USGS fire research in the southeast

September 17, 1999

Wildfires have long played a key role in structuring ecosystems and plant communities in the southeastern United States. From the coastal prairie of Texas and Louisiana to the marshes and pinelands of Florida, many native species have adapted to a natural regime of frequent wildfire caused by lightning strikes. In addition, fires set by Native Americans and European settlers have influenced vegetation patterns in the Southeast for centuries.

The effects of fire are not subtle. Frequent fire can dramatically change the structure of a forest; for example, it can change a forest from one with thick underbrush to one with a parklike, cathedral structure. But while fire is essential to the maintenance of some ecosystems, it has long been viewed solely as a destructive force that needs to be controlled. Over the past several centuries, extensive fire suppression efforts have been one of the most pervasive human impacts on the natural ecosystems of this region, and, according to fire ecologists, altered fire regimes have contributed to the current threatened status of a number of native habitats in the southern United States.

The importance of fire in managing public lands in the Southeast has long been recognized. The largest controlled burn programs in the National Park System are carried out in this region of the United States in Everglades National Park and Big Cypress Natural Preserve in Florida. Prescribed burning on public lands can help maintain the Southeast's unique species and ecosystems, and reduces the risk of catastrophic wildfires.

However much remains to be learned about the effects of fire -- both natural and manmade -- on southeastern woodlands, wetlands and coastal prairies. USGS scientists are studying the fire ecology of the Southeast from a variety of perspectives. Efforts are under way to understand the complex ecological effects of natural fires, such as the massive North Florida fires of 1998, and to determine how fire may best be applied as a management tool to maintain the diversity and vitality of systems in which the natural fire regime has been disrupted.

North Florida

In the summer of 1998, recalls USGS ecologist Dr. Sue Grace, much of North and Central Florida was ablaze. The disastrous Florida fire season that year was largely the result of an unusual sequence of climatic conditions linked to El Niño. The previous winter had been warm and exceptionally wet, stimulating plant growth and limiting the ability of managers to reduce fuel loads through controlled burning.

This pattern changed abruptly with the onset of a severe drought in the spring of 1998. Areas that were too wet for managed burns a few months before quickly became too dry. Lightning activity increased in early summer, but with little accompanying rainfall. In the end, says Grace, the fuel conditions and the weather conditions created a catastrophic event. Throughout June and July some 2,500 fires, many of severe intensity, burned roughly half a million acres of the Florida landscape.

In the fall of 1998, Grace, of the USGS National Wetlands Research Center, joined an interagency team of scientists formed under the leadership of the USGS and the U.S. Forest Service with support from the federal Joint Fire Science Board. The research team, which included representatives from a number of federal and state agencies as well as private institutions, had been mobilized to rapidly assess and study the ecological and economic impacts of the Florida wildfires.

Over the past year Grace's work has focused on how native and exotic plant species have responded in the aftermath of the severe wildfire. Of particular concern have been two categories of plants: endemic species, whose only existing populations occur in Florida, and exotic or non-native species, which often have the potential to invade new areas in the wake of a severe disturbance.

Grace says that in the year following the fires, the plant response has been encouraging. Fire is actually very good for the native plants, she says. Although the 1998 fires were very damaging economically, they were good ecologically. No endemic species were harmed by the fires, and Grace's surveys have revealed larger populations and expanded ranges of a number of threatened flowers and grasses.

One native shrub species, the Rugels pawpaw, increased from only 200 individuals before the wildfires to more than 2,000 afterward. The percentage of plants producing flowers -- and thus creating new seeds -- also increased. Other rarities, including pitcher plants, lilies and native grasses such as the Florida three-awn, were found in locations where they had not previously been known to occur.

Grace notes that plants in Florida have evolved for thousands of years in an area that experiences heavy lightning activity and is also susceptible to seasonal drought. Besides these Florida endemics, she says, we have many other plants that are highly fire-adapted, and flower only after fires. These plants do well in burned areas because more light reaches the soil surface -- they're not shaded out by the overstory vegetation. In these conditions plants such as the pawpaw can flower and set seed in much larger numbers. Some species produce seeds that have been dormant in the soil for years, only able to germinate after a burn when the young plant will have the sunlight and nutrients it needs to grow.

While native plant species appear to have benefitted from the 1998 wildfires in Florida, so far it appears that most exotics have not. "In all our surveys I have yet to see any great expansion of exotic species," says Grace. She notes however that some non-native species may still be poised to take advantage of the new conditions. One in particular that she has been watching closely is cogon grass, a troublesome Florida invader with an affinity for disturbances such as those created by fires and the miles of fire lines plowed through the north Florida landscape to combat the blazes.

Grace says the 1998 Florida fires showed just how susceptible today's landscape is to severe burns. A century ago this region was dominated by open woodlands of longleaf pine, a fire-resistant species. Today, most of the longleaf pine woodlands have been replaced by planted stands of other, more commercially valuable species such as slash pine and loblolly pine, which are more susceptible to fire.

One of the goals of the interagency research team was to assess the value of prescribed burning for reducing timber losses from severe wildfires. The 1998 fires were of such intensity that even some areas that had been recently treated with controlled burning experienced heavy tree mortality. "But overall," says Grace, "prescribed burning gave us an edge. In a lot of the forest stands that were under prescribed burning the fires didn't get as hot or intense, and so they did not climb into the canopy and kill the pines."

A study led by Dr. Ken Outcalt, a research ecologist with the U.S. Forest Service, found that natural stands that had been burned a few months previous to the fires sustained less damage than those in which fuels had a longer time to accumulate. The difference was even greater in pine plantations, where recently burned stands experienced only 5 percent tree death compared to 52 percent in stands with two or more years of fuel accumulation.

South Florida

Farther south in Florida, pine flatlands give way to the pine rockland ecosystems of the Miami Rock Ridge. These forests of South Florida slash pine house a diverse array of plant species including tropical hardwoods and palms, as well as a number of unique native grasses and flowers. Pine rocklands are considered a globally endangered habitat; today they occupy less than 10 percent of their former range in South Florida, with the largest intact parcels in Everglades National Park and Big Cypress National Preserve.

Dr. Jim Snyder, a research biologist with the USGS Florida Caribbean Science Center, is studying how fire may best be applied to maintain and restore these unique ecosystems. Like the longleaf pine flatlands to the north, South Florida rockland forest communities are composed of species that have adapted to, and may be dependent on, the periodic presence of fire.

Pines benefit from fire by reducing competition from hardwoods, says Snyder. Pine seeds germinate and survive better when mineral soil is exposed. And many herbaceous species flower very little unless they have been burned recently. When fire is suppressed, hardwood species quickly outcompete the pines, a closed canopy develops, and many of the unique pineland understory species disappear.

The oldest prescribed burning program in the National Park System was initiated in Everglades National Park in 1958 to preserve native species in pine rockland and surrounding wet prairie ecosystems. Today, South Florida remains one of the most active areas for prescribed burning in the country, as land managers try to maintain a delicate equilibrium between a fire-dependent ecosystem and ever-growing numbers of fire-wary Florida residents.

Since 1996, Snyder has been conducting a long-term study of experimental burns in Big Cypress National Preserve. His goal is to provide information that will help managers fine-tune their prescribed burn programs to best protect native plants and to maintain healthy pine forest systems. Snyder notes that simply trying to duplicate what was once the natural fire regime is not an option any longer. For example, the biggest lightning-caused fires usually occur in May and June under drought conditions; controlled burning is too dangerous an undertaking at these times. The fact is that in South Florida the federal agencies cannot allow the biggest natural fires to burn, says Snyder. Instead, all these forests require prescribed fires.

On Snyder's experimental plots in Big Cypress, USGS and National Park Service personnel are conducting burns in spring, summer and winter, and at frequencies of 3 and 6 years, for a total of six different treatment combinations. On all of the plots fire behavior, as well as short- and long-term responses of pine, hardwood and herbaceous plant populations, is being closely monitored. The experiment will continue for a number of years; the first round of 6-year treatments begun in 1996 will not be burned again until 2002.

Initial results from the first series of burns are now being analyzed. Snyder hopes that the information gathered from this kind of detailed, long-term study will help managers tailor their burn programs to meet specific objectives. For example, he says, if they want to reduce wax myrtle in the understory, their preliminary observations indicate that spring burning will be more effective than winter or summer burning.

Snyder notes, however, that fire management in Florida is a complex task -- particularly where wildland areas and human settlements exist side by side. He notes that using fire to protect remaining patches of pine rockland in the National Key Deer Refuge in the Florida Keys is extremely difficult due to the presence of houses within and adjoining the refuge property. Even huge areas like the Everglades and Big Cypress have to deal with air quality issues, Snyder says. Life is getting more and more difficult for fire managers, he adds.

The Coastal Prairie

The coastal prairie of Louisiana and Texas is one of the Southeast's most endangered ecosystems. The amount of prairie remaining is only about one percent of the original, says Dr. Jim Grace, a research ecologist at the National Wetlands Research Center. And what's left is in small pieces with only a portion of it protected.

Like other habitats in the Southeast, the coastal prairie has long been sustained by fire. Native prairie plants are well adapted to survive blazes that sweep the surface clear of dead grasses and encroaching trees. Without fire the coastal prairie would not exist, says Grace -- woodlands would quickly take over, and the prairies' unique array of native plants and wildlife would disappear.

Most of the coastal prairie has been converted to agricultural land over the past century. One of the difficulties in protecting remaining prairie habitat, says Grace, is that wildfires can no longer spread across wide areas. The prairie is in pieces, and consequently, for fire to continue to play its life-sustaining role it must be carefully managed.

As the prairie habitat becomes fragmented and the natural fire regime disrupted, the danger of invasion by exotic species increases, says Grace. The most immediate threat to the coastal prairie today is an invasive tree species known as Chinese tallow. While this species has been around since the 1700s, it has greatly expanded over the past century. The population explosion has proceeded over the last 80 years, and now it has reached clearly catastrophic levels, says Grace.

In his studies of how fire can be used in coastal prairie ecosystem restoration, Grace has come to focus much of his attention on how the Chinese tallow invasion can be controlled. Once tallow takes over an area, he said, it is extremely hard to eradicate, and its dense stands shade out the native prairie plants. "It's incredible the degree to which tallow eliminates all the herbaceous vegetation," says Grace. "The ground ends up completely bare of other vegetation."

Making matters worse is the fact that Chinese tallow is extremely fire resistant. In a natural community, when there is an accumulation of woody vegetation, fire can come along and burn it out, says Grace. Chinese tallow, however, makes the ecosystem non-flammable. "There's no real way to recover once the tallow stands get extremely dense, unless you apply an herbicide and bring in bulldozers," he says.

But Grace has found that fire may still play a role in halting the tallow invasion. The key, he says, is to apply fire early in the colonization process, before the tallow stands are dense enough to eliminate all the ground vegetation. With a sufficient supply of dry fuel underneath, Grace says, Chinese tallow can be controlled by fire. Though tallow stands can't be eradicated by burning, with careful monitoring and fire management the species can be prevented from taking over new areas of coastal prairie habitat.

Grace says burns conducted during the growing season may be the most effective way to combat tallow and to restore the natural fire dynamics of the coastal prairie. Dormant season prescribed fires are easier to conduct, and they have less impact on some wildlife species, he says. But the natural fire regime was, by and large, one of growing season fires. "These fires," says Grace, "have a much stronger impact on woody plants. A lot of what we are trying to do is re-establish the natural processes and see if native plants can then have some success in competing with exotics."
As the nation's largest water, earth and biological science and civilian mapping agency, the USGS works in cooperation with more than 2000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers. This information is gathered in every state by USGS scientists to minimize the loss of life and property from natural disasters, contribute to the sound conservation, economic and physical development of the nation's natural resources, and enhance the quality of life by monitoring water, biological, energy and mineral resources.

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