Section 1.3: Aspen Health

Outcomes

Students will:

  • Analyze signs and symptoms of an unhealthy tree, and compare its characteristics with a healthy tree.

  • Investigate and describe threats to a trembling aspen and evaluate the impact of each threat.

Key Terms

Canker / Defoliation / Discolouration / Foul Smell / Fungi

See content or Module Glossary for definitions.

Do Trees Get Sick?

Much like with humans, trees and other plants can get diseases and/or be impacted by threats and become sick. Trees do have systems in place to combat diseases and threats but sometimes they are not successful and the tree dies. This process is part of nature and one all living things go through.

When it comes to sample trembling aspens for the TREE program (sample process outlined in Module 4), it is important you do your best to identify a healthy tree from a sick one. Some sick trees will not show signs and symptoms on the outside but when you obtain your core, you will see that the parts of the core are starting to rot. If the sample you collect is unhealthy or rotting, we will be unable to collect any usable data for the TREE program.

*A scientific study published by Canadian Forest Service Publications narrowed in on factors that affect trembling aspen health in the boreal forest of Alberta, Saskatchewan, and Manitoba. The article is the foundation of this section and can be found here: https://cfs.nrcan.gc.ca/publications?id=22828

Signs & Symptoms of Unhealthy Trees

There are numerous pests and diseases that affect trembling aspen. The following lists are some common things to look for when assessing the health of aspen in your area.

Unhealthy bark will have at least one of the following symptoms:

  • Discoloration: Any colour other than a pale green/white.

  • Foul Smell: Wood rotting below the bark will have a foul smell.

  • Cankers: Discolored and depressed bark that is dead (see Figure 17).

  • Fungi: Any fungal bodies, like mushrooms or mold, visible around the trunk or higher in the bark is a clear sign that the tree is infested (see Figure 18).

Unhealthy leaves will have at least one of the following symptoms:

  • Discoloration: Any color other than green. The exception is in autumn when the leaves transition to their fall colours of mostly yellow and occasionally red. Discoloration of unhealthy leaves occur in tiny spots. These spots can grow and merge to cover the entire leaf (see Figure 19).

  • Fungi: Any visible fungal bodies on the leave are a clear sign that the tree is infected.

  • Defoliation: Lack of leaves during warmer seasons over small or large areas of the tree.

Figure 18 shows small brown fungi growing on an aspen tree (seen at the center and left side of the tree).
Figure 18 shows small brown fungi growing on an aspen tree (seen at the center and left side of the tree).
Figure 19 shows leaves from a trembling Aspen affected by Marssonina Leaf Spot. Image by Manfred Mielke.
Figure 19 shows leaves from a trembling Aspen affected by Marssonina Leaf Spot. Image by Manfred Mielke.
Figure 17 shows a tree canker, causing slight discolouration and anomalies. Image by AvalokiteshvaraBudha.
Figure 17 shows a tree canker, causing slight discolouration and anomalies. Image by AvalokiteshvaraBudha.

What Threatens a Trembling Aspen?

The list below contains various threats to a trembling aspen that can cause unhealthy states. Aspen can be harmed by pests (P) or diseases (D). Falling trees, rubbing of neighbouring trees, or human impact (such as contact with a vehicle) can also cause mechanical damage, which make aspens more susceptible to pest invasion or disease.

  • Relatively Serious: Forest tent caterpillar (P) / Armillaria root disease (D) / Large aspen tortrix (P) / Poplar peniophora (D) / False tinder conk (D) / Hypoxylon canker (D)

  • Less Serious: Aspen leaf beetle (P) / Aspen leafroller (P) / American aspen beetle (P) / Poplar borer insect (P)

However, not every threat or disease impacts a trembling aspen strand. The presence of a specific pest is often dependent on specific stand conditions. For instance:

  • Diseases such as the large aspen tortrix and poplar borer appear to prefer aspen stands with larger, more widely spaced trees.

  • Armillaria root disease and hypoxylon canker are both found in the less dense stands, which means these pests may be thinning out trembling aspens.

  • The poplar peniophora disease prefers younger, smaller, and more dense stands.

  • False tinder conk is more prevalent in older, larger, less dense stands.

The Most Common Trembling Aspen Threat

In the prairie provinces, the forest tent caterpillar (FTC; see Figure 20) causes the most harm to trembling aspen. An outbreak of FTC's leads to a reduction in growth and tree mortality through defoliation. Sites affected by FTC defoliation show an increased proportion of weakened, declining, and dead aspen trees.

Figure 20 shows a forest tent caterpillar. Image by Greg Hume.
Figure 20 shows a forest tent caterpillar. Image by Greg Hume.

When an Aspen Gets Sick Lesson

Identify the signs and symptoms of healthy and unhealthy trees and investigate the different threats and diseases that impact them. Lessons can be downloaded and adapted from here: https://bit.ly/3DdJggV

Additional Resources & References for Section 1.3

Resources

  • Factors Affecting Trembling Aspen Health on the Prairies: https://cfs.nrcan.gc.ca/publications?id=22828
    Online article from the Forest Ecology and Management Journal that discusses the results of a survey on the boreal forests of Alberta, Saskatchewan, and Manitoba on the various factors (such as forest tent caterpillars, root diseases, climate) impacting trembling aspen.

  • Natural Resources Canada: Trembling Aspen: https://tidcf.nrcan.gc.ca/en/trees/factsheet/58
    If you click on the insects and diseases tab on the webpage, you can get more details into the various insects and diseases that impact trembling aspen. There is a tab as well for the general description of this type of tree.

References

Brandt, J.P., Cerezke, H.F., Mallett, K.I., Volney, W.J.A., and Weber, J.D. (2003). Factors affecting trembling aspen (populus tremuloides) health in the boreal forest of Alberta, Saskatchewan, and Manitoba, Canada [Webpage Publication]. Retrieved from https://cfs.nrcan.gc.ca/publications?id=22828