Vegetation and Fuels

Wildland fire from both natural and human causes has played a prominent role in shaping the landscapes of North America for millennia. There is an extensive collection of literature on the ecological role of fire in North American ecosystems and widespread understanding of the historical role that human settlement patterns have had in changing the frequency, extent, and location of fire. One universally accepted point is that nearly all of the natural vegetation communities across North America historically burned—many quite frequently. The intensity with which they burned was a function of both the biophysical environment (climate, topography, and vegetation) and the frequency of ignition, both natural and human-caused.

In general, more frequent burning is associated with less intense or severe wildfires. Conversely, infrequent burning generally leads to higher severity fires that consume much of the aboveground live and dead vegetation—the principal fuels in a wildfire.Table 3.2. Fire regime groups and descriptions.
Table 3.2
This pattern arises naturally from the accumulation of fuels between events, absent of any other disturbance or activity that reduces it. Ecologists use the concept of fire regime and fire regime groups (FRG) to characterize the relationship between fire frequency and fire severity and their ecological implications (table 3.2, from Barrett and others [2013] [2]).

Of note is the relatively high frequency of fires in FRGs I and II, which average 35 years or less between fire events and include many of the fire-adapted forest and rangeland types in the United States. Nearly half of the current undeveloped natural vegetation within the conterminous 48 states falls within lands that historically supported FRGs I and II (figure 3.2), totaling about 1.1 million square miles. If we presume that this area previously experienced a fire return interval of 35 years (the upper bound), Figure 3.2. Historical fire regime group values in areas currently dominated by natural vegetation.
Figure 3.2
then a lower-bound estimate of roughly 31,000 square miles (over 20 million acres) would have burned on average each year within these two FRG areas alone. Such estimates provide a sense of perspective when compared to the annual acres burned in the recent decade, 2002 through 2011. The best estimate of annual area burned in counties dominated by FRGs I and II within the conterminous 48 states is roughly 7,800 square miles, or one-fourth of the historical lower bound for this area. Another way of stating this is that the average time between wildfires has more than quadrupled across a significant portion of our Nation.

Increasing the time interval between fires means that many fires occurring today are of higher severity than they were historically. Substantive shifts in vegetation away from fire-adapted species are also occurring. Changes in fire return intervals are not limited to just FRGs I and II. A previous analysis suggested increased fire return intervals throughout the United States except for some areas of the Southwest and Great Basin, where invasive grasses have contributed to reduced fire intervals and radically changed vegetative structure and composition. A second significant observation is that many of the large fires that occur today disproportionately occur in areas that historically were FRGs IV and V. These include many areas where the natural fire regime is relatively infrequent, high-severity fires—the most difficult and expensive to control or extinguish.

The issue is not as severe in areas under active prescribed fire regimes, including southeastern pine forests and some western forests and grasslands. For example, a recent survey reported 7.9 million acres of prescribed fire activity for silvicultural purposes in 2011, 6.5 million acres of which occurred in the Southeast.[3] There also are areas within larger national parks, scattered wildlife preserves, and designated wilderness areas nationwide where natural fire regimes have been successfully reintroduced and maintained for decades.

Understanding these broad-scale changes in fire regimes is essential to crafting an effective national strategy. Fire regimes are intrinsically and fundamentally connected to fuel accumulation, vegetation composition, and subsequent fire behavior when wildfires inevitably occur. More extreme fire conditions can be expected in areas where the time between fires has been extended, unless fuels have been reduced by other means. Human development and suppression can postpone wildfires, but not exclude them, except in unusual circumstances.

Moreover, the confluence of climate factors and the fuel accumulations that result from sustained, vigorous suppression in some locations make exclusion increasingly difficult. The basic biophysical environment remains conducive to wildfire and is unlikely to change in a way that would mitigate wildfire occurrence.[4] Fuels do not simply disappear in the absence of wildfire in fire-adapted ecosystems. Either they accumulate and wait for the next fire to occur, slowly decompose, or some form of active fuels management such as prescribed fire or mechanical treatment is required. Conversely, in those ecosystems where fires have become more frequent, fuels management may be required to protect remaining unburned areas or to alter species composition or structure.

Historical perspective provides a benchmark for areas where returning natural vegetation to near-historical or desired conditions is a primary objective. However, a fundamental challenge in wildland fire management is that restoring historical conditions is neither practical nor desirable in many locations. The degree to which wildfires or fuels management can be tolerated within a given landscape depends upon community values and land management objectives.

Where fuels cannot be managed to match historical levels, then adjustments must be made within human communities to accommodate a new normal in fire occurrence and extent. For forested systems, this likely means a progressive transition from historical FRG I or III to a new FRG IV and less frequent, higher-intensity fires. Higher-intensity fires lead to higher suppression difficulty, increased risks to firefighter and public safety, and more severe social or ecological damage when they occur. Changes in rangeland and shrubland systems also can lead to increased, more continuous fire extent, often with greatly increased rates of spread, which also increase suppression difficulty and risk to firefighters. Additionally, changes in fire frequency can lead to an undesirable mix of new species that move into these systems (e.g., invasive grasses such as cheatgrass or encroachment by woody species such as juniper).

Opportunities Discussion

The primary purpose of hazardous fuels management is to reduce the extent, intensity, and severity of wildfire if and when it encounters a treatment area during the lifespan of the treatments.[5] To be effective, fuel treatments must reduce fireline intensities under the conditions most likely to result in harm. That is, they have to work across a range of weather conditions likely to occur during a wildfire. Depending on the ecosystem, reduced extent, intensity, and severity can have beneficial ecological effects. For example, wildfires burning less intensely may mimic historical fire effects more closely, helping to restore or enhance native, fire-adapted vegetation. In addition, less severe fires damage or kill fewer economically valuable trees and exhibit less soil erosion following fires. Strategically placed fuel treatments can have broader landscape effects that extend beyond the perimeter of the area physically treated, either through affecting fire behavior directly or by facilitating ecologically sensitive containment strategies. Such treatments can affect the spatial distribution of fires, leading to more desirable vegetation composition and structure, which reduces the potential for invasive species and can help preserve structure that is currently limited on the landscape (i.e., sagebrush).

Reduced intensity also means that suppression efforts are more likely to be effective and can be conducted more safely in areas where wildfires are unwanted or threaten communities. Fuel treatments near homes and communities also are an effective, proactive way of reducing the likelihood of structure ignition and enhancing the safety of firefighters and the public.

The three primary means of managing fuels are prescribed fire, managing wildfire for ecological purposes and resource objectives, and non-fire treatments involving mechanical, biological, or chemical methods. Treatments can occur in isolation or in combination, depending on management objectives and resource constraints.


[2] Barrett, S.; D. Havlina; J. Jones; W. Hann; C. Frame; D. Hamilton; K. Schon; T. Demeo; L. Hutter; and J. Menakis. 2010. Interagency Fire Regime Condition Class Guidebook. Version 3.0 [Homepage of the Interagency Fire Regime Condition Class website, USDA Forest Service, U.S. Department of the Interior, and The Nature Conservancy]. [Online], Available:

[3] Melvin, Mark A. 2012. 2012 National Prescribed Fire Use Survey Report. Technical Report 01-12. Coalition of Prescribed Fire Councils, Inc. 19 p. Available at

[4] Some northern hardwood forests may be the exception to this general rule. As human burning has decreased, compositional and structural changes within these forests have caused them to become more fire-resistant so they burn less frequently and less intensely.

[5] Here, vegetation treatments for the primary purpose of reducing hazard are distinguished from treatments that reduce vegetative fuels as a secondary benefit. For example, prescribed fire can be used for the primary benefit of promoting desirable vegetation in areas devoid of significant wildfire hazard (e.g., native rice fields). Many agricultural, silvicultural, and habitat enhancement practices have secondary fuels benefits, but are not conducted for that primary reason.