Northwest Illinois Forestry AssociationWoodland owners sharing ideas on forest productivity

Snags, Edges, and Understory Plants


A "snag" is a dead or dying tree suitable as a perch or nest site or food source for wildlife.

While some snags occur naturally through disease; others are created by foresters through thinning or crop tree release. They are purposely left standing to make homes and perches for birds and mammals.

There are at least 38 species of birds that either excavate nest holes or use existing holes in dead or dying trees.  In addition, at least 29 species of mammals also use tree cavities.

Snags serve birds in many ways. Woodpeckers and small insect-eating birds find food on these dead or dying trees. Birds of prey frequently use snags as hunting perches. Many songbirds that occupy habitat on the forest edge use snags as singing perches.  Woodpeckers often use resonant undecayed portions of snags as drumming sites to signal their territory.  Primary cavity nesters, such as woodpeckers, typically excavate their own nest sites. Secondary cavity nesters, such as screech owls, use natural cavities and abandoned woodpecker excavations.

Wildlife specialists have noted that snags are most useful if they are at least 12 inches in diameter, although some smaller birds use trees as narrow as 4 inches. Sometimes snags are left not for their size; rather they were those crowding Crop Trees. Wildlife specialists also have found that at least three snags per acre is ideal.

Snags will eventually decay, fall to the ground, and return their stored nutrients to the forest soil.


The edge of a forest can be very important to wildlife. This edge area is usually ignored. The neighboring cropland or prairie or pasture ends; shrubs and tall plants fill in the space on the edge of the forest - they grow faster than trees. So, for a few years this space is dense with "weeds" and "brush".

Birds and small mammals need this type of vegetation for cover from predators and severe weather. What's more, frequently, the type of shrubs that occupy this space are seed-bearing or fruit-bearing. Dogwood is an example - it grows in thickets and provides fleshy fruits that many birds eat.

This edge habitat allows wildlife to live and feed close to the forest. They can take advantage of the mast producers, such as oaks, walnuts, and hickories without having to travel great distances.

Ideally, this edge habitat should be at least 30 feet wide without any trees shading it. The plants should produce berries, seeds, and lots of new twigs to chew. Foot traffic, mowing, and grazing should be eliminated to provide safety for wildlife nests.


The understory of a forest is composed of trees, shrubs, and ground plants. The trees are either seedlings struggling to survive or are older trees that have been suppressed by the lack of sunlight.  This section will address the other plants - the shrubs and ground plants.

The development of understory growth is dependent upon several factors: the quality of the soil, the amount of rain, and especially the density of the canopy. Oak forests usually have a thicker understory than a beech-maple forest because oak canopies are more open and oak crowns are not as dense as maple crowns or beech crowns. Canopy density appears to control what grows on the forest floor.

There are different ways that plants have adapted to live within the forest, competing for sunlight and nutrients.

Some try to complete all of their annual growth before the canopy closes over with leaves; they grow in the spring or fall. Examples are bloodroot and may apple.

Some have adapted to live with very little or no direct sunlight; examples of these would be the lichens and mosses.

Some plants start on the forest floor with extensive root systems and send up shoots that reach the forest canopy; these are the vines.  Both virginia creeper (vine) and grape vines can overtop trees and shade the canopy.  Grape vines will deprive trees of enough sunlight that they will die.

Some plants grow in clusters, shading out any competitors - claiming a small section of forest floor and resources.  Examples include dogwood and prickly ash.

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