Scientists work together to solve problems. They develop research studies to help them answer questions. Scientists often create experimental designs to help them answer those questions. With an experimental design, there is a structured process that uses an experiment to test a hypothesis.

In this study, the scientists had a unique opportunity to study wildfire using an experimental design. Due to the uncertain nature of wildfire, setting up an experimental design is difficult; it is hard to know with certainty where and when a wildfire will burn. Experiments with wildfire may also be difficult because of a lack of data on the condition of the forest before the fire.

However, the area that was burned by a wildfire in this study had been used for forest research for many years before the wildfire went through it. Therefore, there was a lot of data available about the land. This circumstance allowed the scientists to gather data for a large-scale wildfire forest management study.

Wildfires can be highly unpredictable. The fire behavior triangle is a way for people to think about how wildfires move and change, or basically how a wildfire behaves.

The fire behavior triangle helps people make predictions about what might happen during a wildfire. The three sides of the triangle are fuels, weather, and topography (figure 1).

Surface fuels—all living and nonliving material near the ground that can be burned by a fire. Dead branches, leaves, twigs, and other debris that pile up on the forest floor, plus low shrubs, are all examples of surface fuel (figure 2). There is also fuel in the canopy of a tree.

The most important factor in the fuels category is moisture level. High moisture content slows the burning process. If fuel moisture is very low, the fuel is consumed more rapidly, which can lead to extreme fire behavior.

Weather—the wind, temperature, and humidity. Wind is the most important factor of weather because it brings oxygen to the fire. Fires need oxygen, heat, and fuel to burn.

Another weather factor is temperature. Generally, fuels ignite faster with higher temperatures. Finally, humidity is also an important weather factor. Low humidity means drier fuels. Drier fuels catch fire more quickly.

Topography—the shape of the land. Topography can help or hinder the spread of the fire. For example, a rocky area can act as a fuelbreak. Elevation is another important component of topography. Higher elevations tend to be windier but colder. The slope of the topography is also useful to understand. For example, fire at the bottom of a steep slope may move more quickly. This quick movement is because the heat rising from fire at the bottom of the slope may preheat the area above the fire. Then the preheated area catches fire more quickly as the fire moves up the slope.

People take care of forests for a variety of reasons. One reason is to prevent severe wildfires that can hurt people and damage land and property. As you read in Thinking About the Environment, wildfire behavior is influenced by the fuels in a forest (both on the surface and in the tree canopy), weather, and topography. People can’t really control the weather or topography of a forest, but they can manage the availability of fuels.

Forest thinning and prescribed fire are two forest treatment tools that are used to help reduce canopy fuels and ladder fuels in the forest. Canopy fuels are fuels in the top portion of the tree, otherwise known as the crown of the tree (figure 3). Ladder fuels are fuels, like grass, shrubs, and low branches, that allow fire to climb from the ground to reach the tree crown.

Forest thinning means that trees are removed from the forest in a planned way (figure 4). Thinning prevents the overcrowding of trees. Overcrowded areas create stress on the trees, which can lead to unhealthy and diseased trees. Additionally, overcrowded areas provide more fuel for wildfire if it burns through that area.

Thinning generally removes smaller trees that can act as ladder fuels. Crown fires, in the tree canopy, move quickly when a forest is overcrowded because the crowns of the trees are closer together (figure 5).

Prescribed fire is a treatment that uses controlled fire. Controlled fire is fire that is set on purpose by trained professionals and monitored by them. Prescribed fire burns surface fuels such as dead and downed wood and twigs and branches that have fallen to the forest floor (figure 6). It also helps to control understory shrubs (figure 7).

Prescribed fire can also help create fuelbreaks. Fuelbreaks are areas in a forest that contain less trees and vegetation (figure 8). The lack of fuel in fuelbreaks means that the wildfire may be slowed down or have nowhere else to go. Prescribed fire helps to reduce overall fuel in a forest and to maintain a healthy forest ecosystem.

In this research, scientists were interested in understanding the impacts of different forest treatments, like thinning and prescribed fire, on forest conditions. Because it would be dangerous to test their hypothesis by creating a wildfire, they collected data on forest treatments like thinning and prescribed fire. When a real wildfire burned unexpectedly through those study plots, the scientists were then able to study how different forest treatments affected the behavior and severity of wildfire.

The study took place on the Klamath National Forest in northern California in an area where long-term research was already being conducted (figure 9). In the late 1990s, scientists began studies in this area to assess different forest treatment methods such as thinning and prescribed fire. The study area contained mostly ponderosa pine and white fir. The scientists visited the plots periodically to assess the effects of the treatments.

Over a century ago, long before these research plots were installed, forests in the study area were quite open. Studies of the tree rings show that in the past this area burned about every 11 years, which helped thin the forest and benefited ponderosa pine. Ponderosa pine is a species that is well adapted to frequent fire.

The forest was logged in the 1920s and many of the larger pines were cut. Larger trees can also be more resilient to fire. Wildfires, which occurred frequently before, were extinguished. Because of this management, smaller ponderosa pine and white fir trees grew back (figure 10). Smaller trees and white fir are more sensitive to fire. This combination of trees was far denser than the original forest.

The research area contained control plots that were not treated. Therefore, the control plots had more trees and greater tree canopy cover than the other treated areas. Controls are important in scientific experiments because they provide a point of comparison. Controls allow a scientist to know that what is being observed is not just a random event. Scientists compared the different forest treatment plots with the control plots to determine the impact of the treatments.

The plots that were treated had four different treatments applied. They were:

• Thinning with an emphasis on keeping big trees (Thin-BT)
• Thinning with an emphasis on keeping pine trees (Thin-P)
• Prescribed fire only (Fire-only)
• Thinning with an emphasis on keeping pine trees plus the addition of prescribed fire (Thin-P-fire)

The scientists repeated each treatment in multiple plots (table 1).

In the summer of 2021, the Antelope Fire, which started with a lightning strike, burned through all the plots in the study area. The study area burned from August 4 through August 9 under a range of weather conditions.

Some of the study units burned while a Red Flag Warning was in place. A Red Flag Warning means that there are extreme conditions such as high wind, low humidity, and dry fuels that make conditions ideal for wildfires to start or for ones already burning to spread more quickly (figure 11).

The scientists realized that, although the research plots had been heavily damaged by the wildfire, there was another research opportunity available. The scientists could study how the wildfire affected the treatment areas in the research site. This information would provide valuable data about how best to manage forests to survive wildfires.

After the Antelope Fire, the scientists inspected the trees from all the plots in the study area, collecting data on fire damage—also known as fire severity. The severity ranged from mild to high.

The scientists gathered data on the amount of tree crown that was scorched or consumed by the wildfire and the height of charring on the bark (figure 12). Trees that had any green needles left on them were considered “live.” The scientists also collected the same data in the untreated control areas. The scientists used all the data they gathered and information about forest treatments in the past to determine if the forest treatments influenced wildfire behavior and severity.

Before the wildfire, the control plots had five to seven times more trees per hectare than the thinned treatment areas. One hectare is about 2.5 times the size of an acre (and just slightly smaller than a soccer field). The treatment areas that included thinning contained 98–120 trees per hectare. After the wildfire, the scientists found that all treatment areas experienced lower fire severity than the control.

The most noticeable decrease in fire severity was for treatment areas that had been both thinned and treated with prescribed fire (Thin-P-fire) (figure 13).

The scientists found some interesting results depending on the condition of the treatment area before the wildfire. Within a treatment area, fire severity increased if the plots had a dense canopy and a large amount of surface fuel (figure 14). Plots with a less dense canopy and/or less surface fuel had lower fire severity.

Additionally, the scientists found that larger trees were less likely to die in the wildfire and had less of their crown affected or killed by the fire.

This research is the first large-scale study with treatment areas for thinning and prescribed fire that burned almost entirely during a wildfire. The scientists found the treatments helped with fire behavior, fire severity, and tree mortality even though it had been 20 years since thinning and 10 years since a prescribed fire for some of the plots. Moreover, all treatment areas did better than the control plots.

The results held true even when fire weather conditions were the most extreme, like during the Red Flag Warning, indicating that thinning and prescribed fire are useful ways to help mitigate fire behavior and severity. Additionally, the findings suggest an advantage to retaining the larger trees when thinning, as the larger trees showed more resilience to the fire.

These results can help land managers and people who work in forest management and with wildfires to understand how thinning and prescribed fire may impact wildfire behavior and reduce wildfire severity.