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| Forests are organized assemblages of trees, other plants and animals, in
complex association with each other and their physical environment. Efforts to
develop an understanding of the capability of land to produce timber have been
an inherent element of forest management for more than a century. As emphasis
in forest management changes from simply trying to grow the "best trees" on "the best sites" toward
maintaining forests in a more natural condition while still utilizing the
resources, it is becoming even more important that we pay attention to the
physical environment that controls forest ecosystems. So, what site factors are important and how do we evaluate them? The answer to this question varies from region to region. While perfect understanding of community-site relationships does not yet exist in any part of the world, enough useful information is available in most regions to enable forest managers to include site characteristics in their management considerations. Figure 4 (page 10) illustrates the essence of site characteristics that affect tree growth, species composition and succession. All of these should play a role in the development of management recommendations. For simplification, the two site types in Figure 4 are labeled only as "loamy soil." However, the implications, are far reaching. Site Type I (loamy soil) has considerably higher nutrient content and moisture holding capacity than does Site Type II (sandy soil). As a consequence, Site Type I not only has a higher yield of current crop (aspen or red oak) than does Site Type II, but it also has a capacity to support two moisture- and nutrient-demanding species (sugar maple and beech) that Site Type Il does not. This has important successional implications for aspen and oak stands currently growing on both site types. Other site factors could easily be substituted for soil texture in Figure 4, e.g., north- vs. south-facing slope or valley bottom vs. ridge (other features in Figure 4 will be explained in subsequent chapters). |
| Figure 4. A schematic representation of two site types (loamy soil and sandy soil), two forest cover types (aspen and red oak), and eight stands. Each stand has unique composition and is defined by a specific combination of overstory and understory species. Each stand also can be considered as a unique ecological or silvicultural opportunity unit. |  |
| In practice, any site factor that is expected to produce similar
differences as illustrated above could be considered as a basis for site type
differentiation. In some regions, various site classification systems have been
developed and serve as valuable tools in forest management. For example, in
Wisconsin and Michigan, forest sites have been classified along a soil
moisture-nutrient gradient (Kotar and Burger, 1996, A Guide to Forest
Communities and Habitat Types of Central and Southern Wisconsin; Kotar, Kovach
and Locey, 1988, Field Guide to Forest Habitat Types of Northern Wisconsin;
Coffman, Alyanak, Kotar and Ferris, 1980, Field Guide to Habitat Type
Classification System for Upper Peninsula of Michigan). Segments of the gradient are referred to as "vegetative habitat types." In the field, individual habitat types are recognized by the presence of characteristic understory species. Keys to diagnostic plants and community descriptions are used to classify a given site. In addition, potential forest dynamics (successional pathways) and management implications are presented for each habitat type. In some regions, especially on eastern National Forests, a system of hierarchical land units is being developed (e.g., Cleland et al. 1992. Field Guide - Ecological Classification and Inventory System of the Huron-Manistee National Forests). In this system, the smallest units, called ecological land types (ELTs), function similarly to site types described in Figure 4. When information on the productivity of individual tree species and on potential community dynamics is available for ELTs, they can be used in a similar way as the habitat types. Where no formal site and community classifications exist, forest resource managers may have to consult available local sources on geology, soils and vegetation ecology in order to differentiate ecologically significant site types on lands they manage. The following is a list of generally recognized factors that directly affect site quality, primarily in terms of available soil moisture and nutrients and, in some cases, also temperature and light: |
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| It is important to recognize, however, that numerous combinations of the above factors (from left and right columns) can result in functionally similar sites. This makes site quality evaluation more difficult in the absence of regional studies. For example, without additional information, it is difficult to estimate which of the paired site factors below results in more favorable moisture and nutrient conditions: |
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