June 2008

FIRE IN THE EVERGLADES

 

WHEN DO FIRES OCCUR?
(photo: Wildfire in the refuge)

Natural fires occur now at the transition from the dry to the wet season. This is because plants and litter get drier during our spring and the first major thunderstorms often have lightning before it rains. By August the number of lightning strikes can be as many as 100s per square mile per storm, but plants and litter are too wet for fire to spread very far.

Fires set by people these days, purposely or accidentally, probably occur mostly in the dry season, especially in drought years. Such fires are likely to be intense, spread quickly, and are particularly dangerous in areas near human habitation. Thus during the dry season we have bans on burning litter and on camp fires.

We do not know when, or even why, indigenous people set fires. Perhaps they set fires whenever they wanted to drive game, to improve forage quality for grazers, to clear brush to make approaching enemy more visible. or to provide a fire break around habitation.



WHERE DID FIRES OCCUR?

Fires occur in all habitats in the Everglades. Although they have been more frequent in dry pinelands, they also occur in wet sawgrass marshes. In the 1989 drought fires were allowed to burn and spread over three-quarters of the Everglades National Park. These fires burned sawgrass ridges and the shallowest sloughs. In marshes the fires burned plant parts that were above the water level. In sloughs that had dried out, muck and peat fires occurred.

Soil cores tell us that muck or peat fires occurred during droughts before we drained much of the Everglades. Soil cores in many parts of the Everglades have layers of charcoal with wetland plant fragments and pollen. This is consistent with the reality of cycles of wet and dry times at many time scales. We know that there have been short El Niño - La Niña cycles, long multi-decadal climate oscillations, and even longer centennial cycles of wetter and drier times long before settlement.

Muck and peat can accumulate an average foot per 225 years. So in the 5000 years of Everglades existence peat should have accumulated to 22 feet deep at least in some spots. Yet the maximum depth in the historic sawgrass plain was about 12 feet. Some hypothesize that the discrepancy is due to soil loss from fire.

WHY DO GLADES COMMUNITIES NEED FIRE?
(photo: A prescribed burn in the Refuge)
 

Unfortunately most of us grew up thinking that wild fires are bad. Smokey the Bear was a powerful education symbol; "Only YOU can prevent forest fires". The movie BAMBI showed forest fires as scary and destructive. Today we recognize that fire is necessary to many natural communities including both tree swamps and grassy marshes in the Everglades.

Many plants are adapted to fire. Plants have evolved a variety of ways to survive fires and flourish. All grasses, sedges, and rushes have their growing tips just below ground. Within two months after a Refuge fire burned sawgrass to the water line, new green growth was already two feet high! Bushes adapted to fire have their growing tips on above-ground branches but also underground on spreading roots and rhizomes. The above-ground parts are killed in fires but the bushes quickly re-sprout from underground growing tips. Slash pine juveniles have dense clusters of needles which protect the inner growing tips. And adult pine trees have such thick bark that they burn only on the outside. Sabal palmetto, our state tree, has its growing tip at the top of the tree and conducting tissue throughout the trunk and even serious burning of the outside trunk does not kill the tree.

Fire adapted plants are often actually helped by fire. Burning quickly releases nutrients from dead litter and killed leaves, which stimulates sudden growth spurts. All plants massively flower and fruit and this benefits insects and wildlife. Some flowers have dormant seeds that are stimulated to germinate and grow only by the heat of fire.

Smaller species of plants are indirectly helped when fire burns back larger species and stops the larger from dominating extensive areas. Potential dominants include saw palmetto in pine woods, wax myrtle in tree islands, and sawgrass in marshes. With frequent fire the dominants get reduced in size and the overall diversity of species is much higher.

Many fire adapted plants actually even promote fires! The green leaves of some grasses and bushes have oils and resins that make them highly flammable. By promoting fire, fire-adapted species favor themselves over non fire adapted species that have growing tips only above ground.

If fire is too infrequent then the process of plant succession results in bushes and trees that are not resistant to fire. As these species grow taller they cast more shade and the understory becomes more moist. This further decreases the likelihood of fire.

HOW DO WE MANAGE FIRE IN THE REFUGE?

The Refuge fire crew faces a continuing balancing act between suppressing fires that may harm big tree islands and promoting fires that maintain the tree island, sawgrass marsh, and water-lily slough mosaic. Bushes and trees on tree islands among sawgrass marsh and water lily sloughs are not fire-adapted. They have their growing tips above ground. Consequently they may be killed by fire during droughts if water levels are kept too low. As Jon Wallace, our Loxahatchee NWR fire expert writes, "In the natural Everglades, before humans started changing the hydrology (mainly by drainage), fires were certainly less intense and did not impact tree islands and hammocks as easily during the natural fire season. That is why at The Refuge we conduct prescribed burns at times when the tree islands will be impacted minimally, while water levels are high." Avoiding fires in tree islands is a special management priority until we can remove the dense growth of old-world climbing fern that festoons some of our tree islands. This thick fern growth allows any fire to climb to the top of the bushes and trees and kill them.

HOW CAN WE BALANCE THE NEED FOR FIRE WITH THE SAFETY FOR HUMANS?

Fire is necessary for the health of many Everglades plant communities but is also dangerous when adjacent to human habitation. To reduce the danger we thin the understory and remove dead vegetation. Then prescribed fire can safely rejuvenate the fire-adapted community. It would be good to have lawns or golf courses adjacent to natural communities as a wider fire break. And in the best of all possible worlds we should require more fire resistant home construction in areas adjacent to our natural plant communities to reduce risk even further during prescribed fires that are so important to The Everglades.


REVIEW QUESTIONS       

a. Draining the Everglades has increased fires
b. Peat and muck fires were never natural
c. Fire suppression may change communities
d. Many plants have adaptations to promote fire
e. Tree island bushes and trees are adapted to fire
f. Most natural fires occur in the dry season