Silvicultural systems are described by the harvest method used. The systems consist of a “planned series of treatments for tending, harvesting and re-establishing a stand” (Helms, 1998).
The system used for a particular stand of trees is determined by the tree species being managed and its growth requirements. Typically, there may be several species being managed within one stand.
Along with the species of tree to be grown, several other factors are considered by forest management planners when deciding on the use of any particular silvicultural system. These include:
- the management objectives for the site and/or stand
- the age and condition of the trees being managed
- the ecology of the site, such aspects as the soil, moisture, and nutrients present
- the effects of expected climate change over the next forest generation
- the effects of management operations on other forest values, such as wildlife, canoe routes and ecologically sensitive sites
In Ontario, forests are managed under one of the following three silvicultural systems:
- the clearcut system
- the shelterwood system
- the selection system
The Clearcut System
Harvesting and forest renewal operations in the clearcut system emulate some of the effects of natural disturbances such as fire, flood, insect and disease-induced mortality, and windstorms.
Most tree species in the boreal forest region, such as black spruce, jack pine, aspen, and birch, are adapted to these types of natural disturbances. They germinate and grow best in full sunlight. Forests tend to regenerate to form relatively pure stands of trees of the same age after experiencing one of these natural disturbances. The life cycles of these species of trees make clearcutting the most silviculturally and ecologically appropriate method of harvesting and regenerating stands.
Over thousands of generations, these species have adapted to co-exist with the effects of periodic wildfires, and have developed methods of regenerating themselves after these extreme but naturally-occurring events.
Traditionally the clearcut system is an “even-aged” process, meaning that all of the trees in a stand are cut more-or-less at the same time and the timber extracted. In practice, not all trees within a cutting area are harvested. Current policies and regulations mandate that some trees must be left standing to protect certain non-timber values (e.g. fish and wildlife habitat) and to emulate a natural disturbance like fire. Shortly after the harvest, many of the trees remaining may die or blow over, providing habitat for wildlife species that live in young forests. The result is that the new regenerating stand is composed mostly of trees of the same young age, thus the use of the term “even-aged”.
Results of a traditional clearcut harvest (no longer used in Ontario)
In addition to individual trees left standing, small patches of trees are also left unharvested. Sometimes, strips may be left standing to provide animal corridors from one edge of the cut to the opposite side. All of these features mimic the stand structure that usually follows a wildfire, and help maintain forest biodiversity for the continuation of natural ecosystem processes.
Clearcutting as practiced today. Note the irregular boundaries, islands and peninsulas of wood and the residual trees left in the blocks.
Regeneration of the trees in this system may be natural or artificial. Often, natural regeneration is encouraged by:
- planning harvests to follow a year of good seed production
- orienting cuts so they are at right angles to prevailing winds or,
- leaving additional uncut strips, patches and individual seed trees
The Shelterwood System
The shelterwood system is a silvicultural practice in which a maturing stand is harvested in a series of two or more cuts over a span of 10 to 30 years. Each of the initial cuts leaves a “thinned” canopy or overstorey beneath which seedlings can regenerate and develop in the partial shade of the remaining trees. Partial shade favours species such as white pine, white spruce, and red oak.
The final cut leaves a healthy crop of well-developed young trees that can grow into a mature stand over a relatively short time span. This system is sometimes used to rehabilitate degraded maple, beech and yellow birch stands in central Ontario.
White pine regenerating under a jack pine overstorey in the Kirkwood Forest of Ontario
The shelterwood system is the appropriate silvicultural approach to be used with tree species that have adapted to natural events that damage or kill only some trees within a stand (e.g. ground fires, insects, and diseases). The result is an “even-aged” process in which there may be two and sometimes three distinct age classes, or cohorts, of trees.
Before any cutting is carried out, trees are marked for cutting or for retention. The first, or preparatory, cut removes part of the original stand to promote the growth and seeding ability of the remaining trees. Regeneration then occurs naturally with seeds from the remaining trees. Once the regeneration has been well established, the remaining mature trees are removed in the final cut.
The Selection System
In the selection system individual trees or small groups of trees in a stand are cut every 10 to 20 years.
Red maple regenerating under the selection system.
The selection system leaves a largely intact canopy suited for shade-tolerant trees, those species capable of germinating and developing to maturity in the shade of larger trees. This is a form of “uneven-aged” management. The trees comprising the stand represent a range of ages, and usually do not constitute distinct age cohorts. This system imitates the natural regeneration processes occurring in stands where major stand-replacing events, such as wildfires and windstorms, are relatively infrequent and trees die individually or in small patches.
The selection system is used most often in the Great Lakes-St. Lawrence forest region with species that can grow in shade or semi-shade. These include sugar maple, beech, yellow birch and eastern hemlock.
As with the shelterwood system, trees are marked for cutting or retention before the harvesting begins. Some trees are retained for:
- genetic purposes
- the provision of food for wildlife (“mast” trees)
- the provision of habitat or structure
- aesthetic reasons
Regeneration takes place naturally from stump or root sprouts or from seed from the remaining forest.
Helms, J.A., ed., 1998. The Dictionary of Forestry. Soc. Amer. For., Bethesda, MD, 210 p.