Success of fire suppression in northern forests

In North America, the belief that fire suppression has substantially reduced the average annual area burned is widely held by resource managers and is often thought to be self-evident. Direct empirical evidence however is essentially limited to just two studies by Stocks (1991) and Ward and Tithecott (1993), that use Ontario government fire records to make comparisons of average annual area burned between areas with and without aggressive fire suppression policies. Numerous subsequent studies have presented the same information, often in a different format (Martell 1994, Martell 1996, Weber & Stocks 1998, Li 2000, Ward & Mawdsley 2000). The proponents of these studies argue that areas without aggressive fire suppression policies have larger average fire sizes and greater average annual area burned and a longer interval between fires and that this is evidence of the effect of fire suppression.

However, the idea that fire suppression can effectively reduce the average annual area burned is the focus of a vocal debate in the scientific literature. In particular, several recent papers have argued against this idea (Miyanishi & Johnson 2001, Miyanishi "et al." 2002, Bridge et al 2005). These papers claim that statistically rigorous techniques for estimating the average annual area burned, called the fire cycle, do not show changes in the fire cycle associated with fire suppression and that the evidence used to support the effect of fire suppression is biased and has been presented in a way that is flawed. Note that none of these papers criticize fire management agencies for being anything less than completely committed to their mandate. Nor do they suggest that fire personnel are not well trained, efficiently deployed or well managed. Instead, these papers simply suggest that despite the resources employed, fire management agencies are simply unable to effectively reduce the average annual area burned.

The impact that effective fire suppression may have on the average annual area burned is important for many reasons, but in particular, its impact is key to the current paradigm of sustainable forest management in many jurisdictions. One of the core aspects of SFM in many jurisdictions is the use of wood supply models to determine sustainable harvest levels. This determination of sustainable harvest levels often assumes that fire suppression has been effective at reducing the average annual area burned. Thus, if current assumptions about the effect of fire suppression are wrong, the impact on SFM could be substantial.

Evidence that fire suppression has been effective

For the most part, studies that support the effects of fire suppression compare either the number of fires or the average fire size between areas with and without aggressive fire suppression policies. Typically, these studies use the same or similar data from provincial fire records for Ontario covering a span of about 20 years.

Proponents of these studies have argued that, firstly, fires are, on average, much larger in areas without aggressive fire suppression policies than in areas with aggressive fire suppression policies because these fires are allowed to spread freely. Secondly, the proponents have argued that far more lightning caused fires are detected in areas with aggressive fire suppression and yet, the average annual area burned is much higher in areas without aggressive fire suppression. It is implied that fire suppression must, therefore, be reducing the area burned by lightning fires.

Recently, Cumming (2005) used novel approaches to analyze multiple components of fire management activity in greater detail than previously done, and confirmed the effectiveness of fire suppression.

Evidence that fire suppression has not been effective

On the other side of the debate, the evidence that fire suppression has not been effective at reducing the average annual area burned has come primarily in two forms. Firstly, in the form of time-since-fire studies, which, it has been argued, do not show detectable changes in the fire cycle that can be associated with fire suppression. Secondly, in the form of criticism of the way that provincial fire records have been used to support the effect of fire suppression.

Numerous time-since fire studies have been done in the temperate, boreal and sub-alpine forests across Canada and the U.S. (reviewed in Johnson 1992, see also Bergeron & Archambault 1993, Johnson & Wowchuck 1993, Johnson "et al." 1995, Weir "et al." 2000). The techniques used in these studies are felt to be well founded in statistical theory (Johnson & Van Wagner 1985, Johnson & Gutsell 1994, Reed 1994, Reed et al 1998) and recent improvements offer a way to statistically compare different periods of time to see if they possess significantly different fire cycles (Reed 1994, Reed et al 1998). While these studies often show a change in the fire cycle at the beginning of the 20th century, this change is usually associated with large-scale climatic factors, such as the end of the little ice age (Johnson 1992, Bergeron & Archambault 1993, Weir "et al." 2000), and not fire suppression. In particular, around Ontario there have been at least six time-since-fire studies that show there has been no change in the fire cycle since 1920 (Heinselmann 1973, Woods & Day 1977, Suffling "et al." 1982, Bergeron 1991, Gauthier "et al." 2000, and Bridge "et al." 2005). In Ontario, active fire suppression activities began sometime in the late 1910's, but these suppression activities are generally thought to be minimal compared with post 1950 when fire suppression began in earnest and technological advances made fire fighting much more effective (OMNR 2002, Thompson 2000).

Comparisons of the average annual area burned between areas with and without aggressive fire suppression policies, it is argued, are biased by the fact that small fires are virtually unreported in areas without aggressive fire suppression policies, where detection often relies on reports from settlements or commercial aircraft. Critics have argued that the number of lightning caused fires in areas with and without aggressive fire suppression policies are in fact quite similar and that the smaller average fire size, and the lower proportion of fires in larger size classes in areas with aggressive fire suppression is clearly a consequence of this bias (Miyanishi & Johnson 2001, Miyanishi "et al." 2001).

Critics have also argued that despite suppression attempts the actual number of large fires in both areas is quite similar (Miyanishi & Johnson 2001, Miyanishi et al 2001). It has been argued that if fire suppression cannot impact the large fires, then it cannot impact the average annual area burned since almost all of the area is burned by only a few large fires.

Finally, studies that compare areas are also often based on averages of annual area burned made over periods of 12 to 17 years (Martell 1994, 1996, Ward & Tithecott 1993, Li 2000). Some have argued that this is too short a time period because the extreme year to year variation in area burned makes such averages highly variable and difficult to interpret (Johnson "et al." 1996, Weir "et al." 2000).

Reasons why fire suppression may not have affected the fire cycle

Several people (Weir "et al." 1995, Johnson "et al." 1995, 1998) have explored reasons why fire suppression may not have affected the fire cycle. In general, they feel that in closed-canopied forests, like the boreal, as little as 3% of the lightning caused fires account for up to 95% of the area burned (Stocks & Street 1993, Johnson & Wowchuck 1993). Most fires remain small, but a few occur under conditions that allow them to increase rapidly in size. It is this small proportion of large lightning caused fires which has the most influence on the area burned and the fire cycle.

In years with a large area burned, fires in these closed-canopied forests characteristically have high intensities, high rates of spread and high duff consumption. In these years, extreme fire behaviour is preceded by a persistent anomalous high pressure system which produces prolonged periods of above normal temperatures and below normal precipitation (Newark 1975, Harrington & Flannigan 1987), and leads to the severe drying of both medium and heavy fuels. Under these extreme conditions, fire behaviour exhibits little difference between aspect, elevation and vegetation type (Anderson 1968, Alexander "et al." 1983, Nimchuck 1983, Janz & Nimchuck 1983, Street 1985, Flannigan & Harrington 1988, Fryer & Johnson 1988). In years with only a small area burned, differences in aspect, slope, elevation and vegetation composition can have a significant effect on the fire behaviour (Alexander & McAlpine 1987, Johnson et al 1998), however, the area burned in these years is insignificant.

The extreme fire behaviour associated with persistent high pressure systems results in large areas burned. It has been argued that during these years, it is unlikely that fire suppression can significantly influence the total area burned because under these conditions fire management agencies are quickly overwhelmed (Weir "et al." 1995).

References

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