Old Growth often precedes earlier stages of stand development.

Herbs, low shrubs, and tree seedlings usually occupy a site after a disturbance

High Shrub and Early Tree Stage
After a few years, the early trees and high shrubs began to dominate the site. The shade produced by these larger plants caused the herbs and low shrubs to disappear. Their energy source, the sun, was taken away. But not before they secured their future in the seed bank and provided food for wildlife. Their stage is over, at least for now. Then the high shrub and early trees began to thrive. Shaded soil altered nutrient production by soil microorganisms to match the demands of the trees.

Tree seedlings and tall shrubs begin to dominate the site.

Stem Exclusion Or Pole Stage
During this stage, the addition of new trees stops or is reduced to a few shade tolerant species. Individual trees with the slightest advantage of height, crown area and water, or nutrient availability begin to dominate. This can be a slow process, especially if there are 20 to 30 thousand trees per hectare. Hunters and hikers refer to this stage as a thicket. It's extremely difficult to walk through and it provides lots of cover for hares, deer and other wildlife in the understory.

This is also a period of rest for the understory. Leaf litter builds up, along with the stems and branches of trees that lose the race for dominance. Low light levels cause all the herbs to disappear from the understory. Even mosses have difficulty staying alive. This is most common in conifer thickets. However, in a thicket of white birch, light will penetrate in early spring and late fall and some mosses and lichens remain in the understory during the stem exclusion or thicket stage of a forest stand. Fungi are particularly well suited for this stage. They do not require sunlight and shade provides the moisture they need to flourish.

The logs that were not consumed by the fire are waiting to be recycled by fungi into nutrients for the trees that survive the thicket stage.


During the pole stage, very little light reaches the forest floor.

Understory Re-initiation Stage
Thirty years later, the stem exclusion stage has ended. The white birch occupied the dominant canopy position. A few balsam fir remained as co-dominant trees and more have become established in the understory. Shade tolerant balsam fir will survive in the understory for many years.
The stand now looked different. The 20,000 stems of white birch and balsam fir per hectare have been reduced to 1800. Growth rates have accelerated due to the reduced competition for nutrients, along with availability of sunlight to convert nutrients to food. The boles of the remaining trees grew rapidly and the dense shade of the thicket caused many lower limbs to fall or self prune.

As soon as light begins to reach the forest floor again, trees and plants begin to grow there.

During the stem exclusion stage, the organic layer becomes better suited for the addition of new plants. Fungi begin to break down any large debris. This, coupled with lots of leaves and smaller debris, means that nutrients for herbs, shrubs and trees are in good supply. A much higher canopy with small openings created from crown damage by the swaying of trees in the wind now exists. This condition may persist throughout this stage of development. These small openings allow sunlight to penetrate to the forest floor. Shade tolerant herbs, shrubs and trees begin to establish themselves. Lady's slipper, witch hazel, red spruce and hemlock begin their cycles.

The old stems left from the fire were now only moss-covered lumps lying throughout the stand. The salamander population had increased as the logs created small pools necessary for egg laying. White birch and balsam fir began to die, creating the necessary holes to provide eastern hemlock and red spruce with the light they need to take their place in the canopy. Much of the original debris left from the fire has been recycled by the white birch and balsam fir. As they die, they contributed organic matter that ensured that eastern hemlock and red spruce had sufficient nutrients to reach maturity.

Mature Tree Stage
The stand now began to take on the characteristics of the original forest prior to the disturbances of wind and fire. Only scattered white birch and balsam fir remained. They had been replaced by longer-lived eastern hemlock and red spruce. Because small groups and single trees fell during the transition between the understory re-initiation and old growth stages, an all-aged forest developed. Low light conditions allowed shade tolerant eastern hemlock and red spruce to become established in the understory while they waited for an opening in the crown. Once a tree fell and created an opening, they were poised to grow and fill the space. These shade tolerant species do not grow as fast as white birch and must be established to out-compete such faster growing species.


Trees begin to die of old age, allowing even more light to reach the forest floor.


The ground was covered with mosses, some ferns and a few herbs. The mosses helped maintain high levels of moisture required for micro-organisms to break down leaves, boles and roots of the fallen trees. They also contributed moisture to the air, which helped maintain a high relative humidity in the understory. This is important for some organisms that inhabit a forest in an old growth stage.

The white-footed mouse had been replaced by the red squirrel, which is thriving on the large amounts of seed being produced by the mature trees. No longer was there large amounts of browse for the white-tailed deer. Slow growing lichens appeared which are only found in forests that have not been disturbed for many years. Fungi thrive in the cool, moist understory of the old growth stand. Fungi have very wide spreading root systems that can transport nutrients from one area to another. This is helpful in the decomposition of large woody debris that is common in this forest stage.

A typical forest nutrient cycle

Lesson 1 Quiz