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
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:
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.
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.
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.
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.