A Look at Forest Health—Simple Steps to Help Evaluate the Woods Nearby

Posted: April 21, 2016

Now—before the trees are fully leaved—is an ideal time to observe factors that help to describe forest health.

In the “procession” of spring blooms and leaves, it will be another three weeks or so before the leaves of trees begin to unfurl. Some shrubs, however, conspicuously have had their leaves for almost a month now. As happy as many of us are to see the first green after the winter months, the green of these early-leafing shrubs is actually a tell-tale indicator that these shrubs are likely invasive species—autumn olive, Russian olive, Japanese barberry, and bush honeysuckle to name a few, as well as small trees like the Bradford pear.

Like the early leafing-out of invasive shrubs, seasonal changes generally reveal clues to woodland health that may have gone unnoticed before. Now—before the trees are fully leaved—is an ideal time to observe some other factors that help to describe forest health. Though a forester will use a more in-depth process to evaluate forest health, there are some simple steps anyone can take to get a basic understanding of whether or not a forest is “healthy.”

One important first step in evaluating forest health is to look at the physical arrangement of the woods—the forest structure. A healthy forest has multiple vertical layers. Beginning with the forest floor and moving up, these layers are: the leaf litter, the understory (composed of herbaceous plants, woody shrubs and young trees usually under about 5 ft. in height), a mid-story (with young trees and shrubs taller than those in the understory), and an overstory (also called the canopy). Trees and shrubs that have died are also part of forest structure. Each layer in the forest structure performs a set of “job duties” for the forest ecosystem, whether it is shelter or food for a variety of animals, regulating the amount of light available to plants below, or enriching the soil through decomposing leaves or logs.

To practice what a forester looks for in evaluating forest structure, begin by simply looking from the ground to the tops of the trees, and then scan the woods from side to side. Are the expected layers of the forest structure there? Are there large gaps—with no living or dead tree structures—between the canopy and what is below? Is there a very dense shrub layer present? Do some shrubs and/or vines develop leaves long before the other trees and shrubs?

Early leaf development is often a clue to identifying invasive species. Landowners can take advantage of this characteristic “self-identification” by invasive plants and take time to mark them with flags, ribbons, or other visible marking for locating later. Killing invasive shrubs is an important activity. At the edge of a woodland, where they are commonly found, their seeds are carried by wildlife and become planted elsewhere. In this way, these shrubs can spread deeper into woodland interiors, where it is especially important to find and remove them. The dense layer formed by these fast-spreading invasive shrubs significantly reduces the amount of light that reaches the forest floor. This reduction in light, in turn, may eliminate or greatly reduce the number and diversity of native plants and considerably impact the successful development of tree seedlings.

Looking further into the forest structure, are dead or dying trees present? In a healthy forest, some dead and dying trees are expected—in fact, they are necessary to the balance of a forest ecosystem. Dead trees that are still standing, known as snags, provide food to insects and animals that eat insects, shelter to a variety of birds and mammals. Dead trees and branches also continue to provide food and shelter after falling to the ground, in addition to helping to build and enrich the soil. Because of this role in the ecosystem, fallen dead trees, also called “coarse woody debris,” are more good indicators of woodland and even stream health.

On the other hand, if a forest has many dead or dying trees, it could be a sign that a pest insect such as emerald ash borer or gypsy moth has invaded. It could also indicate some type of disease that is affecting the trees. Signs of a dead or dying tree include: woodpeckers visiting a particular tree frequently, holes or patches of cleared bark left from their feeding, or tree branches without bark and/or small twigs.

Having trees of different ages is another indicator of forest health. It would seem that trees of different heights and diameters in our woodlands indicate trees of different ages. Instead—due to past harvesting, land clearing during settlement times, and some catastrophic natural events (insects, fire, disease, for example)—most trees in a given Pennsylvania woodland are actually about the same age. Differences in the height and diameter of individual trees is a direct result of light conditions and “room to grow” available to the trees. The species of tree is another factor that leads to differences in height and diameter, as some species grow faster than others. In a forest, there may be patches of trees that are a different age than another patch of trees within the same forest—as a result of light, space, and species—but the overall age of the forest will be about the same.

We can promote woodland health by helping along the growth of younger trees. An easy starting point, with the help of a few guiding questions, is to evaluate whether or not the conditions are right for establishing new trees to grow into the canopy. Are there openings in the canopy that allow light to reach the forest floor? If not, some tree species may not receive enough light to grow; they may be “hanging out” in the understory waiting for the right light conditions to help them grow. Are there tree seedlings present across the forest floor? Are there young trees that are unusually scraggly? If so, it could be an indicator that deer are browsing heavily on the young trees, stunting the growth of the tree.

These are just a few but important ways to look at woods to get a sense of their health. It is very likely that the woods near you could use a little assistance. The necessary help may include: removing non-native and invasive plants to allow growth of native species; protecting young trees from deer damage with fencing and shelters; and creating openings in the canopy so that trees of different ages and species can become established and eventually grow into the canopy. Diversity in woodland species and age is essential in the long term. A healthy forest is diverse and will thereby remain resilient to unforeseen changes that will occur over time.

Contact Information

Leslie Horner
  • Forest Stewardship Program Associate
Phone: 814-867-5982