Changes in the forest over time

Forests are in a constant state of change. Disturbances in the forest--such as windstorms, fires, major pest attacks, land-use conversions, or logging operations--all are important factors that lead to changes in the forest. Yet even without such a disturbance, the composition of the forest will gradually change over time as the forest community matures. The particular changes that take place are influenced by the amount of light, nutrients, and water available to the plants. The gradual change in species composition and replacement of some forest species with others over time or after a disturbance is called succession.
	Disturbance may remove some or all of the existing vegetation, including eliminating a particular species.
	Sun-loving, fast growing plants such as grasses, wildflowers, and trees like aspens and white birches are the first plants to establish in the disturbed area. These plants are called early successional species or pioneer species.
	As the pioneer trees become larger and shade the area, other tree species begin to grow up in their shade. The particular plant community that will replace the early successional species will depend upon the amount of nutrients and water available to the plants. For example, if the early successional species at a site are aspen and white birch and the site is very wet, we would expect balsam fir and white spruce to be the next tree species at the site. If the site had only moderate moisture, we would expect red oak and red maple. If the site had less moisture, we would expect white pine.
	Eventually, shade-tolerant species or species otherwise better suited for the current site conditions may take over as the short-lived pioneer species die out. The community will continue to undergo gradual changes in species composition, which will lead to a final, relatively stable community composed of species highly adapted to the growing conditions. These species are called late successional species, and this community sometimes is referred to as the climax community.
At any point along this successional sequence disturbance may occur, which will then set the forest back to an earlier stage in succession. Various harvesting techniques or pests could accelerate succession if they select against pioneer species.