Management of forest stands may help, team finds

Nearly 19,000 acres burned around the Nebraska National Forest at Halsey in early October, according to the U.S. government. The same month, more than 50 additional wildfires were reported across the state.

With the wildfire season reaching further into autumn and concern growing about the loss of structures, ecosystems and lives, scientists at the University of Nebraska–Lincoln are evaluating how to manage landscapes to be more robust against future fire threats. A study published in the journal Ecosphere compared how two land management techniques may impact Nebraska forests’ resilience against wildfires.

“We are seeing more wildfires in the Great Plains,” said Victoria Donovan, the study’s first author and a former postdoctoral researcher in the Department of Agronomy and Horticulture. “… Because of the large fires that have been happening out West, there has been a push for forest management that reduces wildfire risk and for research on the effectiveness of that management.”

Fires spread both horizontally through a forest and vertically, reaching from the ground to branches and shrubs and even into crowns, or tops of trees. Blazes that spread into tree tops are classified as crown fires and are generally the most intense and ecologically damaging. Many management techniques emphasize removing potential fuels to limit fire from spreading vertically into canopies and from tree to tree.

One such technique, targeted grazing, is a practice whereby cattle graze throughout forest stands. Grazing reduces fuels on the forest floor and prevents young trees from establishing in the forest understory. The other technique modeled in the study, mechanical thinning, includes physically removing lower branches and small trees. Both methods can increase the gap that fire must jump to reach higher strata. Reducing crown size in mechanical thinning also limits how fire can spread between the tops of trees.

“If you have a really large distance between the ground and your canopy fuels, you are going to need a really intense fire to create the flame lengths and heat in order to ignite the crown,” said Donovan, now an assistant professor at the University of Florida. “If you have a more continuous vertical fuel distribution, then it is going to be a lot easier for the fire to transfer from the ground into your tree crown.”

To better understand the techniques’ effectiveness in Nebraska’s ponderosa pine forests, Donovan’s team used models to quantify the probability of crown fire in forests managed with targeted grazing and crown thinning. The researchers first censused a forest to account for how vegetation — or fuel — was vertically distributed. They then simulated the two management techniques by altering vegetation layers in their model to represent each management approach, increasing the distance the fire needed to transfer vertically from the ground to higher branches. The model evaluated how likely crown fire was to occur in those forests under different climate conditions.

“Fire is difficult to study, especially wildfire, because you don’t know when it is going to happen or where it is going to happen,” said Donovan, who conducted the study through Nebraska’s Extreme Fire Research Lab. “Modeling allows us to estimate wildfire outcomes without having to wait for a wildfire to occur.”

The team found that targeted grazing did not substantially reduce vertical fire spread in forests. Mechanical thinning was much more effective, as it increased the average fuel gap by about 24 feet — roughly 12 times greater than by targeted grazing alone. Combining the two techniques increased that gap even more, by up to 24.5 feet. That distance also decreased the likelihood of fires reaching the crown under mild and moderate wildfire conditions.

However, even combining the two techniques failed to avoid crown fire under high-risk wildfire conditions, when weather is most conducive to wildfire spread.

“Our study shows that focusing on management at the scale of a forest stand can only do so much to prevent crown fire under more extreme conditions, particularly as things get more favorable to fire under climate change in the future,” Donovan said.

Donovan emphasized that fire is not inherently bad, as crown fire can benefit diversity and create new habitats for wildlife species. Moreover, ponderosa pine forests possess adaptations, like thick bark, that make them more resilient to forest fires.

When a fire travels through a ponderosa forest, it naturally causes self-thinning by removing lower branches and increases the distance that future fires need to travel from the ground to the canopy, Donovan said. However, because fires have been historically suppressed, that self-thinning has not occurred. Understory fuels have accumulated, creating conditions for high-intensity fires that can harm even fire-resilient trees.

“One of the concerns is that since we’ve suppressed fire in these systems for so long, they’ve become very fuel-dense and homogenous,” Donovan said. “Right now, a lot of our treatments focus on changing the stand structure or individual trees in a stand without thinking about the landscape, how that has been altered by fire suppression, and how that has reduced the resilience of the forested system.”

One potential solution, she said, would involve focusing on how fire spreads across the landscape more broadly. Implementing more variety in land cover, including areas with low amounts of fuel, could limit wildfire spread and intensity.

“You can envision a giant homogenous stand of trees. If you were to light a fire on one end of it, it can just spread through the entire thing unimpeded,” Donovan said. “But if you manage your landscape by creating heterogeneity that would have emerged from mixed-severity fires historically … that could help protect the forest as a whole.”

Agronomy and Horticulture Forestry