Thinning intensity affects fuel reduction success

Aug. 2, 2011, Washington, D.C. – In the
largest study to-date of fuel treatment effectiveness, U.S. Forest Service
researchers have found that intense thinning treatments that leave between 50
and 100 trees/acre are the most effective in reducing the probability of crown
fires in the dry forests of the western United States. The study, the results
of which are published in a recent issue of the Canadian
Journal of Forest Research, provides a
scientific basis for establishing quantitative guidelines for reducing stand
densities and surface fuels. The total number of optimal trees per unit area in
any given forest will depend on species, terrain, and other factors.

“This study proves once again that an ounce
of prevention equals a pound of cure,” says U.S. Forest Service chief Tom
Tidwell. “Thinning dense forests reduces the impacts of the catastrophic
wildfires we’ve already seen this year and expect to see more and more of in
the future. This work helps protect communities, provides jobs and
promotes overall better forest health.”

Decades of excluding fire in the Western
states have resulted in densely packed stands and a build-up of forest-floor
fuels in many dry forests, which can lead to large, continuous crown fires when
wildfires do occur. Crown fires are of particular concern to managers because
they are challenging to suppress and are capable of causing widespread
mortality in stands.

The importance of thinning was illustrated
by the recent Wallow fire in Arizona, which burned more than 538,000 acres. A
system of fuel treatments developed cooperatively by federal, state, and local
governments, as well as private citizens, successfully reduced fire behaviour
and allowed fire fighters to protect thousands of structures and, in many
places, halt the spread of fire.

“Most forest managers understand that dry
Western landscapes need to be heavily thinned to significantly reduce the
threat of crown fires, and our findings now give a sense of just how much
thinning is required,” says Morris Johnson, the study’s lead researcher and
fire ecologist based at the station’s Pacific Wildland Fire Sciences
Laboratory. “We found that thinning at this level reduced tree density, raised
the canopy base height, and reduced canopy density.”

To test how effective fuel treatments in
the Western states are in reducing the probability and severity of crown fires,
Johnson, along with University of Washington researcher Maureen Kennedy and
station research biologist David L. Peterson, used the Fire and Fuels Extension
of the Forest Vegetation Simulator to simulate the effects of four types of
thinning and surface fuel treatments in dry forest types in 11 Western states.
By putting information on weather and fuel conditions into the simulator—which
is the standard computer model used by most federal, state, and tribal agencies—they
generated simulations in 45,162 forest stands that depicted crown fire hazard
and potential fire behaviour based on thinning densities leaving 300, 200, 100,
and 50 trees/acre. The simulations suggested that the effectiveness of
fuel treatments in the West depends on thinning intensity, with the most
intense treatments studied, which leave 50 to 100 trees/acre, being more
effective in reducing the threat of crown fires than less-intense treatments.
Thinning to this level, along with the removal of post-treatment residual
slash, made conditions unfavourable for crown fire initiation and reduced the
probability of active crown fire.

Findings from the study, in addition to
confirming the importance of thinning treatments, can help managers to evaluate
the effectiveness of their fuel treatments.

To read the article online, visit www.treesearch.fs.fed.us/pubs/38390.

To learn more about the Pacific Wildland
Fire Sciences Laboratory, visit www.fs.fed.us/pnw/pwfsl.