Every type of forest is different and there is no commonly accepted definition of an old-growth forest. British Columbian scientists have developed a working definition that’s based on the age when the province’s different forest ecosystems typically begin to develop old-growth characteristics.

Characteristics of old growth can include tree species, tree age, tree size, surrounding forest structure, ecological function, and historical disturbance. While these characteristics vary according to location and species, old-growth forests tend to have more diversity of plant and animal life (including standing dead trees, or snags, and fallen trees) than younger forests. The trees are often larger and the forest canopy is layered with openings that allow light to reach the forest floor, encouraging the growth of ferns, berry bushes and mosses.

Since the 1990s, the Government of British Columbia has used a definition of old growth that is based on the age of trees, biogeoclimatic zones and the frequency of natural disturbances such as wind, fire and landslides. Generally speaking, most of B.C.’s coastal forests are considered to be old growth if they contain trees that are more than 250 years old. Some types of Interior forests are considered to be old growth if they contain trees that are more than 140 years old.

The Biodiversity Guidebook (1995) provides a more detailed and technical definition of an old-growth forest.