Effective forest management and silviculture are intimately linked to an understanding and appreciation of forest ecology. Understanding the biophysical characteristics of forests and forest sites, the silvics or autecology of various tree and plant species, and the relationship of disturbances to forest development are central to developing effective management plans and making reasonable predictions about future conditions of a forest.

Site characteristics

A fundamental need when considering management options for a stand is to consider the relationship between forest site quality, that is its soil and topographic characteristics, and the species of trees that can grow best on that site. Although not without exceptions, there is a strong relationship between key measures of site quality, such as soil moisture and nutrient availability, and the type of tree species that naturally occur on that site. For example, pines tend to occur on soils that are dryer and less nutrient rich than many northern hardwood species. The latter tend to occur on soils that are moist (mesic), nutrient rich, and loamy textured. Some early successional species, such as aspen, occur across a wide range of site conditions. Most species of trees will grow best on mesic, nutrient rich sites, regardless of where they occur naturally. However, if they are not native to a particular type of site, it may take a significant management effort to allow them to establish and maintain dominance on a site. For instance, red pine and white spruce have desirable growth rates when established on northern hardwood sites, but it takes a significant economic investment in site preparation and herbicides to allow them to achieve and maintain dominance in the face of competition from species native to the site.

Table 1: Relative growth potential for major tree species across habitat type groups (only those habitat types where the species occurs naturally are considered).¹

Growth Potential Legend

Very good

Good

Fair

Poor

	Very dry-dry	Dry-dry mesic	Dry mesic	Mesic	Mesic- wet mesic	Wet mesic- Wet
Sugar maple
American Beech
Hemlock
Balsam fir
White cedar
Basswood
Red maple
Yellow birch
White ash
Black ash
White spruce
White pine
Red oak
White birch
Aspen
Red pine
Jack pine
Pin/Black oak

For more details on determining your site potential, including native plant communities, visit Wisconsin Forest Habitat Type Classification System (PDF, 146K) and visit Minnesota's Native Plant Communities provided by the Minnesota Department of Natural Resources.

Species characteristics

Understanding ecological characteristics of plant species is important when considering management objectives. Differences in growth rates will help to determine the amount of effort that might be needed to establish and maintain particular species on a site, even ones that are native to that site. For example, root suckers of bigtooth and trembling aspen have higher early height growth rates and both species tend to regenerate from suckers at high initial densities, ensuring that aspen will take control of a site soon after a harvest. On the other hand, establishing a slower growing species like red pine may require several years of competition control investments to keep early successional shrubs such as hazel or raspberry from dominating the site.

Species characteristics are also important from the standpoint of managing for wildlife habitat and more broadly, for the diversity of plants and animals that may occur in the forest. For instance, hard mast producing species, such as oaks, are favored by many species of birds and small mammals. So insuring that oaks are present in your forest, if the site is conducive to growing them, may be an important consideration.

Tree species can change the characteristics of the site on which they occur. For example, the needle litter of conifers such as red pine are acidic and over time, can reduce the pH of the soil and the availability of certain nutrients. This may be a consideration when converting a site to a species that does not typically grow there, such as the earlier example of a northern hardwood site converted to red pine.

Natural disturbance

	Three-mile Island blowdown (E. Sagor)

Forests are complex and dynamic systems that change in response to disturbances of various types and intensities. Understanding the response of tree species to disturbance is a key to effective forest management. In general, small canopy disturbances, such as removal of a single large canopy tree through selective harvest or perhaps from natural windthrow, will provide opportunities for neighboring trees to grow laterally, but little opportunity for new generation to develop. As opening size increases, again either naturally from a major disturbance or through more harvesting, the probability of new regeneration establishing increases, along with increasing growth of remaining trees. How the forest floor and mineral soil is disturbed will also influence composition of the developing forest. Opening of the canopy along with ample mineral soil exposure will favor establishment of certain species of plants that require less competition in the understory or respond favorably to high soil temperatures. The combination of canopy disturbance and mineral soil exposure can result from natural surface fires that occasionally spread through tree crowns and kill overstory trees, or a windstorm that uproots a number of trees in a patch, or from harvesting that also exposes mineral soil.

Stand characteristics

There are certain descriptors of stand structure that you should understand when reading material in the guides and when considering choices among management practices.

Age structure is a description of the predominant tree age characteristics of the stand. Common categories used to describe age structure include:

• Even-aged: most of the main canopy trees are of one age-class. In natural stands these trees are rarely all the same age, as would be true in plantations. Rather, a stand is said to be even-aged if the main canopy trees all established within a period that is no more than 20% of the typical life span of the species being considered. As the stand develops, new trees may establish in the understory, but until these start to grow into the canopy, the stand is still generally considered even-aged.
• Two-aged: most of the main canopy trees are of two distinct age groups. This might occur if part of the stand was harvested or killed by a natural disturbance, leaving many remnant trees, followed by regeneration of a new group (cohort) of trees.
• Uneven-aged: these stands are composed of more than two distinct age groups. Rarely are they truly all-aged, but rather the distinct age groups (cohorts) are established in response to some past disturbance that left some trees but also provided opportunities for new regeneration over several to many decades.
  

Figure 1. Types of stand age structure (Definitions from: Helms, John A. 1998. Dictionary of Forestry.)

Composition generally is used to describe the species of trees that occur in the stand, for example, red pine or oak. Stands composed of largely one dominant species are said to be mono-specific. Stands composed of two or more species are said to be mixed-species stands. Stand composition can be described more broadly to include description of other vegetation layers, such as red pine with a hazel understory.

	Spore structures of pine needle rust on red pine needles. (S. Katovich)

Large Dead Wood, which until recently was referred to as coarse woody debris, includes snags (standing dead trees) and dead trees and large branches on the forest floor. Trees die naturally in any forest for a variety of reasons. This occurrence, in small to moderate numbers, is not indicative of any particular health concern. In fact, dead wood in the forest provides habitat and food resources for many different species, including small mammals, birds, insects, fungi, and some plants.

Vertical structure refers to the types and distribution of different vegetation layers, from the ground to the top of the canopy. Vegetation layers in most forest stands, at some point in their development, will include canopy trees, subcanopy trees, woody shrubs, and groundlayer vegetation (including tree and shrub seedlings, herbs, and mosses). Additionally, different species of canopy trees (in mixed-species stands) will often form stratified layers with the crowns of faster growing (or older) species rising above the crowns of slower growing (or younger) species.

Horizontal structure refers to the horizontal spatial distribution of trees, other vegetation, and large dead wood across a stand. On one extreme, even-aged plantations have uniform horizontal structure, with an even distribution of trees, canopy cover, understory vegetation, and forest floor conditions. At the other extreme, an old-growth northern hardwood forest that experiences regular small-scale canopy disturbances from tree blowdown, has a high degree of spatial heterogeneity in structure and composition. Some areas in the stand that develop in new canopy gaps, perhaps with exposed mineral soil from uprooted trees, will have abundant herbaceous and shrub vegetation. Other areas in the stand will be dominated by closed canopy conditions, a thick litter layer and few understory plants. The point is that in a plantation horizontal structure will likely be relatively homogeneous across the stand. Conversely, in the old-growth forest, horizontal structure will likely be heterogeneous across the stand. Between these two endpoints, horizontal structure may vary widely depending on stand age, composition, and disturbance and management history.

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¹ From: Kotar, J., Kovach, J., Burger, T., 1999. University of Wisconsin-Madison. Field Guide: Forest Habitat Types of Northern Wisconsin.