By Tom Wheeler
Fire severity—how impactful a fire is to the natural environment—escapes easy explanation. There is no one thing that determines fire severity, rather a variety of factors influence it. But looking across millions of acres of burned land in the West, one thing bears true: fires burned with less intensity on lands that had the highest protections from logging. Why is this?
Wildland fire behavior is sometimes explained using the fire behavior triangle, with fuels, weather/climate and topography all interacting with each other to influence how fast and how hot a fire will burn. Logging affects two sides of this triangle: fuels and weather/climate.
Logging obviously removes some “fuels” as trees and brush are combustible. But this simple truth masks some of the important complexity. How fuels are distributed within an area, both vertically and horizontally, together with their type (fast burning “fine” fuels, like leaves or twigs vs. long-burning “heavy fuels, like logs, which may smolder longer but don’t typically flare up) and moisture all impact fire behavior. While logging removes some fuels, it typically removes the heavy fuels while redistributing finer fuels—the logging “slash” of limbs, tops, branches and needles—from standing trees to the ground. This can leave “jackpots” of fuels that can cause sudden and aggressive flare ups.
Clearcut logging introduces yet another issue: uniform crown structure. As clearcutting produces even-aged forests, the tops or “crowns” of these trees will be approximately the same height. And given the high density of trees typically replanted after a clearcut, these are going to produce an interlocking, dense crown. If fire jumps to the crown, the fuel continuity provided by this uniform crown structure can quickly cause fast-moving and highly impactful fires.
Logging also impacts the likelihood of tree survival. Large, old trees generally are more resilient to fire. Their thick bark helps insulate the trunk from damage and tall branches make fires jumping from the forest floor to the crown more difficult. Of course, logging can both remove big resilient trees and make their future development more difficult, as they are logged before they can reach a nice, ripe age.
Weather and climate are also extremely important to understanding fire behavior. Logging affects both weather and climate. Cutting shade-producing trees can both increase in-forest temperatures and reduce moisture in the forest, producing a parched microclimate. Thinning trees increases in-forest wind speed, which can both create a drier forest through enhanced evaporation rates as well as fanning the flames during a fire to produce a greater intensity.
While it is clear that logging can impact site-specific conditions that intensify fire behavior, logging also is a major source of greenhouse gas emissions. Climate change serves to exacerbate wildfires by broadly changing weather and climate conditions. Logging releases carbon that was once locked up in a forest. While some carbon is stored in forest products, like paper or lumber, the majority of carbon in a tree becomes susceptible to release as greenhouse gases soon after that tree is logged. Only a small percentage of a tree ever becomes a stable forest product; the majority of the rest is typically burned or decomposes along the way, either in slash piles in the forest or as “mill waste” burned to produce heat or energy. By taking carbon that was safely sequestered for many decades and releasing it suddenly in a quick pulse, logging significantly contributes to greenhouse gas emissions—so much so that researchers now report it is the largest source of greenhouse gas emissions in Oregon (and presumably is the largest in the timber-producing areas of California as well).