Northwest Illinois Forestry AssociationWoodland owners sharing ideas on forest productivity

Forest Sites



The sum of all the natural factors that influence forest growth is characterized as "the site".  Foresters often talk about site index, or good sites versus bad sites, or hardwood sites versus pine sites. They refer to the combination of environmental factors such as climate, soils, slope and aspect, and elevation, all of which affect not only which trees grow where, but how fast they grow and their form at maturity. Slope, aspect, elevation, and the local climate determine the micro-climate of a site.

As examples: northeast-facing slopes tend to be cooler and moister than southwest-facing slopes; high-elevation sites are colder and more exposed than sites in the lowlands; steeper slopes shed more rain. Just by knowing the aspect, elevation, and slope of a site, one can get a good idea about its potential productivity and which species will grow well there. The concept of site is one of the most important in forestry because the interaction between a stand and its site determines growth potential. It is always a combination of factors that determine site quality. However, if all other factors are equal within a stand, then the site quality for timber production is a function of the soils, especially their physical, chemical, and biological properties.

Forest Soils

The characteristics of a forest soil are defined by varying combinations of its four main ingredients - mineral particles, organic matter, water, and air. Chemical properties are determined mostly by the type of minerals fro which the soil is derived. For example, limestone-derived soils tend to be more fertile, while soils derived from granite tend to be infertile.

Fertility is largely a function of a soil's chemical properties. Soil pH, a measure of the acid-intensity of a soil solution, is commonly related to fertility. Forest soils tend to be acidic; but more acid soils are generally less fertile. Physical properties of a soil - whether it is coarse or fine - depends not only on the type of minerals present, but also on the particle size.

Fine soils tend to be more productive than coarse soils; however, they usually do not drain well and are more easily eroded. Fine soils are also more susceptible to damage from compaction. The presence of organic matter - rotting debris such as leaves and twigs - influences both the physical and chemical properties of a soil. Soils with a high proportion of organic matter tend to have better structure and are more fertile. Organic matter also helps forest soils hold water. Over 50 percent of the volume in the upper layers of an undisturbed forest soil is made up of pore space that holds water and air. The proportion of air and water is both affected and determined by the physical properties of the soil. Since roots need air to breathe and water to supply the rest of the tree, the soil pore space is extremely important.

Activities that compact the soil, eliminating space for air and water, will lower the productivity of a site. This is why it is so important for timber extraction equipment to stay on established trails; especially on sites with fine-textured soils that tend to compact more readily. Consider, too, that more than half of the feeder roots in a forest are found in the top six inches of soil. Biological properties of a soil refer to the communities of fungi, bacteria, and insects that live on organic matter produced by trees and shrubs. 

Though mostly microscopic, these organisms are absolutely essential to the growth and development of forests. While some can occasionally cause disease in trees, most feed on fallen leaves and woody debris.  Their main role is to recycle nutrients, such as phosphorus, potassium, and calcium, tied up in vegetation and dead animals. Without these decomposers, the forest floor would be littered with debris accumulated over thousands of years, and forest nutrients would be locked up in dead material.

Some organisms, such as a group known as mycorrhizae-forming fungi, cause beneficial infections on tree roots. In exchange for a share of the energy manufactured by the tree, these fungi tremendously improve a tree's ability to take up water and nutrients. The tree and mycorrhizae-forming fungi depend on one another., each supplying the other with sustenance for a small cost. Unfortunately, mycorrhizae roots are very susceptible to changes in environmental conditions, especially those caused by soil compaction. (Hence, forest management decisions must take into consideration the way soil organisms will react to a harvesting operation.) Usually, their activity increases due to higher temperatures on the forest floor; however, soil compaction will inhibit them.



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