The historical fire regimes discussed above are a function of the underlying biophysical environment, natural ignitions, and burning patterns of Native Americans prior to European settlement for hunting, gathering, and agricultural purposes. Present day regimes are also strongly affected by the biophysical influences of vegetation, climate, and natural ignitions, but the human footprint and its effect on fire regimes is radically different than before. For simplicity, one can broadly divide wildland fire into two principal regimes—natural and human-driven. In the natural regime, fire occurrence and extent is primarily driven by environmental variables including vegetation and weather, and natural ignition sources (primarily lightning). The human-driven regime reflects the primary influence of human-caused ignitions and the influence of suppression activities. Much like historical fire regimes, the present-day effects of humans and nature cannot be spatially disaggregated cleanly. That is, both operate within the same geographical landscape. At any particular point on a landscape (or point in time), one or the other may be dominant but not exclusive. The implications of the differences between human and natural causes are clearly important to the concept of designing management options to affect ignitions.
The difference between the natural and human-driven regimes can be illustrated by looking at seasonal patterns of wildfire occurrence and the area burned by fires of different causes. Figure 3.11 depicts the bi-weekly pattern of fire occurrence attributed to three different causes: accidental, incendiary, and natural, compiled from a combination of Federal, state, and local data sets. The most commonly reported causes are accidental, which include debris burning, fireworks, equipment, campfires, and others. Incendiary fires include malicious arson events or other incidents where fires were set intentionally using incendiary devices. Figure 3.11 also indicates the close agreement in time between accidental and incendiary ignitions. In contrast, natural ignitions have a very strong and consistent seasonal pattern that rises in the spring, peaks in the summer, and declines in the fall. The seasonal pattern in area burned as a result of these different causes displays an interesting periodicity in which the area burned due to natural ignitions exceeds that from other causes through late spring and summer (figure 3.12).
Clearly, human ignitions are the predominant cause of wildfires throughout the United States. In the conterminous 48 states, more reported incidents began with human-caused ignitions than from natural ignitions in 98 percent of the counties. The area burned from these human-caused fires exceeds that from natural ignitions in 94 percent of the counties. Only in more remote counties of the West is the pattern reversed.
Programs that target the prevention of human-caused ignitions have the potential to substantively affect wildfire occurrence and extent in essentially every county. There is a need to support fire prevention educational efforts as well as to develop adequate and enforceable state and local ordinances related to wildfire prevention. Examples of the latter include burn permitting systems and enhanced law enforcement efforts focused on fire. There is clear evidence that small investments in fire prevention help reduce the high cost of fire suppression, as well as associated wildfire damages. Such programs are most effective when they focus on the underlying causes of these human-caused ignitions in each location, and tailor the prevention programs to specific causal factors and community dynamics.