Increasing concern over wildfires has prompted a focus by state and federal fire-fighting agencies to stop this trend. For many decades the emphasis has been on fuel modification with success on certain landscapes but limited improvement on others. Since humans are a dominant ignition source over the majority of North America there is reason to believe improvements in fire prevention may be a key. Indeed, the USFS has been reporting fire causes since it began collecting systematic data on fires in 1905, with the explicit purpose of helping land managers design fire prevention programs. This study contrasts fire ignition patterns in five homogenous climate divisions in California over the past 98 years on state Cal Fire protected lands and 107 years on federal USFS lands.
Results/Conclusions
Throughout the state, fire frequency has increased steadily since the early 1900s until a peak around 1980, followed by a marked drop to the present. There was not a tight link between frequency of ignition sources and area burned by those sources and the relationships changed over time. Natural lightning-ignited fires decreased from north to south and from high to low elevation, comprising >70% on USFS forests in the northeast to <1% on coastal Cal Fire lands.
Frequency of human caused fires was positively correlated with population density for the first 2/3 of the record but this relationship reversed in recent decades. We propose a mechanistic multi-variate model of factors driving fire frequency, where the importance of different factors changes over time. Most human caused ignition sources have declined markedly in recent decades with one notable exception, powerline ignitions. Powerlines have been reported ignition sources since 1905 but as human infrastructure expanded into wildland landscapes the probability of powerline ignitions has risen. These typically occur during high wind events and are capable of rapid spread over long distances. One important avenue for future fire hazard reduction will be consideration of solutions to reduce this source of dangerous fires.