A simulation study of more than 45,000 forest stands has provided the scientific basis for fuel reduction guidelines in the western United States. In the largest ever study of fuel treatment effectiveness, U.S. Forest Service researchers have found that intense thinning treatments that leave between 50 and 100 trees per acre are the most effective in reducing the probability of crown fires in the dry forests of the west. Thinning to this level, along with the removal of post-treatment debris known as “slash,” made conditions unfavorable for crown fire initiation and reduced the probability of active crown fire.
Decades of excluding fire in the western states have resulted in densely packed stands and a buildup 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. This year, Arizona and New Mexico have already experienced the worst fires in the states’ histories.
The study provides a scientific basis for establishing quantitative guidelines for reducing stand densities and surface fuels. The total number of optimal trees per acre on any given forest will depend on species, terrain and several other factors.
“This study proves once again that an ounce of prevention equals a pound of cure,” said 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.”