Old-Growth Forests

Old-growth forests are an end-of-succession, climax ecosystem. They are dominated by trees of great age, but occurring within a mixed-species community with an uneven-aged population structure (that is, all tree ages are represented in the community). The physical structure of old-growth forests is very complex, and includes multiple horizontal layers, gaps of foliage within the canopy, great variations of tree sizes, many large, standing dead trees (called snags), and logs lying on the forest floor. In some ecological contexts the term "old-growth forest" might also be used to refer to senescent stands of shorter-lived species of trees, such as cherry, birch, or poplar. However, the usual interpretation is that an old-growth forest is a late-succession or climax ecosystem, with the broad features described above.

For old-growth forests to develop, a long time must pass between events of disturbance that are severe enough to cause a stand-level mortality of dominant trees. Therefore, old-growth forests occur in places or regions where fire, hurricanes, and other catastrophic disturbances are rare. These circumstances are especially frequent where there is a great deal of rainfall throughout the year. Consequently, many of the best examples of old-growth forests are tropical and temperate rain forests.