More research confirms insect damage reduces wildfire severity

pine beetle damageAdditional research about the effects of insect outbreaks on fires confirms that generally, insect damage reduces burn severity.

Researchers from the University of Vermont and Oregon State University studied 81 Pacific Northwest fires that burned in areas affected by infestations of two prevalent bark beetle and defoliator species, mountain pine beetle (Dendroctonus ponderosae) and western spruce budworm (Choristoneura freemani). The fires spanned the years 1987 to 2011.

Insect fire severity

Few of the 81 fires occurred in forests while the needles were still on the trees in the red highly flammable stage of the outbreak shortly after the trees were killed by mountain pine beetles. The researchers recommend more studies in this area.

Aside from the transient red stage the burn severity decreased for more than 20 years following a MPB attack. Forests affected by western spruce budworm (WSB) exhibited a sharp decrease in fire severity immediately after an attack. This decrease is likely due to the fact that the WSB defoliates the tree, removing fuel from the canopy. The MPB kills the tree from the inside, leaving the dying needles on the tree until they fall off in one to two years. It makes sense that fewer fine fuels in the canopy would reduce the fire intensity and make it less prone to transition from a ground fire to a crown fire. But in the WSB-affected forests, the fire intensity slowly increased after 20 years to a neutral condition, then continued to increase in the 5 to 10 subsequent years. The researchers elaborated on that effect:

The relatively rapid increase of the budworm-fire coefficient with time indicates that the thinning effect on fuel profiles is less persistent for the defoliator (WSB) than for the bark beetle (MPB). In addition to relatively lower per-unit-area tree mortality impacts, WSB affects host forests that are more productive than those affected by MPB in the study region , leading to more rapid accumulation of live overstory and understory vegetation. Thus, as time elapses following WSB outbreaks, fuel density and connectivity likely increase in multiple strata, including dead surface fuels and total live biomass, the latter of which is associated with higher burn severity.

The research was conducted by Garrett W. Meigs, Harold S. J. Zald, John L. Campbell, William S. Keeton, and Robert E. Kennedy. The paper is titled, Do insect outbreaks reduce the severity of subsequent forest fires?

Our analysis

There is legitimate cause to be concerned about fires during the one or two year red needle stage after an insect attack, although I think more research studying actual fires is needed in this area. And there is danger from falling snags 5 to 20 years after an attack. Snags are dangerous for firefighters and any structures, hikers, traffic on roads, and any improvements that could be damaged by the falling trees. But as numerous researchers have found, after the needles are on the ground fire behavior, intensity, and severity decrease.

We first wrote about a post-attack decrease in burn intensity in a 2010 article titled Firefighters should calm down about beetle-killed forests.

Rather than panicking and rushing out to cut every tree affected by insects, preventive measures to keep forests healthy could be more effective, such as prescribed fire and thinning.

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Author: Bill Gabbert

After working full time in wildland fire for 33 years, he continues to learn, and strives to be a Student of Fire.

3 thoughts on “More research confirms insect damage reduces wildfire severity”

  1. Hello,

    A recent article in Fire Ecology Volume 10, Issue 2, 2014 has a different view. Long time BC fire managers are seeing higher levels of fire behaviour in these forests. This article seems to support what they are witnessing. The following is from their abstract:

    “Although the dataset is less robust than a strictly experimental approach, fires had faster spread and more crown fire than predicted, with a linear average of 2.7 times higher ROS in best fit models than expected for unaffected pine. The most likely crown fire initiation threshold was ISI 5.5”.

    Perrakis et al.: Modeling Wildfire Spread in MPB-Affected Stands, British Columbia
    Daniel D.B. Perrakis1*, Rick A. Lanoville2, Stephen W. Taylor3, and Dana Hicks1.

    I guess more study is needed.

    1. Your excerpt comes from the abstract of the paper and refers to the time period 1 to 5 years after an attack. Here is the complete abstract:

      The mountain pine beetle (Dendroctonus ponderosae Hopkins; MPB) has killed lodgepole pines (Pinus contorta Dougl. ex Loud.) across 20 million hectares of central British Columbia, Canada, since the late 1990s, challenging land managers as well as fire management personnel. Although recent studies have used models to simulate how MPB might affect fire spread, very little fire behaviour has been documented in MPB-affected stands. We documented rate of spread (ROS) in experimental fires and wildfires in recent MPB-killed stands in British Columbia using interpretations of oblique photographs, airborne measurements of wildfire spread, and experimental burns. Fire spread observations were used to develop ROS models following the empirical approach of the Canadian Forest Fire Danger Rating System (CFFDRS). Sixteen fire runs were examined that occurred in mature MPB-affected pine stands from 1 to 5 years since peak attack. Observations of ROS were associated with corresponding weather measurements from nearby weather stations and non-linear regression curves were fit to paired ROS and Initial Spread Index (ISI) data according to CFFDRS convention. Although the dataset is less robust than a strictly experimental approach, fires had faster spread and more crown fire than predicted, with a linear average of 2.7 times higher ROS in best fit models than expected for unaffected pine. The most likely crown fire initiation threshold (P = 0.5) was ISI 5.5. Fire intensity is likely higher in early post-MPB stands due to increased ROS, lower crowning thresholds, and greater consumption of fine dead branches. Further studies on fire behaviour in MPB-affected stands are needed, but the present findings can help reduce uncertainty in fire and land management decisions in the interim.

      This is how the Conclusions section of that paper begins:

      Although some previous simulation studies suggest otherwise, the observations in this study provide evidence for more rapid crown fire behaviour and very rapid spread in the first few years following MPB attack under moderate fire danger conditions.

      This research only provides information about rate of spread immediately following an attack.

      1. Which makes sense … in first few years there are lots of dead/dying needles in the crown. Later, needle fall makes the crowns slightly less torchable.


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