effect of tree physiology as well (Glitzenstein et al. 1995).
4. Long-term annual fires in the growing season can eradicate woody shrubs and hardwood sprouts (Waldrop et al. 1992). However, a similar effect is not evident for fire return intervals of 2 or more years. Two short term studies investigating effects of fire season on shrub stem densities did not find an anticipated strong negative effect of growing season fires, though burning during the growing season did somewhat inhibit in-growth of new woody stems (Olson and Platt 1995, BROKEN-LINK Drewa et al. 2000).
5. Except for the effect of annual summer fires, long-term experiments have not demonstrated a strong impact of fire season on ground layer species composition in longleaf pine savannas (Waldrop et al 1992, Streng et al. 1993, Kush et al. (2000), Glitzenstein et al. 2003b). Kush et al. (2000) demonstrated a rather striking negative effect of spring and, especially, summer biennial burning on legumes. Given the results of Hiers et al. (2000, 2003) discussed above, this finding is difficult to interpret. After 22 years of biennial fires, Kush et al. (2000) also found the highest overall species richness in winter burn plots, but the differences were relatively minor (114 species in the winter burn plots, 104 species in both spring and summer burn plots).

The inconsistent results discussed above provide little guidance for managers. Probably the most important “take home message” is that fires need to occur frequently regardless of season (Glitzenstein et al. 2003, Gray et al. 2003). Given the probable importance of growing season fire as an evolutionary force, and indications that reproductive biology of at least some species is cued to growing season fire, it seems likely that it is important to include some growing season burns as part of a prescribed burn regime. However, it does not seem necessary, or perhaps not even desirable, for all burning to be done during the growing season.

Effects of Fire Intensity

Fire intensity is a third component of fire regime. However, there is relatively little known about effects of this factor in longleaf pine stands. Glitzenstein et al. (1995) demonstrated differences in tree mortality rates for both oaks and pines in response to variation in fire temperatures and various measures of fire intensity. Williamson and Black (1981) and Platt et al. (1991) documented spatial variations in fire temperatures due to needle deposition patterns, with temperatures under or around clusters of longleaf pine trees generally greater than those further away from pines. Rebertus et al. (1989) demonstrated higher rates of sandhill oak topkill nearer to longleaf pine trees, and suggested that this might be a fire temperature effect. Hierro and Menges (2002) experimentally manipulated fuel loadings, and hence fire intensities, in south-central FL pine flatwoods. Shrub responses were monitored pre-fire and four years post-fire. Results indicated similar responses regardless of fuel loading treatment. Densities of some clonal shrubs, particularly Quercus minima and Ximenia americana, increased markedly after fires. Most other species had returned to pre-fire levels by the end of the study period. Overall shrub species richness increased post-fire. Hierro and Menges (2002) concluded that fire intensity effects on ground layer shrub communities in pine flatwoods “are likely only with extremely intense burns”. More such studies are needed, with data collected on herbs as well as shrubs. Also there is a critical need for predictive models of fire intensity effects on vegetation, incorporating such data as plant physiological responses, soil and duff buffering, and fire weather influences.

In general, data suggests that fire effects in longleaf pine woods are almost entirely beneficial, and indeed critical for the continued survival of this ecosystem. However there is much to be learned about effects of different fire regimes in particular habitats or regimes. It is entirely possible that a prescribed burn regime that is suitable for longleaf pine savannas in the outer Coastal Plain of South Carolina or Florida may not be suitable for an inner Coastal Plain site in Alabama or a west Gulf Coast site in eastern Texas.