Tree mortality

Tree mortality is increasing worldwide including Canada.

Tree mortality refers to the death of forest trees and provides a measure of forest health. The premature death of healthy trees may be caused by drought and other extreme weather events, or by climate-triggered outbreaks of insect pests in weakened forests. A widespread increase in tree mortality has been reported in many forest types in Canada and around the world over the past few decades. A severe drought in 2001–2002 led to the dieback and decline of aspen forests across large areas of western North America. Although tree mortality is hard to project, it is expected to continue to increase with continued climate change, particularly in areas where extreme weather events, such as severe drought, become more frequent. Increased tree mortality has impacts on forest ecosystem functioning, timber supply and carbon balance.

Read how tree mortality and its indicators are defined

Tree mortality refers to the death of forest trees that is not the result of harvesting. The attribution of tree mortality to climate change is difficult. The death of trees often has multiple causes and is highly episodic and patchy, posing challenges for tracking and determining causation.

For Forest Change, tree mortality is estimated using the percentage of annual loss of living tree aboveground biomass in repeated observations within long-term plots established by government agencies and forest companies... Within each plot, every tree is marked and georeferenced. Observations include the status (dead/live) and the diameter (at 1.3-metre height) of each tree. Living tree biomass is estimated by applying mathematical equations that use measurements of tree stem diameter and total tree height (see: Canadian national tree aboveground biomass equations). Tree death due to major disturbances such as fire or harvesting is excluded.

Why tree mortality is important

Tree mortality provides a measure of forest health and affects carbon balance.

Climate-related disturbances such as drought can lead to the premature death of healthy trees that are important to forest ecosystem functioning and wood supply for the forest industry. Tree mortality can emerge abruptly at a regional scale when climate conditions exceed species-specific physiological thresholds or if climate triggers associated outbreaks of insect pests in weakened forests.

Changes in tree mortality may affect forest age structure. Differences in tree mortality between species result in changes in forest composition. Increases in tree mortality also have impacts on local, regional and global carbon budgets: living trees take up CO2 through the process of photosynthesis, while dead trees release CO2 when they fall and decay on the forest floor.

Tracking the effects of climate change on tree mortality provides information on broad-scale die-back processes that have impacts on forest ecology and the forest sector.

What has changed

Tree mortality in many forest types has increased in recent decades and has been attributed to climate change.

A widespread increase in tree mortality has been reported in many forest types in Canada and around the world over the past few decades. Large-scale mortality episodes and forest decline have occurred following significant drought and heat waves, often in combination with outbreaks of insects and diseases. However, the interaction between climatic variation and forest ecological processes are complex, and it is difficult to determine whether climate change was the main cause of increasing tree mortality.

A severe drought in 2001–2002 led to the dieback and decline of aspen forests across large areas of western North America, including the aspen parklands of Saskatchewan and Alberta (Figure 1).

Photo showing dead aspen stands as a result of the severe drought in western Canada in 2001–2002 that resulted in widespread tree mortality.

The 2001-2002 drought resulted in widespread aspen mortality in western Canada

Graph showing  trends in aspen mortality in western Canada for the period 2001–2012, following the severe 2001–2002 drought.

Figure 1 – Trends (2001-2012) in aspen mortality (in percentage of annual loss of aboveground biomass) across a network of 24 study sites in western Canada (Climate impacts on productivity and health of aspen, CIPHA), following the severe 2001-2002 drought

Larger image [25 Kb]

 
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 Mortality
2001 1.24
2002 1.38
2003 1.95
2004 2.08
2005 2.87
2006 3.08
2007 2.26
2008 2.65
2009 2.92
2010 2.38
2011 2.78
2012 2.16

The outlook

Tree mortality will likely increase in areas where extreme weather events become more frequent.

Climate change projections indicate that in some parts of Canada, droughts and other extreme events are expected to become more frequent in the future. These changes could trigger increases in tree mortality and episodes of forest decline in affected areas, posing challenges for forest management and the long-term supply of forest resources and services, including carbon balance.

Sources and references for tree mortality and its indicators

Canadian Forest Service key contacts

Ted Hogg, Research Scientist, Vegetation Climate Interactions, Northern Forestry Centre
Michael Michaelian, Forest Health Technician, Northern Forestry Centre

Adaptation tools and resources

Assisted migration of tree species in Canada – a web page describing the human-assisted movement of plants to more climatically suitable habitats and links to articles, reports and resources

Adaptation Options – an inventory of adaptation options proposed in the literature for the managed boreal forest

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

Find out more
Related Canadian Forest Service research