Natural Disturbances

Status and Trends
evidence of change but direction, interpretation, and biodiversity impacts not fully understood
Concern, rate of change unknown
data not comprehensive, but good data for fire and certain insects
Medium confidence in finding

KEY FINDING 19. The dynamics of natural disturbance regimes, such as fire and native insect outbreaks, are changing and this is reshaping the landscape. The direction and degree of change vary.

This key finding is divided into three sections:

Natural disturbances are discrete, sometimes cyclical, events that cause significant change in ecosystem structure or composition. Size, frequency, severity, seasonality, and duration of the disturbance event determine the impact on biodiversity. Large disturbance regimes are important as they have shaped ecosystems. Although other disturbance agents are important, this key finding focuses on fire and native insect outbreaks which are widespread and particularly important ecological drivers in forests and grasslands. Fire and insect outbreaks affect each other and are influenced by weather, climate, vegetation dynamics, and human management.

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Photo: fire ©
Forest 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.


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


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

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Photo: beetle damage © Epp
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.


Spruce budworm

Map: spruce budworm. Click for graphic description (new window).
Source: adapted from Canadian Forest Service, 200738
Photo: spruce budworm © Thérése Arcand, NRCan, CFS

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.

Thousands km2 of moderate to severe defoliation, 1909 to 2007
Graph: area defoliated by eastern spruce budworm. Click for graphic description (new window).
Source: pre-1909 to 1980 (blue line) adapted from Kettela, 1983;49 1974 to 2008 (red line) adapted from National Forestry Database, 201046 and Strubble, 200850

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

Photo: mountain pine beetle © Leslie Manning, CFS

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

Cumulative area affected
1999 and 2009
Two maps of cumulative area affected by pine beetles. Click for graphic description (new window).
Source: adapted from B.C. Ministry of Forests and Range, 2010;56 Alberta Sustainable Resource Development, 201057
Area affected annually by mountain pine beetle in B.C.
Thousand km2, 1928 to 2009
Graph: area affected annually by mountain pine beetle in B.C. Click for graphic description (new window).
Source: adapted from B.C. Ministry of Forests and Range, 2010;56 Alberta Sustainable Resource Development, 201057

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

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