Monitoring Forest Trees for Symptoms and Signs of Potential Problems

Poor health in trees can be caused by insects, diseases, animal damage, old age, competition with other trees, environmental factors (such as drought), or some combination of these factors. Tree health issues may or may not be problematic, depending on their extent and what is causing them. For example, most organisms that kill trees are native to our forests and occur at some endemic level most of the time. Many foresters do not consider damage from such organisms a problem unless levels become epidemic. New exotic species are of more concern to foresters. Moreover, trees can exhibit characteristics that may look problematic but actually are related to normal tree development. For example, lower branches on a tree may die because of being shaded out by branches higher on the tree.

If you notice an unacceptable number of damaged, dying, or dead trees in your forest or if tree health issues are interfering with your goals and objectives for your site, you can take action to address the problem. The first step in addressing the problem is examining your trees for information. As you examine your trees, it is important to know what you are observing and how to describe your observations to a professional. Also, you must understand that you may not be able to observe exactly what is causing a tree illness or that you may observe only a secondary cause of a tree illness.

Pine branch infected with Marchalina hellenica, a type of scale insect that lives by sucking the sap from pine trees. The insects leave visible secretions known as “honeydew” on the branches. Photo: Diomidis Spinellis, Wikimedia Commons (CC BY-SA 3.0).

Recognizing Symptoms and Signs

Forest health professionals use symptoms and signs as clues to the causes of tree illnesses. To make progress in addressing a tree illness in your forest, you need to be able to recognize symptoms and signs of tree health issues.

  • A symptom is a response by a tree to something attacking it. Examples include off-color needles or leaves, wilting leaves, and epicormic branching (a tree’s last-ditch effort to sprout branches and leaves wherever it can).
  • A sign is an indication of the existence of a pest or disease. Examples include insects in various stages of development (adults, larvae, or eggs); insect tunnels; exit holes in the bark of a tree; and mushrooms growing out of a tree trunk. It is important to note that even signs of a pest are not conclusive evidence that the pest is the agent that is killing the tree. Once a tree is weakened for some reason, such as by drought or flooding or a particular pest, it is vulnerable to attacks by a variety of pests.

As you monitor your forest, answering the following questions can help you identify symptoms and signs of tree health issues:

  • Are whole trees turning brown?
  • Are tree crowns slowly thinning or fading?
  • Is there heavy needle or leaf loss?
  • Is there evidence of insect feeding?
  • Are leaves/needles smaller than normal or otherwise deformed?
  • Are there dead branches?
  • Is growth slowing significantly?
  • Are conks, mushrooms, or other types of fruiting bodies of decay fungi growing out of a tree’s trunk?

If you observe tree health issues in your forest, note where on trees you are seeing the symptoms and signs. For example, did you find an insect on a tree’s bark, on its leaves, or in the soil around its roots? Are the leaves of a tree dying from the top of the tree first, or are they dying from the bottom up? Noticing where a problem is occurring can provide insight in determining its cause. For example, a sun-loving insect eats leaves beginning at the top of a tree. A fungal leaf infection starts in the moist environment of the lower canopy of a tree. 

Once you have examined specific trees, look at the woods as a whole. Is just one tree dying, or are multiple trees affected? If multiple trees are involved, note whether they are in a pocket grouped together. Next, determine whether only one species is affected or multiple species are affected. If only one species of tree is affected in a pocket, the cause of the problem may be a pest damaging the roots of connected trees.

Also, keep in mind that most insects and diseases affect particular species of trees. In contrast, if multiple species are showing signs of trouble, the problem may be from abiotic (nonliving) causes. Examples of abiotic causes include salt damage; herbicides; drought; flooding; and extreme weather events, such as hail, severe wind, and lightning.

Determining the Cause of Observed Symptoms and Signs

Recognizing tree illness symptoms and signs is only the beginning of addressing a tree health problem in your forest. Ultimately, you need to determine what is causing the symptoms and signs you observe and whether that causal agent is something that could kill an entire tree or even multiple trees. Many factors that affect trees in a forest also can affect trees in nearby yards and urban landscapes. The results of your examination of trees in your forest will help you either identify the cause of the problem yourself or be able to explain the problem to a professional. For professional assistance, you can provide details from your observations or bring samples or photos to your local Cooperative Extension System office staff, who should be able to help you identify the cause. Also, a local state forester may be able to visit your site and help you assess the situation. Additional resources for getting forestry help are available as well.

The following articles provide more information about monitoring your forest to keep it healthy:


Kris Tiles, University of Wisconsin-Extension

Chris Schnepf, University of Idaho