Long-term decreasing trend in forest fires in northwestern Canada

Tuomo H Wallenius,Philip J Burton,Juho Pennanen

doi:10.1890/es11-00055.1

Abstract

The annual area of forest burned has decreased in recent centuries over large areas of Fennoscandia, Siberia and temperate North America. To determine if this same trend extends to a sparsely populated region of northern Canada, fire scars on living and dead trees, forest stand ages and charred wood were systematically sampled in 85 study plots in an area of 564 000 km2 in northwestern Canada. A significant negative trend in the occurrence of forest fires was observed: average area burned per year decreased from 2.0% in the first half of the 19th century to 0.33% in the later half of the 20th century. Annually burned areas correlated significantly with a local tree ring based index, July monthly drought code and the Pacific decadal oscillation but not with June-August mean temperature, distance to the nearest road, or the year of road building. None of the climatic indicators or access history (indicative of the start of local fire suppression) could explain the long-term negative trend in fires. Earlier interpretations that humans dominated the causes of forest fires in the past, even in sparsely populated regions, deserve further attention as a possible explanation for the decreasing trend in fires.

Highlights

The ecological literature typically describes the North American boreal forest as a biome that frequently burns in natural lightning ignited forest fires (Rowe and Scotter 1973, Johnson 1992, Payette 1992)
Average annually burned area decreased from 2.0% in the first half of the 19th century to 0.33% in the later half of the 20th century, corresponding to fire cycles of 50 and 300 years, respectively
In the 19th century average annually burned proportion of the landscape varied between 3% and 1% in different decades, corresponding to fire cycles of 30 to 100 years (Fig. 3); over the entire 19th century, the average fire cycle was approximately 60 years

Summary

Introduction

The ecological literature typically describes the North American boreal forest as a biome that frequently burns in natural lightning ignited forest fires (Rowe and Scotter 1973, Johnson 1992, Payette 1992). The majority of fire history studies in North America have been conducted either in the southern part of boreal zone (Weir et al 2000, Bergeron et al 2001) or in more temperate coniferous forests (Heinselman 1973, Heyerdahl et al 2001). Except for the research based on fire statistics and satellite images spanning the last few decades (Stocks et al 2002, Kasischke and Turetsky 2006), relatively few large-scale fire history studies (Yarie 1981, Larsen 1997) have been conducted in northern boreal North America or in ‘‘the true boreal’’ as defined by Brandt (2009). It has been suggested that as a consequence of climatic warming, fires have (and will continue to) become more frequent

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