Below is a synopsis by biologist Gwen Starrett of an article by Hicke et al (2012) re beetle-killed trees and fire.

Link to original article from Forest Ecology and Management: http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1224&context=barkbeetles


What are the effects of bark beetle-caused tree mortality on wildfire?

This is my synopsis of conclusions from a paper (Hicke et al. 2012) that synthesized previously published studies on this topic.

Effects on fire behavior and fuel loading depend on the phases beetle-killed  trees go through after a beetle attack.  The authors describe these as:

1)     Red phase – needles are dry and still on tree

2)     Gray phase – needles off

3)     Old phase – regrowth and understory development

How does fuel loading change through time? Canopy fuel remains unchanged until the gray phase when it decreases.  Then it increases in the old phase due to regrowth.  As needles fall, the fine surface fuels increase then decrease as they decompose.  Coarse fuels increase significantly during the  old phase as larger branches and snags fall.  Ladder fuels increase during the gray and old phases as shrubs and seedlings take hold.

How does fire behavior change with these changes in fuel?  Starting in the red phase and continuing with time, the surface fire behavior properties increase due to surface fuel loads.  These surface fire behavior properties are: rate of spread, reaction intensity, flame length.

Torching potential, the potential for a surface fire igniting a tree or group trees, increases in the red phase because of the low moisture in dead needles.  While needle drop might seem to help this problem, the increased surface loads and no change in canopy base height more than offset the benefits.

The potential for an active crown fire increases in the red phase because of the dead needles.  However, during the grey phase, this potential declines substantially as needles drop and dead branches fall.  Crown fire potential then increases again with regrowth.

How does tree mortality affect the probability of fire occurrence and burn severity?

In the red phase:  The authors expect that crown fires will increase in the red phase due to higher torching potential and active crown fire potential.  Probability of surface fires is not expected to increase.

In the gray phase and old phases:  The authors are uncertain about crown fire probability since likelihood may decrease due to canopy bulk density, but may increase due to increased torching potential.  Surface fire probability increases due to increased surface fuel loads.

Burn severity is unchanged in the red phase but increases in the next phases due to increased surface loads.

The most disagreement in the above conclusions was with the earliest post outbreak phase and crown fire behavior.

How does tree mortality compare to other factors in increasing fire risk?

“The effect of beetle outbreaks on probability of occurrence or burn severity were smaller than other drivers such as climate, tophography, blowdown, and cover type.”  However, these studies have not compared beetle killed- tree areas to unattacked stands with similar conditions.  The authors suggest that more research is needed to tease out the specific factors that best indicate when fires would become more likely or more intense.  For example, fire behavior may change only under some environmental conditions, such as during intermediate wind speeds or specific atmospheric moisture levels.


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