Distribution of tree species

A shift in climate conditions could result in changes in tree species distribution and forest composition.

Climate determines the distribution of tree species and shapes the composition of forests. Changes in climate drive changes in the distribution of tree species, which, in the long term, can affect ecosystem processes and the types of wood fibre available to the forest industry. The migration capacity of trees varies across species but, in general, a shift in species distribution is a slow process.

Climate conditions have already shifted, affecting the distribution of certain tree species in Canada. In the last 50 years, changes have occurred in plant hardiness zones. The climate in an area where a tree species currently lives and reproduces is likely to shift to new locations. In Canada, the rate of projected climate change is expected to be 10 to 100 times faster than the ability of trees to migrate, resulting in impacts on forest health and productivity.

Read how tree species distribution and its indicators are defined

The distribution of a species is the area in which it naturally lives and reproduces during a defined time. Since trees are immobile organisms, it may be decades before noticeable shifts in distribution of tree species due to changing climate can be detected.

Precise data on past tree species distribution is limited, making the assessment of past changes challenging. Indicators that are related to the distribution of species can be backcasted to estimate how climate conditions relevant to species distribution have changed. Plant hardiness zones identify the location of environmental conditions under which plant species can successfully survive and grow. Well known to gardeners, these zones can be used for assessing past changes.

Projecting future distribution is also difficult, because of uncertainty about the capacity of tree species to adapt to new climate conditions and to migrate in order to follow their suitable climate condition. However, temporal and spatial changes in climate suitability zones (or climatic niches) can be modelled. The climate suitability zone is the current climate across the current distribution of a species. Climate suitability zones can be delineated using different modelling approaches based on projected climate data. These models are based on the assumption that present distributions of tree species are in equilibrium with the current climate conditions. They do not project the future distribution of species, but rather where the climate conditions in which the species currently lives and reproduces are expected to prevail in the future.

For Forest Change, past changes in the distribution of individual tree species were not assessed directly. Rather, plant hardiness zones were used to estimate changes in climate conditions related to species distribution. The original Plant Hardiness Zones Map was based on 1931–1960 climate data. The updated maps used data from 1961–1990 and 1981–2010, and different interpolation techniques. The comparison of the zones between the three time periods (1931–1960, 1961–1990 and 1981-2010) shows changes that are consistent with climate change.

The baseline information on species distribution was estimated using its current climatic suitability zone that is based on the climatic reference period 1971–2000. Future tree distribution was not directly estimated. Projected climate suitability zones were modelled using the Canada’s Plant Hardiness Site interactive mapping tool. This tool goes beyond a single hardiness zone map and enables the projection and mapping of the future climate suitability zones for more than 3,000 individual plant species.

Why the distribution of tree species is important

Tree species distribution shapes forest composition and affects ecosystem processes and wood supply.

The distribution of tree species is highly dependent on climate. Changes in climate affect the distribution of plant species and result in changes in forest composition. Tree species differ in the way they interact with their environment (water and nutrient cycling, competition, etc.). Changes in species composition may therefore affect several ecosystem properties. Forest composition also determines the wood fibre types available to the forest industry.

A tree starting to grow today in its current climate suitability zone may experience very different climate conditions when it comes to maturity. Some tree species may not be able to migrate or adapt fast enough to a rapidly changing climate, resulting in impacts on forest health. Forest management therefore needs to take into account shifting climate suitability zones. Careful selection of seed sources and species can contribute to maintaining healthy and productive trees in this changing climate.

What has changed

Climate conditions have shifted and changes in the range and local abundance of certain tree species have occurred.

Climate change has already altered the geographical range of certain tree species. In Canada, tree species migration is a slow process and precise data on past tree species distribution is limited. Assessing past changes in the location of tree species is therefore challenging. However, it is possible to report on past changes in the climate conditions that are relevant to species distribution.

Plant hardiness zones identify the location of environmental conditions under which a species or variety of plant can successfully survive and grow. The updates of the original Plant Hardiness Zones Map indicate changes in the hardiness zones that are generally consistent with changes in climate. These changes are most pronounced in western Canada (Figure 1).

Three maps showing the Plant Hardiness Zones in Canada for the periods: 1931 to 1960; 1961 to 1990; and 1981 to 2010.

Figure 1 – Original (1931–1960; left) and updated (1961–1990; centre and 1981-2010; right) Canada’s Plant Hardiness Zones Maps

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The outlook

Two maps comparing Canada’s current (1971–2001) distribution of trembling aspen with the species’ future (2071–2100) climate suitability zone according to climate scenario RCP 8.5.

Figure 2 – Current (1971–2000) versus projected (2071–2100) climate suitability zone of trembling aspen (Populus tremuloides) in Canada under the Representative Concentration Pathway (RCP)Footnote* 8.5 assuming continued emissions increases
Note: Climate suitability zones were modelled using the Canada’s Plant Hardiness Site interactive mapping tool

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Shifts in climate conditions are expected to outpace tree species migration capacity.

Climate change projections suggest that the climate prevalent in the area where a tree species currently lives and reproduces (i.e., climate suitability zone) will shift to new locations (Figure 2). However, it is difficult to forecast whether tree species will be able to adapt to new climate conditions or to migrate to follow these displacements. The capacity to migrate varies across tree species (see: Vulnerability of Canadian forest tree species to climate change).

In the medium term, the relative proportion of species within a forest could change, but radical changes in forest composition are not expected. Available records on past tree migration rates in response to natural changes in climate give an indication of the potential migration capacity of tree species.

The rate of projected climate change is unprecedented and, overall, is expected to be 10 to 100 times faster than the ability of trees to migrate in Canada. This discrepancy may result in species becoming maladapted, less productive and more vulnerable to insects and diseases.

Sources and references for tree species distribution and its indicators

Canadian Forest Service key contacts

Dan McKenney, Chief, Landscape Analysis and Applications, Great Lakes Forestry Centre
André Beaudoin, Research Scientist, Remote Sensing and Spatial Modelling, Laurentian Forestry Centre
Pierre Bernier, Research Scientist, Forest Productivity, Laurentian 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

Canada’s Plant Hardiness Site – a web application with species-specific models and maps of current and projected climate distribution for more than 3000 plant species

SeedWhere – a web application with a climate similarity mapping tool that can help users match seed sources to planting sites under the current or a future climate

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

Find out more
Related Canadian Forest Service research