Silvicultural Systems

Silviculture has been defined as “the art and science of controlling the establishment, growth, composition, and quality of forest vegetation for the full range of forest resource objectives.”

Successful silviculture depends on clearly defined management objectives. However, silviculture is often confused with managing stands and forests purely for timber.  The term silviculture is also used to manage forests for wildlife, water, recreation, aesthetics, or any combination of these or other forest uses.

There are many silvicultural systems available for managing forests depending on the management objectives. These systems were developed in central Europe over several centuries as man started to manage forest stands in order to regenerate useful species in a way that would ensure some long-term sustainability.

There are basically two types of silvicultural system used in Irish forests: the clearfelling system and the continuous cover system.

CLEARFELLING SYSTEM

Most forest stands in Ireland are managed under the clearfelling system where the entire forest stand is felled at maturity, the timber removed and the area replanted. This system mimics the natural forest cycle where storms or fire devastate forest stands which are eventually regenerated by seed from trees in the surrounding area. Clearfelling and replanting has been widely adopted in Ireland for a number of reasons:

• The afforestation of the country following centuries of exploitation meant that planting forests created uniform even-aged crops of trees that require harvesting at the same time.
• Afforestation for many decades was confined to wet and exposed land which was only suited to conifer species. Conifers are shallow rooting and coupled with restricted rooting depth due to high watertables, many forest stands are growing on sites vulnerable to windthrow.
• Ireland has a very windy climate which is prone to experiencing catastrophic storms, usually the end of hurricanes, occurring at approx 10-15 year intervals. Opening holes in the forest canopy for regeneration exposes stands to the risk of windthrow.
• Natural regeneration of stands is unpredictable and sporadic and with strong vegetation competition there is no certainty of when and if the felled trees would regenerate.
  

  Clearfelled site ready for replanting ( photo A. Pfeifer)

Windthrow is the greatest threat to Irish forests and is perhaps the main reason why foresters have to date chosen the clearfelling system.  The effects of windthrow can be devastating and can significantly reduce the timber value of a crop through stem shattering and breakage; also harvesting costs are greatly increased due to tangling of stems and upturned roots.

Through trial and error it became evident that the clearfelling system was the most appropriate system for the forest conditions that occurred over large parts of the country. It also met the main management objective of producing commercial timber in a predicable and sustainable way.  However, as the nature of the forest estate changes with the planting of better land and a higher percentage of more wind firm broadleaves, and the with a wider range of goods and services that are expected from forests today e.g. – amenity, nature conservation, landscape enhancement, etc.  there is a need to use other silvicultural systems such as continuous cover systems that can deliver these objectives along with timber production.

CONTINUOUS COVER SYSTEMS

Continuous cover systems, sometimes known as shelterwood systems or continuous cover forestry, are the classical silvicultural systems that are used in central Europe. These systems maintain a continuous forest cover with trees being harvested when they reach the required sizes. Natural regeneration of stands is encouraged through the careful felling and opening of the canopy to allow sufficient light to the forest floor to allow seedlings from the overhead trees to develop. Over time, the older trees are removed allowing the young trees to reach the upper canopy. By this method a multi-storied and multi-aged stand structure is formed and a continuous forest cover is maintained in perpetuity. This is a close to nature approach to managing forests as the forest ecosystem remains intact and is increasingly being favoured as an alternative to the clearfelling system, particularly for broadleaves.

Continuous cover stand, Enniskerry, Co Wicklow (photo A. Pfeifer)

In Ireland we have little

experience with continuous cover systems and there is still much to learn. However, foresters are experimenting with these systems and assistance is being sought from European experts who are helping to advise on the intricacies of these systems.

Today, the area of conifer forest being managed under continuous cover is limited (approx 2,000ha). The transformation of even aged stands to an uneven aged structure takes time and it will still be many years before these can be seen in the Irish countryside.

Knowing which species and sites are suited to continuous cover systems is a key factor to their successful introduction and use on a wider scale. Opportunities to manage broadleaves under continuous cover systems, however, are probably greater than conifers due to their deeper rooting systems resulting in greater wind firmness and the fact that they are grown on better drained soils. Many of the existing native broadleaved woodlands have been essentially managed on continuous cover, although they have been seriously exploited in the past, but they have still retained their forest cover.

Continuous cover systems are particularly suited to managing areas for biodiversity and nature conservation as the forest habitat remains intact. They are also suited to recreation areas and forest parks where forest cover is required to preserve the amenity.

The implementation of continuous cover systems requires a high degree of skill in managing the forest, particularly in managing the regeneration. Forest operations tend to be carried out a little and often rather than in distinct phases such as under the clearfelling system e.g. establishment, thinning and final harvesting. When fully operational, continuous cover systems will provide a continuous flow of timber and therefore a regular income to the forest owner, unlike the clearfelling system where the main returns come at the end of the rotation.

While we have still to gain experience with continuous cover systems and the more complex techniques of management required, a good start has been made. The organisation ProSilva Ireland was founded in 2000 in order to develop and promote alternatives to clear-felling in Irish forestry. There are enthusiastic advocates of these systems and forest stands are being managed by both Coillte and private owners under continuous cover. It is unlikely that continuous cover systems will entirely replace clearfelling but they provide the forest manager/owner with another tool to achieve the management objectives for the forest.

Further information is available from the Coford information note : Continuous Cover Forestry