Old-Growth Forest
This type of forest is also described as a primary forest, ancient forest, virgin forest, primeval forest, frontier forest, or ancient woodland.? It is a forest which contains very old and mature trees (and associated advanced structural features) which exhibit unique ecological features..
This type of forest typically contains large and old live trees, large dead trees, sometimes called "snags", and large trunks and/or limbs which have fallen on the ground.? Individual tree mortality creates gaps in the main canopy layer, allowing light to penetrate the main canopy and create favorable photosynthetic conditions for the undergrowth, which is why primary forest undergrowth is more developed than in immature tree stands.
Many worldwide old-growth forest stands are threatened by habitat destruction through excessive logging. The resulting destruction reduces biodiversity, affecting not only the old-growth forest itself, but also indigenous species that rely upon old-growth forest habitat.
Primary forests are often home to rare species, threatened species, and endangered species of plants, birds and animals, making these woodland areas ecologically very important. One example of a rare species reliant upon old-growth forest is the Northern Spotted Owl found in North America. Levels of biodiversity may be higher or lower in primary forests when compared to secondary growth forests, depending on specific circumstances, environmental variables and geographic variations, e.g. what part of the world the forest is located. Logging in primary growth forests is a contentious and highly controversial issue in many parts of the world today.

Definition by Characteristics
Most definitions use forest characteristics to define old-growth forest. Usually the characteristics include presence of mature trees, dead standing snags, a multilayered canopy dominated by large tall trees, and accumulations of large dead woody material on the forest floor.

Stand Dynamics Definition
From a stand dynamics perspective, a primary forest is one in a stage that follows Understory Reinitiation Stages. A review of the stages helps to understand the concept:
Stage 1-Stand-replacing: disturbance hits the forest and kills most of the living trees.
Stage 2-Stand-initiation: population of new trees becomes established.
Stage 3-Stem-exclusion: trees grow higher and enlarge their canopy thus competing for the light with neighboring trees. Light competition mortality kills slowly growing trees and reduces forest density. This allows those surviving trees to increase in size. Eventually the canopies of neighboring trees touch each other and drastically lowers the amount of light that reaches the lower layers. Due to that, the understory or undergrowth dies and only very shade tolerant species survive.
Stage 4-Understory Reinitiation: trees die from low level mortality, from wind damage and diseases for example. Individual canopy gaps start to appear and more light can reach the forest floor. Because of this, shade-tolerant species can establish in the understory growth on the forest floor.
Stage 5-Old-growth: Main canopy trees become mature and more of them die, which creates even more gaps in the canopy. Since the gaps appear at different times, the understory growth trees establish in different time intervals from each another. Furthermore, the amount of light that reaches each understory tree depends on its position relative to the gap. Thus, each understory tree develops at a different speed. The difference in establishment timing and in growth speed create a population of understory trees that vary in size and shape. Eventually, some understory trees shoot up to become as tall as the main canopy trees. Because of this, the gap created by the older dead tree is closed by a younger one that eventually will also die and will be replaced by another tree. This perpetual process is typical for the forest old-growth stage.
This, however, does not mean that the forest will be old-growth forever. Generally, there are three possible scenarios for an old-growth stage forest in the future:
1)The forest will be hit by a new stand-replacing disturbance (e.g. logging, wind, disease, insects, fire, etc.) and most of the trees will die.
2)The woodland will eventually create unfavorable conditions for new trees to regenerate. In this case, the old trees will die and smaller plants will thrive in their place.
3)The regenerating undergrowth trees are different species than the main canopy trees. In this case, the forest will switch back to the Stem-Exclusion stage, but with different species of trees. Woodlands in old-growth stages can be stable for centuries and even thousands of years, but it all depends on its tree composition, man-made conditions, and the general climate of the area. For example, frequent natural fires do not allow boreal forests to be as old as the forests of western North America or those of northeast China.