Effects of Fire Suppression

Shifts in Species Composition

The characteristic overstory of widely spaced longleaf pine trees and diverse grass-dominated ground layer is dependent on frequent understory fires (see: Historic Fire Regimes). In the absence of fire a major transformation occurs leading to the loss of longleaf pine itself as well as the numerous fire dependent ground layer species (Heyward 1939, Lemon 1949, Brockway and Lewis 1997, Kush et al. 1998). Rates of species loss with time since fire appear to vary with edaphic conditions, being most rapid on mesic sites and slowest on very dry or wet sites (Streng and Harcombe 1982, Maliakal et al. 2000). The ultimate fate of fire suppressed longleaf pine stands is somewhat debatable. In the initial stages, such stands are likely to be dominated by hardwood tree and shrub species already present as sprouts in the fire maintained woodland (Menges et al. 1993, Abrahamson and Abrahamson 1996, Maliakal et al. 2000). Menges et al. (1993) documented a variety of long-term successional patterns in central Florida longleaf pine forests. The type of succession was dependent on edaphic factors as well as landscape factors, including propagule availability. Menges (1993) suggested that drier upland sites might eventually converge on some type of xeric hammock community. However, long-term data from a fire suppressed subxeric longleaf pine site in eastern Texas demonstrated that even xeric hammock or upper slope oak-hickory species are ultimately dependent on fire at some return interval. Following approximately 50 years of fire exclusion this site is succeeding to a closed stand dominated by upland laurel oak (Quercus hemisphaerica), American Holly (Ilex opaca) and southern magnolia (Magnolia grandiflora).

Effects of fire suppression on fauna are predictable based on habitat relationships. Species dependent on open woodlands and savannas decline with fire exclusion, whereas species that prefer closed woods increase. Generally the latter are common species in the modern landscape whereas the former are often rare and endangered (e.g. red-cockaded woodpecker, gopher tortoise, indigo snake). Thus maintaining open frequently burned pinelands is critical for animal as well as plant conservation.

Altered Fire Behavior

Low intensity surface fires characterize fire behavior in well-maintained longleaf pine stands with good quality groundcover. On the other hand, fire suppressed longleaf pine sites are generally less likely to burn, but are more prone to high intensity fires when they do burn. Streng and Harcombe (1982) discussed effects of fire exclusion on fire behavior in longleaf pine woodlands. Initially pine litter and fine woody fuels accumulate, increasing the probability of higher intensity fires. Long-term fire exclusion, however, results in invasion of hardwood trees, declines in grasses and other fine fuels, decreased fuel height and aeration. The consequence of these changes for predicted fire behavior depends on fuel moisture, which is in turn a function of time since rain and current weather conditions. Over most of the range of fuel moistures fires are less likely to ignite and spread, and flame lengths and estimated fireline intensities are considerably lower than in frequently burned fire maintained sites. However, at low fuel moistures predicted fireline intensities are considerably higher than in frequently burned sites due to high loading of heavy fuels.

Saw palmetto dominated flatwoods represent an exception to the above scenario (Heyward 1939). Due to the unique properties of this terrestrial palm, communities dominated by this species can burn with high intensity within 2-3 years post fire. Furthermore, dense saw palmetto stands can resist invasion of less pyrogenic hardwoods for considerable periods of time. The consequence is an already flammable base fuel that continues to increase in flammability with time since fire. It is not surprising that the most dangerous and economically destructive fires in the southeastern US in recent years have occurred in fire suppressed saw palmetto flatwoods.