Competitive Exams: Forestry: Silviculture
Silviculture is the art and science of controlling the establishment, growth, composition, health, and quality of forests to meet diverse needs and values of the many landowners, societies and cultures.
Forest regeneration is the act of renewing tree cover by establishing young trees naturally or artificially, generally promptly after the previous stand or forest has been removed. The method, species, and density are chosen to meet the goal of the landowner. Forest regeneration includes practices such as changes in tree plant density through human-assisted natural regeneration, enrichment planting, reduced grazing of forested savannas, and changes in tree provenances/genetics or tree species. “Human-assisted natural regeneration” means establishment of a forest age class from natural seeding or sprouting after harvesting through selection cutting, shelter (or seed-tree) harvest, soil preparation, or restricting the size of a clear-cut stand to secure natural regeneration from surrounding trees. “Enrichment planting” means increasing the planting density (i.e.. the numbers of plants per hectare) in an already growing forest stand."
Silvicultural regeneration methods combine both the harvest of the timber on the stand and re-establishment of the forest. The proper practice of sustainable forestry should mitigate the potential negative impacts, but all harvest methods will have some impacts on the land and residual stand. The practice of sustainable forestry limits the impacts such that the values of the forest are maintained in-perpetuity.
There are five different regeneration methods:
- Single-tree selection-The single-tree selection method is an uneven-aged regeneration method most suitable when shade tolerant species regeneration is desired. It is typical for older and diseased species to be removed, thus thining the stand and allowing for younger healthy trees to grow. Single-tree selection can be very difficult to implement in dense or sensitive stands and residual stand damage can occur.
- Group selection-The group selection method is an uneven-aged regeneration method that can be used when mid-tolerant species regeneration is desired. The group selection method can still result in residual stand damage in dense stands, however directional falling can minimize the damage. Additionally, foresters can select across the range of diameter classes in the stand and maintain a mosaic of age and diameter classes.
- Clearcut-An even-aged regeneration method that can employ either natural or artificial regeneration. Clear cutting can be biologically appropriate with species that typically regenerate from stand replacing fires, such as lodgepole pine (Pinus contorta). Alternatively, clearcutting can change the dominating species on a stand with the introduction of non-native and invasive species as was shown at the Blodgett Experimental Forest near Georgetown California. Additionally, clearcutting can prolong slash decomposition, expose soil to erosion, impact visual appeal of a landscape and remove essential wildlife habitat. It is particularly useful in regeneration of tree species such as Douglas-fir which is shade intolerant.
- Seed-tree-An even-aged regeneration method that retains widely spaced residual trees in order to provide uniform seed dispersal across a harvested area. In the seed-tree method, 2 − 12 seed trees per acre (5 − 30/ha) are left standing in order to regenerate the forest. They will be retained until regeneration has become established at which point they may be removed. It may not always be economically viable or biologically desirable to re-enter the stand to remove the remaining seed trees. Seed tree cuts can also be viewed as a clearcut with natural regeneration and can also have all of the problems associated with clearcutting. This method is most suited for light seeded species and those not prone to windthrow.
- Shelterwood-A regeneration method that removes trees in a series of three harvests:
- Preparatory cut
- Establishment cut
- Removal cut.
- The method's objective is establish new forest reproduction under the shelter of the retained trees. Unlike the seed tree method, residual trees alter understory environmental conditions (i.e.. Sunlight, temperature, and moisture) that influence tree seedling growth.
- Intermediate stand treatments
- Release treatments
- Weeding: A treatment implemented during a stand's seedling stage which removes or reduces herbaceous or woody shrub competition.
- Cleaning: Release of select saplings from competition by overtopping trees of a comparable age. The treatment favors trees of a desired species and stem quality.
- Liberation Cutting: A treatment that releases tree seedling or saplings by removing older overtopping trees.
The goal of thinning is to control the amount and distribution of available growing space. By altering stand density, foresters can influence the growth, quality, and health of residual trees. It also provides an opportunity to capture mortality and cull the commercially less desirable, usually smaller and malformed, trees. Unlike regeneration treatments, thinnings are not intended to establish a new tree crop or create permanent canopy openings.
Common thinning methods:
- Low Thinning (thinning from below or German thinning)
- Crown Thinning (thinning from above or French method)
- Selection Thinning (thinning of dominants or Borggreve method)
- Mechanical Thinning (row thinning or geometric thinning)
- Free Thinning
- Ecological thinning is where the primary aim of forest thinning is to increase growth of selected trees, favoring development of wildlife habitat (such as hollows) rather than focusing on increased timber yields. Ecological thinning can be considered a new approach to landscape restoration for some types of eucalypt forests and woodlands in Australia.
Pruning, as a silvicultural practice, refers to the removal of the lower branches of the young trees so clear knot free wood can subsequently grow over the branch stubs. Clear knot-free lumber has a higher value. Pruning has been extensively carried out in the Radiata pine plantations of New Zealand and Chile, however the development of Finger joint technology in the production of lumber and mouldings has led to many forestry companies reconsidering their pruning practices.