Human Alterations of Fire Regime of Florida Scrub

Habitat fragmentation and active fire suppression have worked together to reduce the frequency of fire in Florida scrub and across the Florida landscape. Lightning ignited fires probably burned over large areas before roads and developments were built. These features, even covering as little as 10% of the landscape, cause a 50% reduction in fire extent (Duncan and Schmalzer 2004).

Even in areas managed actively by prescribed fire, human practices have altered the fire regime. Most prescribed fires in Florida have had agriculture or forestry goals and have been done during the winter months. Only small areas of Florida scrub would likely have burned from lightning ignitions during winter months. Prescribed fires probably have reduced mean fire intensity and would have less variation in fire intensity than uncontrolled fires. Wildfires are likely to burn more land during windy, droughty conditions, whereas prescribed fires tend to be lit during times when control is easier. Prescribed burns are also likely to change the patchiness of burned vs. unburned areas. More uniform burning conditions could translate to less patchiness, or more benign conditions could translate to larger unburned patches. Aerial ignitions, which are becoming more common, could also leave more unburned areas. Prescribed burning is conducted entirely during daylight hours with fires being extinguished before nightfall due to smoke management considerations. Presettlement fires likely burned both day and night, and for longer durations than prescribed fires. Due to lack of monitoring of areas actually burned, we cannot thoroughly evaluate the characteristics of the current fire regime.

Fire Suppression Effects

With fire suppression, several changes occur. Gaps in the vegetation, crucial to many herbaceous plants and some animals (Hawkes and Menges, 1996, Hokit et al. 1999, Greenberg et al. 1994, 1995), begin to close (Hawkes and Menges 1996). The matrix vegetation also becomes taller and thicker (Abrahamson 1984 a, b, Schmalzer 2003). Surface litter builds up (Schmalzer and Hinkle 1996) because litterfall rates exceed litter turnover rates (Lugo and Zucca 1983). The continued growth of tall shrubs tends to suppress herbaceous plants and smaller shrubs (Givens et al. 1984). However, this process is slow and not particularly directional (Menges et al. 1993). As light levels drop, sand pine mortality begins to occur and is not balanced by recruitment (Conway et al. 1997, Parker et al. 1997). Ground lichens also spread, and this suppresses seedling recruitment (Hawkes and Menges 2003).

One effect of continued fire suppression is that potential postfire colonists can be locally eliminated, so that the postfire recovery lacks some species characteristic of recently burned patches in recurrently burned areas (Abrahamson and Abrahamson 1996). This is likely due, in part, to depletion of soil seed banks of obligate seeders (Menges et al. in preparation). In some cases, local extinctions have been documented (e.g., Menges and Quintana-Ascencio 2004). Fire suppression also eliminates gaps that may have to be created by a combination of mechanical treatments and fire (Schmalzer and Boyle 1998).

Fire suppression has particularly devastating effects on Florida scrub jays, causing reduced fitness and, ultimately, territory abandonment (Breininger et al. 1995). Population viability is reduced in low quality, fire-suppressed landscapes (Root 1996, Breininger et al. 1999). Scrub jay abandonment depends on the rate of shrub height growth (and other factors) and occurs more rapidly in coastal than interior scrub (Breininger and Schmalzer 1990, Fitzpatrick et al. 1991). Scrub jay abandonment is probably not due to reductions in acorn supply in fire-suppressed stands, as acorn supply remains relatively constant (Abrahamson and Layne 2002b). Across the landscape, both short-height, recently burned scrub and tall, unburned scrub act as population sinks, receiving excess birds produced in optimal scrub habitat (Breininger and Carter 2002).

Fire suppression also alters subsequent fire behavior. As oaks and other shrubs become small trees, they are both unlikely to carry fires off the ground and unlikely to be killed by fires. These fire resistant clumps of oaks are termed “oak domes” if they occur as patches within a more open matrix (Guerin 1993). Fire suppressed areas become more resistant to restoration using subsequent fires (Duncan et al. 1999).