Some basic concepts of forestry and silviculture will be useful to you in understanding, with respect to forest diseases, how we've gotten where we are today and how we might get where we want to be tomorrow. Students with no prior forestry background may find this information of value for a general understanding of forest management. Of course, it is greatly simplified and cannot substitute for a course in silviculture.
Objectives of Forest Management
In managing, there obviously must be an objective. This is some benefit or benefits that people believe they can better obtain by changing the forest than by leaving it alone.
A major objective in forest management has been and will continue to be timber production. This means maximizing productivity and quality of trees to be harvested for wood products. Forest pathology is strongly rooted in this objective. Viewed from this objective, forest pathology is part of pest management.
Although timber production has only rarely been the sole objective, other objectives are becoming increasingly important to foresters and the public. In addition to traditional objectives such as game, recreation, and watershed, others such as non-game wildlife, biodiversity, esthetics, and forest health are given increasing emphasis. As you learn about forest diseases, consider how they influence these objectives, and how pursuing those objectives influences forest diseases.
Silviculture is the theory and practice of managing forest establishment, structure, composition, and growth to achieve management objectives. Silvicultural systems specify a planned program of treatment during the whole life of the stand, including reproduction, any intermediate cuttings, and harvest.
The following is a description of some silvicultural methods that are or have been common, and which have particular relevance to forest diseases. There are, of course, other systems and the descriptions here are simplified.
In an even aged system, trees are more or less the same age during most of stand development. This can be achieved several ways:
- Clearcutting method:
- Removal of the entire stand in one cutting. Reproduction is obtained artificially, by natural seeding from adjacent stands, or from the cut trees themselves. In the strict sense, virtually all trees are cut, but it is sometimes loosely applied when all non-merchantable trees are left standing.
- Seed-tree method:
- Removal of mature timber in one cutting, except for a small number of trees left singly or in small groups to provide seed for regeneration.
- Shelterwood method:
- Removal of mature timber in a series of cuttings that extend over a relatively short time. This encourages establishment of essentially even-aged reproduction under the partial shelter of seed trees.
In an uneven-aged system, there is a range from seedlings to mature trees at any one time in the stand.
- Selection method:
- Removal of mature timber, usually the oldest or largest trees, either as single scattered individuals or in small groups at relatively short intervals, repeated indefinitely. This encourages the continuous establishment of reproduction.
Wounding of residual trees can be more frequent under uneven-aged management. There are two reasons. First, uneven-aged management generally involves more frequent stand entries (for cutting trees) than does even-aged. Second, regeneration at various stages of development is always present during cutting. Still, intermediate cuttings under even-aged management can also lead to significant wounding.
Other Terms and Practices
Tree sizes are often described with the following terms (figures are approximate):
- young plant established from seed, up to several feet in height
- between seedling and 5" diameter
- 5-10" dbh
- > 10" dbh and suitable for cutting lumber
- Basal area:
- The cross-sectional area of a tree at breast height (see DBH). The term is often used to refer to basal area of trees on a per acre or per hectare basis, incorporating both tree size and density.
- Diameter of a tree at "breast height," or 4.5 feet. In practice, the circumference is actually measured, but a special measuring tape ("d-tape") is used that is marked so that diameter can be read directly.
- The number of years trees are grown before harvest, or a full stand cycle.
- Advance reproduction or regeneration:
- New trees that appear before any special measures are taken to establish them.
- Cutting intermediate in stand development, aimed primarily at reducing stand density to increase growth of residual trees and, if it is a commercial thinning, utilization of trees that would ultimately die of suppression. Thinnings are called precommercial if the trees cut are too small to be merchantable. The term is usually used with respect to even-aged stands, but can refer to even-aged groups that make up the immature portions of uneven-aged stands.
- A partial cut in which only the biggest and best trees are cut. A synonym is selective (not selection) cutting . The residual stand is of poorer quality. The sole objective is immediate profit rather than stand improvement or future productivity and quality. Needless to say, the practice is frowned upon . Unfortunately, it was common in the past, and many poor stands of today are the result.
- Salvage cutting:
- Removal of trees that are dead, dying or badly damaged, where factors other than competition are involved. Usually, the primary goal is to realize their value before they decay. Some variants are:
- Presalvage cutting: Removal of trees highly vulnerable to killing or damage.
- Sanitation thinning (or cutting): Removal of trees that have been attacked or are vulnerable to attack by insects or pathogens in order to reduce inoculum or insect populations so that the pest is less likely to spread to other trees. However, the term is sometimes used with respect to diseases where inoculum reduction is probably not worthwhile.
- Planting out of native range:
- Often, fast-growing trees with desirable qualities are identified and selected for planting in areas outside of the native range. In some cases, trees even grow better and with better form in the new area than at home, probably because of diseases or insects that were left behind. But those diseases, if they are introduced to the new region, or new ones that appear may eventually be far more damaging than diseases in the native range. Usually, this has nothing to do with tree stress.
- Off-site planting:
- Even within the geographic range of a species, certain planting sites may be found to be inappropriate. Often this is because of soil type, elevation, exposure, etc. Certain diseases, especially those related to stress, may appear that do not appear in the natural habitat.
- A standing, dead tree; may function as perch, foraging site or cavity tree for wildlife.
- An area of forest whose site conditions, history, species composition and structure are sufficiently uniform to distinguish it from adjacent areas. It is managed as a unit. May be a few acres to a few hundred acres. Stands may be grouped into forest compartments.
- "Tolerant" and "intolerant" are usually used with reference to shade. Very intolerant trees (jack pine, aspen, black locust) require full sun and do not grow well under an established canopy. Very tolerant trees (balsam fir, sugar maple, beech) can become well established under a closed canopy. Many trees have intermediate levels of tolerance.
Last modified 27 May, 2007