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Forest fires

Forest fires are a natural part of North America's landscape and play an important role in maintaining and regenerating some types of forests (NIFC 2000). Wild fires ignited by lightning are useful for clearing out old and dead trees which are then quickly replaced by robust new trees (CCFM 2000). Such fires open spaces for new seedlings, help increase diversity, clear debris and increase the availability of nutrients (Jardine 1994).

Since the 1970s, the annual area burned by forest fires has grown (see figure). The increase has been due to a number of factors: fuel build-up from past fire protection programmes; changes in policy related to prescribed burning; and increased public access to forests. Climate change has also been implicated. The relative importance of these factors is controversial.

The United States has long had an aggressive policy of fire suppression and, by the 1970s, fires were kept to about 2 million ha a year in the lower 48 states compared to the 16 million ha burned every year in the 1930s (Booth 2000, CEQ 2000, H. John Heinz III Center 2001).

Forest area burnt (million ha/year): North America

Since forest authorities decided to let more natural fires burn themselves out, the area of forest burnt each year has been increasing

Source: CCFM 2000, CIFCC 2001 and NIFC 2000

As a result, species normally eliminated by fire became dominant. Dead trees accumulated during periods of drought, creating excessive fuel loads. Fire suppression prevented natural low-intensity fires from burning this accumulated fuel. The result was increasingly large and disastrous fires (CEQ 2000).

The importance of periodic natural fires began to be recognized in the 1970s. US policies of suppressing all fires before they reached 4 ha in size by 10 am the next day ceased in the late 1970s (Gorte 1996). Decisions were taken not to interfere with fires in wilderness areas or national parks unless people or neighbouring land was threatened (COTF 2000, Turner 2001). In addition, prescribed burning and 'let burn' policies to reduce built-up fuels and protect settlements and businesses were introduced. Such fires are either purposefully lit or are lightning fires that are allowed to burn. Annually, more than 2 million ha are treated by prescribed fire in the United States (Mutch 1997).

These policies have not been without controversy, however. In 1988, parts of Yellowstone - the largest National Park in the United States - were allowed to burn after being struck by lightning. The fires spread quickly because of severe summer drought and high winds. Eventually a decision was made to suppress the fires. At the cost of US$120 million, this was the costliest fire-fighting event in US history (NPS 2000).

The challenge of managing wildfires has been exacerbated by population increases close to fireprone areas. It is estimated that in the 1990s wildfires damaged six times as many homes as in the previous decade (Morrison and others 2000). Wildfires also create smoke hazards and some highways, airports and recreation areas periodically have to close because of reduced visibility. Smoke also constitutes a health hazard, due to the toxic chemicals it contains.

Changes in climate that may bring drier conditions and more severe storms may also play a role in changing fire patterns. In 1989, for example, record fires burned in western Canada and the areas east of James Bay. They were caused by unusual weather conditions and an unprecedented heat wave in the Arctic (Jardine 1994, Flannigan and others 2000). The severity of Canada's 1995 fire season, which burned 6.6 million ha of forestland, was also due in part to extremely dry conditions (EC 1999b).

In the future, North America's annual fire severity rating may well increase due to climate change, which is predicted to increase the number of lightning strikes and the intensity and frequency of windstorms (Jardine 1994). Research into the links between climate and forest change is being intensified.