Examining Fuels Treatment Effectiveness in Rocky Mountain National Park

Key Takeaways

• RMNP has been actively preparing to mitigate wildfire threats to human life and property using fuels treatments. These efforts have been ongoing for several decades.   
• Fuels treatments are not stand-alone defenses against wildfires; rather, they are designed to augment firefighting efforts by reducing fire behavior to manageable levels.  
• Promoting fire-resistant aspen stands in key locations near park assets and infrastructure may be a useful management strategy to increase fire resiliency in these areas.  
• The reduction of forest fuels through slash pile burning is an important aspect of fuels treatments. Modelling suggests lodgepole stands with lower 1- to 100-hour fuel loads tend to be more resistant to extreme fire behavior. 

Rocky Mountain National Park (RMNP) has a comprehensive Wildland Fire Management Program that prioritizes the safety of employees and the public, and the protection of communities, infrastructure, and natural and cultural resources, while restoring and maintaining fire-adapted ecosystems. RMNP’s Wildland Fire Management Program has two program elements that help achieve these goals: firefighter response to wildland fire and a proactive fuels management plan.

Under the Wildland Fire Management Plan, fire staff perform “fuels treatments” near values at risk such as facilities, infrastructure, and the park’s boundary to protect these assets and nearby communities. Fuels treatments reduce the amount of flammable material on the landscape including trees, shrubs, dead and downed vegetation, and grasses. The Wildland Fire Management Program uses several techniques during fuels treatments including:

• thinning trees in high priority areas,
• limbing trees to remove dead and low-level branches, and
• consolidating thinned/limbed material and surface fuels - like dead and downed trees - into slash piles. During the winter, these slash piles are burned to remove excess fuels from the landscape.

Fuels treatments are just one strategy RMNP uses to prepare for, and manage, active wildland fires. Fuels treatments create a safer place to fight fire, create buffer zones to keep fires within park boundaries, establish evacuation routes, and decrease the duration of a wildfire. Over the last decade, fire staff have performed fuels treatments on roughly 6,500 acres of park land as part of on-going fire planning and protection efforts.

Image 1. A comparison of a lodgepole forest before (left image) and after (right image) fuels treatments. Fuels treatments reduce the amount of flammable material on the landscape by thinning trees, limbing trees, and burning excess fuels during prescribed burns.

NPS Photo

In 2019, RMNP partnered with The Ember Alliance and Colorado State University to study the effectiveness of fuels treatments. The research team worked to determine:

1. How much fuel is removed from the landscape during treatments, and
2. How effective fuels treatments are in mitigating extreme wildland fire behavior in these areas.

How much fuel is removed from the landscape during treatments?

To quantify the amount of material removed during fuels treatments, researchers measured forest characteristics (the number of live and dead trees, basal area, and density), different canopy fuels (cover, bulk density, and canopy-base height) and surface fuels (litter, duff, and dead and downed woody material) in forests before and after fuels treatments occurred. At study sites in lodgepole pine forests, fuels treatments reduced the average basal area and density of dead trees by over 70% and increased canopy-base height by an average of 2 feet. Subsequent slash pile burning reduced the surface load of woody material (10- to 1000-hour fuels) by an estimated ~60% (Figure 1). See Definitions below.

Figure 1. The average (bars) and range (brackets) of canopy fuels and surface fuel loads in untreated (control) plots and treated (post-treatment plots). “NA” indicates that canopy fuels were too sparse to estimate crown-base height in two treated plots.

NPS Report

How effective are fuels treatments in mitigating extreme wildfire behavior?

Fire behavior is a term used to describe how fire moves across a landscape. Extreme fire behavior implies the movement is unpredictable, difficult to manage, and dangerous for firefighters and communities. Crown fire - when fire rapidly burns through the forest canopy – is an example of extreme fire behavior. Surface fires describe the behavior of fire burning along the ground and are more easily managed by fire professionals. Oftentimes, surface fires are desirable during prescribed burning events. Other metrics, like fire rate of spread and flame length, can be used to describe fire behavior.

One goal of fuels treatments is to reduce the likelihood of extreme fire behavior near park assets and communities. By modelling fire behavior in pre-fuels treatment and post-fuels treatment forest plots, managers can begin to understand how effective treatments are in accomplishing this goal.

Topography, weather, forest characteristics, and fuel loads all influence how fire behaves. Fire behavior models, such as the Crown Fire Initiation and Spread (CFIS) system and the Fire and Fuels Extension of the Forest Vegetation Simulator (FVS-FFE), take this information into account. Forest and fuels data were derived from measurements taken before and after fuels treatments occurred. To cover a range of scenarios, researchers modelled fire behavior under both moderate and severe fire weather conditions, which includes wind speed and fuel moisture. Utilizing local weather data, moderate fire weather conditions were defined as the conditions observed on the 50th percentile of windiest and driest days occurring in the park during the typical summer fire season. Similarly, severe fire weather was defined as the conditions observed on the 90th percentile windiest and driest conditions observed (i.e., severe conditions are experienced only 10% or fewer days during the fire season).

Figure 2. Predicted fire behavior for plots, before and after fuels treatments, under moderate and severe fire weather conditions. Extreme fire behavior is represented as Active and Passive crown fire.

NPS Report

Under moderate fire weather conditions, areas where fuels treatments occurred were more likely to support surface fires than active crown fires and fuels treatments may reduce the likelihood of crown fire occurrence by 35% (Figure 2). In addition, the average crown fire rate of spread is slower in fuels treatment areas compared to untreated areas. Similar flame lengths (1 vs. 2 feet) are predicted in both treated and untreated zones.

Under severe fire weather conditions, there is a high occurrence of active crown fire in both fuels treatment areas and untreated areas (Figure 2) and a similar crown fire rate of spread is predicted. Flame lengths are modelled to be greater in untreated areas (8 feet) compared with fuels treatment areas (3 feet), which is significant because flame lengths less than 4 feet can generally be attacked directly by firefighters on the ground.

The results of this research suggest that, while fuels treatments are effective at mitigating extreme fire behavior under certain conditions, they are not stand-alone defenses against wildfire. While fuels treatments can reduce fire behavior to manageable levels, they are also designed to assist firefighting efforts. Strategic fuels treatments can create access and evacuation routes and safer places for firefighters to work on the ground, which can help prevent structure loss and fire spread.

What else can be done?

When examining which methods best reduce the likelihood of extreme fire behavior in lodgepole pine forests, researchers found that complete overstory removal to promote more fire-resistant species - like aspen and shrubs – was the most effective. While complete overstory removal in all treatment areas is impractical from both a cost and labor perspective, RMNP staff and research partners are examining ways to promote aspen regeneration in key locations to increase fire resiliency.

Besides overstory removal, researchers identified other effective methods of reducing extreme fire behavior. This includes increasing crown-base height, and removing large, woody surface fuels through slash pile burning – both of which occur during fuels treatments.

RMNP’s Wildland Fire Management Program will continue these strategic and proactive fuels treatments to increase the safety of employees and the public, and the protection of communities, infrastructure, and natural and cultural resources in and near Rocky Mountain National Park.

Definitions

Basal Area

The cross-sectional area of the trunk of a tree or stand of trees at breast height (4.5 feet).(Glossary (usda.gov))

Canopy

In a forest, the branches from the uppermost layer of trees; on rangeland, the vertical projection downward of the aerial portion of vegetation.(Glossary (usda.gov))

Canopy Base Height

The lowest height above the ground at which there is a canopy base height sufficient amount of canopy fuel to propagate fire vertically into the canopy. Canopy base height is an effective value that incorporates ladder fuels such as shrubs and understory trees (Scott and Reinhardt, 2001).

Canopy Bulk Density

The mass of available canopy fuel per unit canopy volume. It is a bulk property of a stand, not an individual tree (Scott and Reinhardt, 2001).

Canopy Cover

Canopy Cover describes the percent cover or cover class of the tree canopy in a stand. Specifically,  the vertical projection of the tree canopy onto an imaginary horizontal surface representing the ground’s surface. (Canopy Fuel Characteristics | NWCG)

Down Woody Material

A tree or part of a tree that is dead and laying on the ground. (Glossary (usda.gov))

Duff

The layer of decomposing organic materials lying below the litter layer of freshly fallen twigs, needles, and leaves and immediately above the mineral soil. (Glossary of Wildland Fire Terminology)

Fuel Moisture (1-, 10-, 100-, and 1000-hour fuels)

Fuel moisture is measured for live herbaceous (annual and perennial) and woody (shrubs, branches, and foliage) fuels and dry (dead) fuels. These are calculated values representing approximate moisture content of the fuel. Fuel moisture in live fuels varies through the growing season and between different climate classes. There are 20 different fuel models, representing the variety of vegetation in the area, that a manager can use when calculating fire danger. Dead fuel moisture is the moisture content of dead organic fuels, expressed as a percentage of the oven-dry weight of the sample. Dead fuel moisture is controlled solely by exposure to environmental conditions and is critical in determining fire potential. Dead fuel moistures are classed by timelag. A fuel’s timelag is the time necessary for a fuel particle of a particular size to reach 63% of equilibrium between its initial moisture content and its current environment.Dead fuels in NFDRS have four time lag classes:

• 1 hour—Fine flashy fuels, dried herbaceous plants or round wood less than 1/4" diameter. Also includes the uppermost layer of litter on the forest floor. Responds quickly to weather changes. Moisture in these fuels varies greatly throughout the calendar day and is principally responsible for diurnal changes in fire danger. It is computed from observation, time, temperature, humidity and cloudiness.
• 10 hour—Round wood 3/4" to 1" diameter and the layer of litter that extends to 3" to 4" below the surface. Moisture in these fuels is computed from observation, time, temperature, humidity, and cloudiness, or may be a standard set of "10-Hr Fuel Sticks" that are weighed as part of the fire weather observation.
• 100 hour—1" to 3" diameter. Moisture in these fuels is computed from 24 hour average boundary condition composed of day length, hours of rain, and daily temperature and humidity ranges.
• 1000 hour—3" to 6" diameter. Moisture in these fuels is computed from a 7-day average boundary condition composed of day length, hours of rain, and daily temperature and humidity ranges.

(Understanding Fire Danger (nps.gov))

Litter

The top layer of forest floor, composed of loose debris of dead sticks, branches, twigs, and recently fallen leaves or needles; little altered in structure by decomposition. (Glossary of Wildland Fire Terminology)