To further understand the concept of an ecological trajectory, let?fs assume you are walking through a 40-acre stand of quaking aspen and paper birch that is about 20 years old. The forester with you notices that part of this stand is on an upland with loamy-sand soil. He points to understory plants that indicate a moderately dry growing site. Besides the tall aspen, there are quite a few shorter red and white oaks, red maple, sugar maple, white ash and basswood. If nature takes its course and no natural disaster or human interference occurs while the hardwoods are maturing, this aspen stand will eventually succeed to a northern hardwood stand in which aspen is a minor component. But if a fire sweeps through the stand or it is clearcut when the aspen are mature, then hardwoods will be suppressed and aspen will continue to dominate the stand.
Another portion of the aspen stand you are exploring is on a lowland with silty clay-loam soil with a high percentage of humus (decaying organic matter). The forester points to shrubs and understory plants that indicate the site is moist to wet during much of the growing season. Beneath the canopy of aspen, balsam fir and white spruce are growing in abundance. In the absence of a natural disaster or clearcut, the ecological trajectory for this stand is toward a spruce-fir stand with a minor component of aspen. If a fire swept through this stand or it were clearcut when the aspen were mature, then aspen would quickly regenerate and the balsam fir and white spruce would again be suppressed.
Do not assume that the tree species currently growing on a site are those best adapted for long-term production. Ask a forester to evaluate the whole plant community (trees, shrubs, and understory plants) as well as soil, moisture, topography, and climate, then recommend which tree species will grow best on the site. Change stand boundaries as needed to encompass sites with relatively uniform growing conditions.
From the above discussion, you probably also realize that if no human management intervenes in a woodland, natural forces still cause change. Each tree species has a natural life span that may be shortened on a poor site or lengthened on a good site. Competition for nutrients, moisture, and sunlight leads to the death of individual trees. Fire, wind, insects, and disease can weaken and eventually kill trees.