Drought

Drought is expected to become more frequent and severe in parts of Canada.

Tree species are adapted to specific moisture conditions. Having less water available through drought has a range of negative impacts on the health of forest ecosystems. Direct impacts include reduced growth, increased tree mortality, and failure to regenerate. Indirect impacts include reduced ability to defend against insects and disease, and increased fire risk. These impacts affect the availability of wood fibre for the forest industry.

Parts of western Canada are experiencing more frequent and more severe droughts. Scientists expect drought to affect new areas across the country and to become even more frequent and severe, which could have far-reaching impacts on Canada’s forests in the future.

Read how drought and its indicators are defined

Why drought is important

Reduced water availability has a range of negative impacts on forests.

Trees are adapted to specific moisture conditions. Drought reduces forest growth and can lead to increased tree mortality and regeneration failure. The distribution of boreal forest and prairie grasslands in western Canada corresponds closely to climate moisture regimes, suggesting that moisture limitations prevent conifer regeneration south of the current boreal forest boundary. Forests may also experience indirect effects of drought through a reduced capacity to defend against insects and disease and through increased fire risk.

Tracking drought will help forest managers better plan the planting stock (e.g., species and provenance) and the silvicultural practices (e.g., planting periods, thinning, etc.) adapted to drought conditions.

What has changed

Certain areas are currently experiencing more frequent and more severe droughts.

Several regions of Canada experienced substantial droughts between 1951 and 2010, but with significant variability between decades. However, during the first decade of the 21st century (2001–2010), exceptional droughts were observed across the country – for example, the 2001–2002 drought in the Prairies (Figures 1 and 2), which caused abnormally high aspen mortality (see Tree mortality).

Similar observations have been reported for many forest types worldwide. With droughts expected to become more frequent and severe in many regions of the world, including most of Canada’s forest regions, there are growing concerns about forest distribution, tree health and regeneration success in the future.

Figure 1 – Two maps, one showing the mean Climate Moisture Index drought in the aspen parkland between 1951 and 2000, and the other map showing the 2001–2002 drought in the same region.

Figure 1 – Mean Climate Moisture Index (CMI) for 1951–2000 and the 2001–2002 drought in the aspen parkland

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Graph displaying annual variability in the Climate Moisture Index in Canada’s aspen parkland between 1891 and 2010. Higher values denote wetter years, whereas lower values denote drier years.

Figure 2 – Long-term changes in the Climate Moisture Index (CMI) in the aspen parkland

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Set of five maps of Canada showing the mean annual Climate Moisture Index (CMI) for the reference period 1981 to 2010 compared to the projected mean CMI for the short term (2011–2040), medium term (2041–2070), and long term (2071–2100) using greenhouse gas scenario RCP 2.6 and again, for the long term, using climate scenario RCP 8.5.

Figure 3 – Reference period (1981–2010) and projected mean annual Climate Moisture Index (CMI) for the short- (2011–2040), medium- (2041–2070), and long-term (2071–2100) under the Representative Concentration Pathway (RCP)Footnote * 2.6 (rapid emissions reductions) and, for the long-term (2071–2100), under RCP 8.5 (continued emissions increases) for Canada

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Graph data
Table listing the Climate Moisture Index (CMI) values for Canada’s aspen parkland for the years 1891 to 2010. Higher values denote wetter years, whereas lower values denote drier years.
Year Climate Moisture Index (CMI)
1891 -8.2
1892 -12.8
1893 -12.7
1894 -18.1
1895 -18.6
1896 -2.6
1897 -13.7
1898 -14.8
1899 0.7
1900 16.8
1901 22.4
1902 7.3
1903 -2.9
1904 7.9
1905 -15.0
1906 -3.9
1907 -1.6
1908 3.7
1909 -2.7
1910 -29.8
1911 -4.6
1912 -3.7
1913 -3.8
1914 -10.2
1915 -6.8
1916 -0.6
1917 -11.9
1918 -19.7
1919 -30.0
1920 5.5
1921 -0.1
1922 -16.9
1923 -5.8
1924 -18.3
1925 -8.5
1926 -11.2
1927 13.4
1928 -11.1
1929 -21.9
1930 -16.2
1931 -11.0
1932 -0.5
1933 -9.5
1934 -0.8
1935 -4.4
1936 -13.4
1937 -15.1
1938 -18.8
1939 -5.9
1940 -7.7
1941 -24.0
1942 4.5
1943 -2.3
1944 -0.3
1945 -15.1
1946 -12.9
1947 -14.5
1948 2.3
1949 -19.4
1950 -12.5
1951 4.9
1952 -0.9
1953 1.5
1954 4.7
1955 10.3
1956 3.7
1957 -18.1
1958 -6.2
1959 -13.9
1960 6.6
1961 -11.2
1962 -10.9
1963 -9.1
1964 -23.1
1965 13.7
1966 -7.2
1967 -7.5
1968 -12.2
1969 -3.6
1970 9.2
1971 -5.6
1972 -7.8
1973 3.0
1974 14.9
1975 -0.2
1976 -7.6
1977 -9.7
1978 1.6
1979 4.4
1980 -15.8
1981 -5.0
1982 -11.9
1983 1.2
1984 -5.0
1985 1.4
1986 5.9
1987 -7.2
1988 -10.5
1989 -6.9
1990 -1.8
1991 -6.9
1992 -12.2
1993 -3.3
1994 0.5
1995 -13.3
1996 0.8
1997 1.8
1998 -11.7
1999 2.6
2000 -0.5
2001 -15.1
2002 -35.8
2003 -5.8
2004 -1.3
2005 4.5
2006 -2.5
2007 1.3
2008 -8.9
2009 -18.1
2010 -2.7

The outlook

Drought is expected to become more frequent and affect new areas.

Projections suggest that the frequency and severity of drought will continue to increase in several areas that are already dry, such as the Southern Interior of British Columbia and the Prairie provinces (Figure 3). For some areas of western Canada, future drying poses the additional concern that prairie-like conditions will expand northward into managed areas of the southern boreal forest, leading to forest degradation and major changes in ecosystem functioning.

Moist regions, such as the Pacific and Atlantic coastal areas, are expected to be less affected, with limited changes in annual CMI values over the next 100 years. However, these moist areas could become more prone to the impacts of seasonal droughts even if the annual CMI indices remain positive.

Increases in drought could have far-reaching impacts on Canada’s forests, both directly, through impacts on tree growth and survival, and indirectly, through drought-related increases in the frequency of fire, insect outbreaks and other disturbances.

How drought and its indicators are defined

Drought is a shortage of precipitation over an extended period, usually a season or more, resulting in an insufficient water availability that has adverse impacts on vegetation, animals and people. Drought differs from aridity, which is a permanent climate feature in regions characterized by low precipitation, such as deserts.

This project uses three indicators for drought:

  • The Climate Moisture Index (CMI) was calculated as the difference between annual precipitation and potential evapotranspiration – the potential loss of water vapour from a landscape covered by vegetation (see equations). Positive CMI values indicate wet or moist conditions and show that precipitation is sufficient to sustain a closed-canopy forest. Negative CMI values indicate dry conditions that, at best, can support discontinuous parkland-type forests. The CMI is best suited to evaluating moisture conditions in dry regions, such as the Prairies.
  • The Soil Moisture Index (SMI) was calculated using daily or monthly weather records of precipitation and temperature, along with elevation and soil water-holding capacity (see equations). Lower values indicate drier conditions. The SMI is designed to provide a direct measure of soil moisture changes within the rooting zone of trees. It is better suited than the CMI for representing seasonal droughts in regions with high winter precipitation, such as the Pacific coast and eastern Canada. However, as this indicator was developed quite recently, it is not as well-tested as the CMI, which was developed in 1997.
  • The Palmer Drought Severity Index (PDSI) was calculated using precipitation, temperature, moisture-holding capacity of the soil, and local infiltration (see equations). Positive and negative values represent conditions that are wetter and drier, respectively, than the long-term (≥30-year) historical mean for a given location. This indicator is most commonly used in agriculture, but can be applied in forestry.
Sources and references for drought and its indicators

Canadian Forest Service key contacts

Ted Hogg, Research Scientist, Vegetation Climate Interactions, Northern Forestry Centre
David Price, Research Scientist, Integrative Climate Change Impacts Modelling, Northern Forestry Centre

Adaptation tools and resources

Forest Change Toolkit – a list of tools and resources for climate change adaptation

Find out more
Related Canadian Forest Service research