One way of evaluating the effects of thinning hardwood forests is to compare the growth and yield of a thinned forest with the growth and yield of 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 development when skeptics doubt whether a new forest will ever become reality.

When a new forest is between 15 and 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.

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 or she has decided to help Mother Nature speed up the natural processes of tree development. After observing which trees have the best potential for future development, the landowner eliminates competing trees before they would die naturally.

This simple operation concentrates the growth potential of the forest into the 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 the yield of valuable forest products. For example, consider the outcome if the above forest was left to develop naturally for 80 years.

About 95 tons per acre of salable material, including 8,000 boardfeet of saw timber, is present on each forest acre.

However, when the same forest is thinned every 10 years starting at age 20, the approximate cumulative yield per acre is 122 tons, including 18,000 boardfeet.

Periodic thinnings have increased this theoretical yield of 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 thinnings at 10 or 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 a higher tree quality from the application of good forest practices.

Ron Fodor is district manager of the Butler County Conservation District.