Impacts on Forest Growth and Health

"Changes in climatic conditions affect all productivity indicators of forests and their ability to supply goods and services to human economies." ⁽⁵⁾

Researchers expect that even small changes in temperature and precipitation could greatly affect future forest growth and survival,⁽⁶⁾ especially at ecosystem margins and threshold areas. Over the last century, Canada has warmed by an average of 1°C.⁽⁷⁾ During the same time period, plant growth at mid to high latitudes (45°N and 70°N) has increased and the growing season has lengthened.⁽⁸⁾ Historic warming has also had an impact on tree phenology. For example, in Edmonton, Alberta, trembling aspen has begun to bloom 26 days earlier over the past 100 years,⁽⁹⁾ and the bud break of white spruce in Ontario appears to be occurring earlier.⁽¹⁰⁾ Plant hardiness zones also appear to have shifted in response to recent changes in climate, with the most significant changes occurring in western Canada (Figure 2).⁽¹¹⁾

Figure 2: Changes in plant hardiness between 1930-1960 and 1961-1990 (modified from reference 11)

Climate models project that future warming will be greatest during the winter months. This trend is evident in the historic climate record for most of the country. For example, over the past century, winter temperatures in the Canadian Rockies have warmed about twice as much as spring and summer temperatures.⁽¹²⁾ Higher temperatures in the winter would have both positive effects on forests, such as decreased winter twig breakage,⁽¹³⁾ and negative effects, such as increased risk of frost damage.⁽¹⁰⁾ Although warmer winters would increase the over-winter survival of some insect pests, reduced snow cover could increase the winter mortality of others.⁽¹⁴⁾

Higher winter temperatures may also increase the frequency and duration of midwinter thaws, which could lead to increased shoot damage and tree dieback (references 15 and 16; see Box 1). A decrease in snow cover could further increase tree dieback due to frost-heaving, seedling uplift⁽¹⁷⁾ and increased exposure of roots to thaw-freeze events.⁽¹⁸⁾

BOX 1: Are winter thaws a threat to yellow birch?⁽¹⁹⁾

In the past, large-scale declines of yellow birch have been documented in eastern Canada. Studies indicate that winter thaws and late spring frosts may partially explain the diebacks. Winter thaws decrease the cold hardiness of birch, thereby increasing the vulnerability of the affected trees. The effect of a winter thaw on birch seedlings is shown in the photograph below. Winter thaw events can also cause breakdowns in the xylem of yellow birch, making it more difficult for water to pass from the roots to the branches. Future climate changes are expected to result in more frequent and prolonged winter thaws, and the likelihood that birch dieback may worsen. The effect of thaw on shoot dieback. The top photo is the control (not exposed to thaw), whereas the bottom photo shows yellow birch seedlings that were exposed to thaw. Photo courtesy of R.M. Cox.

Climate change would impact future moisture conditions in forests through changes in both temperature and precipitation patterns. As the temperature increases, water loss through evapotranspiration increases, resulting in drier conditions. Higher temperatures also tend to decrease the efficiency of water use by plants. In some areas of Canada, future increases in precipitation would help offset drying caused by higher temperatures.⁽²⁰⁾ In other regions, however, decreases in precipitation will accentuate the moisture stress caused by warming. Changes in the seasonality of precipitation and the occurrence of extreme events, such as droughts and heavy rainfalls, will also be important. For example, tree-ring analysis of aspen poplar in western Canada revealed that reduced ring growth was associated with drought events, whereas growth peaks followed periods of cool, moist conditions. ⁽¹⁸⁾

Forest characteristics and age-class structure also affect how forests respond to changes in moisture conditions. Mature forests have well-established root systems and are therefore less sensitive to changes in moisture than younger forests and post-disturbance stands, at least in the short term.⁽⁵⁾ In addition, certain tree species and varieties are more moisture or drought tolerant than others. For example, bur oak and white fir are better able to tolerate drought conditions than most tree types.⁽²¹⁾

While numerous studies have investigated the impacts of elevated CO2 on forest growth and health, the results are neither clear nor conclusive.⁽⁵⁾ Although researchers generally agree that higher CO2 concentrations improve the efficiency of water use by some plants (at elevated CO2 concentrations, plants open their stomata less, thus reducing water loss through transpiration), diverse results have been found concerning the overall effects on plant growth. For example, higher CO2 concentrations have been found to increase the growth of various types of poplar,^{(22, 23)} but have little to no effect on the growth of Douglas fir,⁽²⁴⁾ aspen and sugar maple. ⁽²⁵⁾ The differing results between studies could relate to the species studied, individual tree age, the length of the study period and differences in methodology. It is also important to note that some researchers suggest that any positive response of plants to enhanced CO2 concentrations may decrease over time, as plants acclimatize to elevated CO2 levels.⁽⁵⁾

The uncertainties concerning how trees will respond to elevated CO2 concentrations make it challenging to incorporate this factor into impact assessments. Additional complications arise from the possibility that other anthropogenic emissions will affect forest growth. For example, ozone (O3), a pollutant that causes visible damage to tree species,⁽²⁶⁾ has been shown to offset the potential benefits of CO2 on tree productivity.^{(26, 27)} On the other hand, some suggest that nitrogen oxides, which are released through fossil fuel combustion and high-intensity agriculture, may lead to enhanced forest growth,⁽²⁸⁾ especially in nitrogen-limited ecosystems. Another study found that these growth enhancement factors (e.g., CO2 fertilization, nitrogen deposition) actually had minimal influence on plant growth relative to other factors, particularly land use.⁽²⁹⁾

Overall, the impacts of climate change on forest growth and health will vary on a regional basis, and will be influenced by species composition, site conditions and local microclimate.⁽¹²⁾ In the aspen forests of western Canada, forest productivity may increase due to longer frost-free periods and elevated CO2 concentrations,⁽¹⁸⁾ although an accompanying increase in drought stress could create problems. Productivity in northeastern Ontario may also increase under the combined effects of higher temperatures, increased precipitation, and a longer growing season.⁽³⁰⁾ In contrast, some researchers suggest that climate warming could have a negative impact on the physiology and health of forest ecosystems in the Great Lakes-St. Lawrence region.⁽³¹⁾

Impacts on Tree Species Migrations and Ecosystem Shifts

"Our forest ecosystems will be in a state of transition in response to the changing climate, with primarily negative impacts." ⁽³²⁾

Climate change may result in sometimes subtle and non-linear shifts in species distributions.⁽⁵⁾ As conditions change, individual tree species would respond by migrating, as they have in response to past changes in climate. There is concern, however, that the rapid rate of future climate change will challenge the generation and dispersal abilities of some tree species.^{(33, 34)} Successful migration may be impeded by additional stresses such as barriers to dispersion (habitat fragmentation) and competition from exotic species,^{(35, 36, 37)} and changes in the timing and rate of seed production may limit migration rates.⁽³⁴⁾

It is generally hypothesized that trees will migrate northward and to higher altitudes as the climate warms. The warming of the last 100 years has caused the treeline to shift upslope in the central Canadian Rockies.⁽¹²⁾ Temperature, however, is not the sole control on species distribution, and temperature changes cannot be considered in isolation. Other factors, including soil characteristics, nutrient availability and disturbance regimes, may prove to be more important than temperature in controlling future ecosystem dynamics. The southern limit of the boreal forest, for example, appears to be influenced more by interspecies competition⁽³⁸⁾ and moisture conditions⁽³⁹⁾ than by temperature tolerance. The distribution of trembling aspen in western Canada is also largely controlled by moisture conditions.⁽⁴⁰⁾

Predictions of future changes in species distributions are exceedingly complicated, and results from available studies vary greatly. Predictions of migration rates in northern forests by 11 leading ecologists varied by more than four orders of magnitude.⁽⁴¹⁾ This could be related to the fact that predictions are often derived from models, which require a number of assumptions to be made. For example, many models assume that seeds of all species are uniformly available, and that environmental conditions do not fluctuate between regions, leading to overestimation of future species diversity and migration rates.⁽⁴²⁾ Models also generally do not account for the potential role of humans in assisting species migrations. Model projections should therefore be viewed as indicative of trends, rather than conclusive of magnitude.⁽⁴³⁾

Some key results of recent studies that combined historical trends or climate simulations with ecosystem models are listed in Table 1.

Table 1: Recent research results of forest migrations

It is important to note that species will respond individually to climate change and that ecosystems will not shift as cohesive units. The most vulnerable species are expected to be those with narrow temperature tolerances, slow growth characteristics⁽⁴⁹⁾ and limiting dispersal mechanisms such as heavy seeds.⁽⁴⁵⁾ For example, since trembling aspen has better seed dispersal mechanisms than red oak and jack pine,⁽⁵⁰⁾ it may be more successful at migrating in response to climate change. Differing species' response to anthropogenic emissions may also affect competitive ability,⁽⁵¹⁾ with potentially significant impacts on forest ecosystem functioning.⁽⁴⁹⁾

Impacts on Disturbances

"Increases in disturbances such as insect infestations and fires can lead to rapid structural and functional changes in forests."⁽⁵⁾

Each year, approximately 0.5% of Canada's forests are severely affected by disturbances, such as fire, insects and disease.⁽¹⁾ These disturbances are often strongly influenced by weather conditions and are generally expected to increase in the future in response to projected climate change.⁽⁴⁾

Cumulative impacts arising from the interactions between disturbances are likely. For example, an increase in drought stress is expected to increase the occurrence and magnitude of insect and disease outbreaks.⁽³⁰⁾ Similarly, an increase in defoliation by insect outbreaks could increase the likelihood of wildfire.⁽⁵²⁾ The interaction between fire and spruce budworm in Ontario is described in Box 2. In addition to tree damage, changes in the disturbance regime would have long-term consequences for forest ecosystems, such as modifying the age structure and composition of plant populations.⁽³⁰⁾

BOX 2: Interactions between spruce budworm and wildfire in Ontario⁽⁵³⁾

Wildfires and spruce budworm (SBW) outbreaks are widespread disturbances in the boreal forest. Fleming et al.(53) examined historical records to investigate the interactions between these disturbances in Ontario, and estimate how they will be affected by future climate changes. Spruce budworm outbreaks are thought to increase the occurrence of wildfires by increasing the volume of dead tree matter, which acts as fuel for fires. The researchers documented a disproportionate number of wildfires occurring 3 to 9 years following spruce budworm outbreaks, with the trend being more pronounced in drier regions such as western Ontario, where wood fuels tend to decompose more slowly. The study concluded that drier conditions induced by climate change would cause wildfires to increase in stands with SBW defoliation, as well as increase the frequency and intensity of SBW outbreaks. Spruce budworm: dorso-lateral view of mature larva (Image courtesy of T. Arcand, Laurentian Forestry Centre, Canadian Forest Service)

Forest Fires

"In most regions, there is likely to be an increased risk of forest fires..."⁽⁵⁾

Forest fires are a natural occurrence and necessary for the health of many forest ecosystems. Indeed, without fire, certain tree species and ecosystems of the boreal forest could not persist.⁽⁵⁴⁾ However, fires can also lead to massive forest and property damage; smoke and ash generated by fires can create health problems, both locally and at great distances; and evacuations forced by fires have a wide range of social and economic impacts. Average annual property losses from forest fires exceeded $7 million between 1990 and 2000, while fire protection costs average over $400 million per year.⁽⁵⁵⁾

Studies generally agree that both fire frequency in the boreal forest and the total area burned have increased in the last 20 to 40 years.^{(56, 57, 58)} There is, however, less agreement among studies that examine longer term records, with both decreases^{(59, 60)} and increases⁽⁶¹⁾ reported, reflecting differences in location, timeframes and study methodologies. It is also important to note that although large fires (over 1 000 hectares) account for only 1.4% of forest fires in Canada, they are responsible for 93.1% of the total area burned.⁽⁵⁵⁾ Hence, caution is required when trying to compare studies examining changes in fire frequency and area burned.

Fire season severity is generally projected to increase in the future due to climate change (Table 2). Reasons for the increase include a longer fire season, drier conditions and more lightning storms.^{(62, 63)}

Table 2: Forest fire predictions

¹ RCM, regional climate model

There is relatively high uncertainty associated with most studies of climate change and forest fires, due largely to our limited understanding of future changes in precipitation patterns. Where precipitation increases, forest fire frequency may experience little change or even decrease.⁽³⁾ It has also been shown that warm weather and dry conditions do not necessarily lead to a bad forest fire season. This was exemplified in 2001: despite the extreme heat and dryness, wildfire frequency was down and total area burned was the lowest on record.⁽⁶⁹⁾ Vegetation type will influence changes in future fire frequency and intensity. For example, conifers are more likely to experience intense fires than are deciduous or mixed-wood stands. Hence, species migrations in response to changing climate would also affect future fire behaviour by changing the fuel types.⁽⁷⁰⁾ Some other factors that influence fire seasons include wind, lightning frequency, antecedent moisture conditions and fire management mechanisms.

Insect Outbreaks

Insect outbreaks are a major problem across Canada, with resulting timber losses estimated to exceed those from fire. ⁽⁷¹⁾

In certain regions, defoliation by pests represents the most important factor controlling tree growth.⁽⁷²⁾ The response of insects to climate change is expected to be rapid, such that even small climatic changes can have a significant impact. Insects have short life cycles, high mobility, and high reproductive potentials, all of which allow them to quickly exploit new conditions and take advantage of new opportunities.⁽¹⁴⁾

Higher temperatures will generally benefit insects by accelerating development, expanding current ranges and increasing over-winter survival rates.⁽¹⁴⁾ For example, insect pests that are not currently a problem in much of Canada may migrate northward in a warmer climate. Warmer conditions may also shorten the outbreak cycles of species such as the jack pine budworm, resulting in more frequent outbreaks,⁽⁷³⁾ and increase the survival of pests like the mountain pine beetle, that are killed off by very cold weather in the late fall and early spring.⁽⁷⁴⁾ However, an increase in extreme weather events may reduce insect survival rates,⁽¹⁴⁾ as may a decrease in winter snow cover.

Climate change would also have indirect effects on forest disturbance by pests. For example, extended drought conditions may increase the sensitivity of trees to insect defoliation,⁽³⁾ as would ecosystem instability caused by species migrations. Projected increases in anthropogenic emissions (e.g., CO2, O3) may further reduce tree defences against insects and diseases.^{(75, 26)} Climate change may also affect insect outbreaks by altering the abundance of insect enemies, mutualists and competitors. For example, warmer weather may have differing effects on the development rates of hosts and parasitoids,⁽³⁴⁾ as well as the ranges of predators and prey.⁽⁷⁶⁾ This could alter ecosystem dynamics by reducing the biological controls on certain pest populations.

Extreme Weather

The frequency and severity of extreme weather events, such as heavy winds, winter storms and lightning, are projected to increase due to climate change.

The impact of extreme climate events on forests and the forest sector was clearly demonstrated by the 1998 ice storm that hit eastern Ontario, southern Quebec and parts of the Maritime Provinces. Damage from the ice storm in areas of Quebec was comparable to that of the most destructive windstorms and hurricanes recorded anywhere.⁽⁷⁷⁾ Long-term economic impacts have been evident in the maple sugar industry, with almost 70% of the Canadian production region affected by the storm.⁽⁷⁸⁾ Researchers are still working to quantify the actual costs.⁽⁷⁹⁾ Ice storms are not uncommon events, but the intensity, duration and extent of the January 1998 event was exceptional.⁽⁷⁸⁾ Nonetheless, such storms may become more frequent in association with milder winters in the future.⁽³⁾

Wind damage can result from specific events, such as tornadoes and downbursts, or from heavy winds during storms. In the Great Lakes area, downbursts are a key wind disturbance that can affect thousands of hectares, with both immediate and long-term impacts.⁽⁸⁰⁾ Heavy winds can also cause large-scale forest destruction through blowdown. For example, a heavy storm in New Brunswick in 1994 felled 30 million trees, resulting in losses of $100 million.⁽⁸¹⁾ Factors such as tree height, whether or not the tree is alive, and stand density affect whether a tree is just snapped or completely uprooted by heavy winds.⁽⁸²⁾ Wind events may also have consequences for other forest disturbances, such as fires and insect outbreaks. For example, researchers have found that spruce beetle reproduction is favoured in blowdown patches.⁽⁸³⁾

A warmer climate may be more conducive to extreme wind events, although there is much uncertainty on this issue.⁽⁸⁴⁾ Given the localized nature of these events, and the fact that wind phenomena are generally poorly understood, reliable modelling of the frequency of future wind events is not available at this time.⁽⁸⁰⁾

Social and Economic Impacts

The biophysical impacts of climate change on forests will translate into many different social and economic impacts (Table 3), which will affect forest companies, landowners, consumers, governments and the tourism industry.⁽⁸⁵⁾

Table 3: Examples of socio-economic impacts of climate change⁽⁸⁵⁾

The magnitude of socio-economic impacts, such as those listed in Table 3, will depend on 1) the nature and rate of climate change; 2) the response of forest ecosystems; 3) the sensitivity of communities to the impacts of climate change and also to mitigation policies introduced to address climate change; 4) the economic characteristics of the affected communities; and 5) the adaptive capacity of the affected group.⁽⁸⁶⁾

Exports of forest products are an important component of the Canadian economy, valued at $47.4 billion in 2001.⁽¹⁾ A greater degree of warming at higher latitudes may mean that Canadian forests experience greater impacts on productivity as a result of climate change than forests of many other countries.⁽⁸⁷⁾ However, because of uncertainty regarding the magnitude and even the direction of many of these impacts, it is extremely difficult to assess Canada's future competitive ability in international markets. If Canadian forests were to experience faster tree growth and greater wood supply⁽⁸⁸⁾ and global timber shortages occur as predicted, due to population and economic growth,⁽⁸⁹⁾ Canada's forest industry could benefit. Climate change may require changes in international trade policies and the pricing of forest products,⁽⁹⁰⁾ which are generally based, at present, on the assumption of a stable climate.

First Nations are extremely concerned about the impacts of climate change on Canada's forests.⁽⁹¹⁾ Since more than 90% of reserves are located on forested lands, forests play a vital economic and cultural role for many First Nations communities.⁽¹⁾ The projected impacts of climate change on forests, especially with respect to increased disturbances and species migrations, could threaten the sustainability of some of these communities.