Forests for Old Fields

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. If you make a bad guess on drainage, pay more attention to the other tests as you go to the right in each column of table 1.

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 jnches 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.

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.

The height of dominant trees, the ones that grow without crowding or shading, at any given age is really the best measure of site quality.  We used 50 years as the standard age, and estimated from growth curves the height of any stand where age was less or more than 50 years. This actual or estimated height at 50 years is known as site index. A stand, for instance, whose current height and age indicated a probable height of 75 feet at 50 years was assigned a site index of 75.

Obviously, comparison of tree heights of two stands, say 10 and 40 years old, would not be a fair comparison of the richness of two soils; but if the height of the 10-year-old stand were calculated from the curve to what it would be in 40 more years, and the height of the 40-year-old stand were calculated to what it would be in 10 more years, the two heights could be compared on a fair basis. This we did by assigning a site index to each stand.

When we placed the site index of each stand in one of the six positions according to its soil, the group site index averages fell into regular order roughly by steps of 10 from 50 to 100. Black locust stands on sites whose subsoil was slowly drained, sticky, compact, and bluish-colored or drab-mottled below 8 inches averaged only 50 in site index—much too low for profitable black locust growth. Stands on sites with good to fast drainage, with crumbly, loose, reddish-brown subsoil, averaged roughly 100 in site index. This arrangement of soil and tree growth brought an order to soil facts that showed how tree growth responds to soil.

The differences in thrift of black locust stands were pronounced. We found knotty, straggly, tight-barked, runty stands; and straight, tall, fluted stems with bulging roundness that seemed to split the bark. All these differences stood out more and more clearly as we measured trees and looked at the soil. The soil was different, too.

A slow-growing stand in Ripley County, Ind., near Osgood, was on a ridge top where the drying winds swept the moisture right out of the soil. To make the site still worse, only a shallow soil lay over bedrock— not much chance for good timber here; only 32 feet high at 25 years. A fast-growing grove in a little valley churchyard near Paris Crossing, Jennings County, Ind., had a deep, mellow silt loam soil (Cincinnati silt loam). Its subsoil was well aerated; its color was a golden brown. The site was sheltered from excessive wind; its soil was moist—small wonder that the trees were 13 inches in diameter.and 90 feet high at 40 years.   We began to use simple relations of soil stickiness, compactness, and color to drainage and aeration to explain tree differences.

These simple relations have great value to forestry, but they were not found in a hurry. As early as the late 1920’s, Tom Bushnell, chief of the Indiana Soil Survey, was arranging soils by drainage groups.

Richard Bradfield, working at Cornell University, found that roots of apple trees penetrated the soil only so far as it was well oxidized. If the ratio of oxidation to reduction (measured electrically) was high, the roots penetrated deeply and the trees were thrifty. If the ratio of oxidation to reduction was low, the roots died at a slight depth and the trees were slow growing and unthrifty. Dr. Bradfield, of course, knew about the relation of drainage to aeration; he wanted a quick test for apple soil.

His work adds another link to the chain of site evidence. A well-drained soil is a well-aerated soil, and a well-aerated soil is a well-oxidized soil.  But the iron oxides in a well-oxidized soil are red and brown, whereas the iron oxides in a reduced soil are blue or green. Therefore the subsoil of a well-drained, well-aerated, well-oxidized soil is red or brown, and the subsoil of a poorly drained, slowly aerated, reduced soil is bluish or drab. The degree of drainage and aeration accordingly are indicated by subsoil color.

Milne, a South African soil scientist, in 1936 gave the name “catena” (Latin for “chain”) to groups of soils varying systematically in drainage. Soil surveyors have worked painstakingly and long, mapping and describing soils, and their work continues. Even now they do not fully agree on what a catena really means; its application is new. Some say it is a hydrologic sequence—a high-sounding expression meaning arrangement by degree of soil moistness; some say it is an order of drainage; but whatever they finally decide, black locust growth defines it as a range of usable soil water.

The arithmetic of soil water is simple. Total rainfall less runoff water less evaporation water equals ground water. Obviously, useful water in soil does not depend alone on how much rain falls, or altogether on how much runs off the surface, or even on how much runs into the soil; but it depends also and importantly on how much is evaporated.

A very little observation soon convinces one that a south slope loses more water by evaporation than a north slope because it gets more direct sunlight, and that a wind-swept upper slope or ridge loses more water by evaporation than a lower slope or cove because more moisture-absorbing air passes over it. But evaporation varies so much that a way of evaluating it by sites had to be found for hilly land. At this stage of the search for an answer to site prediction on abandoned fields, we had attempted to find it by measuring the subsoil. The application of topography, with its three parts—aspect, exposure, and position—in making site predictions remained to be studied.

How could the effect of topography on tree growth be measured? That is the question we asked ourselves. Trees themselves answer that question. After all, what instrument will take rainfall, temperature, evaporation, and soil data in all their variations, throw them into an equation, and come out with a perfect answer? Trees do it. So to find out how topography affects growth of yellow poplar we examined 77 second-growth and old-field stands in the hilly area of the Central States.

We separated the site indexes of the 77 stands that occurred on slopes into hot and cool groups. (Hot slopes were defined as S., SW., SE., and W. slopes; cool slopes were defined as N., NW., NE., and E. slopes.) The difference between the average hot and cool slope in terms of site quality was 10 site-index points.

But this 10-point difference represented the combined influence of the three elements of topography: Aspect, exposure, and position. Accordingly, we divided the 10 points roughly into 3 parts, attributing 3 site-index points to each—aspect, exposure, and position.

Furthermore, the extreme site-index range between the average exposed ridge and the average sheltered cove was 33 points. Accordingly, we distributed this total difference of 33 points by steps of 3 as in table 2.

This table enabled us to assign a site index to any topographic effect in hilly terrain from cove to ridge. The figures therefore should be subtracted from 100 to give the actual site index attributable to topography.

On the very dry sites it may be necessary to plant pines to add a moisture-conserving litter cover to the soil. Hardwoods then replace the pines naturally if seed trees are near.

The trees to plant on an abandoned field soil that is drier than normal depends on depth of the surface soil or.depth to subsoil. Depth to subsoil for black locust of average or better than average site index was found to be 14 inches or more; for black walnut, 16 inches or more; and for yellow poplar, 24 inches or more.

Tree growth depends largely on the degree of site dryness or wetness.  When the site indexes of the species studied were arranged in order from lowest to highest, three bands of site condition stood out:  Dry sites, normal sites, and wet sites. Abandoned fields fell into the normal site group and into normal sites temporarily dry because of loss of litter and surface soil. Yellow poplar stands were found almost altogether in the sites of normal moisture. Black locust stands had a much wider range; a few of them persisted with poor thrift in both dry and wet sites.

1. It is a dry site if its soil is coarse, deep sand, loose shale, or rock, in any position other than a flat where the water stands near the surface.
      It is a dry site if bedrock is nearer than 24 inches to the surface or if erosion has removed the surface soil down to less than 4 inches from a tight subsoil.
2. It is a wetsite if it is seasonally waterlogged.
3. It is a normal site if it is neither dry nor wet.

We can dispose of the dry and wet sites at once by turning to table 3.  There you will find data on the original species that grew on dry and wet sites and the species that we recommend for them.

If your site is neither dry nor wet it falls into either the flat to gently rolling class or the hilly class. If it is in the flat to gently rolling class we need not worry about topographic effect. And by gently rolling I mean slopes of less than 25 percent and hills not over 50 to 75 feet high. Now look the site over and estimate the rate of drainage according to the first table. Give it a number from 1 to 6, expressing how fast you think rainfall will run through the soil.

In any case if it is a normal site, take your spade, a soil auger, or, better still, a post-hole digger, and dig a hole about 3 feet deep. Examine the subsoil—incidentally, the subsoil here is the tight layer usually 1 to 3 feet below the surface—and classify it according to one of the horizontal columns of the first table. At the right you find the site index.

If your field is a normal site in the hills you must estimate its negative points by placing it according to the second table. Suppose the site index that you get from the first table is 90; and further suppose your field is a lower north slope sheltered from excessive wind. In the second table you will find your field described by the third line (slope—cool—sheltered—lower—6). Simply subtract the 6 from the 90 and get 84, the site index. This figure indicates that on a scale of 100-foot height growth in 50 years, you can expect your stand to be 84 feet high.

Finally when you have the site figure, turn to the third table and there find the species that grew on the original site and the species we recommend for planting.

John T. Auten, a silviculturist with the Research Branch of the Forest Service, has been engaged in forest soil investigations since 1929. He has been a soil analyst for the Iowa soil survey and professor of chemistry and soils at the Pennsylvania State Forest School. Dr. Auten is a graduate of the University of Illinois and Iowa State College.