Forest fire prevention forces hard choices
We need to alter our approach as our climate changes, Robert Gray writes.
The area burned by wildfires this summer in B.C. is enormous by any measure. Over 1.2 million hectares as of Aug. 22, and there are still several weeks of fire season remaining. This wildfire season adds to an ever-increasing area burned over the past decade. Will we ever see an end to summers with this much area on fire? According to climate scientists, not likely. Climate predictions include longer fire seasons, more significant droughts (meaning more dead trees and thus more fuel for fires), more fire starts, and larger and more severe fires.
A reasonable question is whether or not at some point enough area will be burned that it will actually start to impede fire activity. The answer depends on future climate and what we as society do with regard to vegetation and dead fuels. There is not much we can do about the climate predictions — unfortunately, we’re locked into this climate pattern for the foreseeable future. The only place we can make a difference is with what is available to burn when fire returns the next time.
Within a few short years of a high-severity fire (one that kills the majority of the forest), the dead trees start to fall. With increased sunlight hitting the forest floor, grasses, forbs and shrubs take hold and create a continuous layer of fine, flashy fuel capable of quickly spreading a fire once lit.
If another fire occurs in quick succession, it is referred to as a re-burn. This subsequent fire tends to spread slowly but can burn very intensely as it consumes the downed trees and often kills regenerating trees. Once again, grasses, forbs and shrubs return, and once they are fairly continuous, can support another fire. If most of the downed wood was consumed in the re-burn, the next fire is not likely to grow very large or to burn very intensely.
Fires burning in grass, forbs and shrubs are highly susceptible to increases in relative humidity and are quickly extinguished during a typical summer’s night increase in humidity or are halted by abrupt changes in topography or fuel breaks. To survive, they need to smoulder in a remaining log or stump close to the burning perimeter of a fire. The next day, as the temperature rises and humidity falls, the smouldering fire moves out of the log and into unburned areas of cured grass, allowing the fire to spread for another day. If there are no logs or stumps for smouldering, the fire burns out.
Historically, when much of the west was burning frequently, many fires burned in fine, flashy fuels including grass, forbs and shrubs, and even small changes in topography and fuels were effective barriers to fire spread. Large fires were relatively infrequent. So how do we emulate that process?
This is where there are very difficult compromises that need to be made. Traditionally, our approach to post-burn landscapes is to salvage any remaining economic value from burned stands, often leaving logging slash and replanting the site. This pattern simply perpetuates the problem. Downed logs offer important wildlife habitat, and forests that quickly regenerate following fire contain valuable timber. However, if we are on the cusp of our new normal, we may have to emulate the re-burn model over large areas. This means heavily thinning recently burned areas (leaving some dead standing trees and few logs), prescribed burning them and replanting to very low density. It will be difficult to embrace the re-burn model, but we need to carefully consider the benefits.