13 Managing Fire in Forests
Smoky Bear says, “Only YOU can prevent Forest Fires!” But, is Smoky correct? Should you prevent forest fires? Let them burn? Prescribe fire for your forest? The debate over control of fire in forests is, pardon the pun, indeed hot. Fires have occurred in many forest ecosystems for millennia, usually started by lightning. Humans have used fire in forests for millennia as well, to encourage berry and other fruit and vegetable production, to move game animals so that they were easier to kill, and presumably to make movement through forests easier. Now more and more people are building their homes in forests, each wanting to live in a place where they can enjoy the solitude and beauty of forests around them each day, so fire becomes a very significant risk to life and to homes. As our population grows and our economy grows, the expansion of the urban forest fringe increases. We need merely look to examples in Oakland, California; Sydney, New South Wales, Australia; and the Front Range of the Rocky Mountains in Colorado to see examples of fires that have led to human death and property destruction. So, is Smoky correct? Should we prevent forest fires? Perhaps in some places, but forests have burned and will burn again when there is sufficient fuel, the weather is correct, and there is a source of ignition. Why? Despite Smoky’s best efforts the forest area burned in the United States has increased steadily over the years (Stephens and Ruth 2005).
There are several conditions that interact to increase not only the probability of a fire occurring but also the size of a fire once it starts. First, conditions that cause lightning vary from year to year, but on average probably have not changed significantly over the past 60 years (Stephens 2005). As indicated earlier, the number of people living in and around forests has increased and human-caused ignitions, either by arson or by accidents have increased substantially in the western and southeastern United States (Stephens 2005). Second, because Smoky has been somewhat effective in keeping fires out of many forests, fuels have begun to build up in some forest systems. Without periodic fire in many dry forests, tree densities increase, some trees experience greater moisture stress due to competition for water, and we can see tree death through competition mortality or through insect outbreaks related to tree stress. All of these activities add fuel to the forest, so that when conditions are dry and an ignition source is provided (lightning, a match, a spark from a chainsaw), then the fire can spread rapidly (especially if there is any wind) and extend over huge areas very quickly. In addition, because the fuel loads are now often higher than they have ever been historically, the severity, or effect on the ecosystem, of the fires can be unusually high (Miller et al. 2009). The best conditions for fire spread are hot, dry, windy weather. As climate change produces more variable and extreme weather patterns, we can expect to see more extreme conditions that support the spread of fire in some (but not all) forests. Indeed, Westerling et al. (2006) found that there is a relationship between earlier and warmer spring and summer temperatures, and the frequency of large wildfires. So what can we do? Fuel loads have increased in many places, climate is changing, more people are providing more sources of ignition, and epidemics of insects are killing more and more trees. We cannot let fires burn uncontrolled everywhere, but we cannot control many wildfires under current conditions once they begin. Given the uncertainty associated with what future conditions might be, strategically located active management of some forests to increase their resistance or resilience to fire may be our only reasonable approach to managing this situation (Millar et al. 2007).