Old-growth forests are natural forests that have developed over a long period of time, generally at least 120 years (DNR definition and consistent with definitions for the eastern United States), without experiencing severe, stand-replacing disturbance—a fire, windstorm, or logging. Old-growth forests may be dominated by species such as sugar maple, white spruce, or white cedar that are capable of reproducing under a shaded canopy. These old-growth forests can persist indefinitely. Old-growth forest may also be dominated by species such as red pine, white pine, or red oak that do not reproduce as well under shade and that require disturbance to open the canopy. These old-growth forests will eventually be replaced by the more shade-tolerant tree species in the absence of disturbance. Typical traits of Minnesota old-growth forests include:
- Some trees are at least 120 years old (often at least 2–3 feet across).
- Large, dead standing trees and branches (snags) are common.
- Large fallen trees and branches lie on the ground.
- The forest is a mix of young, old, and middle-aged trees (multi-aged).
- Small openings (canopy gaps) are visible between the tree crowns.
- Dirt piles and holes from tipped-over trees (tip-up mounds and pits) dot the ground.