Don't believe false assumptions on thinning forests

George Wuerthner /


Recently The Bulletin had an editorial chiding Oregon Wild and Cascadia Wildlands for opposing the D-Bug timber sale near Diamond Lake. Fortunately, these groups are challenging a bad timber sale that will likely fail to achieve its goals of fire-risk reduction, will entail unnecessary environmental damage and result in a net financial loss to tax payers.

The most important factor to understand is that not all forest species have the same fire regimes. Lodgepole pine is the dominant tree in the D-Bug timber sale area. Lodgepole pine has a reputation as “asbestos forests” because getting them to burn is so difficult. You could take a flame thrower to these forests in most years and fail to get a significant blaze. Fire suppression and fuels do not play an important role in determining when, and even if, lodgepole pine forests will burn — a fact well established in the fire ecology literature.

Wildfires in lodgepole forests tend to occur when low humidity, extended drought, and most importantly, high winds all happen at the same time in the same place with an ignition source like a lightning strike. However, since the combination of these factors is extremely rare for any particular lodgepole pine stand, large blazes typically are spaced hundreds of years apart.

A further false assumption is that dead trees from beetle-kill significantly increase fire probability. It is only for the first years following a beetle outbreak when trees have red needles that flammability may be increased — and even this conclusion has been disputed by some research and again depends on weather conditions.

Dead trees are surprisingly resistant to burning. Once the needles and small branches fall off of beetle-killed trees — typically within a few years — they are not very prone to ignition. Despite all the dead trees in previously burned forests or beetle-kill lodgepole stands, under most conditions, a past fire or beetle kill actually halts or slows the advance of any new fires — precisely because dead trees don’t burn readily.

It may seem counter-intuitive, but green trees are more likely to burn under extreme fire conditions than dead trees. It is fine fuel (needles, small branches, etc.), not large logs that carries most fires. Any examination of forest fires around the West confirms this — most fires burn through green forests.

There are good reasons for this observation. Under extreme fire conditions, live trees can become extremely dry and highly flammable. Internal moisture content of live trees can sometimes even approach 1 to 2 percent — in Yellowstone National Park in 1988, for example, tree moisture content was 1 percent. By contrast, kiln-dried lumber often has a 10-12 percent moisture content. Plus green trees contain flammable resins. Thus green trees, with low moisture content and an abundance of needles and branches — the fine fuel that drives blazes, are highly combustible.

The science on thinning effectiveness is inconclusive. Indeed, in some instances, thinning can increase fire spread because it opens forests to greater drying and greater wind penetration — both factors that increase fire spread. Strategic thinning in the immediate vicinity of a home may help reduce fire risk, however, the advisable distance is usually only 100-200 feet. Any further distance results in little additional benefit in terms of home protection.

The most effective way to diminish fire risk to homes is to reduce the flammability of structures. Metal roofs, removal of flammable materials around homes like firewood, screened vents on attics and other measures will go a long ways towards ensuring that if, in the very small probability that a fire were to occur in this area, the homes surrounding Diamond Lake would survive the blaze.

Rather than chiding environment groups for challenging bad timber proposals, we can be thankful that these watchdog groups are there to prevent government waste and environmental damage.