Wildfires spread farther where woody plants replace grasses

A team of researchers led by the University of Florida (UF) has recently found that, as woody plants such as trees or shrubs replace grasses, spot fires can occur farther away from the original fire perimeter. This so-called “woody encroachment” is becoming a significant problem not only in grasslands, but also in wetlands and savannahs.

During the past decade, over 61,000 wildfires have decimated about 7.2 million acres annually. Once wildfires start spreading, extinguishing them is challenging due to issues such as spot fires, where winds carry lofted sparks and ignite new fires outside the original fire perimeter. Moreover, the greater the potential spot fire distance, the more difficult it becomes to monitor, control, and suppress wildfires.

“Spot fires are one of the most common reasons why houses burn in a wildfire,” said lead author Victoria Donovan, an assistant professor of Forest Management at UF, who conducted this study as a researcher at the University of Nebraska-Lincoln. 

“It’s not typically because the flames from wildfire reach a house, but that embers from that fire land on roofs, travel through home ventilation systems, or land on other combustible material on or near the home, and ignite the house from there. They’re a big concern for structural damage.”

Donovan and her colleagues examined three phases of wood encroachment: a largely grassland area, a grassland with new forested growth, and a dense forest. By using a mathematical fire simulation program and taking into account the various conditions in the Loess Canyons Experimental Landscape in south Nebraska, they found that prescribed fires – which are often used in Florida to keep woody plant growth under control – could contribute to a significant reduction of spot fires. 

“Our study shows that the risk of spot fire is much lower when you’re burning under the weather conditions used for prescribed fire, regardless of encroachment phase, compared to waiting for the potentially more extreme conditions we can see during wildfires,” Donovan said. “This tells us that using prescribed fire early to control encroachment and reduce fuel load is a lot safer than waiting for a wildfire to occur.”

Moreover, the investigation revealed that it is not only spot fire distance that increases wildfire risk from woody plants. Another major problem is that shrubs and trees can grow much taller than grasses. 

“Think about putting out your campfire on the ground by pouring water on it, and compare that to trying to put out a fire a couple stories above you,” Donovan explained.

The concerns this study highlights are universal and show marked similarities regardless of the type of land where the wildfires occur. In Florida, for example, fire suppression has led to a massive encroachment of shrubs, creating dense forest stands that are very different from the open savannah systems in which more wildfires occurred in the past.

Although Florida has recently become a model for prescribed fires, there is still a high degree of hesitancy among some private landowners. However, using prescribed fire as a controlling process for woody encroachment remains less risky than allowing the encroachments and waiting for wildfires to occur.

“Across the country, data has shown that fire is inevitable. Using prescribed fire allows us to decide what we want a lot of that fire to look like,” Donovan concluded. The study is published in the journal PLoS ONE.

Wildfires and climate change

Wildfires are a natural part of many ecosystems, but they have become more destructive in recent decades, and this trend is partially attributed to climate change. Here’s how climate change is contributing to an increase in wildfires:

Warmer Temperatures

Climate change has led to increased temperatures and earlier spring melt. This results in hotter, drier conditions that can prolong fire seasons. Additionally, warmer temperatures aid the spread of pests like the pine beetle, which can kill trees and create more potential fuel for fires.

Changes in Precipitation

While some areas are experiencing increased rainfall, others are becoming drier due to shifts in weather patterns caused by climate change. In areas that are becoming drier, these changes can contribute to drought conditions that make forests more susceptible to severe wildfires.

Increased Lightning Activity

Warmer conditions can lead to more thunderstorms, which in turn increase the likelihood of lightning – a major cause of wildfires.

Stronger Winds

Climate change can also cause changes in wind patterns, and high winds can help spread wildfires.

The relationship between wildfires and climate change is a kind of positive feedback loop. On one hand, climate change creates conditions that can increase the risk of wildfires. 

On the other hand, wildfires themselves can contribute to climate change. They release large amounts of carbon dioxide (CO2) – a greenhouse gas – into the atmosphere, thereby exacerbating global warming. Moreover, when forests that act as carbon sinks are destroyed by fires, they are unable to absorb CO2, further compounding the problem.

Mitigating climate change, therefore, could help reduce the risk of future wildfires. This would involve reducing greenhouse gas emissions through actions like shifting towards renewable energy sources, improving energy efficiency, and protecting and restoring forests. 

Adaptation measures, such as better land management practices and the development of more fire-resistant infrastructure, could also help communities become more resilient to wildfires.

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By Andrei Ionescu, Earth.com Staff Writer

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