Old-Growth Forest Ecosystem Characteristics

Old Trees

• As their name implies, old-growth forests include old trees. The actual age of the oldest trees depends on the type and location of the forest. Coniferous old-growth forests in the Pacific Northwest are usually over 350 to 750 years old, according to the U.S. Forest Service. The age of Eastern deciduous old-growth forest trees depends on the life span of the dominant tree species, but can range from at least 100 years for red pine stands to over 285 years for hemlock-yellow birch stands. Some forest types are composed of much longer-lived trees, such as the bristle-cone pines of the arid mountains in the western United States. These trees can live to be over 4000 years old.

Tree Size

• In some types of old-growth forests, large trees are a defining characteristic. Old-growth Douglass fir trees in the Pacific Northwest can grow up to 5 feet in diameter and hundreds of feet tall. However, not all old trees are large, and not all large trees are old. In some locations, such as dry areas in eastern and southern Oregon, local conditions slow tree growth, so trees over 200 years old can be as little as 20 feet tall, according to Oregon Wild.

Snags

• Trees in undisturbed forests eventually die on an individual basis. One of the characteristics of old-growth forests is the scattered occurrence of standing dead trees, called snags. Snags serve important roles in old-growth forest ecosystems, providing a food source and nesting site for many types of insects, birds and mammals.

Downed Logs

• While some old-growth trees die and become snags, others fall to line the forest floor with large downed logs. Unlike forests that have been logged in the past, undisturbed old-growth forests are often marked by downed logs. The logs decompose slowly over time, returning nutrients to the soil while providing food and habitat for other organisms in the process.

Canopy Gaps

• The trees of an old-growth forest form a canopy that can shade the forest floor, making it difficult for shade-intolerant species to grow. When individual trees die, their crown disappears, leaving a gap in the canopy. Sunlight reaches the forest floor through canopy gaps and helps fuel new growth.