As part of a woodland inventory, site quality should be evaluated to help predict how well important tree species will grow. The rate that a tree grows depends partly on genetic characteristics and partly on site factors, including soil fertility and texture, moisture, climate, slope, and aspect (direction a slope faces). Soil maps with interpretive information about tree growth are very useful when available, but soil maps are more commonly available for farmland, not for woodland. To judge site quality, foresters are more likely to evaluate the condition of existing trees, measure site index based on tree age and height, or identify understory plants indicating site quality.
Experienced foresters know what a good quality tree of a particular species should look like on a good site. Mature trees on good sites tend to be taller, have straighter stems, fewer limbs on main stems, and long merchantable stem heights, compared to trees of the same species on poor sites. When a majority of mature trees on a site exhibit desirable characteristics, a forester would judge that site to be good quality.
The difficulty in judging site quality by the condition of existing trees is that a stand with poor quality trees may not indicate a poor site. Trees may be poor because the best quality trees were harvested through repeated cuts (high-grading), leaving trees that were genetically inferior or damaged. A stand may have been lightly stocked at one time and pastured. Open grown trees develop short stems with large crowns, but their poor quality appearance may be due to low stocking density, not to site quality. Previous wind or ice damage also may have damaged crowns of trees throughout a stand, leaving trees in poor condition.
Foresters often judge site quality based on the total height that dominant and co-dominant trees will grow in a given time?\usually 50 years in the Lake States. Trees are expected to grow taller on good sites than on poor ones in the same time period. This measure of site quality is called site index. Site index curves have been constructed for many tree species so that site quality may be determined for a stand of trees larger than saplings if average tree age and average total tree height are known.