Schoennagal, T., T.T. Veblen, and W.H. Romme. 2004. The interaction of fire, fuels and climate across Rocky Mountain forests. Bioscience 54(7):661-676
RELEVANT TO: |
FIRE |
TREE CUTTING |
|
SPECIES |
DESCRIPTION
This article discusses the relative influence of fuels and climate on wildfires across three functional groups of forests of the Rocky Mountains (including Utah) defined according to historical variability of fire size, severity, and frequency: High severity fire forests, mixed severity fire forests, and low severity fire forests. For each of these types, current research is synthesized, lessons learned from recent large wildfires are summarized, and the ecological appropriateness and potential effectiveness of fuels reduction treatments is discussed.
MAJOR FINDINGS
- In high-severity fire forests (spruce-fir, spruce-fir-Douglas fir, lodgepole pine PNV groups):
- Infrequent, high-severity, stand-replacing fires dominate the historical and contemporary fire regime in these forests.
- Climatic variation, through its effects on the moisture content of live fuels and larger dead fuels, is the predominant influence on fire frequency and severity.
- Dense trees and abundant ladder fuels are natural in subalpine forests and do not represent abnormal fuel accumulations.
- Fire suppression has had minimal influence on the size, severity, and frequency of high-elevation fires.
- Mechanical fuel reduction in subalpine forests would not represent a restoration treatment but rather a departure from the natural range of variability in stand structure.
- Given the behavior of fire in Yellowstone in 1988, fuel reduction projects probably will not substantially reduce the frequency, size, or severity of wildfires under extreme weather conditions.
- In mixed-severity fire forests ( ponderosa pine-Douglas fir, Douglas fir, grand fir -- Douglas fir, and Southwest mixed conifer PNV groups):
- The historical fire regime in these forests is complex, including both low-severity surface fires and infrequent high-severity crown fires.
- Both fuels and climate have major influences on the frequency, severity, and size of fires.
- Fire suppression has had variable effects on fuel densities in mixed-severity fire regimes, with the greatest impacts on sites that formerly supported open woodlands.
- The occurrence of high-severity crown fires is not outside the historical range of variability, although their size and frequency may be increasing.
- Extreme climate and weather conditions can override the influence of stand structure and fuels on fire behavior.
- Fuel-reduction treatments (mechanical thinning and prescribed burning) may effectively reduce fire severity under moderate weather conditions, but these treatments may not effectively mitigate fire behavior under extreme weather conditions and may not restore the natural complexity of historical stand and landscape structure.
- In low severity fire regime forests (ponderosa pine):
- The historical fire regime in these forests was characterized by frequent, low-severity surface fires.
- Historically, the frequency, size, and severity of fires were largely controlled by spatial and temporal variation in fine fuels.
- Fire suppression has significantly increased tree densities and ladder fuels in low-elevation ponderosa pine forests.
- As a consequence of this change in stand structure, unprecedented high-severity fires now occur.
- Fuel-reduction treatments involving mechanical thinning and prescribed fire are likely to be effective in mitigating extreme fire behavior and restoring this forest type to the historical fire regime.
- A "one size fits all" approach to reducing wildfire hazards in the Rocky Mountain region is unlikely to be effective and may cause ecological damage in some cases.
QUESTIONS RAISED FOR THE THREE FORESTS
- How do the three Forests plan to manage in order to minimize anticipated impacts of global warming and increasingly frequent and severe fire regimes?
RELEVANCE TO FOREST MANAGEMENT
- In high-severity fire forests, fire suppression has had minimal influence on the size, severity, and frequency of high-elevation fires. Mechanical fuel reduction in subalpine forests would not represent a restoration treatment but rather a departure from the natural range of variability in stand structure.
- In mixed-severity fire forests, fuel-reduction treatments (mechanical thinning and prescribed burning) may effectively reduce fire severity under moderate weather conditions, but these treatments may not effectively mitigate fire behavior under extreme weather conditions and may not restore the natural complexity of historical stand and landscape structure.
- In low severity fire forests---dry ponderosa forests---fire suppression has significantly increased tree densities and ladder fuels in low-elevation ponderosa pine forests. As a consequence of this change in stand structure, unprecedented high-severity fires now occur. Fuel-reduction treatments involving mechanical thinning and prescribed fire are likely to be effective in mitigating extreme fire behavior and restoring this forest type to the historical fire regime.