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

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Sugar
maple |

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American
Beech |

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Hemlock |

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Balsam
fir |

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White
cedar |

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Basswood |

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Red
maple |

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Yellow
birch |

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White
ash |

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Black
ash |

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White
spruce |

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White
pine |

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Red
oak |

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White
birch |

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Aspen |

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Red
pine |

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Jack
pine |

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Pin/Black
oak |

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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
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![[photo] Three-mile Island blowdown (E. Sagor)](../../img/ecology/3mi_island_sagor_tn.jpg) |
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Three-mile
Island blowdown
(E. Sagor) |
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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.
![[graphic] Illustration of uneven-aged, Two-aged, and Even-aged stand types](../../img/silv/standtypes.jpg)
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.
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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.
1
From: Kotar, J., Kovach,
J., Burger, T., 1999. University
of Wisconsin-Madison. Field
Guide: Forest Habitat Types
of Northern Wisconsin.