Crash and Burn: How Tornado Damage Affects Fire Behavior

Aerial view of simulated tornado damage at the Piedmont National Wildlife Refuge near Round Oak, Georgia. Photo by U.S. Forest Service.
Aerial view of simulated tornado damage at the Piedmont National Wildlife Refuge near Round Oak, Georgia. Photo by U.S. Forest Service.

Tornadoes and fires are powerful natural disturbances that can kill trees and cause long lasting changes in community composition. “However, most disturbances are neither rare nor catastrophic,” says U.S. Forest Service scientist Joseph O’Brien. “There is a continuum of disturbance severity in most ecosystems, although the interactions among these disturbances haven’t received much study.”

One of the most obvious interactions between wind damage and fire is that fallen trees become fuel, and can increase the likelihood or intensity of fire. “The impacts of simultaneous wind and fire disturbances are poorly understood,” says O’Brien, a research ecologist at the Southern Research Station Center for Forest Disturbance Science. O’Brien and his colleagues recently studied interactions between wind damage and fire behavior. The study was led by Jeffery Cannon of the University of Georgia, and was recently published in Forest Ecology and Management.

The researchers established six study plots in a pine-oak stand in the Piedmont National Wildlife Refuge in central Georgia. Half the sites were left untouched as a control, while tornado damage in the other half was simulated by pulling trees down with a winch. “In this area, tornadoes typically come from the south,” says O’Brien. “We winched the trees to fall towards the north, as if they had been toppled by the wind.” O’Brien and his colleagues also winched the trees during tornado season (between March and May) and created gaps of about 40 yards across.

The downed trees were left in place, and after about a year, a prescribed fire was conducted. The simulated tornado damage dramatically changed fire behavior, but the differences were not due simply to the extra fuels. Many of the large downed tree trunks were not even consumed in the fire, probably due to its low intensity. “Our study suggests that subtle changes in the types of fuel and its physical arrangement led to changes in combustion patterns,” says O’Brien.

The simulated tornado disturbance caused finer, more flammable fuels such as small branches in the tree crowns to clump together. During the prescribed fire, these areas burned hotter and more fuels were consumed. The largest ecological effects of fire were probably centered on these areas of intense fire. The study shows that wind damage and fire influence each other in a variety of ways, some of which are not immediately obvious. For example, the timing of fire could interact with environmental conditions such as drought and cause larger fuels, like the tree trunks, to also burn. This would probably amplify the patterns O’Brien and his colleagues identified.

“We found that wind and fire interact synergistically,” says O’Brien. “Together, these disturbances can create the potential for more intense fires.”

Read the full text of the article.

For more information, email Joseph O’Brien at jjobrien@fs.fed.us.

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