Summer 1998 was one of the worst fire seasons for Florida: between mid-May and mid-July, some 2000 wildfires burned over 494,000 acres of forest in the north and central parts of the state, with total damages estimated as high as $880 million. Over 225,000 acres of southern pine plantations were destroyed, and 370 businesses and residences ruined or damaged.
Historically, periodic wildfire or prescribed burning controlled the buildup of "rough" -- the undergrowth vegetation that can fuel catastrophic fires -- in flatwood pine forests. Low-intensity prescribed fire every few years is the ideal means for controlling rough and promoting the germination and growth of native and plantation pines.
"Southern pine ecosystems are dependent on a one to five-year fire cycle," said Wade, research forester with the SRS Disturbance and Management of Southern Pine Ecosystems unit in Athens, GA. "Without fire to control pests and brush and promote seed germination, the pines in these ecosystems would eventually be replaced by hardwoods."
As human population in the South has grown, it has become increasingly difficult to set fires in forests along the wildland-urban interface because of home safety and health concerns --- and, in some cases, public misconceptions about fire ecology. Yet without prescribed burning, the undergrowth in pine flatwoods quickly becomes almost impenetrable, the perfect fuel for high-intensity fires that can endanger people and property.
"The 1998 wildfires in Florida prompted interest in developing new strategies for managing hazardous fuels," said Wade. "Our study looked at thinning and herbicide treatments as methods to provide protection from catastrophic fires."
Brose and Wade collected data from three sites where three different treatments -- prescribed burning, whole-tree thinning, or herbicides -- were applied to five different ages of rough. The three treatment sites were dominated by longleaf and slash pine, with gallberry (an evergreen shrub) and saw palmetto the main species in the understory rough.
The researchers entered data from the sites into a computer modeling system designed to simulate the flame length and rate of fire spread for each combination of treatment and age of rough. The fuel models developed by the system were merged with weather data to simulate the effects of a summer fire under normal and under drought conditions for each of the treatment/age combinations.
The results showed that prescribed burning is still the best approach for immediate protection against catastrophic wildfire, with thinning the next best approach. "Unfortunately, the fire protection provided by both thinning and prescribed fire is relatively short-lived," said Wade. "Gallberry and saw palmetto are well-adapted to disturbance and start sprouting again quickly. As these fuels rapidly redevelop within three years after treatment, the likelihood of dangerous fire returns."
The herbicide treatments, however, have the opposite initial effect. For the first year after application, shrubs, though dead, are still standing and highly flammable. In the second year of herbicide treatment, the shrubs and undergrowth have almost disappeared, dramatically lowering the potential intensity of fire; the only fuel left is a blanket of pine needles and herbaceous plants beneath an open stand of trees. These conditions continue for at least six years – enough time for the trees to become merchantable.
The problem with the herbicide treatment is that litter increases on the forest floor if the forest is not regularly burned. A low-intensity fire during a drought could burn the roots of pines, resulting in more damage than a higher-intensity fire in a stand managed with prescribed fire.
"Combining treatments may be a way of capturing the strengths of different practices and avoiding some of their weaknesses," said Wade. "An example is using an initial prescribed fire followed by herbicides. The fire reduces the height of the rough and provides ecological benefits, while the herbicides keep the forest environment open for a longer period of time."
The authors call for more research in this area of fuels management, citing certain limitations in the Florida experiments. Several follow-up studies, funded by both the Joint Fire Science program and the National Fire Plan, are currently underway to determine the practicality of combinations such as periodic fire and a single herbicide application, as well as the impact of combination treatments on factors such as soil composition and wildlife.
The full text of the article is available from the SRS website at http://www.srs.fs.fed.us/pubs/viewpub.jsp?index=4443 .
For more information: Dale Wade at (706-559-4307) or dwade01@fs.fed.us