Study identifies windstorm risks for Southern pine forests

Growing trees across a central swath of the Southeastern United States comes with an elevated risk for windstorm damage, according to new research from the University of Georgia.

Using a modeling process that accounts for a variety of variables—in this case, factors such as terrain, depth of bedrock, soil types and frequency of severe storms—researchers have developed a series of maps to help guide landowners. The goal, said lead author and postdoctoral researcher Christine Fortuin, is to better assess levels of risk over decades, which can help inform decisions such as when to thin trees or clear cut a pine stand.

The study was recently published in the journal Landscape Ecology.

“We do effectively have a Southern tornado alley, from southern Mississippi up through northern Alabama and into Arkansas and Oklahoma. There’s some who postulate that it’s part of a larger tornado alley across the Great Plains,” said Fortuin. “Most of these tornadoes go from the southwest to the northeast, and it’s interesting to see the intensity of the tornadoes that go through the region are really high. So, it’s not just about the frequency but also the intensity of the storms.”

The process, called “GAMSel” or generalized additive model selection, resulted in heat maps showing areas of elevated risk from Texas to North Carolina. Pockets of higher risk—representing an elevated risk of tornadoes or higher winds—bubbled up along a rough line stretching from Oklahoma through central Arkansas, Mississippi and Alabama.

To create the maps, Fortuin and the collaborators on the project used ground-level variables, such as slope and soil characteristics, to weigh how these factored into tree damage severity from tornadoes or severe thunderstorms.  The likelihood of severe damage based on these factors, in combination with the frequency of wind events, was then mapped using GIS software to determine wind damage risk hot spots.

The project is part of a larger suite of studies looking at the effects of severe storms on forests across the Southeast. As the intensity of storms in the region increases, said Fortuin, there’s an increased need for updated management practices and guidance for landowners to reduce timber loss.

“Especially in the wake of hurricane Michael, there has been an increase in interest and funding to work on wind disturbances—especially through the U.S. Forest Service Southern Research Station,” said Fortuin, “And they’re interested in how management of stands might be interacting with damage so we can make some recommendations for stand management based on what we know.”

By looking at variables that specifically affect tree growth, the study could pave the way for new insights in risk modeling, said Rob Hoyt, the Dudley L. Moore Jr. Chair and professor of risk management and insurance at UGA’s Terry College of Business. “There well could be lessons learned that would be valuable in other, broader contexts,” he said.

Often, said Hoyt, damage by tornadoes gets overshadowed by hurricanes. But the annual cumulative damage they cause is nothing to ignore.

“We’ve seen growing numbers of severe convective storms impacting our properties, as well as increasing fatalities associated with these storms. Roughly, on average, convective storms have been causing $17 billion in insured losses—that is pretty close to what hurricane damage losses have been,” he said. “People don’t always appreciate that because we see more damage from fewer events—for example, Katrina did huge damage—compared with the accumulation of a lot of damage from more convective events.”

The paper is one piece of a larger puzzle for land managers, said co-author Cristian Montes. Now, researchers will use the maps to develop more targeted management guidelines for landowners with pine stands in higher risk areas.

“One of the major things I’m concerned about is, how do you include climactic variation in that decision-making process? What is the range of expected yields you could have at the end of each rotation given climactic events?” said Montes, associate professor at Warnell and co-director of the school’s Plantation Management Research Cooperative, for which the study was created. “Next, you add what are the chances you’ll be hit by a tornado, hurricane, beetles or fungi, as related to climactic variables.”

Traditional forest measurement tools can help us forecast tree growth and yield, but there’s always some amount of variation based on these variables. These updated models allow researchers to reduce that variation—saving landowners time and money.

“Just like, when you look at the weather, there’s a 20% chance that it’s going to rain. We’re saying there’s a 20% chance you’re going to get hit by a tornado,” added Montes. “When you include that in the decision-making system, that makes it a very powerful tool.”