Understand soils for forest management success

Forests are ecosystems and consist of living (biotic) and non-living (abiotic) components.

Current conditions in any forest ecosystem are a product of the site conditions (soils, water, and climate) and past management activities. In Northeast Michigan, many forests are very different today than they were before the Great Cutover of the early 20th century. Those forests have changed over the past 100-plus years, and will continue to change over the upcoming decades.

To conserve forests into an uncertain future, we must understand how forests once functioned and how they once looked (structure and composition). We then must compare historic conditions to current conditions and think about what caused observed changes. At the most basic level, that understanding starts with an appreciation of forest soils.

A long-winded textbook definition for soil is as follows: a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons, or layers, that are distinguishable from the initial material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment.

As stated above, soils are comprised of layers or horizons. The four most commonly encountered horizons (from above to below) are: 1.) O (organic) horizon, 2.) A (topsoil) horizon, 3.) B horizon, or subsoil, and, 4.) C horizon, or parent soil.

The uppermost horizon is the organic horizon (O). That horizon is very thin and usually pretty dark and made up of living and decomposed materials. Tiny plants, fungi, insects, bugs, and other invertebrates are found in that horizon. As those organisms and leaves, twigs, and other organic material decompose, the resulting nutrients feed that layer. One reason leaves should not be raked is because they provide cover for organisms inhabiting that horizon and provide nutrients as they decompose.

The next layer is the A horizon, or topsoil. Depending on the forest soil type, the topsoil can be of varying depths. In dry, upland coniferous forests, that horizon is thinner compared to the A horizon found growing northern hardwoods. Agriculture in our area is mostly confined to areas that historically grew northern hardwood forests because of the depth of that soil horizon.

The B horizon, or subsoil, is rich in minerals that were transported down in the soil column from above. The C horizon is called the parent material and is the geological layer from which all soils above developed.

Many different forest types with different soil horizon characteristics are found in Northeast Michigan. The U.S. Department of Agriculture’s Web Soil Survey is a useful tool to understand soils: websoilsurvey.sc.egov.usda.gov.

Users can find their property location and query soils using the Web Soil Survey. Information provided include suitability for growing different tree species, along with potential soil management limitations. That type of information can go a long way to make tree and shrub plantings successful and to understand forest changes.

Unfortunately, many forest landowners are not provided soils information before they manage their land. Too often, the wrong plant species are planted on a soil type suitable for another plant species.

Forests are not agricultural fields, and simply applying fertilizers to achieve goals is not a panacea.

Adding fertilizers to forest soils can, in fact, have multiple adverse effects.

First off, on many soils, much of the fertilizer applied runs off during rain because of soil type characteristics. That runoff harms nearby ecosystems, especially lakes, ponds, and streams. Applying fertilizers on forest soils can also benefit non-native plant species.

While most non-native plant species are shade-intolerant and respond to increased light levels with increased rates of growth, a study published in 2008 in the journal Biological Invasions also showed effects of enhanced soil nutrients, especially on sandy soil.

So, what is the take-home message for forest landowners?

There are some things that are relatively fixed in a forest: soils, water, and climate, for instance. Successful forest conservation starts with an understanding of those site factors and the land use history of a property.

Once the natural capabilities of a property are known, ownership goals can be developed. Subsequent planning and management can then adapt ownership goals and actions to those site characteristics.

For more on forest soils and the concept of site index, see this website from North Carolina State University: content.ces.ncsu.edu/forest-soils-and-site-index.

For more about soils and tree planting, see this website from Penn State Extension: extension.psu.edu/forest-landowners-guide-to-tree-planting-success

Greg Corace is the forest and wildlife ecologist for the Alpena-Montmorency Conservation District. For more information, email greg.corace@macd.org.


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