A review of the stages helps to understand the concept

1. Stand-replacing: disturbance hits the forest and kills most of the living trees.

2. Stand-initiation: population of new trees becomes established. 3. Stem-exclusion: trees grow higher and enlarge their canopy, thus competing for the light with neighbors. Light competition mortality kills slowly growing trees and reduces forest density. This allows surviving trees to increase in size. Eventually the canopies of neighboring trees touch each other and drastically lowers amount of light that reaches lower layers. Due to that, the understory dies and only very shade-tolerant species survive.

4. Understory reinitiation: trees die from low level mortality, such as windthrow and diseases. Individual canopy gaps start to appear and more light can reach forest floor. Hence, shade-tolerant species can establish in the understory.

5. Old-growth: Main canopy trees become older and more of them die, creating even more gaps. Since the gaps appear at different times, the understory trees will be at different growth stages.

Furthermore, the amount of light that reaches each understory tree depends on its position relative to the gap. Thus, each understory tree grows at a different speed. The difference in establishment timing and in growth speed create a population of understory trees that are variable in size. Eventually, some understory trees grow to become as tall as the main canopy trees, thereby filling the gap. This perpetuation process is typical for the old-growth stage. This, however, does not mean that the forest will be old-growth forever. Generally there are three possible futures for old-growth stage forest: (A) The forest will be hit by a new stand-replacing

disturbance and most of the trees will die. (B) The tree community will eventually create unfavorable conditions for new trees to regenerate. In this case, the old trees will die and smaller plants will create woodland. (C) The regenerating understory trees are different species than the main canopy trees. In this case, the forest will switch back to Stem-Exclusion stage, but with different tree species. The forest in old-growth stage can be stable for centuries but the length of this stage depends on the forest's tree composition and climate of the area. For example, frequent natural fires do not allow boreal forests to be as old as coastal forests of western North America. Of importance is that while the stand switches from one tree community to another, the stand will not necessarily go through old-growth stage between those stages. Some tree species have relatively open canopy. That

allows more shade-tolerant tree species to establish below even before Understory Reinitiation stage. The shade-tolerant trees will eventually out-compete the main canopy trees in stem-exclusion stage. Therefore, the dominant tree species will change, but the forest will still be in Stem-Exclusion stage.

Tree species succession may change tree species composition once the old-growth stage has been achieved. For example, an old boreal forest may contain some large aspen trees, which may die and be replaced by smaller balsam fir or black spruce. Consequently, the forest will switch back to Understory Reinitiation stage.[11] If old growth stage is seen as an end point of stand development, it can be easily evaluated using structural or static attributes. However, in some forest ecosystems this can lead to decisions regarding the preservation of unique stands or attributes that will disappear over the next few decades because of natural succession processes. Consequently, using stand dynamics to define old-growth forest is more useful in cases where the species that constitute old-growth forest can have long life span or in ecosystem where succession is very slow.