Canadian Forest Service Publications

The potential and realized spread of wildfires across Canada. 2014. Wang, X.; Parisien, M.-A.; Flannigan, M.D.; Parks, S.A.; Anderson, K.R.; Little, J.M.; Taylor, S.W. Global Change Biology 20(8):2518-2530.

Year: 2014

Issued by: Northern Forestry Centre

Catalog ID: 35498

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1111/gcb.12590

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Given that they can burn for weeks or months, wildfires in temperate and boreal forests may become immense (eg., 100 – 104 km2). However, during the period within which a large fire is ‘active’, not all days experience weather that is conducive to fire spread; indeed most of the spread occurs on a small proportion (e.g., 1 – 15 days) of not necessarily consecutive days during the active period. This study examines and compares the Canada-wide patterns in fire conducive weather (‘potential’ spread) and the spread that occurs on the ground (‘realized’ spread). Results show substantial variability in distributions of potential and realized spread days across Canada. Both potential and realized spread are higher in western than in eastern Canada; however, whereas potential spread generally decreases from south to north, there is no such pattern with realized spread. The realized-to-potential fire-spread ratio is considerably higher in northern Canada than in the south, indicating that proportionally more fire-conducive days translate into fire progression. An exploration of environmental correlates to spread show that there may be a few factors compensating for the lower potential spread in northern Canada: a greater proportion of coniferous (i.e., more flammable) vegetation, lesser human impacts (i.e., less fragmented landscapes), sufficient fire ignitions, and intense droughts. Because a linear relationship exists between the frequency distributions of potential spread days and realized spread days in a fire zone, it is possible to obtain one from the other using a simple conversion factor. Our methodology thus provides a means to estimate realized fire spread from weather-based data in regions where fire databases are poor, which may improve our ability to predict future fire activity.

Plain Language Summary

This research looked at why fires spread. Fires need hot, dry, windy weather to spread significantly beyond their current burn area. But they do not always spread in these conditions, so there are other factors at play. The researchers looked at daily weather data and fire progression maps for the entire forested landmass of Canada over three decades. They found differences across the country in how many days of conducive weather translated into wildfire spread. In western Canada there are more days conducive to spread and more days of actual spread than in eastern Canada. As you move from south to north, there are fewer days conducive to fire spread, but more of those days result in actual spread. Other factors affecting actual spread are the proportion of coniferous (evergreen) trees (which burn more easily than deciduous trees), the human impacts on the landscape (which can decrease spread by breaking up forest areas), and intense droughts. They found that each “fire zone” area of Canada has a distinct relationship between days conducive to spread and days of actual spread, so that the likelihood of fire spread can be predicted from the daily weather. This provides a way to estimate fire spread in areas where fire databases are poor, which may improve our ability to predict fire activity.