Key finding overview
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.
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.
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
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
Mountain pine beetle damage
Large-scale insect outbreaks are an important natural disturbance regime in Canada. Changes in patterns of outbreaks of some insect species are evident but they are not uniform, with some increasing in severity, some decreasing, some showing no sign of change, and many without long-term data. Insect outbreaks and fire each affect the other and both are influenced by climate. For example, the suppression of wildfire has caused changes in forest structure in some areas, increasing their susceptibility to outbreaks of some insects. At the same time, insect outbreaks can influence fire dynamics, for example, increasing wildfire intensity in post-outbreak stands.
The spruce budworm, native to Canada’s boreal and mixedwood forests, is one of Canada’s most prevalent and influential insect defoliators. Of the four species that occur in Canada, the most widespread is the eastern spruce budworm. Its preferred hosts are balsam fir and white and red spruce, but it can also defoliate black spruce.39 It is most damaging to older, denser forest stands although during severe outbreaks all host stands are vulnerable.40 Together with fire, the eastern spruce budworm is the dominant natural disturbance in the boreal forest.41 Cycles of spruce-budworm outbreaks, recurring approximately every 30 to 55 years,42 influence species composition, age-class distribution, successional dynamics, and forest condition, thereby playing an important role in shaping forest ecosystems.43, 44 Outbreaks occur somewhat synchronously over extensive areas, but outbreak duration varies regionally.45 The last peak outbreak was in 1975, when over 510,000 km2 were defoliated nationally.46
Western spruce budworm affects a much smaller area. The last peak defoliation was in 2007, when about 8,600 km2 were defoliated nationally.46 Severity of attack is low, for example, 95% of affected area in B.C. was classified as light in 2008.47 One study mapped historical attack in the Kamloops Forest Region and found an increase in attack over the four outbreaks between 1916 and 2003, particularly after 1980.48
Area defoliated by eastern spruce budworm east of the Manitoba border and in Maine, U.S.
There is no consensus on whether there has been a change in frequency of eastern spruce budworm outbreaks.44, 45, 51, 52 An overall increase in the area it has defoliated is apparent for Ontario and Quebec, however, which represented 98% of the area affected during the last peak outbreak.46, 49 There is no consensus on whether this constitutes a trend. At the same time, the severity of outbreaks in New Brunswick decreased between 1949 and 2007.53 Studies that conclude there have been changes in the pattern of attack have attributed them to fire suppression, forest harvesting practices, temperature increases in the spring, insecticide spraying, and less reliable reconstructions of historical outbreaks.44, 54, 55
Mountain pine beetle
The mountain pine beetle is native to western North America and at least four large-scale outbreaks have occurred in B.C. in the last 120 years.25 The disturbance has changed in the last decade, however, with an infestation of unprecedented intensity in B.C.58, 59 In 2005, it spread to Alberta,60 where it has spread rapidly, including to jack pine/lodgepole pine hybrids.61, 62 Attack results not only in changes to the forest, but can result in changes in water temperature and flow patterns, and increased soil and stream bank erosion.63 Beetle-killed stands are also more vulnerable to fire,64-67 and the combination of increased insect attack and past fire suppression can lead to an increase in intense, stand-replacing wildfires.68 The infestation appears to have peaked in B.C., likely because most host trees in the central plateau have already been attacked, and because variable terrain and greater tree diversity have slowed the spread in other areas.58
Host availability, climate, and forest management practices all influence mountain pine beetle dynamics.25 Changes that have contributed to the current infestation include:
- The proportion of older age classes of lodgepole pine stands, which are more susceptible to attack, increased from 17% in the early 1900s to 55% in 2002,64 largely as a result of fire suppression,25, 64, 67, 70 and harvest practices that change forest structure.64, 67, 71
- Climate has changed since 1920 to become more suitable for the beetle.72 Warmer winters73 have led to increased beetle survival. Temperatures in spring and late fall also affect mortality.71 For example, earlier onset of spring has increased spring survival.58, 72, 74