Long-term examination of methods for controlling wildfires in sagebrush steppe ecology

10 years following prescribed fire treatment. Credit: SageSTEP

A new study lead by a scientist from Oregon State University is the first long-term examination of techniques to restrict the spread of wildfires in the sagebrush steppe ecosystem, which dominates portions of the western United States.

The number, size, and intensity of wildfires in the sagebrush ecosystem, which spans large portions of Nevada, Oregon, and Utah, as well as portions of California, Idaho, Washington, and Wyoming, have increased dramatically in recent years, primarily due to climate change and the spread of invasive grasses.

Researchers investigated a variety of approaches for reducing fire intensity. Herbicide treatment demonstrated limited long-term advantages, prescribed fire reduced the fire behaviour metrics they monitored but increased invasive grasses, and mechanical thinning lowered the majority of fire behaviour metrics without significantly increasing invasive grasses.

Lisa Ellsworth, the study’s lead author and a range ecologist in Oregon State’s College of Agricultural Sciences, stated, “It’s a very magnificent ecosystem, but it’s incredibly fragile.” It was designated as one of the most imperilled ecosystems in North America due to its sensitivity to climate change, invasive species, and altering fire regimes.

Ellsworth, together with scientists and managers from the federal Bureau of Land Management, U.S. Forest Service, U.S. Department of Agriculture, U.S. Geological Survey, University of Idaho, and Utah State University, participated on a 10-year research of various ecosystem fire suppression techniques. Today, their results were published in the journal Ecosphere.

In the background is a sagebrush environment that has been invaded by western juniper. In the foreground, scientists are measuring plants where juniper has been removed. The credit goes to SageSTEP.

Along 435 miles of roadways in sagebrush habitat in southeast Oregon, southwest Idaho, and northern Nevada, the Bureau of Land Management is constructing and maintaining fuel breaks. The completion of the project is anticipated to take 10 to 15 years.

Ellsworth expects that the research will contribute to the Bureau of Land Management’s effort.

She stated, “I feel the burden of time in these systems.” We must implement ways to protect our sagebrush steppe areas in good health and break out of this invasive grass and fire feedback cycle.

The objective of this study is to fill a knowledge gap on the long-term impacts of adopting various approaches to mitigate fire-induced losses in sagebrush ecosystems.

Historically, it was anticipated that wildfires occurred every 50 to 100 years in this habitat because native plants grow slowly and are dispersed, limiting their capacity to spread rapidly. This historical cycle has been significantly interrupted by the proliferation of exotic annual grasses, which cover a larger portion of the area and dry out faster than native grasses, so allowing the ignition and spread of additional fires. This has resulted in fire occurrences that are more than double the norm of the past.

Prior to the mowing of a sagebrush patch, it was treated with a mowing treatment. The credit goes to SageSTEP.

The recently released study centred on fuel treatments, or activities that decrease or disperse combustible material in an effort to lessen fire intensity. The researchers examined three fuel treatments: herbicide application, managed fire, and mechanical thinning, or mowing, which removes sagebrush and other plants’ top growth.

The researchers then utilised a fire modelling programme to see how the various treatments affected fire behaviour. They analysed three fire behaviour metrics: rate of spread, flame length, and response intensity, a measure of the amount of heat per unit of fire surface.

Six locations, varying in size from around 50 to 200 acres, were examined in five states: Oregon, Washington, Idaho, Nevada, and Utah. The Sagebrush Steppe Treatment Evaluation Project (SageSTEP) was initiated in 2005 to assess options for restoring the Great Basin’s ecology.

Among their conclusions:

Prescribed fire resulted in the highest reduction of the three fire behaviour parameters because it was the only treatment to significantly reduce the overall fuel load on the sites. However, research conducted by Ellsworth and others has shown that exotic grasses quickly establish themselves in burned areas following managed fire.

Mechanical thinning decreased the length of the flame, an essential statistic since it can facilitate firefighter access. Surprisingly, the researchers discovered that the strategy was virtually as successful as prescribed fire in lowering the severity and rate of the reaction. However, the reduction in modelled fire spread and reaction intensity diminished after the third year, although the drop in modelled flame lengths persisted for the whole decade. Ellsworth referred to the mechanical method as a “very solid trade off” for minimising the majority of fire behaviour parameters without increasing invasive grasses, in contrast to managed fire.

The researchers determined that herbicide treatments were ineffective for lowering fuels and/or fire behaviour metrics. Targeting broad-leaved shrubs, the herbicide tebuthiuron had no initial effect for the first two years and a small effect after that. Through ten years, dead shrubs remained standing, therefore the fuel structure was not considerably affected. Imazapic, a herbicide designed to diminish annual grasses, had only a short-term (two to three years) effect on lowering fuels for fire, resulting in a marginal reduction in the predicted fire spread.

Further information: L. M. Ellsworth et al, Fuel reduction treatments reduce modeled fire intensity in the sagebrush steppe, Ecosphere (2022). DOI: 10.1002/ecs2.4064

Journal information: Ecosphere

Source: Oregon State University