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.

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. Next, let us decide whether your field is a dry, wet, or normal site.

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.

2. 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.

3. It is a wet site if it is seasonally waterlogged.
4. 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—coolsheltered—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.

THE AUTHOR

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.