WSU researchers found that recent wildfires have been getting hotter, occurring more often and burning more acreage.

In some of the areas the researchers studied, climate change caused fires. In other areas, the cause was fire suppression, said Erin Hanan, lead author and postdoctoral researcher at WSU.

When people, such as firefighters, suppress forest fires, this results in a buildup of plants on the ground. Without fires burning and clearing away the plants, the plants continue to grow and amass. These dry plants are at a higher risk of burning, which causes larger forest fires, said Jennifer Adam, WSU civil and environmental engineering professor. 

The fires caused by climate change also tend to be larger because climate change lowers the moisture in the climate, which limits plants’ water consumption. With less water, these plants will become drier, and this condition is favorable for fires, Adam said.

“The 2020 fires were fascinating because we’re starting to see more fires in places that have not had fires for a very long time because they have wetter climates,” she said.

Old methods of forest management such as controlled burns are not always an option because they can sometimes harm the ecosystem, nearby communities or infrastructure. Another option is to cut down trees, which is known as thinning, Adam said.

Thinning is expensive to do, hands-on and often lacks economic incentive, she said.

“We actually have people at WSU who are trying to develop a market for thinning materials. Vikram Yadama, a civil engineer at WSU, is trying to understand how thin diameter trees can be used for building materials,” Adam said.

Additionally, the study found if the soil is relatively moist over time, the fire is limited in how much it can burn because the vegetation is too wet to burn. In forests with relatively moist soil, fire can dry vegetation and lead to larger, more severe fires, Hanan said.

If the soil is arid or dry, the landscape is very dehydrated and fuel-limited, meaning there is not enough fuel to carry a fire. In extremely dry places, climate change can limit the amount of vegetation available to burn, Hanan said.

In semi-moist soil, greater fire risk is caused by a combination of climate change and fire suppression, she said. These areas are not necessarily at greater risk of burning, but there are multiple factors contributing to the cause of wildfires.

Hanan said the study used a model to record the daily precipitation, temperature and humidity of certain areas. The data was used to simulate climate change influences on plant growth, plant build-up and the dryness of plants. 

The simulation could be compared to the areas that the researchers studied with similar plant growth and aridity to determine the likelihood of wildfires, she said. They could then see how to lower the chance of fires in those areas.

Researchers considered past climate records when determining what portion of daily temperature fluctuations are due to climate change, Hanan said. The climate change records were from the National Science Foundation-funded initiative, FireEarth. 

The data was collected from Idaho mixed-conifer watersheds with varied terrain, said Adam, co-leader of the FireEarth project. Watersheds are areas where water flows to streams and rivers that eventually channel out to bodies of water.

While the research was performed in Idaho, the data is applicable to other parts of the western U.S. The environments share similar forest types, terrain and climates, Adam said.