An edge effect refers to how the local environment changes along some type of boundary, or "edge."

As humans we deal with many types of environmental "edges," such as the cold blast of air upon entering an air-conditioned building from a hot street, or entering a treed, shaded city park after biking along a broad, treeless avenue. The edge is the change in temperature and humidity that we feel, and an edge effect is our reaction to the change in temperature (e.g. putting on a sweater, or searching out a patch of sun).

The edge effect we most often hear about in forestry refers to the forest edges that are created when we harvest trees, particularly when we choose to clearcut. Tree canopies provide the ground below with lots of shade, while a clearcut allows sun to reach the ground; tree canopies maintain a cooler, moister environment below, while a clearcut is warmer and drier when the sun is out. As time passes and a stand of young trees emerges on a clearcut, the environment in the young stand changes and the edge begins to fade. As a mature forest develops, the edge fades away. The edge effect is the result of two different conditions influencing the plants and animals that live on the edge.

Some animals, deer and elk for example, like the forest edges because they can find food in the clearing and hiding cover in the trees— they are called "edge species," because they can thrive on the edge and benefit by having various habitats near one another. Other animals, like the spotted owl, prefer areas with a lot of canopy and few edges. Edges for this and other plants and animals, can be dangerous. Research suggests, however, that the dangerous edges occur when a forest shares an edge with agricultural or suburban development lands, and not when the forest lands remain in forest use— the edge between an old and young forest is not an obstacle as is an edge between a forest and an agricultural field, or a forest and a suburb.

If we are managing our land with most plants and animals in mind, then should we be mindful of our edges caused by harvesting timber? It may be that edges are less important to the success of our forest animals than is the total area of forest habitat suitable for them. While there is no ideal forest patch size for all animals, we do need to manage our forested landscape to include a mix of forest types, and a sufficient area of each type, that will meet the needs of the greatest number of species.

When we plan to harvest trees from an area, we need to consider what will result from the removal of an older forest structure and subsequent creation of a younger one. Will we hurt animals that we want to protect by reducing habitat? Will we provide forage for animals that we want to grow strong? Will fragile plants disappear ? Will desirable plants flourish in their new environment? That is life in the forest mosaic, and as we learn more about plants and animals in the forest, we'll be better able to both protect our forest resources and harvest trees.