Tropical storm season is about the worst time of year for a marsh to burn because of the possibility of a tropical storm surge flooding the soil.
Such a surge is stressful, but unburned plants generally survive because their leaves extend above the water and so can transport oxygen down to the roots. “This snorkel ability is what sets wetland plants apart from upland plants,” Andy Nyman, LSU AgCenter associate professor in the School of Renewable Natural Resources.
But if a marsh was recently burned, the roots lack a "snorkel" and can be damaged, even if the storm surge is only a few inches deep. That's why managed burns are done when the soil is flooded, and lower water levels are predicted for the next week or so, Nyman said.
A fire that happens when the soil is flooded is called a cover burn because only the aboveground cover is burned away, whereas the roots remain healthy. Within a few days, new leaves sprout from the roots and grow several inches in a week or two.
The ideal time for a managed burn is just as a cold front is about to pass.
“Burn when south winds have flooded the soil and just before the north winds will drain the marsh,” Nyman said, adding that by the time the next cold front floods the soil, the new leaves are tall enough to rise above the water and function as snorkels.
A second major difference between summer fires and winter fires is that atmospheric conditions in winter generally cause the smoke to rise, spread and quickly dilute, whereas atmospheric conditions in summer generally cause the smoke to stay close to ground, linger and accumulate.
Nyman said there are two possible reasons why this summer fire has attracted so much attention.
“Most New Orleans residents probably are unaware of the frequent marsh fires each winter because weather conditions then allow the smoke to rise and leave the area.”
The second reason is that this fire might be making more smoke than a typical cover burn because soil is also burning.
One reason to have a managed burn program is to reduce the fuel available for summer-time lightning fires.
“It’s possible, though unlikely, that marsh grasses can be so productive in some places and in some years that they can burn more than once a year,” Nyman said.
An ideal burning program would burn one-third to one-fifth of the area each year so that there always would be a mix of early succession and robust late succession. Managed marsh burning is becoming harder to plan because the acceptable wind directions decline as suburbs spread and the fear of lawsuits increases.
“There is much less marsh burning in southeast Louisiana because the marshes there are so fragmented by ponds, lakes, canals, bayous, roads, etc. Marsh fires are much more common in southwest Louisiana where the marshes are less fragmented, and a single lightning strike can start a fire that can burn for thousands of acres,” Nyman said.