How Well Do Fuels Treatments Work? A Study in Rocky Mountain National Park

Rocky Mountain National Park's Wildland Fire Management Program puts the safety of staff and the public first, protects communities, infrastructure and natural and cultural resources, and restores and maintains fire-adapted ecosystems. It has two parts: firefighting when wildfires start, and a proactive fuels management plan.

In brief

• The park has been using fuels treatments for several decades to reduce wildfire threats to life and property.
• Fuels treatments are not stand-alone defences: they are meant to help firefighters by bringing fire behaviour down to manageable levels.
• Encouraging fire-resistant aspen near park facilities may help make those areas more resilient to fire.
• Burning slash piles matters: modelling suggests lodgepole stands with lower loads of 1- to 100-hour fuels resist extreme fire behaviour better.

What fuels treatments are

Near values at risk — facilities, infrastructure and the park boundary — fire staff reduce the flammable material on the ground and in the trees: trees, shrubs, dead and fallen vegetation and grasses. The methods:

• thinning trees in high-priority areas;
• limbing trees, removing dead and low branches;
• gathering the cut material and surface fuels, such as dead and fallen trees, into slash piles, which are burned in winter.

Treatments make safer places to fight fire, buffer zones that keep fires inside the park, and evacuation routes, and they shorten wildfires. Over the past decade, staff have treated roughly 6,500 acres.

Image 1. A lodgepole forest before (left) and after (right) fuels treatment — thinned, limbed, and with excess fuel burned. NPS photo.

The study

In 2019 the park worked with The Ember Alliance and Colorado State University to ask two questions: how much fuel do treatments remove, and how well do they reduce extreme fire behaviour?

How much fuel is removed

The researchers measured forests before and after treatment: live and dead trees, basal area and density; canopy fuels (cover, bulk density and canopy-base height); and surface fuels (litter, duff, and dead and fallen wood). In lodgepole pine:

• treatments cut the average basal area and density of dead trees by more than 70 percent;
• they raised canopy-base height by an average of 2 feet;
• burning the slash piles then cut the surface load of woody material (10- to 1,000-hour fuels) by an estimated 60 percent.

Figure 1. Average (bars) and range (brackets) of canopy fuels and surface fuel loads in untreated (control) and treated plots. "NA" means canopy fuels were too sparse to estimate crown-base height in two treated plots. From the NPS report.

How fire behaviour changes

Fire behaviour describes how fire moves across a landscape. Extreme behaviour is unpredictable, hard to manage and dangerous — crown fire, racing through the canopy, is an example. Surface fire, burning along the ground, is easier to manage, and is often what prescribed burns aim for. Rate of spread and flame length also describe behaviour.

Topography, weather, forest structure and fuel loads all shape fire. The researchers ran two models — the Crown Fire Initiation and Spread (CFIS) system and the Fire and Fuels Extension of the Forest Vegetation Simulator (FVS-FFE) — on the before-and-after data, under two sets of local weather, including wind speed and fuel moisture:

• moderate: the park's 50th-percentile windiest, driest summer fire-season days;
• severe: its 90th-percentile days — conditions met on 10 percent or fewer of fire-season days.

Figure 2. Predicted fire behaviour in plots before and after treatment, under moderate and severe fire weather. Extreme behaviour is shown as active and passive crown fire. From the NPS report.

The severe-weather flame lengths matter because flames under 4 feet can generally be attacked directly by firefighters on the ground.

The lesson: fuels treatments can curb extreme fire behaviour in some conditions, but they are not a stand-alone defence. They bring fire down to manageable levels and support firefighting — creating access, evacuation routes and safer places to work — which helps stop fires spreading and buildings being lost.

What else can be done

The most effective way to reduce extreme fire in lodgepole pine was removing the overstory entirely to favour fire-resistant aspen and shrubs. That is impractical everywhere, on cost and labour, so the park and its partners are looking at encouraging aspen in key locations. Raising canopy-base height and burning large woody surface fuels — both already part of fuels treatments — also helped. The park will continue its strategic fuels work.

Terms

Fuel moisture and time-lag classes. Dead fuel moisture — the water in dead fuel as a percentage of its oven-dry weight — depends only on the surroundings and is critical to fire potential. Dead fuels are classed by time lag, the time a piece of a given size takes to get 63 percent of the way to balance with its surroundings:

Sources

• National Park Service, Rocky Mountain National Park, "Examining Fuels Treatment Effectiveness in Rocky Mountain National Park." https://www.nps.gov/articles/examining-fuels-treatment-effectiveness-in-rocky-mountain-national-park.htm
• Definitions as cited there: U.S. Forest Service glossary; National Wildfire Coordinating Group, Glossary of Wildland Fire Terminology and "Canopy Fuel Characteristics"; Scott and Reinhardt (2001); NPS, "Understanding Fire Danger."
• The original gives the 10-hour class as "3/4" to 1"" diameter; the class runs from ¼ inch, where the 1-hour class ends, as NPS's "Understanding Fire Danger" gives it.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.