The typical forest ("stand") starts life with a relatively large number of small trees, usually thousands or tens of thousands per acre. The number of trees decreases as they grow older and larger, at first rapidly but more slowly with each passing decade. An "old-growth" forest might have only a hundred or so very large dominant trees per acre.
This continual diminution in numbers is primarily the result of a rigorous natural selection and is the expression of one of the most fundamental biological laws: self-thinning. Those trees that are more vigorous or best adapted to the environment are most likely to survive intense competition for light, moisture, and nutrients. Growth in height is the most critical factor in competition, although those tress that increase most rapidly in height are almost invariably the largest in all dimensions, especially in size of crown. As their taller associates crowd the weaker trees their crowns become increasingly misshapen and restricted in size. In this constant attrition the weaker trees are progressively submerged and the strongest forge ahead.
Structural classifications
We can differentiate among individuals into different strata or crown classes and as in this example from a longleaf pine ecosystem from southern Georgia, into different lifeforms as well.
However, four standard crown classes are usually recognized. These include:
Dominant: Trees with crowns extending above the general level of the crown cover and receiving full light from above and partly from the side; larger that the average trees in the stand, and with crowns well developed but possibly somewhat crowded on the sides.
Codominant: Trees with crowns forming the general level of the crown cover and receiving full light from above, but comparatively little from the sides; usually with medium-sized crowns more or less crowded on the sides.
Intermediate: Trees shorter than those in the two preceding classes, but with crowns extending into the crown cover formed by the dominant and codominant trees; receiving a little direct light from above, but none from the sides; usually with small crowns considerably crowded on the sides.
Overtopped (Suppressed): Trees with crowns entirely below the general level of the crown cover; receiving no direct light either from above or the sides.
The structure of a forest ecosystem can change in response to many factors. Changes in site conditions can lead to dramatically different structures. For example, in the southeastern United States there are more developed shrub layers on wet sites than mesic to dry sites. Time (also known as succession) is another factor that can lead to changes in forest structure (in addition to changes in forest composition). The figure below shows the various pathways of change over time in a mixed-oak forest ecosystem of southeastern Ohio. The amount by which the population changes in size during one stage or over one interval of time is termed as net recruitment.
