Discovering the Forests of Alaska , Oregon, and Washington

Stages in the Life of a Forest

For inventory and mapping purposes, scientists recognize five basic stages in the life of a forest. These stages are defined below. In nature, forests do not fit into these classes as neatly as the table below suggests. A forest changes gradually from one stage to the next. Often young forests in the seedling stage will have some pole-size, mature, or old trees. A forest with many old trees often has patches of young trees, too.

Ecosystems can be studied in terms of their structures, composition, and processes. Examples of structures include:

Seedling

Trees are 0 to 10 years old. Most or all trees are less than 10 years old, and the new forest is very open and may seem more like a meadow. In a forest with old trees, the seedling trees start in the shade of older trees or in openings created by fire, wind, or people.

Pole and sapling

Trees are 10 to 40 years old. Most or all trees are in this age group, and the forest canopy is often closed, shading the forest floor. Tree diameters are not large yet.

Young

In softwood species such as Douglas-fir and pine, trees are 40 to 80 years old. In hardwood species such as maple and oak, trees are 40 to 60 years old. The forest is taller and the trees are bigger than a pole and sapling forest, yet the growth rate is still rapid. Forests may have a closed canopy or may have some small openings.

Mature

In softwood species, trees are 80 to 140 years old. In hardwood species, trees are 60 to 140 years old. Trees have reached the mature size for their species. The growth rate of mature trees has leveled out, and mature trees grow at a slower rate than young trees.

Old mature

Trees are 140 years or older. Some tree species such as Douglas-fir and western redcedar are very long-lived, and may live to 500 or 1,000 years old if no fire, storm, disease, insect outbreak, or timber harvest kills them. Tree species such as red alder have much shorter lifespans.