Forests for Old Fields

by JOHN T. AUTEN

THE 10 million acres of abandoned fields in the Central States, reforested with the right kind of trees, could produce enough lumber to build 150,000 new 6-room houses each year.

What is the right kind of tree to plant on an old field? The answer is not easy, but I shall tell you what we foresters in the Department of Agriculture have learned about trees and soil. We do not know it all, but we can help your planting average.

Anybody who wants to plant the right tree in the right place has to know something about the soil. He finds that the original stand that might serve as a guide is gone; often much of the surface soil is eroded away, and the old field is covered with briars, brush, and weeds. Many of the natural clues to the original forest have been lost.

Fortunately, we were able to follow a few remaining clues through several years of study and observation to some helpful conclusions. One of the most significant of our findings is that subsoil (the soilsman calls it the B horizon) and topography hold the final answer to the kind of tree that should be planted.

We began this study of soil and trees in the Central States about 15 years ago. At that time the best way to find out what happens to soil under cultivation seemed to be a comparison of virgin wood and nearby field soil. Accordingly, we located and examined 22 remnants of virgin hardwood forests and adjacent fields. One of the most interesting differences that appeared between woods and field soil lay in their ability to absorb water. Sometimes the virgin-wood soil absorbed water as fast as it could be poured and measured, but the field soil absorbed water very slowly. Furthermore, even though high, dry sites exposed to wind absorbed a great deal of water, they produced scrubby hardwoods; and if the soil was coarse and excessively drained also, they produced pines. Moist-cove and north-slope sites were occupied by thrifty stands of red oak, white oak, white ash, walnut, and yellow poplar.

These discoveries brought out two important facts: First, litter-protected, porous woods soil absorbed much more rainfall than bare, compacted field soil; and second, some trees required more water than others. putting these two facts together, we concluded that cultivation of woods soil makes the site a drier site and temporarily shifts the possible tree cover from high-moisture-requiring desirable hardwoods toward less desirable dry-site species or pines.

But notwithstanding the great differences in rate of water absorption of woods and field soil, the virgin-wood soil and adjoining field soil were alike beneath the surface. Their subsoils had the same color and the same degree of compactness and stickiness—in fact, only the surfaces were different.

Now the question arose: Why were some subsoils drab-colored and others brown? Why were some subsoils mottled and others not, some compact and others loose? More important than any: Why were some kinds of trees growing on drab, tight soils and others on brown, loose soils? Only further research could answer such questions and the best place to find the answers seemed to be where planted trees had succeeded and failed on many different kinds of soil. Accordingly, we studied 135 black locust plantations for soil and growth differences. We examined the subsoil of each plantation for plasticity (stickiness), compactness, and color. We looked the plantation over for general lay of the land and nearness to streams; then we estimated how rapidly we thought the land would drain.

You will probably find the subsoil of your field pretty well described by some one horizontal row in table 1. For instance, if the subsoil is very sticky, it will be very compact when dry. If it is drab or light gray, it probably will be mottled not far below the surface.

The quality of drainage and aeration indicated in the first table by the various degrees of stickiness, compactness, and color grew in importance as our plantation study progressed. We found very striking examples of the effect of subsoil on trees.

An example is a stand of yellow poplar in the Waterloo Forest, Ohio. At 19 years of age it had grown little more than 2 inches in diameter and 10 feet in height. The site was a lower slope along a stream, sheltered and cool. It should have been an excellent site; the trees should have been 8 to 10 inches in diameter and 40 feet high. They would have been except for one soil condition—a tight plastic clay subsoil. In contrast, a yellow poplar stand in a deep, cool cove in Wolfe County, Ky., was only 33 years old; yet it averaged 104 feet in height. The reason: A very deep, well-aerated soil with no tight subsoil.

Any one of the subsoil properties of the first table may serve to describe a soil, but the other three make the site estimate more reliable. We found out all these facts about soil as we went along. But that was only half of the story. The next step was to learn how fast the trees grew on the different soils.