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.

What Can Cause a Tree to Change?

Like any living thing, trees can be impacted by threats that can cause changes in their life cycle. Trees do have systems in place to combat threats but sometimes, just as much as humans can get sick, so to do trees. While being sick is not necessarily a bad thing, it can have a negative impact on the tree and cause it to rot or die in some cases. On the other hand, sometimes what the sick tree produces, can be beneficial for other organisms and be incorporated into their life cycles. Changes to our life cycles is part of nature and one all living things go through.

*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

What Kind of Trees Does TREE Need?

The research question for TREE is looking to understand how the environments across Canada impact an average trembling aspen tree. Therefore, when it comes to sample trembling aspen for the TREE program (sampling process outlined in Module 4), it is important you do your best to identify a healthy tree from a sick one.

Some trees impacted by change will not show signs and symptoms on the outside but when you obtain your tree core, you will see that the parts of the core are starting to rot. If the sample you collect appears sick or is rotting, we will be unable to collect any usable data for TREE research. The best practice for sampling is to identify which trees are healthy (continue reading for signs and symptoms) and to not sample from unhealthy trees, as these trees are in a vulnerable state. Sampling from these trees can make the tree worse so it is best to leave them be, giving them time to heal or let nature take its course.

In other cases, sometimes the core is healthy but you notice there is an infestation or something appears off with the tree. Make note of this in your Sample Form so the researchers are aware such as with Figure 17.


Figure 17 shows signs of an insect infestation on a trembling aspen that should be noted on the Sample Form. See the “saw dust” and discolouration around it. Image by Amanda Pfeiffer.
Figure 17 shows signs of an insect infestation on a trembling aspen that should be noted on the Sample Form. See the “saw dust” and discolouration around it. Image by Amanda Pfeiffer.

Signs & Symptoms of Changed 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.

Impacted bark may 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 18).

  • 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 19).

Figure 19 shows small brown fungi growing on an aspen tree (seen at the center and left side of the tree).
Figure 19 shows small brown fungi growing on an aspen tree (seen at the center and left side of the tree). Image by Amanda Pfeiffer.
Figure 18 shows a tree canker, causing slight discolouration and anomalies. Image by AvalokiteshvaraBudha.
Figure 18 shows a tree canker, causing slight discolouration and anomalies. Image by AvalokiteshvaraBudha.

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 20).

  • 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 20 shows leaves from a trembling Aspen affected by Marssonina Leaf Spot. Image by Manfred Mielke.
Figure 20 shows leaves from a trembling aspen affected by Marssonina Leaf Spot. Image by Manfred Mielke.

Connect with Indigenous Groups

Indigenous groups in your area may have knowledge about changing trees. Check out our Indigenous Engagement Starting Points resource to learn how to connect: https://bit.ly/3eENsyn

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:

  • Insects 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 21) 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. You can explore more in our When as Aspen Gets Sick Lesson.

Figure 21 shows a forest tent caterpillar. Image by Greg Hume.
Figure 21 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 the link: https://bit.ly/3DdJggV

When Sampling

Remember, when you are sampling for the TREE program, do your best to locate an average healthy tree. If you come across a tree impacted by a pest or disease, best practice is to show the tree compassion and sample a different tree.

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