One way to evaluate the effects of thinning hardwood forests is to compare the growth and yield of a thinned forest with an unthinned forest.

The first 10 to 15 years of forest development is a dynamic period. During this "brushy stage," there are as many as 10,000 woody stems per acre, in a wide variety of commercial species and brush. This is the period of forest development when skeptics doubt whether a new forest will ever become reality.

When a new forest is 15 to 20 years old, certain predictable patterns can be identified. Most of the brushy species have died naturally or are in a subdominant position in the forest. The new forest has clearly emerged, and the trees now have measurable characteristics. Natural mortality is high.

At 20 years, the total number of trees will range from 1,400 to 2,500, most of which are commercial species. The larger trees on better sites will be about 7 inches in diameter.

The normal development of such a forest during the next 60 years is one of constant struggle among individual trees. Each is competing for sunlight, moisture and soil nutrients.

About 90 percent of the trees present at age 20 will die in the next 60 years. The average annual growth rate culminates between 50 and 70 years for the dominant forest trees.

Individual tree growth begins to taper off and, at age 80, the dominant trees with good growing conditions will average about 15 inches in diameter. Those trees will reach maturity at 18 to 24 inches in diameter at about 140 years of age.

When a forest landowner instigates a thinning program, he has decided to help nature speed up the natural process of tree development. After observing which trees have the best potential for future development, the landowner eliminates competing trees years before they would die naturally.

The simple operation concentrates the growth potential of the forest into better trees, thus increasing their rate of development. A more thrifty and healthy forest is the result.

The younger the stand age when thinning is begun, the greater the increase in yield of merchantable forest products. For example, consider the above forest developing naturally to 80 years of age. Approximately 95 tons per acre of merchantable material, including 8,000 board feet of saw timber, is present on each forest acre. When this same forest is thinned every 10 years starting at age 20, approximate cumulative yield per acre is 122 tons, including 18,000 board feet. Periodic thinning has increased the theoretical yield of merchantable saw timber by more than 40 percent.

A planned thinning program has the potential for reducing the length of time it takes to grow mature trees. When thinning is begun at age 10 or 20 and is followed by periodic thinning at 10- to 15-year intervals, the time required to grow a tree to a desired diameter can be reduced by nearly 50 percent.

Added to this is higher tree quality from the application of good silvicultural practices.