Natural Disturbances

Photo: fire © iStock.com/cfarish
Forest fire

Fire

Fire plays an essential role in ecosystems, cycling nutrients, influencing species composition and age structure, maintaining productivity and habitat diversity, influencing insects and disease, and influencing the carbon flux. Due to the ecological influence of fire, patterns of past fires have shaped the forest of today. Changes in fire dynamics affect fire patterns (size, frequency, seasonality, severity, or type) and can result in significant changes to ecosystems.

Area burned by large fires
Map and graph: area burned by large fires. Click for graphic description (new window).
Source: adapted from Krezek-Hanes et al., 20102 Data for 1959 to 1994 from Large Fire Database, in Stokes et al., 20031 and for 1995 to 2009 from the Canada Centre for Remote Sensing

Large fires (greater than 2 km2) make up only 3% of all fires but account for 97% of the total area burned.1 Over 90% of large fires occur in the boreal forest,2 where extreme fire weather conditions are common and suppression efforts are lower.1, 3, 4 Fire occurrence varies across years and across regions and is influenced by weather, climate, fuels, topography, and humans.4-6 Between 1959 and 2009, the total annual area burned ranged from 1,500 km2 to 75,000 km2.2

Although a long-term decline in frequency and area burned by large fires is evident since the 1850s, particularly in eastern Canada,7-10 annual area burned increased overall from the 1960s to 1980s/1990s. This has been attributed to greater forest use by humans, better fire detection, and increased temperatures over the last 40 years.1, 3, 11, 12 The short-term decline from 2000 to 2009 may be the result of other climatic factors such as large-scale ocean circulation patterns from the North Pacific Ocean which entered a cool phase in the mid-1990s.5, 8, 13, 14 Fire activity is most strongly linked to temperature3,6, 15 and as temperature increases, so should fire activity.

Seasonality

The fire season runs from April to mid-October.2 The time of year that fires occur can affect forest regeneration capacity and intensity.16 Humans cause approximately 65% of fires (large and small) in Canada; however, with most fires being smaller than 2 km2, human-caused fires represented only 15% of the total area burned from 1959 to 1997.1, 17 These fires occurred mainly in the spring and close to human settlements. The majority of boreal and taiga fires are caused by lightning and tend to occur later in the fire season.1, 5, 18 These are often more severe because the fuel is dry, producing fires of great severity and intensity, and they are less likely to be suppressed.19 Evidence from other countries, such as the western United States, indicates a lengthened fire season with wildfires starting earlier in the spring.20 This is thought to be occurring in Canada as well.

Loss of fire as a disturbance agent

Over the last century humans have had a significant influence on fire. Land conversion and fire suppression have resulted in the almost complete loss of large fire as an important disturbance agent in the Mixedwood Plains, Prairies, and Atlantic Maritime ecozones+.2 The success of fire suppression since the 1970s21, 22 has also affected other areas. For example, in the B.C. interior it has led to in-filling of grasslands and ponderosa pine forests with Douglas-fir and other trees and shrubs and increased the amount of fuel, making the forests more susceptible to fires of greater intensity,23, 24 and increasing their vulnerability to insect outbreaks.25 Active suppression now covers 90% of the Boreal Plains, 64% of the Boreal Shield, 41% of the Boreal Cordillera, 20% of the Taiga Plains, and 2% of the Taiga Shield.4 The negative ecological consequences of fire suppression have been recognized and management authorities have started to reintroduce controlled burns on a limited basis in parts of Canada. Fire suppression is a balancing act between maintaining ecological function and protecting human life and property.26

Change in risk of wildfire

July Drought Code, 1901 to 2001
Map: change in risk of wildfire. Click for graphic description (new window).
Source: adapted from Girardin and Wotton, 200927

Drought variables are correlated with fire activity and may be used to reconstruct fire history or predict future risk of wildfire.28-30 Change in the risk of wildfire between 1901 and 2002 was inferred using the Drought Code, an index of water stored in the soil. This index is one of the measures used by fire management agencies to monitor risk.27, 31 Results, based on changes in soil moisture, showed decreasing risk of wildfire south of Hudson Bay, in the eastern Maritimes, and in western Canada, largely due to significant increases in precipitation that resulted in a significant reduction in drought. In contrast, the Taiga Shield, Arctic, and northern Taiga Plains showed an increased risk of fire.27, 31 This analysis only considers climate variables and does not include other factors such as human management and ignitions, insect outbreaks, and vegetation changes.31

Globe

Global Trends

Globally, the total area burned annually has been increasing since the 1950s.32 Both fire weather severity and area burned are expected to continue to increase in Europe,33 Russia,34 Canada and the United States,6, 15, 35 South America, central Asia, southern Africa, and Australia,36 due to increasing temperatures.3, 37