Indicator: Forest diseases

Tree diseases can reduce growth, decrease stand productivity and wood quality, and cause tree death. In a commercial context, this is detrimental; however, in a non-commercial setting, disease can lead to increased biodiversity and nutrient cycling.

  • The environment plays an important role in the disease process, mediating interactions between the host tree and pathogens.
  • Native forest pathogens, such as the root disease agent Armillaria ostoyae, can be widespread and have serious impacts, ranging from volume loss to death.
  • Historically, exotic introduced pathogens have had significant impacts on native trees. For example:
    • White pine blister rust, caused by Cronartium ribicola, has caused significant losses of five-needle pines.
    • Annosus root rot, caused by Heterobasidion irregulare, has the potential to have huge impact on red pine plantations.
    • Dutch elm disease, caused by Ophiostoma novo-ulmi, has caused significant losses to urban elms in many cities.
Armillaria spp. at the base of an infected birch tree
Golden-yellow fruiting bodies of Armillaria spp., the causal agents of Armillaria root rot, at the base of an infected birch tree.

Tree disease occurs as a result of prolonged interaction between a tree and a stress factor that affects normal functioning.

Stress factors are often biological and include pathogens, which are primarily fungi. Pathogens can infect the roots, stems or leaves of trees.

Trees can also be injured by environmental stress, such as drought, which can affect growth and, when prolonged, cause death.

Why is this indicator important?

  • Forest diseases can cause significant loss of volume over an extended period of time and these losses may not be immediately obvious if infections are not lethal.
  • If a host becomes stressed by changes in environmental conditions (e.g., drought), pathogens can have greater impacts, resulting in greater losses to disease.
  • Forest diseases can be difficult to manage economically at the landscape level.

What is the outlook?

  • Drought has affected aspen and spruce productivity in the Prairie provinces in the recent past, and the changing climate may lead to more severe impacts, in turn reducing the area of these species.
  • The oak wilt pathogen, Bretziella fagacearum (previously Ceratocystis fagacearum), has not been reported in Canada, but it is widespread in the United States and has recently been observed near Detroit, close to the Canadian border. The Canadian Food Inspection Agency regulates the importation of oak material.
  • Annosus root rot is increasing in eastern Canada and if untreated it may soon affect every red pine plantation, resulting in losses of high-value trees.
  • Forest disease is a natural ecosystem process and disease impacts will continue.

What reporting frameworks does this indicator support?

Sources and information
  • Bérubé, J.A., Dubé, J., et al. 2017. Incidence of Heterobasidion irregulare aerial basidiospores at different locations in southern Quebec. Canadian Journal of Plant Pathology 40, 34–38.
  • Canadian Food Inspection Agency. Ceratocystis fagacearum (oak wilt): Fact sheet.
  • Cheng, L., Huang, J.G., et al. 2017. Drought causes reduced growth of trembling aspen in western Canada. Global Change Biology 23, 2887–2902.
  • Ennos, R.A. 2015. Resilience of forests to pathogens: An evolutionary ecology perspective. Forestry 88, 41–52.
  • Hogg, E.H., Michaelian, M., et al. 2017. Recent climatic drying leads to age-independent growth reductions of white spruce stands in western Canada. Global Change Biology 23, 5297–5308.
  • Price, D.T., Alfaro, R.I., et al. 2013. Anticipating the consequences of climate change for Canada’s boreal forest ecosystems. Environmental Reviews 21, 322–365.
  • Ramsfield, T.D., Bentz, B.J., et al. 2016. Forest health in a changing world: Effects of globalization and climate change on forest insect and pathogen impacts. Forestry 89, 245–252.
  • Worrall, J.J., Rehfeldt, G.E., et al. 2013. Recent declines of Populus tremuloides in North America linked to climate. Forest Ecology and Management 299, 35–51.
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