Managing Forest Fire and Fuels in a Warmer Climate
Photo: National Interagency Fire Center
By David L. Peterson
Excerpted by permission from Northwest Woodlands, Spring 2010, a publication of the World Forestry Center
Will a warmer climate affect wildfire in the northwestern United States? The simple answer is "yes," but those effects will be somewhat different in different places. A look at the historical record can improve our understanding of fire-climate interactions.
We know that more forest area burns during years when the weather is warm and dry than when it is cool and wet. Not only is this logical, it is borne out by historical data going back to the early 1900s.
Furthermore, we know that fire occurrence in the Northwest varies over time based on specific climatic phenomena. The EI Niño Southern Oscillation produces alternating patterns of warm, dry weather (El Niño) and cool, wet weather (La Niña) on a cycle of about three to seven years. Historical fire data indicate that more forest area does indeed burn during El Niño years and less during La Niña years. Historical fire data and fire histories also indicate that considerably more area bums during the warm phase of the 30-year Pacific Decadal Oscillation than during the cool phase.
This historical perspective on fire provides a window into the future of fire in the Pacific Northwest. Although fire will always be more common in the interior portion of the region, a warmer climate could bring more fire to the westside of the Cascade Range where summers are typically dry and will probably become drier.
Warmer climate means more wildfire
If future climate resembles the climate now experienced during the warm phase of the Pacific Decadal Oscillation, then we can expect … more area will burn on average. We can also expect longer fire seasons. The typical fire season of July and August will probably stretch into June and September, extending the time during which precautions are needed for fire prevention and fire suppression.
The biggest concern for the future will be an increase in extreme weather events, which can lead to conditions that produce large and rapidly spreading wildfire. Average warm conditions are not a big concern. High temperature, low humidity and high wind are the combination of factors that, when coupled with an ignition source and adequate fuels, can produce fires. These rare but extreme weather events will drive changes in fire that we will observe in the future.
Manage fuels to reduce fire severity
Forest landowners cannot do anything about … weather and topography. But they can do something about the third leg-fuels. Four principles define the scientific basis for reducing fire hazard by reducing specific fuel components of a forest. The objective of these principles is to reduce fire severity, not to eliminate fire from the forest. These principles were developed for dry interior forests (e.g., ponderosa pine, Douglas-fir, mixed conifer), but they can be applied to forests in wetter locations as well.
Manage for forest resilience
Fuel reduction activities are highly compatible with managing for resilience to climate change, where resilience is defined as the ability to return to previous structure and function. Density management enhances the vigor of individual trees, thus improving their resistance to fire damage, insect attack and fungal pathogens. This leads to forests that can experience many fires with low mortality of overstory trees.
Managing for resilience is also compatible with managing for sustainable timber production, habitat for deer and elk, good water quality and many other values. Managing for resilience does not necessarily change management plans or activities very much. If you are thinning forest stands, removing surface fuels and otherwise enhancing tree vigor, then you are already doing most of the things necessary to prepare for a warmer climate and more fire. You may want to thin to lower densities and remove more surface fuels than you would have done in the past in order to account for increased stress in a warmer climate.
A warmer climate will almost certainly increase the prevalence of wildfire in forests throughout the Northwest. However, it should be possible to prepare for these changing conditions by managing for resilience through science-based principles of fuels management. Active management that includes resilience to climate change as one of several possible objectives will give forest owners more options in the future, and timely reduction of hazardous fuels will provide insurance for sustainable forest management.
DAVID L. PETERSON is a research biologist with the US Forest Service Pacific Northwest Research Station, Seattle, and lives on a tree farm in Skagit County, WA. He can be reached at [email protected].
Excerpted by permission from Northwest Woodlands, Spring 2010, a publication of the World Forestry Center
Will a warmer climate affect wildfire in the northwestern United States? The simple answer is "yes," but those effects will be somewhat different in different places. A look at the historical record can improve our understanding of fire-climate interactions.
We know that more forest area burns during years when the weather is warm and dry than when it is cool and wet. Not only is this logical, it is borne out by historical data going back to the early 1900s.
Furthermore, we know that fire occurrence in the Northwest varies over time based on specific climatic phenomena. The EI Niño Southern Oscillation produces alternating patterns of warm, dry weather (El Niño) and cool, wet weather (La Niña) on a cycle of about three to seven years. Historical fire data indicate that more forest area does indeed burn during El Niño years and less during La Niña years. Historical fire data and fire histories also indicate that considerably more area bums during the warm phase of the 30-year Pacific Decadal Oscillation than during the cool phase.
This historical perspective on fire provides a window into the future of fire in the Pacific Northwest. Although fire will always be more common in the interior portion of the region, a warmer climate could bring more fire to the westside of the Cascade Range where summers are typically dry and will probably become drier.
Warmer climate means more wildfire
If future climate resembles the climate now experienced during the warm phase of the Pacific Decadal Oscillation, then we can expect … more area will burn on average. We can also expect longer fire seasons. The typical fire season of July and August will probably stretch into June and September, extending the time during which precautions are needed for fire prevention and fire suppression.
The biggest concern for the future will be an increase in extreme weather events, which can lead to conditions that produce large and rapidly spreading wildfire. Average warm conditions are not a big concern. High temperature, low humidity and high wind are the combination of factors that, when coupled with an ignition source and adequate fuels, can produce fires. These rare but extreme weather events will drive changes in fire that we will observe in the future.
Manage fuels to reduce fire severity
Forest landowners cannot do anything about … weather and topography. But they can do something about the third leg-fuels. Four principles define the scientific basis for reducing fire hazard by reducing specific fuel components of a forest. The objective of these principles is to reduce fire severity, not to eliminate fire from the forest. These principles were developed for dry interior forests (e.g., ponderosa pine, Douglas-fir, mixed conifer), but they can be applied to forests in wetter locations as well.
- First, remove surface fuels, especially the fine surface fuels, to reduce flammable material that will carry fire across the forest floor. This is especially important following timber harvest and thinning operations, which create even more fuels. Surface fuels can be removed by hand, with machines or through prescribed burning.
- Second, raise canopy base height to increase the distance that a surface flame would need to extend to reach the lower canopy. This is typically accomplished by removing the smaller trees to reduce ladder fuels, although ambitious landowners can increase canopy base height by pruning the lower branches.
- Third, decrease crown density to reduce the amount and continuity of living and dead fuels in the understory and upper tree canopy, thus reducing the capability of fire spread in aerial fuels. This is also accomplished by removing the smaller trees.
- Fourth, leave as many large trees as possible, because these trees, which have thicker bark and higher crowns, are the most resistant to fire. The desired tree density will vary by location, species and expected fire behavior, but will generally need to be less than 100 trees per acre to effectively reduce crown continuity.
Manage for forest resilience
Fuel reduction activities are highly compatible with managing for resilience to climate change, where resilience is defined as the ability to return to previous structure and function. Density management enhances the vigor of individual trees, thus improving their resistance to fire damage, insect attack and fungal pathogens. This leads to forests that can experience many fires with low mortality of overstory trees.
Managing for resilience is also compatible with managing for sustainable timber production, habitat for deer and elk, good water quality and many other values. Managing for resilience does not necessarily change management plans or activities very much. If you are thinning forest stands, removing surface fuels and otherwise enhancing tree vigor, then you are already doing most of the things necessary to prepare for a warmer climate and more fire. You may want to thin to lower densities and remove more surface fuels than you would have done in the past in order to account for increased stress in a warmer climate.
A warmer climate will almost certainly increase the prevalence of wildfire in forests throughout the Northwest. However, it should be possible to prepare for these changing conditions by managing for resilience through science-based principles of fuels management. Active management that includes resilience to climate change as one of several possible objectives will give forest owners more options in the future, and timely reduction of hazardous fuels will provide insurance for sustainable forest management.
DAVID L. PETERSON is a research biologist with the US Forest Service Pacific Northwest Research Station, Seattle, and lives on a tree farm in Skagit County, WA. He can be reached at [email protected].