Mountain Pine Beetle Outbreaks and Changes to Wildfire Risk and Behavior in Ponderosa Pine Forests on the Colorado Front Range | Colorado State Forest Service

Author:

Chad Julian, Fire Mitigation Specialist, Colorado State Forest Service

Editors:

Amanda West Fordham, Ph.D., Associate Director, Science and Data Division, Colorado State Forest Service
Teddy Parker-Renga, Associate Director, Communications and Communities Division, Colorado State Forest Service

Introduction

The current mountain pine beetle outbreak on the northern Front Range of Colorado is impacting ponderosa pine forests. As forest conditions rapidly change, the goal of this article is to provide a condensed overview on the current state of knowledge concerning how increased tree mortality from these bark beetles has the potential to alter fire behavior and wildfire risk to communities. It is important to acknowledge that while these bark beetles and wildfires are important ecologically to Colorado’s forests, ongoing drought and increasing stress on trees are affecting both disturbances and require more monitoring data to better understand their interactions.

The Colorado State Forest Service consulted with academic, federal, state and local agency scientists, staff and land managers to synthesize existing peer-reviewed literature, technical papers and on-the ground knowledge and observations in these forested systems to produce this Science & Data Byte. This article can serve as a starting point for better communication, understanding and research of ongoing interactions between mountain pine beetles and wildfire on the Front Range of Colorado. It can support the development of adaptation strategies to mitigate risk and promote forest health for current and future generations.

Setting

The outbreak of mountain pine beetle (Dendroctonus ponderosae) is predominately impacting lower elevation forests on the Front Range of Colorado, particularly those within the wildland-urban interface where there is a mix of private and public ownership. Ponderosa pine forests along the Front Range have historically had occasional pine beetle outbreaks and experienced relatively high fire frequencies (e.g., 5-50 years) that kept overall tree density much lower and allowed most mature trees to survive.

Since the early 20th century and Euro-American settlement, these ponderosa pine-dominated forests have been affected by land-use changes and fire suppression policies. These changes have resulted in increases in fire intensity and severity compared to pre-settlement conditions, owing to the buildup of predominantly live and dead fuels following successful fire suppression, in addition to other land-use changes such as grazing and timber extraction. A shifting climate is amplifying these changes and altering forests’ ability to resist and recover following these disturbances.

The article RMRS-GTR-373, Principles and practices for the restoration of ponderosa pine and dry mixed-conifer forests of the Colorado Front Range, offers additional information and background on ponderosa pine forests in this area of the state.

Given changes in the fire regime and the proximity to private lands, homes represent a major value at risk in forests experiencing the mountain pine beetle outbreak on the Front Range (Figure 1). In addition, these forests are an important setting for outdoor recreation for residents and visitors alike, sustain abundant wildlife and contain critical infrastructure, such as power lines, reservoirs and waterways. These forests also provide numerous significant ecosystem services that, included with recreation, wildlife and critical infrastructure, may experience impacts from wildfire in forests affected by mountain pine beetles.

It is important to note the majority of the focus of this Science & Data Byte will be placed on wildfire risk and fire behavior interactions with mountain pine beetle-caused mortality. Regardless of current or future forest conditions resulting from or contributing to outbreaks, home hardening and defensible space treatments within the home ignition zone remain the most effective strategies for reducing wildfire risk to structures’ vulnerability to firebrands and surface fire.

Pre-outbreak conditions

Ponderosa pine forests historically endured a high-frequency, low-severity fire regime that has shifted to a low-frequency, high-severity fire regime because of changes in forest structure and fuel loading, including increased horizontal and vertical fuel continuity. Due to these changes, ponderosa pine forests in Colorado have a significant fire deficit, which leads to higher intensity and overall higher impacts from severity when fires occur. The article by Hagmann et al. (2021), Evidence for widespread changes in the structure, composition, and fire regimes of western North American forests, provides further discussion on changes in forest conditions and fire regimes.

Ample ignition sources, a preponderance of continuous surface fuels (e.g., needles, cones and grasses) and alignment with strong winds on the Front Range produce high to very high burn probabilities year-round. This is amplified by periodic drought conditions and ever-increasing temperature regimes.

Due to development of homes and other structures within the wildland-urban interface, high to very high annual burn probability, and altered forest structure and fuel loading, lower elevation ponderosa pine forests on the Front Range have a higher wildfire risk for built infrastructure compared to many other forests in Colorado even without the current mountain pine beetle outbreak.

During outbreak conditions (both red and gray phases are present)

The foliage on mountain pine beetle-killed trees will turn yellow and then red, or could just turn red the following year after successful attack (i.e., red phase). The red needles may persist in the crown for up to three years depending on exposure to prevailing winds and precipitation events.

Red needles in the canopy have lower foliar fuel moisture content compared to green needles—approximately an order of magnitude lower. Annual burn probability is unaltered, however, as there already is an availability and high connectivity of surface fuels creating conditions for fires to ignite.

Weather remains the primary determinant of fire behavior, with amplifying conditions related to high temperatures, low relative humidity, high wind speed, dry fuels and drought. A considerable proportion of red needles in the canopy may facilitate a transition in fire type from surface to tree crowns via either single or group tree torching under a wider range of fuel moisture conditions and lower windspeeds.

Changes to active crown fire risk influenced by mountain pine beetle-caused tree mortality are variable, as active crown fires are primarily a function of the rate of fire’s spread and crown density, which have complex interactions with the amount and timing of tree mortality and associated changes to crown fuel availability, within-canopy wind flow and surface fuel load. During winter on the Front Range, high wind events are frequent, dry conditions are common and the snowpack is patchy in lower elevation ponderosa pine forests. When these weather and climate conditions align with red needles in the canopy, the change to crown fire risk is clear—it increases. There are lower thresholds for ignition and spread due to lower fuel moisture content in the red needles.

The intensity and timing of the mountain pine beetle outbreak are critical:

High tree mortality in a short period of time (e.g., 80% mortality over two years) can lead to a higher probability of crown fire activity for a given set of weather conditions due to a high proportion of red needles in the canopy (i.e., lower foliar moisture content).
High tree mortality over a longer period of time (e.g., 80% mortality over 10 years) could lower the probability of active crown fire activity initially by decreasing crown fuel density, but it could elevate passive crown fire risk over time as a result of surface fuel accumulation as branches and debris fall from dead trees, as well as new vegetation growth.
Low to moderate tree mortality over variable time periods may result in variable active crown fire risk. It is not known what threshold of mortality is needed to increase active crown fire risk. In these situations, the risk is similar to what is present before the outbreak of mountain pine beetle activity within the forest.

Post-outbreak conditions

After trees die from mountain pine beetle attack, gray phase conditions begin to develop as red needles fall to the forest floor, as soon as one year after the infestation (Figure 2), dominating the post-outbreak environment potentially for decades.

Ponderosa pine trees are prolific needle producers, so tree death and subsequent needle fall will increase fine dead surface fuel loading until needles decompose, potentially contributing to increased fire behavior. At the same time, active crown fire potential is likely reduced in areas dominated by dead standing trees due to lower crown fuel density.

Dead standing trees fall at varying rates, which can increase dead surface fuel loading, mostly in the largest size classes of trees. The net effects of increasing the amount of large-diameter surface fuels on fire behavior are complicated, likely varying by phase of combustion and the effects of long-term weather trends on fuel moisture dynamics.

Grasses and shrubs can increase fuel loading and height as the canopy falls away. As the ponderosa pines and other conifers die and decompose on the Front Range, Gambel oak may increase in areas south of Interstate 70 (this species is not common north of I-70). This Gambel oak could shift the cover type from a forest toward a shrub/grass-dominated landscape. The changes toward shrub/grass-dominated landscapes can increase the annual burn probability.

Regardless of changes to fuels and cover type from mountain pine beetle mortality, weather remains the primary factor influencing fire behavior. Amplifying conditions are related to high temperatures, low relative humidity, high wind speed, dry fuels and drought.

Effects on fire suppression

When the effects of mountain pine beetles are present in a forest, increasing fireline intensity and branding, changing fuel arrangements and easier surface-to-crown fire transition where green trees are present may require changes in fire management strategies and tactics. Significant numbers of dead snags and fallen trees also may create hazards to life safety and accessibility issues.

Current operational fire behavior models do a poor job of incorporating the complex effects and interactions of bark beetle-caused tree mortality on predictions of fire behavior. As such, fire managers are likely to experience surprises or unanticipated events during wildfires burning in bark beetle-affected forests.

When referring to the wildland fire behavior triangle (topography, weather and fuels), forest and fire managers are only able to influence fuels. Treated areas where fuels have been reduced may burn less severely than untreated areas, as noted in this study from the Colorado Forest Restoration Institute. Additionally, the article RMRS-GTR-365, Visualization of heterogeneous forest structures following treatment in the southern Rocky Mountains, is helpful for understanding the different treatment prescriptions and how they may differ in treatment outcomes.

Differences in potential impacts to fire behavior and WUI risk between outbreaks in lodgepole pine and ponderosa pine forests

Lodgepole pine

Crown fuel load (i.e., needles and fine branches) lower compared to ponderosa pine trees of the same diameter
Non-prolific needle production
Well adapted to regrow following infrequent fires at high fire intensity
Wind season aligned with snow on the ground; small window in late summer/early fall where wind season can align with cured fuels
Winter snowfall dominant
Historically longer fire return intervals: low fire frequency, but high intensity
Fewer homes and smaller wildland-urban interface

Ponderosa pine

Crown fuel load (i.e., needles and fine branches) higher compared to lodgepole pine trees of the same diameter
Prolific needle production
Adapted to persist through frequent fires at low to moderate fire intensity; forest conditions now often support much higher fire intensity than forests can survive
Wind season aligned with no snow and dry grassy fuels
Summer rainfall dominant
Historically shorter fire return intervals: high fire frequency, but low to moderate intensity
More homes and greater wildland-urban interface

Reviewers and experts consulted:

Mike Battaglia, Ph.D., Research Forester, USDA Forest Service Rocky Mountain Research Station
Dan Beveridge, Fire & Fuels Manager, Colorado State Forest Service
Ethan Bucholz, Ph.D., Forest Monitoring Program Manager, Colorado State Forest Service
Paul Dennison, Wildland Fire Senior Program Manager, City of Boulder Open Space and Mountain Parks
Max Erikson, Supervisory Forester, Fort Collins Field Office, Colorado State Forest Service
Frank Falzone, State Conservation Forester and TSP Coordinator, Natural Resources Conservation Service
Chiara Forrester, Ph.D., Forest Program Director, The Watershed Center
Allen Gallamore, Area Manager, Colorado State Forest Service
Chad Hoffman, Ph.D., Associate Professor, Forest and Rangeland Stewardship Department, Warner College of Natural Resources, Colorado State University
Chazz Lakin, Wildfire Resilience Coordinator, Colorado State Forest Service
Boyd Lebeda, Fire Behavior Analyst, Colorado Division of Fire Prevention & Control
Janae Malpas, Wildfire Mitigation Program Specialist, Colorado State Forest Service
Todd Neel, Deputy Director, Renewable Resources/State & Private Forestry and Tribal Relations, USDA Forest Service Rocky Mountain Region
Wesley Page, Ph.D., Forest Prescribed Fire/Fuels Specialist, Arapaho and Roosevelt National Forests, USDA Forest Service
Ben Pfhol, Supervisory Forester, Boulder Field Office, Colorado State Forest Service
Rocco Snart, Wildland Fire Management Planning Branch Chief, Colorado Division of Fire Prevention & Control
Nick Stremel, Resource Specialist-Fire, Boulder County Parks and Open Space
Dan West, Ph.D., Forest Entomologist, Colorado State Forest Service
John White, Wildland Fire Planner and Recovery Coordinator, Jefferson County Sheriff’s Office
Brett Wolk, Associate Director, Colorado Forest Restoration Institute, Colorado State University

Technical papers and literature to reference for the Front Range

General changes

Hagmann, R. K.; Hessburg, P. F.; Prichard, S. J.; Povak, N. A.; Brown, P. M.; Fulé, P. Z.; Keane, R. E.; Knapp, E. E.; Lydersen, J. M.; Metlen, K. L.; Reilly, M. J.; Sánchez Meador, A. J.; Stephens, S. L.; Stevens, J. T.; Taylor, A. H.; Yocom, L. L.; Battaglia, M. A.; Churchill, D. J.; Daniels, L. D.; Falk, D. A.; Henson, P.; Johnston, J. D.; Krawchuk, M. A.; Levine, C. R.; Meigs, G. W.; Merschel, A. G.; North, M. P.; Safford, H. D.; Swetnam, T. W.; & Waltz, A. E. M. (2021). Evidence for widespread changes in the structure, composition, and fire regimes of western North American forests. Ecological Applications. 31(8): 24-.

Hart, S. J., et al. (2015). Area burned in the western United States is unaffected by recent mountain pine beetle outbreaks. Proceedings of the National Academy of Sciences.

Hoffman, C. M., et al. (2013). Progress in understanding bark beetle effects on fire behavior using physics-based models. Colorado Forest Restoration Institute.

Keane, R. E., et al. (2022). Modeled interactions of mountain pine beetle and wildland fire under future climate and management scenarios for three western US landscapes. Fire Ecology.

Jenkins, M. J., et al. (2014). Interactions among the mountain pine beetle, fires, and fuels. U.S. Department of Agriculture, Forest Service.

Stephens, S. L., et al. (2022). Mass fire behavior created by extensive tree mortality and high tree density not predicted by operational fire behavior models in the southern Sierra Nevada. Forest Ecology and Management, 518.

Tinkham, Wade T.; Dickinson, Yvette; Hoffman, Chad M.; Battaglia, Mike A.; Ex, Seth; Underhill, Jeffrey. (2017). Visualization of heterogeneous forest structures following treatment in the southern Rocky Mountains. Gen. Tech. Rep. RMRS-GTR-365. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 72 p.

U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. (2009). Fire and bark beetle interactions (PNW-GTR-784).

U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. (2016). Synthesis of knowledge of extreme fire behavior: Volume 2 for fire behavior specialists, researchers, and meteorologists (PNW-GTR-891).

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. (2014). Post-epidemic fire risk and behavior (RMRS-P-70).

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. (2015). Practical tools for assessing potential crown fire behavior and canopy fuel characteristics (RMRS-P-73).

Ponderosa pine specific

Addington, Robert N.; Aplet, Gregory H.; Battaglia, Mike A.; Briggs, Jennifer S.; Brown, Peter M.; Cheng, Antony S.; Dickinson, Yvette; Feinstein, Jonas A.; Pelz, Kristen A.; Regan, Claudia M.; Thinnes, Jim; Truex, Rick; Fornwalt, Paula J.; Gannon, Benjamin; Julian, Chad W.; Underhill, Jeffrey L.; & Wolk, Brett. 2018. Principles and practices for the restoration of ponderosa pine and dry mixed-conifer forests of the Colorado Front Range. RMRS-GTR-373. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 121 p.

Sieg, C. H., Linn, R. R., Pimont, F., et al. (2017). Fires following bark beetles: Factors controlling severity and disturbance interactions in ponderosa pine. Fire Ecology, 13, 1–23.

U.S. Department of Agriculture, Forest Service, Southern Research Station. (2014). Bark beetle outbreaks in ponderosa pine forests: Implications for fuels, fire, and management (SRS-GTR-198, Chapter 12).

U.S. Department of Agriculture, Forest Service, Southern Research Station. (2019). Impact of bark beetle infestation on fuel loads and fire behavior in “old-stage” southwestern ponderosa pine (SRS-GTR-213, Chapter 14).

Lodgepole pine specific with principles and concepts that transfer to ponderosa pine

Klutsch, J. G., et al. (2011). Evaluating potential fire behavior in lodgepole pine-dominated forests after a mountain pine beetle epidemic in north-central Colorado. Western Journal of Applied Forestry, 26(3), 101–109.

Linn, R. R., et al. (2015). Modeling spatial and temporal dynamics of wind flow and potential fire behavior following a mountain pine beetle outbreak in a lodgepole pine forest. Agricultural and Forest Meteorology, 204, 79–93.

Moriarty, K., et al. (2019). Firefighter observations of “surprising” fire behavior in mountain pine beetle-attacked lodgepole pine forests. Fire, 2(2), Article 34.

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. (2015). Effects of bark beetle attack on canopy fuel flammability and crown fire potential in lodgepole pine and Engelmann spruce forests (RMRS-P-73).